**1. Introduction**

The sixth report of the Intergovernmental Panel on Climate Change (IPCC) (AR6) published in 2021 identifies the Mediterranean region (MED) as one of the areas or hotspots most affected by climate change on a global level [1]. Spain is located in this region. This country is already suffering from the impacts (social and economic losses) of the effects of climate change, particularly those related to meteorological phenomena, such as floods, droughts, heat and cold waves, sea storms and forest fires, among others.

Furthermore, Spain has a wide variety of climates within its territory, which shape different landscapes, depending on a series of physical elements, such as the geographic position, altitude, relief, proximity to the sea, the vegetation and fauna and flora of each territory.

In terms of water resources, this variety of climates implies a structural problem for Spain, in which two types of area can be distinguished: humid Spain, (the north-east, north and centre of the peninsula), which receives large annual amounts of precipitation with surplus water resources; and dry Spain (the east, south-east and south of the peninsula), where the average annual rainfall is very low, leading to deficits in the availability of water resources [2]. Paradoxically, some of the optimal lands for growing crops are found in parts of Spain with a water resource deficit, where the soil is rich and favourable for agricultural activities and where irrigated crops predominate over rain-fed crops [2].

The most important transfer in Spain constructed in the twentieth century is the Tagus– Segura Transfer (hereafter, TTS). This is the most important infrastructure given the volume of water it transfers, the areas it supplies and the political and media repercussions [2,3].

**Citation:** Cañizares, A.O.; Cantos, J.O.; Baños Castiñeira, C.J. The Effects of Climate Change on the Tagus– Segura Transfer: Diagnosis of the Water Balance in the Vega Baja del Segura (Alicante, Spain). *Water* **2022**, *14*, 2023. https://doi.org/10.3390/ w14132023

Academic Editor: Adriana Bruggeman

Received: 25 May 2022 Accepted: 22 June 2022 Published: 24 June 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

The introduction of the TTS has contributed a significant amount of water resources for both urban supply and for agriculture (irrigation) in south-east Spain. This contribution of water resources is considerable but insufficient, as it has only fulfilled the transfer volumes contemplated in the Preliminary Project of the Transfer once (water year 2000/2001) [4–6].

It is worth highlighting that in the 1990s (emergence of climate change hypothesis) to the present (2022) (complete confidence in climate change), the successive situations of rainfall droughts that affected the centre and south-east of the peninsula revealed that the TTS was vulnerable to extreme weather situations. In other words, the absence of rainfall in the Tagus headwaters for prolonged periods of time has a serious impact on the TTS [4–6]. Some authors calculated a period of NO transfers of water resources for approximately 15 months. Furthermore, these authors point out that by applying the same methodology, it could be said that the TTS would have been inoperative for a total of 59 months (5 years), from the hydrological year 2004/2005 to 2017/2018 [4].

As noted above, the TTS is vulnerable to extreme atmospheric situations (droughts). This problem is negatively aggravated considering the climate scenarios of emissions and effects on temperatures and precipitation in Spain [2,3,7–9], and, in specific terms, in the south-east peninsular region [10–12], from 2020 to 2050 and from 2015 to 2100, especially in river basins such as the Tagus and Segura.

Therefore, the relationship between the effects of climate change and water resources deserves special attention, given that the variations in climate that are occurring on a global scale are generating a series of effects in the Spanish territory, which is supported by rigorous scientific data. Some of the most concerning effects are the variation in atmospheric dynamics [12]; the increase in temperatures and the variations in rainfall [7,10–13]; the increase in temperature and the increase in sea level [14–18].

All of this has direct repercussions on water planning, given that the headwaters and resources in the basins are highly important for the development of agricultural activity, as they favour the accumulation of water resources in reservoirs and aquifers. Future water plans (third water planning period (2022–2027) and those of the following decades) should simultaneously contemplate solutions to address the reduction in the volumes of useful rainwater and the occurrence (ever more frequent) of intense or torrential rains leading to floods that cause increasing economic damage [10].

Within this context, one of the principal ways to obtain water resources which are not subject to climate variations is by increasing the so-called non-conventional sources, particularly wastewater treatment and desalination.

The former is subject to the water consumption of the population in a year. In this respect, there is a directly proportional relationship: the more water consumed by the population, the greater the availability of treated water, and vice versa. Regenerated water undoubtedly constitutes a buffer for the water resources of the basin and has been incorporated in the current water plan, acting as a complementary source to the resources of the TTS. However, from the end of the twentieth century and the beginning of the twenty-first century, the need to reuse treated water has arisen, giving rise to the passing of Royal Decree 1620/2007 of 7 December, establishing the legal framework for the reuse of treated wastewater, thereby promoting the development of the reuse of treated water and incorporating it into the water resources plan, provided that public health and environmental protection can be guaranteed and establishing the necessary requirements to enable or prohibit the use of treated or regenerated water, according to the afore-mentioned regulation [19,20].

Similarly to treated wastewater, desalinated water does not depend on climate variation. It only depends on its own daily and annual production capacity. Desalination is playing an increasingly prominent role in the hydrological planning of the basins. In Spain, the promotion of desalination began with the repeal of the Ebro Transfer project, through the passing of Royal Decree Law 2/2004 of 18 June, which modified Law 10/2001 of 5 July of the National Hydrological Plan and the implementation of the A.G.U.A. Programme,

and subsequently with the passing of Law 11/2005 of 22 June which modified Law 10/2001 of 5 July of the National Hydrological Plan [4,21].

This led to the planning and construction of large desalination plants along the Spanish Mediterranean coast. The largest desalination plant is that of Torrevieja. It has a current capacity of 80 hm3/year and is managed by a state entity (ACUAMED). It has the largest capacity in Spain and one of the largest in Europe [22].

Desalination is characterised by being a strategic source and has been used in situations of severe drought in Spain, cushioning the effects of drought and providing a complementary water resource to the water of the TTS [4,5]. In fact, in situations of severe drought and when no transfers have been carried out, the desalination plant of Torrevieja has operated at full capacity, substituting the role of the transfer, for urban supply and irrigation [4,5].

While the rest of Europe uses desalinated water basically for urban supply, Spain is the pioneer in the use of desalinated water for agriculture and irrigation, given the water scarcity of the region [8,22].

Desalination in Spain has emerged in response to the transfer policy promoted by the former hydrological policy [5,6], which does not meet today's sustainability objectives. The desalination is accepted (socially) by the different economic sectors, and in many territories of the Spanish Mediterranean coast it has become a principal resource.

In short, the commitment to managing demand and the use of resources in a way that does not generate a situation in which territories are eternally dependent in terms of water is an essential and irreversible process [23]. It is necessary to break away from the traditional paradigm, based on the continuous supply of resources, which has no place in a scenario of climate change with less rainfall and a reduction in groundwater resources [23]. The growing use of "non-conventional" water resources will become a need in the coming decades on the Spanish Mediterranean coast, within the paradigm of demand and the sustainable use of water [23] (Figure 1).

**Figure 1.** Scheme of the old and new paradigm in water planning in study area. Source: own elaboration.

The working hypothesis of the research is because the Tagus–Segura water transfer is being affected by the effects of climate change, especially regarding the quantity of water resources. To verify this hypothesis, a series of secondary objectives were set to corroborate this approach:


#### **2. Materials and Methods**

#### *2.1. Background to the Tagus–Segura Transfer (TTS)*

Beforehand, it should be put into context that the origin of the Tagus–Segura Transfer project dates back to the First National Hydraulic Works Plan (1933), which basically sought to correct the imbalances between the Atlantic and Mediterranean coasts which, through the so-called "Plan de Mejora y Ampliación de los Riegos de Levante" (Extension and Improvement Irrigation Plan in Spanish Levante region), was based on the transformation of a total area of 338,000 hectares, over the provinces of Murcia, Valencia, Alicante, Almería, Albacete and Cuenca [25].

After the severe drought of 1967, the TTS project was approved in 1969, the works were completed, and the diversion started operating in 1979. The diversion is a canal with a length of 286 km and a flow rate of 33 m3/s. It links the Bolarque reservoir, in the Tagus basin, with the Talave reservoir, on the river Mundo, the main tributary of the Segura [25]. The cost of the construction of the diversion and post-transfer systems was estimated at ESP 90,000 million (La Verdad newspaper, 18 February 1998), equivalent to EUR 540,910,984 today.

According to the General Proposal for the Joint Management of the Water Resources of Central and South-eastern Spain, Tajo–Segura System, the final objective was to transfer an annual volume of 1000 hm3. Of this, 640 hm3/year would be used for irrigation. This objective would be met in two phases: a first phase, with a transfer of 600 hm3, and a second phase, with a transfer of 40 hm3. With these estimated volumes, it was expected to transform a total of 90,000 new hectares and complete the allocations of 46,816 existing deficit hectares. The latter were already under cultivation but did not have sufficient volumes of water for optimal irrigation. This was to be solved with the arrival of transferred water.

These planned volumes of water generated a great expectation that led to the transformation of rain-fed crops into irrigated crops (new irrigation), and the area benefited increased to 135,361 hectares. The area contemplated in the TTS project was 136,816, so that some authors indicate that "miraculously, it seemed that the objectives outlined in the preliminary draft had been achieved" [25]. However, the expansion of the surface area occurred during the years of construction of the TTS. Therefore, the increase in irrigated area was only justified using groundwater (indigenous resources) of the territories [25]. This implies that, for example, as in the case of the Vega Baja del Segura, most of the aquifers are overexploited. Consequently, the exploitation of the groundwater resources of the region, which are protected by the official basin organisation (Demarcación Hidrográfica del Segura), is currently prohibited.

Furthermore, this explains why the water from the aqueduct has been insufficient to supply the demand of the Segura basin, as indicated in the respective hydrological plans (2015–2021) and (2022–2027) [26,27], as the water from the aqueduct has been able to maintain, as far as possible, part of the transformed areas. Moreover, to make the best possible use of the water received from the aqueduct for new irrigation, modern irrigation techniques have been introduced, such as drip irrigation.

An interesting point in the Preliminary Project of the Tagus–Segura Transfer is that the demand in the Tagus basin was 1447 hm3/year and the own adjustable resources amounted to 8152 hm3/year, while the demand in the Segura basin was 1045 hm3/year and the available resources (basin) amounted to 820 hm3/year, revealing the deficit of the basin.

With the construction of the aqueduct, demand in the Tagus basin would be maintained, although the basin's resources would decrease by 1000 hm3/year due to the transfer of water from one basin to another, estimated at 7152 hm3/year. As for the Segura basin, demand would remain the same, but the available basin resources would increase thanks to the volumes of water transferred amounting to 2120 hm3/year (offer in the Segura basin). These figures have never been reached, as the Preliminary Project did not contemplate the possibility of an increase in demand in both basins (Tagus and Segura) or the absence of some transfers, for months or even years. In fact, the values estimated in the Preliminary Project have never been obtained with the transferred water.

#### *2.2. Area of Study*

Given that the research is focused on the Tagus–Segura water transfer, the chosen study area is divided into three parts. These three parts coincide with the presentation of the sections in the Results section (Figure 2).

**Figure 2.** Study area: (**A**) Upper Tagus; (**B**) Segura Hidrological Basin; and (**C**) Bajo Segura Region (Alicante). Source: Valencian Cartographic Institute (ICV) and Demarcación Hidrográficadel Tajo (DHT) and Segura (DHS). Own elaboration.

The first area of analysis focuses on the river Tagus basin, specifically in the headwaters of the river Tagus, belonging to the sub-basin known as the Upper Tagus. The source of the river Tagus, the presence of two large reservoirs (Entrepeñas and Buendía) and the beginning of the hydraulic infrastructure of the Tagus–Segura Transfer are in this area (Figure 2). The effects of climate change calculated and estimated by the official basin organisation itself are also considered, in order to indicate the behaviour or trend in this sector.

The second zone corresponds to the Segura catchment area. The aspects analysed are those corresponding to the water balance (supply and demand) to ascertain whether the water transfer has made it possible to eliminate the existing deficit in the Segura basin, as was proposed in the Preliminary Project for the water transfer. Once the water balance is known, special attention is paid to the area corresponding to the province of Alicante. To this end, the UDAs (Agricultural Demand Units) corresponding to this region were selected. The purpose of this analysis is to find out the amount of existing gross or irrigable surface area, and to compare it with the net or irrigated surface area in this region, according to the data provided by the new Hydrological Planning Cycle of the Segura River Basin (2022–2027).The gross and net demand of the previously selected UDAs is analysed below. Gross and net demand is directly related to gross and net surface areas. Therefore, the demand makes it possible to know the volume of water necessary to supply these areas to obtain an optimal irrigation for the crops (calculated by the official basin organisation).

Lastly, the effects of climate change in the Segura basin are analysed, calculated by the basin organisation with respect to rainfall, evapotranspiration, surface runoff and aquifer recharge; this provides information on the water future of the Segura basin (which is structurally deficient).

The third area of analysis is centred on the province of Alicante (Valencian Community), specifically in the region known as Bajo Segura or Vega Baja del Segura. The choice of this area is justified by the fact that it is a region directly dependent on the water resources of the Tagus–Segura water transfer.

In this respect, when the water from the aqueduct reaches the Ojós reservoir, three water diversion channels start from this reservoir, which correspond to the so-called "Post-Transfer Infrastructure". These three channels take different directions. The first heads towards the province of Alicante, passing through the north of the district of Vega Baja del Segura, as far as the Crevillente reservoir (BajoVinalopó district and the Júcar basin). The second channel heads towards the Pedrera reservoir (Bajo Segura district). From this point, another canal splits into two branches: one heading north-east towards the town of Los Montesinos; and another heading south, passing through towns such as San Miguel de Salinas, Orihuela and Pilar de la Horadada (Bajo Segura district). This channel ends when it reaches the municipality of Cartagena (Murcia). The third canal heads southwards, crossing the Murcia Huerta, Algeciras, among other localities; it ends in Almería (Andalusia).

Given the great length of the water transfer and its implications for large areas of land, it has been decided to limit the analysis to the province of Alicante, specifically to the Bajo Segura or Vega Baja del Segura region (made up of 27 municipalities).

The territory of the district of Vega Baja is included within the Segura basin in terms of hydrological planning but is influenced by the Tagus basin as it receives resources from it through the Tagus–Segura Transfer. This reveals the need to analyse the current situation of each basin in relation to the effects of climate change that are visible in Spain, and their impact on the available water resources.

Along these lines, the district of Vega Baja del Segura implements a multi-source system in water management: surface water (River Segura), groundwater, the Tagus– Segura Transfer, the post-transfer, the reuse of treated wastewater and desalination. This situation is ideal in areas with a natural scarcity of water resources [24]. However, the system is vulnerable to the effects of climate change, due to the reduction in surface water resources predicted for this part of Spain. Therefore, it is necessary to reflect on possible future solutions to guarantee supply in this territory with natural rainfall scarcity.

#### *2.3. Data Source and Analysis*

This article analyses the deficiencies of the Tajo–Segura Transfer and its implications in the Segura River basin, and, especially, in the region of Bajo Segura or Vega Baja del Segura, motivated by the effects of climate change.

The methodology applied is based on the hypothetical-deductive model. The hypotheticaldeductive method is one of the most accepted methods currently in the scientific field,

especially applied to social sciences such as geography, the approach given in this article. The method consists of a working hypothesis that, based on the analysis of a series of available data, allows the hypothesis to be corroborated or not. The steps of the hypotheticaldeductive method are: (1) data collection; (2) data evaluation; (3) hypothesis generation; (4) diagnosis; and (5) final conclusion and/or proposal.

The working hypothesis of the research is based on the fact that the Tagus–Segura water transfer is being affected by the effects of climate change, especially with regard to the quantity of water resources. In order to confirm these aspects, information and data have been compiled from the institutions, bodies and official associations (Figure 3).

**Figure 3.** Diagram/outline of the methodological steps in this paper.Source: own elaboration. The innovative contributions of this paper are shown in the shaded areas.

First, the Hydrological Plan of the Tagus Basin (2022–2027) was consulted to compile information regarding the available water resources of the basin. Particular focus was placed on precipitation, surface runoff and the volume of water stored in the Entrepeñas and Buendía reservoirs. The predicted effects of climate change on the Tagus basin were also identified, specifically in the headwaters or sub-basin of the Tagus.

To obtain the data of the volumes of water stored in the Entrepeñas and Buendía reservoirs individually and jointly, the website of the Ministry for the Ecological Transition and the Demographic Challenge (MITERD) through the Centre for Public Works Studies and Experimentation (CEDEX) was consulted. This organisation has all the yearbooks of the volumes of all the basins in Spain, by day and month, depending on the time series chosen. In this way, the years of operation of the TTS have been established as a time series (1979–2021). The principal element in the search was to identify the reserve figure in hm<sup>3</sup> at the beginning of each month in order to analyse the evolution of the water stored in each reservoir or jointly (Entrepeñas–Buendía), so as to establish whether there are currently sufficient volumes of water to transfer via the TTS to the irrigated lands of south-east Spain, and whether these volumes are similar to the theoretical transfer volumes established in the Preliminary Project of the TTS.

After ascertaining the situation in the Tagus headwaters with respect to the precipitation, surface runoff and volume of reservoir-stored water, information and data were collected regarding the Hydrological Plan of the Segura Basin (1st Cycle 2015–2021) and the Hydrological Plan of the Segura Basin (2nd Cycle, 2022–2027).

In the case of wastewater treatment, the data were provided by the Regional Department of Agriculture, Rural Development, Climate Emergency and Energy Transition based

on the data of the Entidad Pública de Saneamiento de Aguas Residuales (Public Wastewater Treatment Entity) (EPSAR) of the Region of Valencia, which refer to the wastewater treatment plants (EDARs) installed in the district, their daily production capacity, their annual production capacity, the water treated and the water regenerated, among other information of interest.

With respect to desalination, data on production capacity and the water produced over the last four to five years were provided by the Torrevieja desalination plant. The public entity ACUAMED provided the data on the Torrevieja desalination plant and the cost of production, the energy cost and the future expansion projects contemplated for the desalination plant.

The different declarations of drought and the adaptation mechanisms adopted were also consulted for the period 2015–2018 to determine how the price of desalinated water produced by the Torrevieja plant was established at EUR 0.30 m<sup>3</sup> and the measures adopted by the basin in response to an extreme atmospheric event.

An analysis of these statements reveals a fundamental aspect that is significant for the future of TTS and traditional irrigation models in relation to water sources and extreme events. Therefore, it appears that HTM-dependent irrigation models enter a situation of pre-warning of drought long before the traditional irrigation models that use the basin's resources. In addition, traditional irrigation models do not enter a drought early warning situation until rainfalls fall to half of the drought early warning values in TTS crops. This justifies greater adaptation to extreme weather phenomena, such as droughts, and therefore resource-dependent irrigated crops.

Finally, in relation to the TTS and the volumes of water assigned and consumed in each irrigation community, the study establishes whether with this volume of water profits have increased or decreased, depending on the greater or lesser amount of available water. To do this, four key socio-economic indicators were taken into account: the area of production, the average budget per inhabitant, recruitment in agriculture and unemployment in agriculture for the district of Vega Baja del Segura.

In this respect, the volumes of water transferred in 2011, 2015 and 2019 were also included, together with the desalinated water produced by the Torrevieja plant (ACUAMED) to establish a relationship between the availability of water and the previously mentioned socio-economic indicators.

A geographical information system called QGIS was used for the cartographic part. The layers used to elaborate the location map of the area of study were downloaded in shape (shp.) format from the Valencian Cartographic Institute (ICV), the National Geographic Institute (IGN), the Demarcación Hidrográfica del Segura (DHS), the Demarcación Hidrográfica del Tagus (DHT) and the Demarcación Hidrográfica del Júcar (DHJ) (Table 1).

This analysis seeks to determine the water balance and to estimate the deficit that exists. It also proposes alternative measures to increase the water supply and a new hydraulic plan for the hydrographic basins of Spain.


**Table 1.** Sources and official documents consulted.

#### **Table 1.** *Cont.*


**Table 1.** *Cont.*


Source: own elaboration.

The scientific contribution of this article is based on the analysis of the most current data offered by basin organisms, the effects of climate change and its involvement in water resources. First, it is shown that the current water situation at the head of the Tagus is not the same as in 1969. This implies a reduction in water resources in the Tagus basin and, consequently, lower volumes of water transferred. These data are demonstrated by the analysis of a short series of precipitation and runoff (1940–1979 and 1980–2018). To complete this analysis, we obtained the existing data of water embalmed in Entrepeñas and Buendía (separately) of the series of operation of the Tajo–Segura Transfer (1979–2020). In order to proceed with a transfer, it is necessary that the volume of water in the reservoirs of Entrepeñas–Buendía (as a whole) exceeds a certain amount at the beginning of each month.

The analysis of the water volumes packed in these reservoirs, both individually and separately, shows a significant reduction in the water resources stored, caused by the effects of climate change due to the lack of rainfall in the headwaters or the volume of rainfall. One of the effects demonstrated by the climate change that occurs in Spain is based on the reduction in rainfall in inland areas as opposed to large discharges that occur on the coast. This has a serious impact on the basins' water resources, since if it does not rain at the head of the rivers, the basins' water resources are diminished.

The analysis of the data from the Segura basin has allowed us to know the water balance (offer and demand) and the existing deficit, despite the water contributions of the Tajo–Segura Transfer. The data obtained have allowed us to know the situation in the province of Alicante, adjusting it to the region of Bajo Segura.

Another new aspect of the work corresponds to the construction of a water balance (supply and demand) and its water deficit, adjusted to the region of Bajo Segura in Alicante. The data show that the existing deficit is not as high as the above claims and that there is water for irrigation in the region. In addition, a table was prepared to estimate the situation for future scenarios in the Vega Baja of the Segura River (2030 and 2050). This table has been compiled as follows: the calculation of these quantities was established on the basis of the following method of analysis: (1) analysis of the official water demand and supply data calculated by the CHS; (2) analysis of the official data on the effect of the decrease in rainfall on water resources (AEMET, CEDEX) in the study area, especially of the resources coming from surface water (Segura River and Tajo–Segura water transfer); (3) knowledge of the territorial dynamics of the study area (for calculating future demands of the different water uses), which is what the field work was used for; (4) calculation of substitution flows for surface water and water transfer with non-conventional water (reuse and desalination), knowing that the reuse of wastewater has a ceiling (depending on the recent evolution of treated flows and total urban expenditure) and that the great asset is desalination for urban and agricultural use. Furthermore, in the current energy transfer process, it is estimated that the final cost of desalinated water will decrease in the coming years, depending on the installation of solar energy sources to feed the Torrevieja plant.

The most relevant of this analysis are two specific issues: (a) it is identified that the highest amount of water demand for irrigation cultivation corresponds to the trans-formed areas from rain-fed to irrigated and is dependent on the waters of the Tagus–Segura Transfer. It is also observed that irrigated crops dependent on basin water resources have a greater resistance to extreme weather (droughts) than those dependent on the transfer. In addition, it has been shown on numerous occasions that the transfer is an infrastructure vulnerable to such situations. (b) The data provided by the basin body include the volume of concessional water allocated to each irrigation community dependent on TTS water in the province of Alicante. However, actual consumption data by irrigation communities are much lower than the volume of water allocated in 1969.

The fact that they do not receive the amounts of water allocated in 1969 is justified by the reduction in water available at the head of the Tagus, hence the importance of having made its analysis earlier. The data show that these irrigation communities will never receive such volumes of water allocated in 1969, when the climate and water reality was quite different from the current one (2022).

However, farmers in the region continue to think that they do not receive such amounts of water for political-social reasons, forming the so-called "water wars". Examples of this are exaggerated claims that it is "the end of Europe's market garden", "without water there is not agriculture" or "that there is a deficit of 1000 hm3/year in the Segura basin". Farmers are therefore still waiting for water that they will never receive again, hence the importance of this article to demonstrate the real situation with rigorous scientific data.

The last aspect to be highlighted is that the deficit in the region of Bajo Segura can be solved by increasing the volume of water from wastewater treatment and desalination, increasing its annual production capacity, as proposed by the basin body. There are also other methods of reducing the deficit, such as the use of modern irrigation systems or the reduction in less productive, if extremist, cultivated land.

All these issues justify the scientific and novel contributions of the article.

#### **3. Results**

#### *3.1. The Effects of Climate Change in the Headwaters of the Tagus (Upper Tagus)*

According to the Hydrological Plan of the Tagus Basin (2022–2027), the current average rainfall in all Spanish areas of Tagus basin is 594 mm (1980–2018 series). These data are something different in the Upper-Tagus sub-basin, where the great reservoirs that regulate the Tagus–Segura Transfer are located and have experienced a decrease between the 1940–79series (655 mm.) and the 1980–2018 series (568.5 mm.).Therefore, now the average rainfall is lower than the average rainfall for the entire Tagus basin.

Many studies highlight the reduction in rainfall in the headwaters of the Tagus basin, calculated at a decrease of 12% in the period 1980–2018 [8,9]. This justifies the Tagus basin plan with the rainfall data (average and maximum) and surface runoff (average and maximum) for the sub-basin of the Upper Tagus, the headwaters of the Tagus and the beginning of the Tagus–Segura Transfer [28].

It should first be clarified that there are three series in the Hydrological Plan of the Tagus Basin: 1940–2018 (long series), 1940–1980 (old short series) and 1980–2018 (current short series). Only the last two short series were considered, using the mean values. The headwater rainfall was calculated from the SIMPA model, which is the hydrological reference model for the calculation of water resources in the hydrological plans. For the calculation of rainfall, the model establishes average data for each planning area within the basin, and when there are no long series in the meteorological observatories of the area, it fills in data from the longest series corresponding to observatories close to the planning area.

The results clearly show the effects of climate change in the headwaters of the Tagus basin in relation to precipitation. Although in some months there is an increase, the general monthly trend corresponds to a considerable decrease. In general terms, it can be observed that the mean obtained in the short series (1940–1979) was 655 mm and in the most recent short series (1980–2018) the mean drops sharply to 568.5 mm of precipitation in the headwaters of the Tagus. This represents a decrease of -86.5 mm of precipitation, which represents a decrease of 13.2%, at present (Table 2). Consequently, these means are substantially different, which implies evidence of the impact of climate change in the Tagus headwaters [28].


**Table 2.** Calculations of average monthly/annual precipitation decrease and percentages in the Upper Tagus (series 1940–1979 and 1980–2018).

Source: Hydrological Plans for the Tagus Basin (2022–2027; 2015–2021; 2009–15). Monthly collated data *Iberian Climate Atlas* (AEMET, 2011). Own elaboration. Color: Identify decreases (red and negative) and increases (green and positive).

Meanwhile, the Hydrological Plan of the Tagus Basin (2022–2027) also reports a series of data related to surface runoff. In this respect, the reduction in rainfall has a direct effect on the surface runoff of the Tagus Basin. These volumes of surface water are used for the transfer.

Therefore, surface runoff functions as an indicator of the impact of climate change, in relation to the reduction in mean precipitation in the headwaters of the Tagus. Adding the monthly averages of surface runoff for the short series (1940–1979) gives a total surface runoff of 657.4 hm3 for the 30-year series. For its part, the sum of the monthly averages of surface runoff for the short series (1980–2018) gives a total result of 380.8 hm<sup>3</sup> in the 30-year series. This implies the reduction of a total of 276.6 hm3 in thirty years, which is a percentage reduction of 42.1% (Table 3) [28].

**Table 3.** Calculations of average monthly/annual runoff decrease and percentages in the Upper Tagus (series 1940–1979 and 1980–2018).


Source: Hydrological Plan for the Tagus Basin (2022–2027). Own elaboration.

Another aspect that highlights the decrease in rainfall and surface runoff in the Upper Tagus due to climate change is the volume of water stored in the Entrepeñas and Buendía reservoirs, particularly when analysing the historical series of 1979–2020, coinciding with the years of the operation of the TTS.

In the water year 1979–1980, the volume of water stored in the Entrepeñas reservoir reached a value of 6308 hm3/year. This figure had fallen to 4083 hm3/year in the water year 2019–2020. Meanwhile, in the Buendía reservoir, in the year 1979–1980, there was a total volume of 14,268 hm3/year, and in the water year 2019–2020 this figure had fallen sharply to 3613 hm3/year. However, it should be remembered that the volume of water authorised for transfer depends on the sum of the two reservoirs (Entrepeñas and Buendía). In this respect, and following the same line of analysis as in the individual cases, the total joint volume of water of the Entrepeñas and Buendía reservoirs in water year 1979–1980 amounted to 20,576 hm3/year, with 7696hm3/year in water year 2019–2020, representing a decrease of 63% (Table 4, Figure 4).

Moreover, it should be noted that in the years of severe drought, as a whole, values of between 5000 and 9000 hm3/year have been reached, revealing their vulnerability to extreme atmospheric events.

Therefore, two conclusions may be drawn: (a) the impact of the effects of climate change on the headwaters of the Tagus is inevitable, as a reduction in rainfall directly affects the surface runoff and the volumes of water stored in reservoirs; and (b) the TTS is adversely affected in this respect, given that the theoretical volumes calculated to transfer to south-east Spain correspond to volumes of water that existed in and prior to 1979–1980, which are not attainable in the present day. This implies the need to carry out a review of the calculations made in the Preliminary Project of the Transfer with current data, given that it is inconceivable that the theoretical water allocations can continue to be planned based on calculations made over forty years ago, when the climate reality was completely different to that of today.


**Table 4.** Comparison between water stored in the Entrepeñas–Buendía reservoir and volumes of water transferred by the Tagus–Segura Aqueduct.

Source: yearbooks of gauges in Spain. Own elaboration.

**Figure 4.** Graph comparison between water stored in the Entrepeñas–Buendía reservoir and volumes of water transferred by the Tagus–Segura aqueduct. (**a**) Evolution of water reservoirs in the Entrepeñas–Buendía reservoir (1979–2018); (**b**) evolution of volumes of water transferred by the Tagus–Segura aqueduct (1979–2018). Source: yearbooks of gauges in Spain. Own elaboration.

Finally, the PHCT (2022–2027) reports the percentage of change in the quarterly runoff, calculated in average (RCP 4.5) and high (RCP 8.5) climate change scenarios for the Upper Tagus. For the months of October to December, a reduction of 14% is calculated in an RCP 4.5 scenario and 20% in an RCP 8.5 scenario, coinciding with the months of most rainfall [28]. This implies that the rainfall trend, the surface runoff and the volumes of reservoir-stored water in the Upper Tagus are in continuous decline, constituting a serious problem for those dependent on the water from the TTS.
