A Historical Review of the Land Subsidence Phenomena Interaction with Flooding, Land Use Changes, and Storms at the East Thessaly Basin—Insights from InSAR Data

Abstract

The Thessaly Plain, Greece’s largest alluvial basin, has undergone significant geological, hydrological, and anthropogenic transformations. This study synthesises historical records, geological and hydrogeological studies to assess the evolution of the East Thessaly Plain, focusing on land use changes, groundwater management, and environmental challenges. Intensive agricultural practices, particularly from the 1970s onward, have led to groundwater overexploitation, land subsidence, and declining water quality. The overexploitation of the aquifers, exacerbated by extensive irrigation and inefficient water management, has resulted in critical groundwater shortages and widespread subsidence, particularly in the Larissa–Karla and Titarisios Cone systems. Additionally, recent extreme weather events, including Medicane Daniel (2023) and Medicane Ianos (2020), have highlighted the region’s vulnerability to hydrological hazards, with extensive flooding affecting urban and agricultural areas. The re-emergence of Lake Karla as a flood retention area underscores the unintended consequences of past drainage efforts. Remote sensing, geodetic surveys, and historical records have been examined to assess the interplay between groundwater withdrawals, land subsidence, and flood risks.

1. Introduction

The Thessaly Plain, one of Greece’s most extensive alluvial basins, has undergone complex geological, hydrological, and anthropogenic transformations throughout its history and has been shaped by dynamic geomorphological and climatic factors that have influenced land subsidence patterns, agricultural practices, and water resource management over millennia. Greece’s primary sector relies heavily on the Thessaly Plain, primarily through land farming, since it is the nation’s second-largest producer of grains (~25% of Greece’s total agricultural production and 5% of the country’s GDP). Even though 44.9% of the territory of the administrative region of Thessaly is mountainous, 17.1% semi-mountainous and only 36% lowland, the name Thessaly is closely associated with the synonymous plain [1,2,3]. The region is divided into East and West Thessaly (Figure 1).

Natural processes and human interventions, including drainage projects, flood control measures, and the extensive use of groundwater resources for irrigation, have had profound implications on the region’s aquifer systems, leading to overexploitation, land subsidence, and declining water quality. The economic and social development of Thessaly has historically relied on agriculture, with land use patterns evolving alongside shifts in water availability. Overexploitation of the aquifers in the Thessaly region has been observed throughout the years. These overexploitation phenomena have been associated with the transition from a period of mild exploitation of groundwater resources (1970) to a period of progressive intensification which culminated in the 90s [1]. Irrigated crops and especially wheat cultivation experienced an increasing trend over time (1978–2010) corresponding to an increase in production and the need for water resources [2]. During that period, the region’s agricultural sector transitioned from traditional farming to modern, industrial-scale irrigation, significantly impacting both surface and groundwater resources.

This unsustainable exploitation of the groundwater reserves has resulted in land subsidence and the appearance of ground fissures in several parts of Thessaly, impacting both infrastructures and residential areas. Studies utilising geodetic techniques, satellite imagery, and in situ observations have revealed significant vertical displacements, mostly linked to groundwater over-extraction. In recent decades, Thessaly has also faced increasing challenges related to extreme weather events, including storms and floods, exacerbated by climate change. The devastating impacts of recent storms, such as Cyclone Daniel in 2023, have underscored the region’s vulnerability to hydrological hazards.

This paper synthesises the existing literature, historical records, and recent scientific studies to provide a comprehensive analysis of the historical, geological, hydrographic, and socio-economic evolution of the Thessaly Region, with a particular focus on the Eastern Thessaly Plain.

2. Land Use Evolution of the East Thessaly Plain

In prehistoric times, an extensive lake, known as Voivida, existed in the East Thessaly Plain, which covered a significant portion of the basin (Figure 2). This lake was created 500,000 years ago and covered the southeastern part of the Plain of Thessaly [3]. The water height in the Plain of Thessaly, and specifically in Lake Voivida, fluctuated over time, resulting in extensive areas being available for cultivation and the establishment of human settlement in lower regions.

Figure 2. The red dots represent the Neolithic settlements around Karla Lake. The blue line is the suggested lake level according to Grundmann [4]. The purple colour is the recreation of the suggested lake extent according to the study by Alexakis [5]. The light green colour represents the Larissa Plain. Reprinted from Journal of Archaeological Science, Vol. 38, Issue 1, Dimitrios Alexakis, Apostolos Sarris, Theodoros Astaras, Konstantinos Albanakis, Integrated GIS, remote sensing and geomorphologic approaches for the reconstruction of the landscape habitation of Thessaly during the Neolithic period, pp. 89–100, Copyright ©2011, with permission from Elsevier.

Figure 2. The red dots represent the Neolithic settlements around Karla Lake. The blue line is the suggested lake level according to Grundmann [4]. The purple colour is the recreation of the suggested lake extent according to the study by Alexakis [5]. The light green colour represents the Larissa Plain. Reprinted from Journal of Archaeological Science, Vol. 38, Issue 1, Dimitrios Alexakis, Apostolos Sarris, Theodoros Astaras, Konstantinos Albanakis, Integrated GIS, remote sensing and geomorphologic approaches for the reconstruction of the landscape habitation of Thessaly during the Neolithic period, pp. 89–100, Copyright ©2011, with permission from Elsevier.

Multiple historians, such as Herodotus, Apollodoros and Stravonas, have mentioned that a few millennia ago, Olympus and Ossa formed a single mountain range. The waters could not find an outlet to the sea, making the Thessalian basin a vast lake [6]. Excavations in eastern Thessaly have revealed settlements from the Middle to the Late Neolithic and Bronze Age—but also from later eras, namely the Geometric, Archaic and Classical times until the late Roman and Byzantine times. The first tools and animal remains of the Middle Palaeolithic era were found on the banks of the Pineios in 1958 near Larissa, dating from 50,000 to 30,000 BC. From 30,000 BC, remnants of human activity in Thessaly reappeared after the end of glaciation, during the Mesolithic Period in the Cave of Theopetra in the Prefecture of Trikala, which is a human settlement that existed from the Palaeolithic Age until the Neolithic [6].

In the plain, indications of prehistoric human settlements during the Neolithic era (7000–1700 B.C.) have been found in hundreds of small hills, known as “magoules” [3]. Some of the “Magoules” of the Neolithic Period continued to be inhabited until the Bronze Age (4000–1100 BC), while in other cases new ones were created. Indications of permanent establishments and grain cultivation have also been found at Argissa, Larissa (charred wheat seeds), dated 7000 BC [6]. Excavations carried out from 1889 to 1906 revealed multiple Neolithic settlements, located in the lowlands, extending from the Valley of Tempi and the passage of Elassona-Meluna to the Pagasetic Gulf [7]. These settlements were constructed along an imaginary line, which likely represents the old shores of Lake Voivida (or Karla) (Figure 2) [4]. One of these settlements is located in Palaioskala, on the east shore of the old Lake Boibida extending to the foothill of Mavrovounio mountain, dating between the 5th and 4th millennium B.C. [3]. These findings are verified by ancient literary sources, accounts by 18th-century travellers and geological observations [8,9,10,11].

Settlements from later eras have also been located northeast of the old lake, namely Thermokipia (Neolithic, 7th–6th millennium B.C.) and Tsiggenia (Bronze Age, 1700–1600 B.C.). Tombs dating back to the Mycenaean era (1750 to 1050 B.C.) have also been found. From the Archaic (800 B.C.) to the late Roman period (284–305 A.D.), agriculture was a key pillar of the economy in the region, which depended on agricultural production and cattle breeding [12]. Evidence of this is farming complexes, such as the one excavated in Tserli, from the 2nd century B.C. (Hellenistic period) [3]. Thessaly was of particular importance during the Middle Ages, at a time when agriculture was the main, if not the only, economic factor [3]. During that period, Thessaly continued to depend economically on farming and animal breeding, which was performed under the control of rich Roman families living in big cities [13]. The grain production was increased, and large quantities were exported [14]. It should be mentioned that the location of the settlements dating back before and during the Middle Ages indicates that these areas were not completely covered by water but were accessible.

Changes in the hydrographic network and geomorphological observations suggest that there were periods when extensive areas of land were available for cultivation and settlement establishment, while in other periods floods or rising waters limited the use of the area [8,9,10,11]. This dynamic situation explains the location of settlements of different eras in areas that were once covered by water. This hypothesis is also supported by satellite imagery and ground-based studies since 1984, highlighting the possibility of significant land use despite the presence of water bodies in the area [8,9,10,11]. Even from the post-Byzantine period, a lakeside settlement of the 11th–12th century which extends to the hills of Mavrovouni has been found [3].

During the Turkish occupation (1371–1822 A.D.), important agricultural and livestock development and trade centres were developed in Thessaly, mainly in the mountainous areas, which had privileges of autonomous activity (Ambelakia, Tsaritsani, Rapsani, Pelion, Kastania, Argithea, etc.) but also in lowland areas (Tyrnavos, Trikala, Zarko, etc.) [6]. From 1881 to 1930, the incorporation of Thessaly into Greek territory was accompanied by efforts to exploit the Plain of Thessaly to ensure food self-sufficiency. Political disagreements significantly affected the course of development of the region, with the first attempt at expropriation and land reclamation projects failing due to lack of funding and political instability. The study of the Italian engineer Nobile in 1913 was the first integrated approach to solving water problems, but geopolitical developments and wars delayed the implementation of the proposed projects [3].

Until the late 19th century, the eastern Thessaly Plain, and more specifically, the Larissa Plain, were sparsely populated, with marshlands occupying the majority. Seasonal floods took place in large areas and did not allow their exploitation, limiting their use for grazing. The gradual influx of new populations, the expropriation of the Turkish estates and the drainage, flood protection and irrigation works carried out before and after the Second World War drastically changed the situation [15]. Nearly all of the lowlands were used for cultivation when the Larissa Plain was drained, and only Lake Karla and Lake Nessonis were mainly maintained [16] (Figure S1). The former is located southeast of Larissa, near the northern slopes of Pelion, on the border of the prefectures of Larissa and Magnesia. Lake Karla was used for fishing, irrigation and cultivation purposes. The establishment of the Public Power Corporation (PPC) in 1950 and the hydroelectric projects in Tavropos further boosted agricultural and industrial development [17].

In the 1970s there was a transition from a period of mild exploitation of groundwater resources to a period of progressive intensification, which resulted in the intensification of groundwater exploitation, exceeding the limits of the system’s endurance, culminating in the 90s. During that period the Greek agricultural sector was taking its first steps towards industrialisation and facing European competition. At the end of August 1962, Lake Karla was drained through an 11 km long tunnel with a design capacity of 8.5 m³/s (22 million m³ per month), which channelled the lake’s water into the Pagasetic Gulf. The decision to drain Lake Karla was also influenced by the presence of swamp areas, which resulted in a strong presence of insects and flooding of the nearby farms. The reclaimed area (approximately 324 km²) was given to farmers (Figure 3). However, drying the lake impacted the local communities, ecosystem, and microclimate of the area. The fish and birds disappeared, and the underground water aquifer withdrew due to overexploitation for irrigation purposes. Thus, in 2010, an effort began to restore the lake, which was finalised in October 2018 [18,19]. It should be noted that prior to the lake’s drainage, the majority of the water needs for the surrounding areas were covered by the lake’s reservoir. Nonetheless, limited boreholes did operate in the artesian aquifers.

Another major milestone in the 2000s was the completion of the Smokovo Reservoir in 2003, which was the first major multipurpose project in the Pineios basin. The reservoir contributes to the irrigation of about 1012 km², the water supply of 55,000 inhabitants, the production of hydroelectric power and the flood protection of the area. In addition, it acts as a water reserve for periods of water scarcity and supports aquifers that were burdened by overpumping. Despite the completion of this project, Thessaly continues to face challenges in water resource management, as many of the planned projects remained unexecuted, limiting the potential for sustainable development in the region [17]. At the same time, smaller water storage projects were constructed, although the absence of large new reservoirs limited Thessaly’s ability to effectively manage floods and droughts, as well as to restore aquifers that are threatened [17]. From 1978 until 2010, durum cereals showed an increasing trend, while the cultivation of other cereals (barley, wheat, oats, etc., mostly dry lands) showed a significant decrease. For the same period, maize and olive crops show an increasing trend, while for vineyards there is a 30% decrease in the area under cultivation [20].

Figure 3. Photo: “Karla Lake drained in 1962”. Photographer: Takis Tloupas. Vania Tloupa Archive. Photograph obtained from Time Machine, https://www.mixanitouxronou.gr/i-ikona-pou-thimizi-erimo-itan-o-defteros-megaliteros-idroviotopos-stin-ellada-apoxiranthike-to-1962-prokalontas-ikologiki-katastrofi-simera-ginete-prospathia-gia-anadimiourgia-tis-limnis-foto-vi/ (accessed on 9 February 2025) [21].

Figure 3. Photo: “Karla Lake drained in 1962”. Photographer: Takis Tloupas. Vania Tloupa Archive. Photograph obtained from Time Machine, https://www.mixanitouxronou.gr/i-ikona-pou-thimizi-erimo-itan-o-defteros-megaliteros-idroviotopos-stin-ellada-apoxiranthike-to-1962-prokalontas-ikologiki-katastrofi-simera-ginete-prospathia-gia-anadimiourgia-tis-limnis-foto-vi/ (accessed on 9 February 2025) [21].

In recent years, arable crops have dominated the plain, with cotton overwhelmingly predominant, although with declining trends in recent years in favour of corn and cereals (Figure 4). Wheat cultivation has been limited mainly to less irrigated areas and has been significantly decreased. The once-thriving tobacco cultivation has been almost abandoned [15]. In 2020, 79.6% of the total cultivated and fallow lands in the Region of Thessaly were arable crops. Trees occupied 13.9%, while horticulture land and vines occupied smaller percentages (~1%). (Figure 5). The exploitation for the Prefecture of Larissa is similar to that of Thessaly, with crops on arable land constituting the largest portion, covering 79% of the total land, followed by land under trees (compact plantations) at 14%. On the other hand, concerning the total irrigated crops in the Prefecture of Larissa, the majority of them are again crops on arable land (79%), followed by land under trees (compact plantations) (17%). Horticulture land and vines comprised 2% each of the total exploitation, respectively.

These trends in agricultural land use highlight the ongoing transformation of the Thessaly Basin, driven by both historical shifts and modern economic pressures. While past drainage and irrigation projects expanded cultivable land, recent fluctuations in crop distribution reflect changing water availability and farming practices. Researchers expect continued urban and irrigated land expansion until 2030 [22].

Figure 4. The River Basin District of Thessaly and main land uses from the Coordination of Information on the Environment (CORINE) Land Cover 2018. Reproduced from Alamanos et al. (2022), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) [23].

Figure 4. The River Basin District of Thessaly and main land uses from the Coordination of Information on the Environment (CORINE) Land Cover 2018. Reproduced from Alamanos et al. (2022), licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) [23].

Figure 5. Land use (%) of the Prefecture of Larissa for the (a) total cultivated and fallow lands and the (b) total irrigated crops.

Figure 5. Land use (%) of the Prefecture of Larissa for the (a) total cultivated and fallow lands and the (b) total irrigated crops.

3. Hydrographic Network

The Thessaly Plain comprises the biggest alluvial basin of Greece and belongs to the water district EL08, which occupies approximately 13,141 km² and has an average elevation of 427 m [24]. The Thessaly water district is mainly supplied with surface water from the Pineios River, the longest river in Greece. Almost all of Pineios rivers and streams originate from the mountainous Thessaly and flow into the Aegean Sea. Therefore, the entire mountainous Thessaly can be considered as Pineios River’s catchment area since its geographical boundaries almost coincide with the watershed [25,26] (Figure 6). The average annual runoff of the Pineios River is estimated at 3.97 × 10⁶ m³ with the largest volume of water (82.01%) flowing during winter and spring [25]. The Pineios River basin has an area of 11,062 km² and occupies 81.94% of the Thessaly water district [24] (Figure S2). In addition to the Pineios River, in the eastern plain, the second biggest river is Titarisios [24]. Additional information about the evolution of the present-day hydrographic network of the Pineios River, as well as the division of the Thessaly water basin (EL08) according to the Water Management Plans, can be found in the Supplementary Material.

4. Geological Setting

4.1. Wider Study Area

The geological structure and evolution of the wider region of Thessaly have been studied extensively in the past by multiple researchers [8,9,10,11,25,27,28,29,30,31,32,33,34,35,36,37,38,39]. In the context of the division of Greece into alpine zones, Thessaly belongs to the Pelagonic zone [36]. The main contributing factor to the formation of the geomorphology in Thessaly is the Alpine orogeny, which created the mountainous complex [40]. The mountains surrounding the Thessaly basin originate from a series of compressive tectonic events, the last of which occurred from the Eocene to the Middle Miocene. Due to the vertical movements of the tectonic blocks, tectonic trenches and mountains were simultaneously created. These movements caused erosion phenomena of the rising mountains and deposition in the neighbouring submerged trench during the Quaternary [36].

The evolution of Thessaly’s geological formations during the Neogene and Quaternary was complex. Lake sediments of the Neogene have been deposited on the surface of the hills in the central part of Thessaly. These lake sediments are found at a considerable depth within the tectonic trenches, where they have been covered by thick layers of Quaternary river sediments [41]. These findings reveal the existence of a large lake in the region of Thessaly during the Neogene period, the water level of which might have reached an altitude greater than 300–400 m. Later vertical movements of individual tectonic trenches took place during the Quaternary, and smaller lakes were formed. This explains the deposition of the sediments of the Quaternary (river lakes) on the sediments of the Neogene [24].

It should be noted that Thessaly is divided into the East and West Plains. An Oligo-Miocene sequence occurs only in the western region, while a Pliocene sequence does not occur at all and vice versa [33]. The existence or absence of these sequences is attributed to paleogeographical reasons, as well as the geotectonic evolution of Thessaly. The geological map of the broader area of (East) Larissa Plain is presented in Figure 7.

The present-day orographic structure of Thessaly and its surroundings is oriented NW-SE, which is the result, firstly, of the compressional phases that built the Hellenides fold-and-thrust belt during the Oligocene–Miocene and, subsequently, of the Pliocene–Early Pleistocene tectonic inversion associated with a NE-SW crustal extension [31,44,45]. In the Middle Pleistocene, however, the geodynamics of the Aegean Region abruptly changed, being characterised by an approximately N–S stretching direction [34,46,47] and the formation of new, roughly E-W trending structures, like the Tyrnavos Basin and Gonnoi Horst in northern Thessaly [35] or the Almyros and Vasilika Basins [31] in southern Thessaly. Researchers believe the major consequence of the new tectonic regime was the progressive disappearance of a Villafranchian Thessalian Lake, which at that time was likely covering most of the two largest basins (Karditsa and Larissa) and the lower parts of the interposed Central Hills [35]. Coastal deposits of this regional-scale Early–Middle Pleistocene lake have been largely mapped by researchers along the western reliefs of the Larissa Plain [31]. As a result of these tectonic movements, throughout the interior of the Greek arc, including the wider Thessalian region, a tensile tectonic regime takes place [31].

Overall, Thessaly is an intense fault zone of a basin-and-range-like morphology, consisting of many parallel, complex and contrasting, high-angle fault sections that set the limits for the Neogene and Quaternary basins of the wider region [48]. The main directions of the faults located in the Thessaly region are NE-NW (predominant), NW-NE, E-W and N-W [31,33,34] (Figure S3). The most significant tectonic elements that characterise the Thessaly region are the Larissa trench, the Titan–Chalkodoni anticline, and the Trikala–Karditsa basin. Currently, the listed active faults of the Thessaly region can be found in the National Observatory of Athens (NOA) active fault database, which was constructed with material extracted from at least 110 scientific papers published in international journals as of 1972 [49,50]. The latest update can be found online at https://doi.org/10.5281/zenodo.13168947 [51].

Researchers have split the Thessalian faults into two systems, the north and south, because even though they have a similar tectonic regime, the activity of the fault zones of the northern part is significantly more limited than that of the southern part [52] (Figure 8).

4.2. East Thessaly Plain

The stratigraphy of the Eastern Thessaly Plain is known from the results of drilling and geophysical studies [37,38,54,55,56,57] as well as more recent morphotectonic studies [34,39]. These studies have revealed that the Eastern Thessaly Plain represents a tectonic depression of the Quaternary age. Basement rocks (mostly carbonate rocks and schists) are generally found at depths ranging from 50 to 250 m, though deeper tectonic depressions have been identified, where the basement extends to depths of 500–700 m. In certain areas, however, the basement rocks emerge slightly above the ground surface. The alluvial deposits (Al) of the Quaternary cover the majority of the basin and change gradually from coarse-grained (central and western part of the plain) to fine-grained (eastern part of the plain), according to geophysical surveys [38] and the geological maps of IGME [58,59]. These sediments overlay the basement rocks, except along the western margin of the plain, where they overlay a narrow strip of Neogene sediments [60].

The west-southernmost zone is comprised of Neogene conglomeratic sediments, overlying the flysch and limestones. According to the geological maps of IGME [58,59], the Neogene post-alpine sediments consist of normally consolidated and, in some cases, over-consolidated clays, pebbles, and sand. The upper part of the Neogene sediments is composed of fluvioterrestrial formations (Pl-Pt), including red clays, clayey sandy material with pebbles, or coarse-grained material and breccioconglomerates, with visible thickness up to 350 m. Their lower part is formed by lacustrine to brackish deposits (M), containing marls, hard marls, and even travertine limestones with thicknesses up to 100 m [58,59]. Fluvial–lacustrine materials, old talus cones and scree, and torrent terrace materials of the Pleistocene and Holocene are also found locally in the area.

In the northwestern part of eastern Thessaly (Tyrnavos, Platanoulia, Ag. Sofia, Dendra, Vryotopos, Ampelonas, Falani, shown in Figure S1), the main source of coarse-grained deposits is the Titarisios River. The Titarisios basin, also known as the Titarisios cone, is bounded by the Pineios River in the northeast. At the same time, in the northwest it is surrounded mostly by carbonate rocks of the karst aquifer of Tyrnavos–Damasi–Koutsocheros and, in a smaller part, their underlying gneiss of the Pelagonic zone. When the coarse deposits of the Titarisios River reach the Pinieus River, they lose their homogeneity, sink to greater depths and are interrupted by layers of clay and other fine-grained materials [54]. In the western margins of the lowland area, Titarisios river deposits do not exceed 50–70 m [29,61,62]. Further east, towards Pineios, the deposits’ thickness significantly increases and can exceed 200 m [61,62].

The central part (Gyrtoni, Omorfohori, Melissochori, Platykampos, Chalki) and the southeastern part of the Eastern Thessalian Plain (Achillion, Melia, East Platykampos, Modestos, Armeniο, Kypseli, Melissa, Kastri, Eleftherio, Namata, Plasia, area of former Lake Karla, Niki, Chalki, Kileler, Stefanovikio, Rizomylos, etc., shown in Figure S1) is characterised as an alluvial lowland area composed of quaternary sediments, mainly clays, sandy clays and marls, with a large thickness of 200–500 m. More recent geophysical studies [37,55,56,57] in the area from Modestos to Stefanovikio have identified alluvial deposits and fluvial–terrestrial Quaternary sediments reaching 160 and 360 m, respectively. The authors suggested that the fluvio-terrestrial Quaternary sediments consist of conglomerates and marl alternations. The thickness and granulometry of the quaternary deposits covering the Karla–Stefonovikeio Plain up to Larissa varies locally and exceeds 400 m in places. Towards the plain’s eastern limits, the deposits become finer. The hard ground of the quaternary deposits in the area of the old lake up to the boundary Kalamaki–Armenio–Stefanovikio–Rizomylos–Ag. St. George consists of metamorphic rocks and mainly schists and marble. In the area of the former Lake Karla, the marble growing east of the Mavrovouni–Pelion Mountain come into direct contact with the alluvial deposits in a long zone at the NE boundary, from the village of Kato Kalamaki to the village of Kanalia. Marble formations also appear in the area of Kanalia village [63].

Representative cross-sections of the central part of the East Thessaly Plain stratigraphy from Alexopoulos et al. (2024) [37] are presented in Figure 9.

5. Hydrogeological Setting of the East Thessaly Plain

Multiple hydrogeological studies have been conducted for the East Thessaly Plain [24,25,54,61,62,63,64,65,66]. The hydrogeological conditions of the Thessaly water basin have also been defined and evaluated during the assembly of the Water Management Plans [65,66]. In the plans, the East Thessaly water basin was divided into three aquifer systems. These systems are the groundwater system of Larissa–Karla (EL0800110), the groundwater system of the Titarisios cone (EL0800220) and the groundwater system of Taousanis–Kalou Nerou (GR0800130). Characteristics of the three aforementioned systems are presented in

Table 1. Water balance for the aquifer systems [63,66].

	ID	Name	Geology	Type of Aquifer	River Basin	Average Recharge (hm3/yr)	Average Withdrawals (hm3/yr)	Irrigation (hm3/yr)
1	GR0800220	Titarisios cone	Quaternary deposits	Coarse-grained	Pineios	90	72.5	52
2	GR0800110	Larissa–Lake Karla	Quaternary deposits	Coarse-grained	Pineios	60	88.5	83.0
3	GR0800130	Taousanis–Kalou Nerou	Quaternary and Neogene deposits, Cretaceous limestones, Gneiss Schists	Coarse-grained, Karstic	Pineios	40	45	11

.

The authors of the Water Management Plans mentioned that in the Thessaly Plain (west and east), four main types of aquifers develop.

• Unconfined piezometric surface aquifers;
• Confined (under-pressure) aquifers;
• Karst aquifers that develop in carbonate rocks (marble, limestones);
• Fractured rock aquifers (gneiss, shales, ophiolites, flysch).

According to researchers, the first two types of aquifers develop in the alluvial formations of the plain and the Pliopleistocene deposits, possibly also in the Neogene, which locally form the bedrock of modern deposits. The capacity for those aquifers depends on grain size (granulometry) and hydraulic conductivity [63,66]. As far as lowland aquifers are concerned, the large-capacity and practically exploitable aquifer systems are the ones developing in the coarse-grained alluvial deposits.

5.1. Titarisios Cone Groundwater System

The Titarisios cone groundwater system is located in the northern part of the eastern Thessaly basin and belongs to the catchment area of the Pineios River. It covers an area of 310.2 km² and develops in the coarse deposits of the homonymous river [63]. In the southern and eastern parts of the basin, the Pineios River also passes. The Titarrisios River deposited coarse materials (boulders, gravel and sand), creating a huge cone in the NW part of the Thessaly basin, in the area of Tyrnavos–Ampelona. This cone reaches the Omorfochori–Platykampos area, where it is found at great depths. In that area, these coarse deposits are covered by the fine-grained deposit materials of the Pineios River, which were deposited before leaving the basin through the straits of Rodia, mainly in the central part and until the former area of Karla. The coarse-grained aquifers that develop in the quaternary deposits are unconfined aquifers [63]. However, downstream from Titarisios River, in the part where the coarse-grained deposits are covered by fine-grained, the aquifers become successively confined (or semi-confined) [24].

In both the Water Management Plan [63,66] and individual studies [20,67], the quantitative water assessment of the Titarisions basin is classified as critical because the withdrawals exceed the renewable water reserves (Figure 10). According to both the first Water Management Plans (2009–2015) and the first revision (2015–2021), the annually renewable water reserves from infiltrations and lateral transfusions from the Damasi–Titan karst system are estimated to be 90 × 10⁶ m³/year, while approximately 72.5 × 10⁶ m³/year are pumped from the system for water supply and irrigation purposes. It is estimated that approximately 7 × 10⁶ m³ are drawn annually from permanent reserves [63]. In the system of the Titarisios basin, the main water pumps of the municipality of Larissa are located.

5.2. Larissa–Lake Karla Groundwater System

The alluvial basin of the Larissa–Karla system covers an area of 617.9 km², and its groundwater system develops in the eastern part of the Pineios River basin. Near the alluvial supply zones, the alluvial material consists of coarse-grained materials in the areas of Chalki, Armenio, Stefanovikio, Rizomylos and Velestino. Coarse-grained materials are also located at the NE and E margins of the tectonic trench originating from the streams Vagiorema, Kaliakouda, Klima, Begiatiko and Xeria. According to the Water Management Plans [63], the alluvial aquifers developing in the quaternary deposits are either unconfined or semi-confined. In the alluvial basin, two aquifer systems are identified, particularly one system in the narrow area around the village of Chalki and a second one in the wider area of the villages of Armenio, Stefanovikio, Rizomylos, and Velestino. In both of these areas, aquifers develop within sand and gravel deposits with admixtures of clay materials, since they are closest to the cones and scree from the rivers and streams. In the lowland part, there is an aquifer in the area of Armenio–Stefanovikio–Rizomylos and Velestino. In that aquifer, a large number of boreholes are located. In the rest of the lowlands, no significant aquifers develop in modern deposits, except for some small, sparse, coarse-grained aquifers under the existing clay layers. A 1974 geophysical research [38] in the northern part of the Karla basin identified the existence of a large thick clay layer in the section of the Platykambos area and Omorfochori, which does not allow lateral recharge from the upstream, rich aquifer of Titarisios (GR0800220), resulting in no significant underground aquifer development. Moreover, the supply from precipitation is relatively small due to the prevalence of finer materials in the upper surface layers. In the remaining section, the alluvial aquifers are laterally recharged from the aquifers of the Pliocene hills and cones formed by streams such as Kousbasaniotis, Xerias, etc. Hydraulic communication also does not exist between the large karst aquifers that develop in the Mavrovouni–Pelion marble and the alluvial aquifers. In the area of Armenio, Sotiriou and Stefanovikio, prior to the drilling of a large number of boreholes for the exploitation of the alluvial aquifer, springs existed. These springs are now completely dried up [63,66]. At the same time, the alluvial aquifers that develop in the wider lowland area of Karla are overexploited. In the wide area of Velestino, Rizomylos and Stefanovikio, a great number of irrigation boreholes have been drilled.

The quantitative water assessment of the Larissa–Karla system, in both the Water Management Plan [63,66] and individual studies [20,67], is also classified as critical due to the withdrawals exceeding the renewable water reserves (Figure 10). The annual renewable water reserves of the system are estimated at 60 × 10⁶ m³, while withdrawals through boreholes reach 87 × 10⁶ m³/year. An annual quantity of 27 × 10⁶ m³/year is withdrawn from permanent reserves.

5.3. Taousanis–Kalou Nerou Groundwater System

The groundwater system of Taousanis–Kalou Nerou (EL0800130) covers an area of 922 km² and is located between the two Thessaly Plains, hydrogeologically separating them through a series of 100–200 m altitude Pliocene hills. These hills are composed of lake-origin limestones, marl, sandstones, conglomerates and sands [63]. The Pliocene deposits’ bedrock is generally ophiolites, schists, gneisses, and marble found at depths of 100–300 m. In places, this bedrock emerges within the hills. Among the Pliocene hills, in the northwestern part of the system, alluvial deposits are found, forming the plains of Mavrovounio and Koutsochero–Kastro–Mandra. The bedrock of the Koutsocheros–Mandra Plain is marble. The aquifers in the system develop within the alluviums of the above-mentioned small plains and the sand-sandstone and conglomerate layers of the Pliocene hills and in the local carbonate formations. Aquifers are heterogeneous and are not continuous, neither in the horizontal nor the vertical direction [63]. For the above reasons, aquifers are developed only locally and in the high permeability layers. Due to the above conditions, recharge of the aquifers is extremely difficult. The main recharge is made by the direct infiltration of rainwater, which, due to the geological structure, is very small. In the NW boundary of the groundwater system, there is an interdependency with the aquifer of the Damasi–Titan karst system (GR0800070). In the limestones of Myra–Kalo Nero, karst aquifers develop, which are discharged through small springs. An underground lateral discharge of the aquifers takes place mainly to the north, towards the Plain of Koutsocheros and the Pineios River (in the area of Amygdalia–Larissa) [63].

In the Water Management Plan [66], the quantitative water assessment of the Taousanis–Kalou Nerou groundwater system (EL0800130) is classified as critical because the withdrawals exceed the renewable water reserves (Figure 10). The annually renewable water reserves are 40 × 10⁶ m³/year, while approximately 45 × 10⁶ m³/year are pumped from the system. It is estimated that approximately 11 × 10⁶ m³ are drawn annually from permanent reserves. The monitoring system in the specific groundwater system is sparse, and only five boreholes in total exist. The water level measurements, in combination with pumping and system recharge, indicate signs of overpumping of the groundwater system. According to the 1st revision of the Water Management Plans (2015–2021), there is a trend towards stabilisation of the underground water level [66].

6. Over-Pumping Activities in East Thessaly

Over the last few decades, Thessaly has been experiencing the effects of a complex water problem, which will worsen with the ongoing climate crisis. The rapid agricultural development and the anarchic management of the available water resources from 1970 until today resulted in the depletion and/or withdrawal of surface and groundwater reserves. To irrigate the continuously expanding area, the farmers exploited the underground aquifers, which was unsustainable and subsequently led to their gradual depletion (150–300 m). The lack of irrigation water and the increasing cost of pumping, especially in eastern Thessaly, led either to the abandonment of crop areas or to the replacement of irrigated crops with non-irrigated crops with lower income for producers. The 2021 floods further increased the complexity and graveness of the problem.

The extent of the exploitation pattern can be understood through the result of the SAMY ΙΙ project, led by the Hellenic Survey of Geology and Mineral Exploration (HSGME). During the project, a total of 21,894 boreholes for irrigation purposes were inventoried. The prefectures of Larissa (8411), Karditsa (6312), Trikala (2800) and Magnesia (2594) accounted for 92% of the total water wells of the Thessaly Water District [68].

It should be noted that in the groundwater systems, a groundwater monitoring network DYPIN operated by the Hellenic Survey of Geology and Mineral Exploration (HSGME) [69] and National Water Monitoring Network [70] are used to measure the groundwater level (Figure 11). The datasets obtained from the aforementioned monitoring systems enable the quantitative status characterisation of the water systems. According to the measurements acquired, all three of the aforementioned systems are classified as in a bad quantitative state (Figure 10).

7. Flood History of Thessaly

Thessaly is one of the most flood-prone regions of Greece, with floods being a recurrent phenomenon, the severity and frequency of which have increased during the past several decades. The evolution of global warming and climate change is expected to further increase the occurrence and severity of extreme events such as the Daniel storm. Flooding phenomena in Thessaly were also recorded from ancient times. For example, the ancient geographer Stravon (64 B.C.–24 A.D.) mentioned that the Pineios River had overflowed, and a lake-like field was created [71]. The historical record of floods in Thessaly highlights the region’s long-standing vulnerability to flooding events, which have caused significant damage to infrastructure, agricultural land, and human lives over the centuries. An overview of major flood events from 1540 to 2020, predominantly affecting the eastern Thessalian Plain, is presented in Table 2. These events are often triggered by intense and prolonged rainfall leading to river overflows, such as those of the Pineios River (Figure 12). Simultaneously, due to the serious issues of water shortage, declining water tables, and water quality, water management has largely concentrated on acquiring water, with little attention dedicated to flood prevention and control.

Table 2. Historical and recent floods, mostly in the eastern Thessalian Plain, events from 1540 to 2020 [71,72].

Year	Description
1540	Thirty days of rainfall resulted in the flooding of Damasi, Larissa
1647	Twenty-four-hour rainfall in Trikala and Larissa resulted in flooding with over 800 dead
1684	The flood destroyed part of Larissa, some riverside villages and a large part of arable land.
1729	Larissa and other areas of Thessaly (Trikala, Moscholouri) suffered significant damages from the great flood of Pineios.
1777	The flood affected three districts of Larissa and two small settlements around it. Casualties and destruction of houses as well as grain in the plain were reported.
1804	The Pineios and Koumerkis rivers overflowed, resulting in the inundation of Larissa
1806	Flood in Larissa
1811	Intense and prolonged rain caused Pineios to overflow. People and animals were drowned, and houses and crops were destroyed.
1826	Overflow of the Pineios River causing distractions in Larissa and the surrounding areas
1836	Extensive flooding in Thessaly Plain affected Larissa, drowning a large number of livestock as well as damaging property and infrastructure.
1872	Pineios overflowed after intense and prolonged rainfall that caused flooding in a large part of the Thessaly Plain, with a noteworthy impact on agricultural land
1882	An unprecedented flood hit Larissa when Pineios overflowed. The flood resulted in a large number of flood victims and significant damages.
1883	An intense 48 h of rainfall resulted in 3 human fatalities, 20 destroyed houses, and incalculable damages.
1902	The flood of the Pineios River affecting Larissa and Koutsochero, impacting infrastructure and properties
1903	The flood of the Pineios River affecting Larissa
1907	A long and strong storm resulted in overflow of the Lithaios and Agia Moni rivers, resulting in flooding of the city of Trikala and the wider area. Human casualties ranged from 100 to 300, with crop damage and dead animals also reported.
1908	Flood in Pineios River
1920	The flood in Larissa affected parts of the city near the river
1948	The flood in Larissa affected property and infrastructure
1963	The flood of the Pinions River affected Pineiada and other parts of the Thessalian Plain
1987	Inundation affected the area of Karla Lake, where the water level increased for a few days, damaging agricultural production.
1994	More than 70 houses in about 20 communities were totally destroyed by the flood. More than 200 houses suffered several damages and 90 minor damages. Almost 80 km2 of agricultural land (cotton fields) were flooded.
2018	The flood of the Pineios River affected agricultural land near Koutsochero and other locations
2020	Unprecedented rainfall phenomena affected the whole of Thessaly. The city of Karditsa was flooded, while the village of Metamorfosi was completely submerged in water and mud. Villages in the Palamas and Farsala areas were flooded (Hurricane Ianos).
2023	Medicane Daniel hits Thessaly, causing extended flooding and damages to infrastructure, human, and animal life loss
2023	Two weeks after Medicane Daniel, Storm Elias hit further, causing again overflowing of rivers and widespread floods in the region of Thessaly

Table 2. Historical and recent floods, mostly in the eastern Thessalian Plain, events from 1540 to 2020 [71,72].

Year	Description
1540	Thirty days of rainfall resulted in the flooding of Damasi, Larissa
1647	Twenty-four-hour rainfall in Trikala and Larissa resulted in flooding with over 800 dead
1684	The flood destroyed part of Larissa, some riverside villages and a large part of arable land.
1729	Larissa and other areas of Thessaly (Trikala, Moscholouri) suffered significant damages from the great flood of Pineios.
1777	The flood affected three districts of Larissa and two small settlements around it. Casualties and destruction of houses as well as grain in the plain were reported.
1804	The Pineios and Koumerkis rivers overflowed, resulting in the inundation of Larissa
1806	Flood in Larissa
1811	Intense and prolonged rain caused Pineios to overflow. People and animals were drowned, and houses and crops were destroyed.
1826	Overflow of the Pineios River causing distractions in Larissa and the surrounding areas
1836	Extensive flooding in Thessaly Plain affected Larissa, drowning a large number of livestock as well as damaging property and infrastructure.
1872	Pineios overflowed after intense and prolonged rainfall that caused flooding in a large part of the Thessaly Plain, with a noteworthy impact on agricultural land
1882	An unprecedented flood hit Larissa when Pineios overflowed. The flood resulted in a large number of flood victims and significant damages.
1883	An intense 48 h of rainfall resulted in 3 human fatalities, 20 destroyed houses, and incalculable damages.
1902	The flood of the Pineios River affecting Larissa and Koutsochero, impacting infrastructure and properties
1903	The flood of the Pineios River affecting Larissa
1907	A long and strong storm resulted in overflow of the Lithaios and Agia Moni rivers, resulting in flooding of the city of Trikala and the wider area. Human casualties ranged from 100 to 300, with crop damage and dead animals also reported.
1908	Flood in Pineios River
1920	The flood in Larissa affected parts of the city near the river
1948	The flood in Larissa affected property and infrastructure
1963	The flood of the Pinions River affected Pineiada and other parts of the Thessalian Plain
1987	Inundation affected the area of Karla Lake, where the water level increased for a few days, damaging agricultural production.
1994	More than 70 houses in about 20 communities were totally destroyed by the flood. More than 200 houses suffered several damages and 90 minor damages. Almost 80 km2 of agricultural land (cotton fields) were flooded.
2018	The flood of the Pineios River affected agricultural land near Koutsochero and other locations
2020	Unprecedented rainfall phenomena affected the whole of Thessaly. The city of Karditsa was flooded, while the village of Metamorfosi was completely submerged in water and mud. Villages in the Palamas and Farsala areas were flooded (Hurricane Ianos).
2023	Medicane Daniel hits Thessaly, causing extended flooding and damages to infrastructure, human, and animal life loss
2023	Two weeks after Medicane Daniel, Storm Elias hit further, causing again overflowing of rivers and widespread floods in the region of Thessaly

Flood occurrences in the Pineios River drainage basin in Thessaly have also been examined by Bathrellos et al. (2018) [73] over the period 1979–2010. Statistical analysis revealed that the number of flood events peaked in 1994, with October being the most flood-prone month. While October had the highest number of events, November recorded the largest flooded area. Spatial analysis revealed the clustering of flood events in areas with historical marshes and lakes, particularly in the Karditsa–Enippeas River and Larissa–Karla drainage basins.

Figure 12. Photo: “Larissa Road, a few days after the October 1883 flood”—Photo Gallery Macedonia, Photographer: Leontaridou 1883. This photograph is exhibited in the Municipal Gallery of Larissa—G. I. Katsigras Museum. Photograph obtained from https://www.larissanet.gr/2018/03/26/spanies-fotografies-apo-tin-plimmyra-tou-pineiou-to-1883/ (accessed on 9 February 2025) [74].

Figure 12. Photo: “Larissa Road, a few days after the October 1883 flood”—Photo Gallery Macedonia, Photographer: Leontaridou 1883. This photograph is exhibited in the Municipal Gallery of Larissa—G. I. Katsigras Museum. Photograph obtained from https://www.larissanet.gr/2018/03/26/spanies-fotografies-apo-tin-plimmyra-tou-pineiou-to-1883/ (accessed on 9 February 2025) [74].

An important step towards the assessment and prevention of flood phenomena was performed with the creation of the Flood Risk Management Plans (FRMPs). These plans are based on European Directive 2007/60/EC, adopted by the European Council on 18 September 2007, which focuses on assessing and managing flood risks. This directive aims to reduce the negative consequences of flooding on human health, the environment, cultural heritage, and economic activity. The FRMPs include several stages, such as a preliminary flood risk assessment, identification of areas of potentially significant flood risk, flood hazard maps, and flood risk maps [75].

According to the Risk Management Plans for Eastern Thessaly, multiple parts of municipalities located in the lowlands were declared as areas in a state of emergency due to flooding (

Table 3. Municipalities in Eastern Thessaly that have been declared in a state of emergency due to flooding.

Municipality	Years
Tyrnavos	2016, 2018, 2019, 2021
Larissa	2018
Kileler	2017, 2018
Riga Fereou	2016, 2018, 2020

). The extent of flooded areas for a return period of (a) 50 years, (b) 100 years, and (c) 1000 years according to the Flood Risk Management Plans are presented in Figure 13.

The flooding extent, risk and hazard in both Larissa and Lake Karla have also been assessed by Kypraiou (2012) [6]. According to the research findings, Larissa faces significant flood risks due to the slow-moving flow of the Pineios River, which does not allow for deepening or erosion of its riverbed. Instead, sediment deposition occurs, leading to flooding during heavy rainfall. The study highlights poor river management as a major issue, with key contributing factors including deforestation, urban expansion, illegal land use, industrial and domestic waste discharge, and unregulated interventions such as sand extraction, bridge extensions, and canal constructions. The flood risk assessment showed that the highest flood risks are concentrated along the Pineios River, affecting both urban settlements and transportation networks. Larissa itself was classified as having medium to high flood risk, with nearby towns also at moderate risk. Critical infrastructure, including roads and railways, crossed through high and medium flood risk areas. During extreme flood events, agricultural lands around Larissa are primarily affected. The lack of designated buffer zones around the river further exacerbates the problem, as industries and settlements continue to encroach upon flood-prone areas. Regarding Lake Karla, the lake’s surface area varied significantly over time due to rainfall, Pineios River overflow, and underground springs, while water losses occurred through evaporation and seepage. The author commented that the flat topography of the lakebed made it highly sensitive to even minor hydrological changes, primarily expanding westward due to the constraints imposed by surrounding mountains to the east.

8. Recent Storm and Flooding Events

Building upon the historical context of flooding in Thessaly, it is evident that the region’s susceptibility to extreme flood events has persisted and even intensified in recent years. While the Flood Risk Management Plans represent a significant step towards mitigating flood risks, the frequency and severity of recent events highlight the evolving challenges posed by climate change and extreme weather phenomena. Notable examples of recent extreme weather phenomena include Medicane (Mediterranean tropical-like cyclone) Ianos in 2020 and Medicane Daniel in 2023, both of which resulted in catastrophic flooding, widespread damage to infrastructure, loss of human lives, and severe impacts on agricultural productivity.

8.1. Medicane Ianos

Medicane Ianos was a rare Mediterranean tropical-like hurricane that impacted Greece on 17–20 September 2020. This rare hurricane-like storm swept across Greece through central Thessaly, hitting mainly areas around the cities of Karditsa and Farsala. It brought high winds reaching 120 km/h, torrential rain and flooding.

Due to this extreme phenomenon, on the 19th of September 2020, the Copernicus Emergency Management Service—Mapping was activated to assess the extent and impact of the phenomenon. The study (EMSR465) focused on areas located in Western Thessaly and, more specifically, in the municipalities of Trikala, Karditsa, Farsala, Palamas, Farkadona, and Sofades. Extensive damages were reported in agricultural land, urban areas of Farsala, Mouzaki and Karditsa cities and also on the road network of the wider area. At least two people died, and two were declared missing. The Fire Service received 630 calls for help and proceeded to 450 rescue operations and 120 floodwater pumping operations in urban areas [76].

According to Lekkas et al. (2020) [77], large swathes of agricultural land and residential areas were inundated due to the storm’s heavy rainfall and river overflow. Floodwaters in some areas reached depths close to one metre. Major disruptions occurred in transportation, including damage to the road network and temporary halts to traffic, particularly on routes connecting Larissa and neighbouring areas. Landslides were also reported, contributing to the challenges in accessing affected regions. The agricultural sector suffered heavily, with significant losses to crops and damage to farmland.

8.2. Medicane Daniel

Three years later, another intense Medicane hit the Thessaly region, Medicane Daniel. On Monday, 4 September 2023, Medicane Daniel moved inland over the Balkans, triggering intense rainfall and thunderstorms. Daniel was an extreme phenomenon in which 3.7 billion m³ of rainwater fell over the area of the Pineios riverbed, of which almost 3 billion m³ fell within 48 h (5 and 6 September) [1,67]. The extreme precipitation phenomena were followed by extensive sudden floods, which resulted in the loss of human lives, livestock, crops, land and assets. Just 20 days later, another phenomenon of great intensity occurred, Cyclone Elias [1].

Storm Daniel also hit Turkey, Bulgaria, and Libya. Extensive damages were caused in the latter, particularly in the coastal cities of Derna and Susah. The storm led to flash floods, severe infrastructure damage and loss of life. Specifically, in Derna, two upstream flood-control dams collapsed, triggering a flash flood which destroyed a significant portion of the city’s buildings, urban infrastructure, and bridges, leaving behind 8.8 million tonnes of debris. A total of 5898 casualties were reported, 8000 people missing, 44,800 displaced individuals and 18,838 houses either damaged or destroyed, with an estimated $1.8 billion in damages. Agricultural losses included the destruction of crops (~162 km²), the loss of topsoil on 4.6% of agricultural land, and the death of 74,363 livestock animals (3.2% of the region’s livestock reserve) [78].

8.2.1. Precipitation

During Medicane Daniel, the most substantial daily rainfall totals were observed in central Greece since 2006. At least two rain gauges in the area recorded over half a metre of rainfall in less than 24 h. Throughout Thessaly, numerous monitoring stations reported receiving between 400 and 600 millimetres of rainfall within the same 24-h timeframe. Rainfall, due to Medicane Daniel, reached up to 47% of the annual average precipitation in just four days. The storm’s intensity was equivalent to a return period of up to 150 years for the Pineios basin [79].

Handelsvereeniging Amsterdam (HVA) estimated the volume of rainfall (in m³) that fell on the various areas within the Pineios River Basin during Medicane Daniel, on the basis of daily rainfall data from the National Observatory of Athens (Institute for Environmental Research). They also estimated the water levels and discharges at the surface water monitoring stations to understand the response of the surface water system [1,67]. The time series indicated that the majority of the monitoring locations demonstrate an almost instantaneous response to rainfall, with very short, high peak discharges (Figure 14).

8.2.2. Flooded Areas

The first assessment of the affected area, by Handelsvereeniging Amsterdam (HVA) [1], showed that on 6 September, around 80 km² were flooded. One day later, on 7 September, less than 60 h after the rainfalls had begun, around 700 km² of land were inundated.

In response to the severe flooding in the Thessaly region, the Copernicus Emergency Mapping Service was activated on the 5 September 2023 (EMSR692) [80]. For the activation, multiple analyses over time to monitor the evolution of the flood extent and its impact on the population and land in key areas of interest were conducted. Using high-resolution satellite data from GeoEye-1, SPOT6, and WorldView-2, repeated assessments from 10 September to 22 September 2023 captured the progressive evolution of water in the flooded areas (Figure 15).

Overall, in the Thessaly region, the event was extended to approximately 859.06 km², and the population affected was 37,950. For eastern Thessaly, three areas of interest were identified, namely Larissa (AO3), Stefanovikio (AO4), and Kalamaki (AO7). In Larissa, the flooded area decreased from 151.83 km² on 10 September to 35.25 km² by 19 September, while in Stefanovikio, it declined from 126.14 km² on 10 September to 84.12 km² by 19 September. These analyses also highlighted the extensive damage to agricultural lands. Approximately 125 km² and 95 km² of arable land were affected in Larissa and Stefanovikio on 19/9, respectively, and 130 km² in Kalamaki on 22/09. The progressive evolution of the flooded area extent and the affected population in the aforementioned areas of interest are presented in

Table 4. The progressive evolution of the flooded area extent and the affected populations in Larissa (AO3), Stefanovikio (AO4) and Kalamaki (AO7).

Area	Date of Analysis	Satellite/Data Used	Flooded Area (km2)	Potentially Affected Population
Larissa (AO3)	10/09	GeoEye-1	151.83	14,000
Larissa (AO3)	12/09	SPOT6	126.28	14,000
Larissa (AO3)	14/09	SPOT6	91.52	14,000
Larissa (AO3)	15/09	SPOT6	86.97	14,000
Larissa (AO3)	17/09	SPOT6	52.60	13,000
Larissa (AO3)	19/09	WorldView-2	35.25	12,000
Stefanovikio (AO4)	10/09	GeoEye-1	126.14	800
Stefanovikio (AO4)	12/09	SPOT6	140.08	1000
Stefanovikio (AO4)	14/09	SPOT6	125.42	1000
Stefanovikio (AO4)	17/09	SPOT6	115.80	750
Stefanovikio (AO4)	19/09	WorldView-2	84.12	650
Kalamaki (AO7)	22/09	GeoEye-1	136.15	800

.

Satellite-based flood detection by Leivadiotis et al. (2024) [82] revealed that the most extensive flooding occurred on 10 September, covering an area of 516.52 km² with maximum water depths reaching up to 9.49 m, especially near riverbanks. The damage assessment indicated that irrigated lands were the most affected, with over 95% of this land type inundated at the peak of the flood.

Buildings in the area were also affected by the flooding. Depending on the location of the construction of these buildings, the characteristics of their construction, and their exposure to recent catastrophic events, the types of failure patterns observed varied. Buildings located at a short distance from the active bed of rushing rivers experienced stability issues due to erosion and undercutting phenomena [72].

According to the Handelsvereeniging Amsterdam (HVA) report [1], in the Stefanovikio area, Lake Karla was flooded with hundreds of millions of cubic meters (m³) of water (Figure 16). The area around Lake Karla differs from the other sub-catchments of the Pineios River Basin since its water is not discharged into the Pineios River. On the contrary, part of the water of the Pineios River is diverted towards Lake Karla to support agricultural development. The Lake Karla area is a closed basin, where water from the dyke breaches near Gyrtoni accumulated during the passing of the Daniel Storm. As a result, the lake became, temporarily, the largest lake in Greece and covered approximately 150 km² of agricultural land, with 1000 inhabitants being affected. On the 5th of September, the water level of Lake Karla was 46.3 m, increasing approximately 1.2 m in the span of two days. This increase correlates to a volume of approximately 40 million m³. The estimations of the total inundated land outside Lake Karla vary.

On 20 September, Lake Karla was estimated to inundate an area of 125–150 km². The volume of water in the inundated areas (excluding Lake Karla) was estimated at 450–500 million m³, which implied that approximately 30% of the inundations in the Lake Karla sub-catchment originated from the Pineios River. The area Lake Karla covered after the floods was almost equal to the area it occupied before it dried up in 1962 [83]. It may take 1 ½–2 years for the water to be fully discharged and for the area to fully recover. Since the area has no natural outlets, the water levels in the area decreased much more slowly compared to other inundated regions. By the 19th of September, 12 days after the event, the inundated areas were still approximately 100 km² and after storm Elias on the 25th of September, they increased once again [1]. The floodwaters from Storm Daniel in Thessaly were removed through a combination of natural drainage and evaporation, pumping operations, and emergency engineering interventions. In some cases, the authorities created breaches in dykes and embankments and directed the water into lakes, reservoirs, and open fields to speed up drainage. The water from the Lake Karla area can also be removed through the existing tunnel which was constructed initially to dry the lake.

8.2.3. Causes

One of the major causes of the flooding phenomena was the overflow of rivers. The speed of water flow on the riverbed was high, which was a result of the clearance of forest systems in semi-mountainous areas near riverbeds from 1955 to 1965 [67]. These land clearances expanded after the start of the European Union’s grant for cultivated areas. Moreover, some of the newly recovered land was found mainly in the natural overflow zones of the rivers. As a result of the expansion, streams were blocked, lands with steep slopes were levelled, meanders were destroyed, and riverbeds were straightened [67]. Moreover, while in the past, major irrigation projects were scheduled, they were never completed. In addition, the capacity of the existing networks of drains, rivers and streams was not able to discharge the volumes of water from these floods. Therefore, there was hardly any attenuation of the surface runoff, and the river discharges could be characterised as flash floods.

8.3. Storm Elias

From 25 to 29 September 2023, 2 weeks after Storm Daniel, Storm Elias descended on Central Greece. The rainfalls that followed resulted in the overflowing of rivers and widespread floods in the region of Thessaly again.

On the 20th of December 2024, the Copernicus Emergency Management Service Risk and Recovery Mapping was activated to provide the Ministry of Rural Development and Food with a delineation product of the current flooded area for the planning of the necessary recovery measures (EMSN184) [84]. The total flooded area in the Karla Lake region increased from 98.15 km² on 18 December 2023 to 106.97 km² on 19 December 2023. Floodwater depths averaged around 2.3 to 2.4 m, with a maximum depth of 3.8 m near Karla Lake [85]. Three months after the Daniel and Elias storms hit, an area north and northwest of Karla Lake was still flooded.

9. Land Subsidence

The intensification of flooding phenomena in Thessaly, particularly in recent years, not only underscores the region’s vulnerability to extreme weather events but also reveals the interconnected nature of hydrological and geological hazards. The extensive flooding caused by storms such as Daniel and Elias has not only led to immediate damage to infrastructure, agricultural land, and residential buildings but may also have long-term implications on the region’s geotechnical stability. Vice versa, flooding can be more severe in areas formerly affected by geohazards, such as land subsidence, by acting on a terrain already damaged or deformed by the geocatastrophic phenomena.

The areas most affected by flooding, including Larissa, Lake Karla, and the broader Thessalian Plain, have also exhibited notable patterns of land subsidence. Land subsidence patterns have been mapped in both the East and West Thessaly plains. The deformation patterns in the Western Plain have been studied more extensively through both ground truth data and InSAR monitoring [86,87,88,89,90]. At the East Thessaly Plain, the land subsidence phenomena have been observed since 1980, when numerous ground raptures appeared, causing extensive damage to linear infrastructure and settlements [60]. In other countries such as the Netherlands, large-scale subsidence commenced, due to the oxidation of organic soils, around AD 1000, when vast wetlands in the western part of the coastal deltaic plain were reclaimed for agricultural purposes. Frequent river flooding and sea ingressions accompanied the land subsidence, as well as further land loss [91]. However, in Thessaly, no records of such cases have been documented due to the manifestation of different mechanisms.

Multiple studies have been conducted in Eastern Thessaly to examine ground subsidence and the appearance of fissures using various methods (Table S1). These studies employed techniques such as ground observations [60,92,93,94,95,96,97], precise levelling and GPS surveying [60], global navigation satellite system (GNSS) [37,98], and advanced remote sensing methods like PSI (persistent scatterer interferometry) [43,99,100,101], SBAS (small baseline subset) [101], or a mix of the last two [102]. These methods enabled researchers to detect and quantify vertical ground displacements, subsidence rates, and structural deformations across different timeframes and study areas. Most of the studies were conducted in the 2010s.

The study areas covered a wide range of locations within Eastern Thessaly, including Larissa, the Larissa Plain, Lake Karla, Rizomilos, Stefanovikio, Melia, Kastri, Niki Village, and other agricultural zones. The maximum recorded subsidence occurred between Modestos and Stefanovikio, with annual rates up to 22.9 mm/year [37]. In contrast, the minimum displacements were observed in the Larissa city centre, where conditions remained relatively stable with only a gradual subsiding trend from 1992 to 2017 [43,98] (Figure 17). It should be noted that in the majority of the aforementioned areas, co-occurring subsidence and uplift processes take place, such as Larissa, linked to seasonal fluctuations of the groundwater table, have been identified. The swelling and shrinking of expansive clay soils, influenced by seasonal variations in groundwater levels, was identified as a key driver of deformation. Nonetheless, a gradual subsiding trend is observed [100]. In some cases, some rebound phenomena with significantly lower values were observed during high precipitation periods, mainly at the NE of the basin [99].

The authors have attributed these subsidence phenomena and the appearance of ground fissures to several causes, with the most common being sediment compaction due to the overexploitation of groundwater [37,43,60,92,93,98,99,100,101,103]. These phenomena have also been attributed to natural compaction [29], fault movements [100] and interseismic strain accumulation along active fault zones in the specific areas [101,103], as well as aseismic tectonic creep along the zone of NW/SE direction [92]. Since, in their majority, the ground fissures were correlating with Quaternary and active normal faults, they caused much nuisance and concern among the population and scientists for future seismic activity. In particular, they had been regarded as evidence of aseismic tectonic creep, and even as premonitory phenomena of oncoming strong earthquakes, while several studies on this topic were funded by OASP, the Greek Earthquake Planning and Protection Organization [92].

Researchers have conducted field surveys in which cracks have been observed in various cities and villages across Eastern Thessaly [60,92,93,94,95,96,97] (Figure 18). These cracks have been primarily associated with ground subsidence phenomena and fissures. Well casing protrusion was probably the first effect of ground subsidence observed in Thessaly. For instance, a protrusion of 20 cm in 4 years and bending of ~200 m deep pipes at a depth of 75 m at Chalki and a protrusion of 50 cm at Magoula were recorded [92]. In most cases, there was no sign of ground fissures or of differential settlements of buildings or roads, indicating that associated land subsidence was rather uniform or with a small gradient of deformation. Notable instances include Larissa, where cracks were reported on house walls and concrete yards, as well as Rizomilos, where extensive damage affected house walls, concrete yards, and even the highway. In Melia, similar issues were documented, including small sinkholes, caused by piping phenomena, alongside structural cracks. Kastri experienced cracks on houses, house walls, and concrete yards, while in Stefanovikio, cracks were identified on the national road. Additionally, in Chalki, Magoula, and Niki Village, cracks were reported, particularly in concrete structures and residential areas. Ground fissures were also observed in the NATO airport in Larissa. Keramopoulou (2014) [104] documented some of these ground fissures and created maps for the villages affected by these phenomena (Figure S4).

The human-induced land subsidence has increased the impact of floods, particularly in lowland areas like Larissa and Lake Karla, because the land surface was lower and the natural drainage was reduced or inverted. Similar cases are reported worldwide, such as in the Ravenna region. More specifically, the reclamation of the swampy areas for agriculture in the Ravenna region resulted in the alteration of the natural landscape. The drying of these swamps, combined with river channelling and groundwater extraction, increased land subsidence. Initially, land subsidence had a natural rate of 2–3 mm per year, which increased to 110 mm/year by the early 1970s. The land subsidence also exacerbated flooding risks, with historical sites and urban areas experiencing permanent inundation [106]. It should be emphasised that in the case of the East Thessaly basin, in addition to the consolidation of the soil layers occupying the marshlands, the drying of the lake was accompanied by the construction of many additional pumping wells which further worsened the overexploitation of the aquifers.

10. Discussion

The Thessaly Plain’s complex geological and hydrogeological evolution, coupled with extensive human interventions, has had profound implications for land use, water management, and environmental stability. Historical records and geological studies indicate the East Thessaly Plain has undergone multiple geomorphological transformations, including shifts in hydrographic networks, lake formations, and tectonic activity. The interplay between natural processes and anthropogenic factors has significantly influenced the current landscape and its vulnerabilities.

Due to the expansion of irrigated agriculture, which intensified from the 1970s onwards, farmers relied increasingly on groundwater extraction due to inadequate surface water availability. The evolution of the land use changes in the last three decades can be seen in Figure 19 and Figure 20.

The total cultivated and fallow lands, as well as the total irrigated crops for each region and Prefecture of Greece, ranging from 1990 to 2022, were obtained through the Hellenic Statistical Authority (ELSTAT) database [107,108]. It should be noted that data from 1990 until 2009 were sparse. The extent of land use for the Prefecture of Larissa is presented because the majority of Eastern Thessaly’s crops and pumping boreholes are located there. In the Prefecture of Larissa, the total cultivated agricultural and fallow land show fluctuations over time, with a notable decrease in 2011, with a peak of approximately 2300 km² (Figure 19). A steady decline is observed thereafter. The same trend is observed for the crops on arable land, which reached a peak of 1944 km² in 2011. Horticultural land remains relatively stable over time with minor fluctuations. There was a noticeable increase in fallow land in 2013, before experiencing a slight decline. An increase in fallow land, possibly indicating changes in agricultural practices, crop rotation, or land conservation efforts.

On the other hand, the trend observed for the total irrigated crops in the Prefecture of Larissa is different from the chart of the total cultivated and fallow lands (Figure 20). The total irrigated crops show fluctuations over the years. A notable increase is observed in the early 2000s. However, there was a period of decline and stabilisation between 2010 and 2020, followed by a significant increase in the most recent years, surpassing previous peaks. The crops on arable land follow a somewhat similar trend. The rest of the agricultural exploitation types show stable trends with minor fluctuations. One of the reasons for the shifts in irrigated crops was the economic burden of pumping water from greater depths, resulting in a transition from irrigated to non-irrigated crops.

With the increase of irrigated crops and the intensification of groundwater extraction, the aquifer recharge rates failed to keep up with withdrawals, resulting in long-term water level declines. These declines are more significant in the Titarisios Cone and the Larissa–Lake Karla groundwater systems.

Concerning the former, the piezometric graphs of the area from 1974 to 2006, the general level drop of the aquifers of the water basin amounts to 10–15 m. However, from 1980 to 1985, it indicated no overexploitation of the underground aquifer since satisfactory replenishment was achieved (Figure 21). From 1985 to 1990 and from 1999 to 2002, a downward trend is observed, interrupted by a period of equilibrium in the years from 1990 to 1999. From 2002 to 2003, a water level increase is seen, at the same levels as the ones of the period 1990–1999, which continues until 2006. From 2011 to 2017, a trend towards stabilisation at low levels and signs of recovery can be seen locally. This observation is transient and is probably linked to the annual fluctuations of irrigated areas and to the inability, due to a general crisis, to cover the increased cost of water pumping from greater depths. The past water level fluctuations generally do not follow the natural discharge and charge rates of the system and indicate a problem of overpumping [63,66].

Concerning the Larissa–Lake Karla groundwater system, an overexploitation pattern from 1970 to 2000 is observed. During this period, extensive groundwater extraction for irrigation led to overpumping, resulting in a steady drop in aquifer levels, with declines from 10 to 100 m recorded between 1974 and 2016.

From 2001 to 2017, overall, a trend of stabilisation at low levels and local signs of recovery are observed (Figure 22). This observation is transient and is probably linked to the annual fluctuations of irrigated areas and to the inability, due to a general crisis, to cover the increased cost of water pumping from greater depths. Nonetheless, several piezometers still present signs of overpumping.

The trends in land use and groundwater depletion in East Thessaly exhibit a clear correlation, highlighting the impact of agricultural practices on water resources. The increase in irrigated crops in the early 2000s coincides with a progressive decline in groundwater levels in both the Titarisios Cone and Larissa–Karla groundwater systems. However, a notable decline in irrigated land after 2010 appears to be linked to a temporary stabilisation of groundwater levels, as seen in the piezometric data. From 2011 to 2017, a trend of stabilisation at low levels and signs of local recovery can be observed, possibly due to reduced water withdrawals driven by economic constraints and the rising cost of pumping from greater depths. The periods of groundwater table decline align with peak irrigated land usage, while subsequent reductions in irrigated crops correspond to slight rebounds in water levels. Despite these temporary recoveries, the overall trend suggests that unsustainable groundwater extraction continues to strain the aquifers, as signs of overpumping persist in several monitoring stations.

The unsustainable extraction rates have caused significant water depletion, land subsidence, and increased salinity in certain areas, further compromising agricultural productivity. Land subsidence is one of the most visible consequences of groundwater over-extraction in the Eastern Thessaly Plain. Multiple geodetic and remote sensing studies have documented significant vertical displacements, particularly in the eastern Thessaly Plain. Subsidence rates of up to 22.9 mm/year in the area between Modestos and Stefanovikio, along with extensive ground fissures, have been linked to groundwater over-extraction and sediment compaction.

Nevertheless, the correlation between ground deformations and active fault movements raises concerns about potential seismic risks, as observed during the 2021 Tyrnavos earthquake sequence. The activation of the Tyrnavos fault and the subsequent Mw 6.3, Mw 6.0, and Mw 5.2 earthquakes demonstrated the region’s seismic vulnerability, with widespread infrastructure damage, liquefaction, and surface deformation recorded.

Flooding remains another major challenge for Thessaly, exacerbated by both historical land use changes and climate change. The recent extreme weather events, including Hurricane Ianos (2020) and Cyclone Daniel (2023), have underscored the region’s susceptibility to hydrological disasters. In particular, the 2023 floods, which resulted in over 850 km² of inundated land, highlighted the inadequate capacity of existing drainage infrastructure and the challenges posed by large-scale land subsidence. The re-emergence of Lake Karla as a major flood retention area illustrates the unintended consequences of past drainage efforts, as the area now struggles to manage excess water in the absence of natural outlets. The delayed recession of floodwaters further underscores the hydrological imbalances created by decades of groundwater mismanagement.

It should be mentioned that following Medicane Daniel, the Greek government issued to HVA the job of assessing the impacts of the floods, as well as creating a master plan that proposes mitigation and adaptation measures to flooding. HVA noted that many flood control projects lack proper ownership and management by a specific authority, which results in problems concerning their maintenance and optimal use of water resources. Additionally, HVA indicated numerous unauthorised dams and other constructions, which have been built by individual farmers, resulting in increased flood risks and inefficient water distribution. Among the proposed methods, HVA experts pointed out the need for adaptation and update of existing studies for under-construction dams, the implementation of discussed canal projects, as well as the expansion of Lake Karla. The need for a centralised or well-coordinated water management authority to oversee water allocation, groundwater management, and flood prevention strategies was also emphasised. Finally, in the master plan, the experts stressed the importance of public awareness and education to inform residents about flood risks and water conservation measures. It should be noted that a body responsible for water management would also aid in the mitigation and adaptation of land subsidence [109].

Despite the development of the Water Management and Flood Risk Management Plans, the current mitigation strategies appear insufficient in addressing the root causes of Thessaly’s water-related challenges. The persistent issues of aquifer depletion, land subsidence, and flood vulnerability suggest that more integrated and proactive measures are required. The HVA master plan took an important step towards the right direction and proposed a more structured approach to flood mitigation and water management. However, the issue of land subsidence remains insufficiently addressed. Without clear strategies for sustainable groundwater management, the region will continue to face significant geotechnical and geocatastrophic risks alongside flooding challenges.

11. Conclusions

The East Thessaly Plain has experienced significant transformations due to geological, hydrological, and anthropogenic influences, leading to substantial challenges in water resource management, land subsidence, and agricultural sustainability. The overexploitation of groundwater systems has resulted in progressive water table declines, increased salinity, and land subsidence. These effects are compounded by extreme weather events, such as Storm Daniel and Storm Elias, which have exacerbated flooding risks and accelerated environmental degradation.

The land subsidence phenomena observed in Eastern Thessaly are primarily attributed to prolonged groundwater depletion and sediment compaction, with occasional influences from tectonic activity. These processes have led to severe structural damage to infrastructure, roads, and settlements, posing long-term risks to both urban and rural communities. The unpredictability and delayed visibility of subsidence further complicate its management, making proactive mitigation efforts crucial. Remote sensing techniques, such as InSAR and GNSS, have proven invaluable in monitoring vertical displacements, enabling early detection of subsidence patterns and providing critical data for regional planning. The integration of these methods into a comprehensive water management strategy can help mitigate further environmental deterioration. Moving forward, Thessaly requires a multi-pronged approach to sustainable water management and land use planning. Integrated strategies that combine improved groundwater recharge methods, the restoration of natural floodplains, and the enhancement of water storage infrastructure are essential. Policy interventions, including stricter regulations on groundwater extraction and incentives for water-efficient agricultural practices, will also be necessary to mitigate the long-term effects of overexploitation.