Water Harvesting and Groundwater Recharge: A Comprehensive Review and Synthesis of Current Practices

Abstract

This review examines state-of-the-art practices in water harvesting and groundwater recharge, emphasizing their crucial role in sustainable water resource management. Groundwater, a crucial resource for drinking and agricultural water, is facing depletion due to the combined effects of urbanization, climate change, and unregulated extraction. This paper provides a comprehensive overview of contemporary knowledge on water harvesting and recharge, using a systematic four-step methodology to identify and analyze relevant studies from databases like Google Scholar, Scopus, and ScienceDirect. It categorizes water harvesting techniques, such as rainwater, flood water, and groundwater harvesting, and evaluates their effectiveness in enhancing recharge. Both natural and artificial recharge methods are explored, highlighting their role in improving groundwater levels and water quality. This study also reviews recharge estimation techniques and their applicability across hydrogeological contexts. This paper promotes a balanced approach to address issues of water scarcity by integrating water harvesting into groundwater management strategies. This integration aims to build resilience against climate change-driven environmental damage. Recommendations are provided to enhance the sustainability of these practices, with a particular focus on arid and semi-arid regions where water demand is rising. Overall, this study underscores the significance of water harvesting and recharge in promoting long-term resource sustainability.

1. Introduction

Groundwater is a vital source of domestic, agricultural, and industrial supply as well as for ecosystems in many parts of the world, especially in arid and semi-arid regions. A brief review of the literature shows that groundwater provides some or all of the drinking water for 50% of the world’s population and accounts for 43% of all water used for irrigation. In addition, it is estimated that a total of 2.5 billion people rely solely on groundwater resources to meet their basic daily water needs [1,2]. However, over-pumping and/or overexploitation of aquifers to satisfy various competing water uses, climate change, etc., are causing groundwater levels to decline. Groundwater depletion has several effects, including (a) lowering of the water table; (b) reduction in water in streams and lakes; (c) ground subsidence; (d) increased costs to the user; (e) deterioration of water quality and ecological damage, etc. Excessive groundwater depletion affects large regions of North Africa, the Middle East, South and Central Asia, Northern China, North America, and Australia, as well as in various local areas around the world [1,2]. The groundwater depletion rate is accelerating around the world, and the consequences are continually worsening, highlighting the need for an objective analysis of the problem and consideration of possible solutions. As a result, the depletion of groundwater is an issue of global interest.

In many developing countries, such as those in sub-Saharan Africa (e.g., Ethiopia, Kenya, etc.), groundwater serves as a main source of drinking water for many urban dwellers estimated at 30% [3] due to its better quality and minimal treatment needs [4]. However, urbanization increases/accelerates groundwater demand while reducing the groundwater recharge areas [5], leading to the decline and complete extraction of groundwater [6]. This issue is particularly severe in arid and semi-arid regions, which make up over 40% of the Earth’s surface, causing significant groundwater depletion globally [7,8,9]. Recharge is essential to sustain water resources and build resilience against climate change and shifting water demands [2]. Among the internationally proposed measures to tackle the problem are groundwater recharge (GWR) and surface water harvesting. This can be conducted both naturally through rainfall and artificially through human intervention.

According to Yannopoulos et al. [10], rainwater harvesting systems are being increasingly adopted by African countries. However, despite the rapid expansion of these systems, progress remains slow due to several factors: (a) low and seasonal rainfall, (b) a limited number and size of impervious roofs, (c) the high cost of constructing catchment systems relative to typical household incomes, (d) a lack of cement and properly graded sand in certain regions of Africa, and (e) insufficient water resources for the construction industry, which adds to the overall costs. Water harvesting (WH) systems are increasingly expanding in Africa (e.g., Kenya, Ethiopia, South Africa, Mali, etc.) [3].

WH is the collection, storage, and use of rainwater or surface runoff to supplement or replace other water sources [11,12]. It involves capturing water via systems like rooftop catchments or small dams, storing in tanks or reservoirs, and distributing it for drinking, irrigation, livestock, or groundwater recharge [13].

WH plays a key role in groundwater recharge and management [7,13,14,15,16,17]. Captured surface runoff percolates into the ground, replenishing aquifers and helping maintain groundwater levels, especially in overexploited areas. By reducing reliance on groundwater for domestic, agricultural, and industrial uses, it slows groundwater depletion and supports aquifer recovery. Rainwater harvesting, in particular, offers relatively clean water for recharge, diluting contaminants and improving the groundwater quality. Integrating WH into management strategies promotes resource sustainability, supports ecosystems, and maintains stream flows and wetlands.

The effectiveness of WH for GWR depends on local hydrogeology, system design, and regional water management practices [18]. When well-implemented, it supports integrated water resource management by enhancing groundwater availability, quality, and sustainable use.

The focus of this manuscript is a thorough review and presentation of current knowledge and practices in the field of WH and GWR. The authors used peer-reviewed scientific articles from international journals on WH and GWR.

2. Methodology

In the preparation of this paper, the authors reviewed peer-reviewed international articles on WH and GWR, specifically selecting studies published in English from 2000 to 2024. A limited number of older articles published before 2000 (1974–1997) were included for their relevance in showcasing historical approaches and trends related to the topic. The literature search utilized popular databases such as Google Scholar, Scopus, Microsoft Academic, ScienceDirect, and ResearchGate, in addition to specialized water-related journal websites. To identify relevant publications, a structured four-step process was implemented (Figure 1):

• Initial Identification: A systematic search was conducted utilizing a combination of keywords, citations, and related articles to compile a diverse range of relevant publications. The keywords employed in the search include “groundwater”, “water scarcity”, “arid”, “semi-arid”, “water harvesting”, “effective water harvesting”, “water harvesting system”, “water harvesting techniques”, “rainwater harvesting”, “flood water harvesting”, “groundwater harvesting”, “qanat system”, “underground dams”, “groundwater recharge”, “sources of water”, “natural recharge”, “artificial recharge”, “surface spreading”, “injection well”, “induced infiltration”, “recharge estimation”, “infiltration”, and “runoff curve number”.
  A total of 166 articles were gathered from various databases in August 2024: 68 articles from Google Scholar, 38 articles from Scopus, 17 articles from Microsoft Academic, 13 articles from ScienceDirect, 15 articles from ResearchGate, and 15 articles from other water-related journal websites. After the collection, duplicate entries were automatically removed, resulting in 107 unique articles.
• Abstract Screening: The abstracts of the 107 identified publications were reviewed to assess their relevance and eliminate papers that were not closely related to WH and GWR. As a result, 20 articles were excluded during the abstract screening process.
• Full Text Analysis: The remaining 87 publications were thoroughly examined to evaluate their relevance and quality concerning the research topic. This process involved a detailed analysis of the content of each paper, resulting in the exclusion of 11 articles.
• Final Selection: After conducting a thorough analysis of the full texts, a final selection of 76 publications was made. Of these, 72 articles were published between 2000 and 2024, while the remaining four articles date back to 1974, 1988, 1996, and 1997. The selected articles were reviewed in detail, and the information collected was systematically organized.

This article filtering process is briefly summarized in Figure 1, while Figure 2 shows the publication year distribution of the selected articles.

3. Review of the Recent Literature

Groundwater depletion and contamination pose significant global sustainability challenges [19,20]. As the most accessible freshwater resource, groundwater is vital for drinking, irrigation, and industry [1].

WH systems are critical for improving water resources worldwide, in general, and in arid and semi-arid regions in particular [4,6,7,10,13,14,16,17,21,22,23,24,25,26,27,28]. Techniques like percolation tanks and check dams (dams on small gully) enhance GWR and improve water quality, particularly in semi-arid regions [4,28]. Alataway and El Alfy [14] emphasized that rainwater harvesting (RWH) and GWR strategies are effective in mitigating water scarcity in arid environments facing rising demands and climatic challenges. Similarly, Ertop et al. [13] addressed growing water scarcity caused by population growth, climate change, and unplanned industrialization, advocating RWH as a practical solution for irrigation and domestic uses of water management.

RWH offers multiple benefits, including cost-effectiveness, flood mitigation, GWR, and supporting sustainable agriculture [17]. Huang et al. [17] advocate for globally systematic RWH implementation, emphasizing the use of local knowledge and materials to develop decentralized, community-based systems as alternatives to centralized water supplies. Hussain et al. [16] highlight RWH through recharge wells as a solution for urban flooding and groundwater depletion. Lasage and Verburg [22] analyze various WH methods, examining their physical, economic, and governance characteristics. They propose a decision-making framework for selecting suitable techniques based on local conditions, concluding that larger structures are more cost-effective but require complex governance, while smaller systems are easier to implement but may incur higher costs per unit of water stored.

Water turbidity has a significant effect on the efficiency of GWR in reservoirs. Increased turbidity levels result in higher sedimentation rates within sand columns, infiltration ponds, micro-dam reservoirs, etc., used for recharge, which in turn reduces the flow rate of water infiltrating into the aquifer [29]. This highlights the necessity for utilizing high-permeability sand columns in recharge reservoirs and implementing strategies to prevent turbid water from entering these systems. By doing so, optimal GWR rates can be maintained, helping to alleviate problems such as land subsidence and seawater intrusion that arises due to excessive groundwater extraction.

Gwenzi and Nyamadzawo [30] discuss the hydrological impacts of urbanization and roof water harvesting in water-limited catchments, with a focus on sub-Saharan Africa. They highlight how urbanization alters hydrological processes, increases surface runoff, and reduces GWR. Research on GWR methods, both natural and artificial, have been extensively explored by several studies [18,31,32,33]. GWR estimation techniques are examined in works like [34,35,36,37,38,39]. These studies employ various methods based on factors such as hydrological and climatic conditions, hydrogeological conditions (soil and geological characteristics), land use patterns, data availability, scale of study (e.g., local or regional and short-term or long-term), and research objectives. The diversity in approaches reflects the complexity of GWR processes and the need to adapt methods to specific environmental and study area contexts.

Gee and Hillel [35] explore the complexities of estimating natural GWR in arid regions, emphasizing the variability of recharge rates, limitations of common methods, and significance of the vadose zone. Recharge variability depends on factors like climate, soil type, vegetation, and topography, which can lead to inaccuracies if estimates are based solely on average precipitation. Traditional methods, such as water balance calculations and simulation models, often yield unreliable results due to challenges in measuring components like evapotranspiration and soil moisture. The vadose zone, lying between the soil surface and the water table, plays a critical role in recharge, with water sometimes moving rapidly through narrow pathways and bypassing larger volumes of soil. This creates concentrated recharge events that are difficult to predict. Gee and Hillel [35] advocate for direct measurement techniques such as lysimetry and tracer tests, to improve accuracy over traditional methods.

Other studies have evaluated various methods, tailoring them to hydrogeological conditions and data availability. Sibanda et al. [36] used techniques like chloride mass balance, water table fluctuation (WTF), Darcian flownet computations, and groundwater age dating. Similarly, Islam et al. [37] employed methods including WTF, water budget, Darcy’s law, empirical relationships, groundwater models, and tracer techniques. Sun et al. [38] and Cambraia Neto and Rodrigues [39] assessed methods such as base-flow separation, WTF, and sequential water balance, highlighting their ease of use, data accessibility, and robustness. These studies underline the importance of selecting appropriate estimation techniques based on the specific context of the region and the reliability of input data.

4. Water Harvesting Types and Techniques

WH refers to the process of collecting and storing rainwater or surface runoff on the ground, underground, in the soils, or in reservoirs for future use [13]. This practice can be implemented at various scales, from small household levels to large-scale municipal or watershed projects. The primary objective of WH is to capture and conserve water for purposes such as drinking, irrigation, GWR, and other needs, particularly in regions experiencing water scarcity or regions having longer periods of dry seasons. WH offers a range of benefits [40] including (a) drought mitigation, (b) efficient water use, (c) flood management, (d) soil conservation, (e) groundwater improvement, and (f) energy conservation. Zhao et al. [41] also pointed numerous benefits in the form of economic, environmental, technological, and social advantages.

WH, a time-honored practice, encompasses three key components that are essential for effectively capturing and utilizing rainwater or runoff [13]. These components are the collection system, transportation system and a storage system. Collection System is the part of the system where rainwater or surface runoff is initially gathered. This system typically includes surfaces such as rooftops or catchment areas that direct water into a designated collection point. Once the water is collected, it must be transported to a storage location, which is facilitated by a transportation system that often involves pipes, channels, or gutters to guide the water from the collection point to the storage unit [13]. The final component is the storage system, where the water is stored for later use; this can be in tanks, cisterns, or other reservoirs that hold the water until it is needed for drinking, irrigation, or other purposes.

WH can be classified into three main types: (1) rainwater harvesting, (2) flood harvesting, and (3) groundwater harvesting, which involve both surface and subsurface storage reservoirs. The primary WH structures employed include short term options such as contour bunds, trapezoidal bunds, semi-circular hoops, rock catchments, and ground catchments. Additionally, long term structures like dugout ponds, farm ponds, irrigation dams, silt/sand detention dams, percolation dams are commonly practiced, alongside subsurface dams. These systems collectively enhance water resource management and contribute to sustainable water use, particularly in areas facing water scarcity.

4.1. Rainwater Harvesting (RWH)

RWH is the process of capturing and storing rainwater from precipitation, specifically rain that falls directly onto rooftops, surfaces, or catchment areas [10]. This practice involves a systematic collection method, utilizing components such as gutters, downspouts, and storage tanks to gather and store rainwater efficiently. The primary objective of RWH is to provide a sustainable water supply for a variety of uses, including domestic needs, irrigation, and other applications, particularly during dry periods or regions with water scarcity.

The design of RWH systems typically targets smaller, more frequent rainfall events, emphasizing adequate storage capacity and effective filtration mechanisms to ensure water quality. These systems are versatile and can be implemented in both urban and rural areas, adapting to varying rainfall patterns. In urban areas, RWH focuses on localized collection from rooftops and impervious surfaces, reducing reliance on municipal water supplies and mitigating urban flooding. In rural areas, the techniques are often employed to enhance agricultural productivity and provide consistent water access for communities.

RWH also plays a critical role in water resource management and climate change adaptation strategies [42]. By capturing rainwater at its source, it helps reduce surface runoff, prevents soils erosion, and supports GWR. Moreover, it contributes to building resilience in regions prone to droughts and water shortages. The adoption of RWH systems is increasingly encouraged as a sustainable solution for addressing global water challenges, promoting efficient water use, and supporting environmental conservation [10,42,43].

4.2. Flood Water Harvesting (FWH)

FWH involves the collection and management of excess water from flooding events, such as heavy rainfall, storm surges, or river overflows. This method employs various structures such as check dams, percolation tanks, and infiltration ponds to temporarily store and regulate large volumes of floodwater. The primary objectives of FWH include mitigating the adverse impacts of flooding, managing surplus water, and enhancing GWR during and after flood events [44,45].

Unlike RWH, FWH systems are designed to handle significantly larger and more rapid inflows of water. As a result, these structures often require robust engineering and durable materials to withstand the high pressures and dynamic conditions of flood events. For example, check dams help slow down water flow and allow it to percolate into the ground, while percolation tanks are used to store floodwater for infiltration into aquifers, improving GWR [46]. Additionally, FWH pits are implemented to collect and contain water temporarily, preventing uncontrolled runoff that can cause soil erosion and damage infrastructure.

This method is particularly relevant in regions prone to seasonal flooding or areas with intense and erratic rainfall patterns. FWH often involves large-scale landscape management, requiring coordinated efforts at both community and governmental levels. Beyond mitigating flood risks [47], this technique provides numerous benefits, such as improving water availability during dry periods, enhancing soil moisture for agriculture, and restoring ecological balance in flood-prone areas [48].

FWH is increasingly recognized as a valuable climate adaptation strategy in the face of changing rainfall patterns and extreme weather events caused by climate change. By capturing and managing floodwaters effectively, this approach not only addresses immediate water-related challenges but also contributes to long-term water security and sustainability [49].

4.3. Groundwater Harvesting (GWH)

GWH involves the extraction and sustainable utilization of water stored in underground aquifers, typically accessed through wells, boreholes, or springs. This method is an essential part of water resource management, particularly in arid and semi-arid regions where surface water is scarce [49]. GWH also includes the use of specialized techniques and structures such as subsurface dams, Qanat systems (combination of shaft and tunnel system) and specialized wells, to enhance GWR and ensure long-term water availability [50,51].

Subsurface dams are built within riverbeds to obstruct the flow of ephemeral streams during seasonal rains, by trapping water in the sediment below the ground surface. Subsurface dams reduce water loss from evaporation and provide a sustainable source of water for aquifer recharge. The stored water can then be accessed later through wells or other extraction methods, making it a reliable source for drinking, irrigation, and other uses [49]. Qanat systems are used in the Middle East and North Africa and are remarkable engineering systems consisting of a series of underground tunnels and vertical shafts. These structures transport groundwater from higher elevation aquifers to the surface using gravity alone, eliminating the need for mechanical pumps. Qanats are highly efficient in regions with sloping terrains and help deliver water to arid plains for agriculture, drinking, and domestic use [50]. Specialized wells such as infiltration wells and recharge wells are designed to facilitate the percolation of surface water into the underground aquifers [52]. These wells enhance GWR by capturing rainwater or runoff and directing it into the aquifer system. In regions with declining water tables [37], recharge wells are increasingly used to replenish groundwater reserves and prevent further depletion [20,52].

By minimizing water loss through evaporation and improving water storage efficiency, GWH contributes to sustainable water management practices. Despite its advantages, GWH faces challenges such as over-extraction, contamination, and the need for proper maintenance of harvesting structures. Sustainable management practices, such as monitoring water levels, regulating extraction, and promoting aquifer recharge, are crucial to ensure the long-term viability of groundwater resources [46].

5. Groundwater Recharge

5.1. Benefits of Groundwater Conservation

Groundwater conservation is essential for both the environment and society due to its far-reaching impacts on sustainable water supply, agricultural support, ecosystem conservation, and drought resilience. Additionally, it plays a critical role in preventing land subsidence, protecting water quality, adapting to climate change, preserving cultural and social values, and enhancing GWR. As a vital resource, groundwater supports numerous livelihoods, ecosystems, and communities, making its conservation a top priority in water resource management.

One of the most effective strategies for groundwater conservation is GWR [48], which involves replenishing aquifers to maintain their sustainability and functionality [53]. This can occur naturally, through precipitation and infiltration, or artificially, by directing water into aquifer systems through recharge structures [48,53].

The identification of areas suitable for artificial recharge is crucial and should be based on specific factors (among others), areas such as (1) declining groundwater levels, (2) de-saturated aquifers, (3) inadequate water supply (wells and hand pumps fail to provide sufficient water), and (4) poor ground water quality.

By focusing on these factors (in addition to geological and hydrogeological factors), GWR efforts can enhance (1) the availability and quality of groundwater, (2) support long-term environmental sustainability, (3) mitigate subsidence, and (4) improve human well-being [49]. Additionally, such measures contribute to climate change adaptation [48], providing resilience against the increasing challenges posed by changing precipitation patterns and water scarcity [54]. Groundwater conservation and recharge are integral to protecting this critical resource and ensuring its availability for future generations [20].

5.2. Water Sources for GWR

GWR is a process that replenishes underground aquifers through both natural and artificial means. To ensure effective recharge, it is crucial to identify and evaluate the availability of adequate water sources. These sources serve as the foundation for designing recharge systems, particularly in regions facing water scarcity or aquifer depletion. The primary sources of water for GWR include (1) rainfall, (2) rainwater collected from rooftops, (3) surplus water from natural streams and springs, and (4) properly treated municipal and industrial wastewater.

Rainfall or precipitation is the most common natural source of GWR. Rainwater percolates through the soil and gradually replenishes aquifers, making it an integral part of the hydrological cycle. However, the effectiveness of precipitation as a recharge source depends on several factors, such as land slope, soil type, geology, vegetation cover, and rainfall intensity. While in situ precipitation occurs naturally at all locations, it may not always provide enough water for effective recharge, especially in arid or semi-arid regions where rainfall is infrequent or insufficient.

In urban and densely populated areas, rainwater collected from large roof surfaces can significantly contribute to GWR. Rooftop RWH systems collect and channel rainwater into recharge structures such as pits, trenches, or wells. This method is particularly effective in areas with limited open land for natural infiltration and helps reduce surface runoff, which often causes waterlogging and urban flooding. By utilizing roof areas, rainwater can be captured and redirected to replenish aquifers, making it a valuable strategy for urban water management.

Natural streams, rivers, and other surface water bodies are another important source of water for GWR. During periods of high flow or seasonal surpluses, water from these sources can be diverted into recharge systems like check dams, percolation tanks, or infiltration basins. This diverted water helps replenish aquifers while minimizing surface water wastage. However, it is essential to ensure that the diversion of water does not violate the rights of downstream users or disrupt aquatic ecosystems. Proper planning and stakeholder engagement are necessary to balance recharge activities with other competing water needs.

In addition to natural sources, properly treated municipal and industrial wastewater can be utilized for GWR [20]. Advanced wastewater treatment processes remove harmful contaminants, making the water safe for recharge purposes. Treated wastewater can be directed into aquifers through recharge wells or basins, providing a sustainable way to reuse water and reduce pressure on freshwater resources. This approach is particularly beneficial in urban and industrial areas where water demand is high, and alternative sources are limited.

In regions where rainfall and natural water sources are inadequate for effective recharge, alternative water sources can be explored and transported to recharge sites. For example, surplus water from nearby reservoirs, rivers, or other catchments can be conveyed through pipelines or canals to areas needing recharge. While this solution enhances recharge efficiency, it requires careful infrastructure planning to minimize water losses during transportation and ensure cost-effectiveness [20]. Kebede et al. [48] pointed out that globally, GWR projects are increasing by around 5% each year.

5.3. Groundwater Recharge Methods

5.3.1. Natural Groundwater Recharge (GWR)

Natural GWR is a process through which water from precipitation and other surface sources infiltrates the ground, replenishing underground aquifers [33]. This occurs when rainwater or melted snow seeps into the soil and permeable rock layers, gradually making its way to the groundwater table. The efficiency and rate of natural recharge are influenced by a variety of factors, including soil type, geology, land cover, topography, depth to groundwater table, vegetation and climate conditions [2]. Favorable conditions for natural recharge include permeable soils, fractured rock formations, perennial rivers and streams, forest cover, and relatively level land with minimal slope, all of which enhance the infiltration of water into the subsurface.

However, natural GWR can be significantly reduced or obstructed due to a variety of factors, many of which are driven by human activities and environmental changes. Urbanization is one of the main contributors to reduced recharge, as the expansion of impervious surfaces such as roads, pavements, and buildings prevent water from infiltrating into the soil. Instead, water flows off these surfaces into storm drains, reducing the amount of water available for aquifer replenishment [55].

The presence of vegetation plays a crucial role in facilitating natural recharge. Trees and plants promote infiltration by allowing water to percolate through the soil, while their roots create pathways that enhance soil permeability. When vegetation is reduced, for example, through deforestation or land clearing, there is a significant reduction in the amount of water absorbed into the ground, leading to lower recharge rates. Similarly, certain agricultural practices, such as excessive tillage, can compact soil, decreasing its ability to absorb water. The use of chemical fertilizers and pesticides further degrades soil health and reduces permeability, impeding the natural recharge process [33].

In addition, excessive groundwater extraction can outpace natural recharge rates, leading to a decline in the water table. Over-extraction can also reduce the effectiveness of recharge, as deeper groundwater levels make it harder for infiltrating water to reach aquifers. Climate change further exacerbates these challenges by altering precipitation patterns and intensities. For instance, an increase in the frequency and intensity of droughts, along with shifts in rainfall distribution, can reduce the overall availability of water for recharge. Areas that experience heavy rainfall over short periods may see more surface runoff than infiltration, further limiting the water that reaches the aquifer.

Another critical factor affecting natural recharge is the discharge of polluted water into natural waterways. Polluted water can block soil pores with impurities, reducing the soil’s infiltration capacity. Industrial discharges, untreated wastewater, and contaminated surface water flows prevent water from percolating effectively into the ground, thereby hindering natural recharge processes [25].

5.3.2. Artificial Groundwater Recharge (GWR)

Artificial GWR is a method of replenishing groundwater reservoirs by deliberately modifying the natural movement of surface water, treated wastewater, rainwater, etc., using appropriate civil/hydraulic engineering techniques [16,21]. This process is typically implemented in situations where natural recharge is insufficient to meet groundwater demand throughout the year and store water for later use. By employing artificial recharge, water managers can enhance the sustainability of aquifers and ensure the availability of groundwater for various uses.

The primary objectives of artificial recharge techniques are to [18] (1) enhance natural recharge: replenishing aquifers by capturing and directing precipitation and surface runoff into the ground; (2) stabilize aquifer levels: contributing to maintaining consistent groundwater levels; (3) control contamination of water supplies: protect groundwater quality and reduce the risk of pollutants infiltrating aquifers (recharged water quality is controlled); and (4) prevent freshwater degradation: minimize salt water intrusion in coastal areas.

These techniques are especially valuable in areas with high water demand, limited natural recharge, or increasing pressure on groundwater resources due to climate change, urbanization, and population growth.

Groundwater levels can be improved through various artificial recharge methods, including spreading, recharge/injection wells, and the induced infiltration method (see

Table 1. Summary of artificial GWR techniques and types (gathered from the reviewed papers).

Artificial GWR Methods
Direct Methods						Indirect Methods
Main Type	Sub-Type	Example *	Main Type	Sub-Type	Example	Main Type	Sub-Type	Examples *
Surface	Spreading (In- and Off Channel)	Flooding	Subsurface	Injection well	Recharge Well	Induced infiltration	Induced Recharge	Pumping Well
		Ditches and Furrows			Dug Well			Collector Well
		Recharge Basins			Recharge Shaft (Vadose zone wells)			Infiltration Gallery
		Run-off conservation structures			Recharge Pit		Aquifer Modification Techniques	Bore Blasting
		Stream-channel modification			Connector wells			Hydro-fracturing
		Surface Irrigation					Groundwater Conservation Structures	Subsurface dam
								Fracture Sealing/Cementation Technique

* Note: Terminologies related to artificial GWR techniques are not standardized. In the reviewed papers terminologies vary from region to region and within a country.

). According to Mukherjee [32], the first two methods—spreading and recharge/injection wells—are classified as direct techniques, with spreading involving the application of water directly to the surface, while recharge/injection wells operate below the surface. In contrast, the induced infiltration method falls under the indirect category, as it utilizes the natural processes of infiltration from nearby water sources to enhance GWR. Examples of induced infiltration method include induced recharge, aquifer modification, and groundwater conservation structures [28,32]. Zhang et al. [56] presented an in-depth review and analysis on historical development, the current situation, and perspectives of managed aquifer recharge.

I.

Direct Methods

(a)

Surface Spreading Method

The surface spreading method is a widely used technique for artificial GWR [57]. In this approach, water is diverted from streams or released from reservoirs into shallow basins or trenches. Recharged water then infiltrates through the bottom of these basins or trenches, percolating through the unsaturated zone (vadose zone) and gradually migrating to the water table. The rate of recharging is influenced by several factors, including (1) the permeability of the material between the basin bottom and the water table, (2) the depth of water table in the basin, and (3) the biological, geochemical, and physical changes occurring within the materials through which the water moves [31]. For optimal recharge efficiency, the area chosen for the surface spreading method have gently sloping terrain that avoids significant gullies or ridges. Additionally, the vadose zone—the soil and rock above the water table—should be permeable and free from clay lenses, as these can obstruct water movement and reduce infiltration. The permeability of the geological formation plays a critical role in ensuring that water easily reaches the aquifer.

Several sub-techniques are commonly employed within the surface spreading method. These include [32,57,58] Flooding, Ditches and Furrows, Recharge Basins, Run-off Conservation Structures (Bench Terracing, Gully plugs, Contour bunds, Contour trenches, Percolation tanks, etc.), Stream Channel Modification, and Surface Irrigation.

The surface spreading method is particularly effective in areas with high permeability in the vadose zone and is a cost-efficient solution for enhancing groundwater resources when natural recharge is insufficient [32].

• (b)

  Subsurface method

The subsurface method is employed in regions where the soil exhibits low vertical permeability, which hinders natural infiltration of water into the ground [31,32]. This technique involves the construction of structures such as injection wells, recharge wells, dug wells, shafts, and pits that are placed below the surface [16]. These structures facilitate the direct recharge of groundwater by allowing water to be introduced into the subsurface layers, overcoming the limitations posed by the surrounding soil conditions. As a result, the subsurface method is particularly effective in areas where traditional surface recharge techniques may not be viable due to insufficient permeability.

II.

Indirect Methods

(a)

Induced Recharge

Induced recharge is an indirect method of artificial GWR that involves pumping water from an aquifer that is hydraulically connected to surface-water sources. By creating a flow gradient, this process encourages the surface water to infiltrate and recharge the groundwater reservoir. One of the notable benefits of induced recharge is the natural filtration effect: as surface water passes through the aquifer materials, its quality often improves before being discharged from the pumping well, provided the hydrogeological conditions are favorable. This natural filtration process can significantly enhance the overall quality of the groundwater [32]. A few examples of induced recharge are described below [29].

Pumping Wells: An induced recharge system typically pumping wells strategically located near perennial streams that are hydraulically linked to an aquifer through permeable materials in the stream channel. A prime location for a well is often at the outer edge of a bend, where the hydrological conditions are particularly conducive to recharge. However, a critical consideration in this system is the chemical quality of the surface water source, as it directly impacts the effectiveness and safety of the recharge process [32].

Collector Wells: These are used to extract significant water from riverbed or lakebed deposits and waterlogged areas. In cases where the phreatic aquifer (unconfined aquifer) near the river has limited thickness, horizontal wells are often more effective than vertical wells. Collector wells equipped with horizontal laterals and infiltration galleries can considerably enhance induced recharge from the stream, making them a preferable option for maximizing water extraction in suitable locations [29,32].

Infiltration Gallery: Infiltration galleries are specialized structures designed to access groundwater reservoirs beneath riverbed strata. These galleries consist of horizontal perforated or porous pipes with open joints, enclosed in a gravel filter envelope [32,57]. They are installed in permeable, saturated layers with a shallow water table and a continuous source of recharge. Typically positioned at depths of 3 to 6 m [32], infiltration galleries collect water through gravity flow. Key considerations when designing an infiltration gallery include the desired yield and economic feasibility of the project [32,59].

• (b)

  Aquifer Modification Techniques

Aquifer modification techniques aim to enhance the physical characteristics of aquifers, improving their ability to store and transmit water. While these methods are primarily used to augment water yield rather than serving as traditional artificial recharge structures, they are increasingly recognized as effective approaches for boosting groundwater storage within aquifers [29,32].

Bore Blasting is an effective technique for hard crystalline and consolidated strata [59,60]. Through detailed hydrogeological investigations, suitable sites are identified where aquifers are underperforming or drying up. In this method, blasting holes are drilled to the required depth of the aquifer—whether it is unconfined or confined. Explosive charges are arranged in rows or circular patterns and detonated simultaneously to maximize the impact. The controlled blasts increase the secondary porosity and permeability of the aquifer, allowing for greater water infiltration and storage capacity [31,32].

Hydro-Fracturing is a modern alternative to traditional blasting methods, particularly in cases where blasting has yielded inconsistent results. This technique is designed to enhance secondary porosity in hard rock formations by applying hydraulic pressure to a specific zone within bore wells. The process involves injecting high-pressure water into the borehole to initiate and expand fractures in the rock. Additionally, it clears clogged fissures and improves connectivity with nearby water-bearing strata, significantly increasing water flow and accessibility. Hydro-fracturing is particularly effective for improving the hydraulic properties of aquifers in areas with limited natural recharge potential.

• (c)

  Groundwater Conservation Structures

Groundwater conservation structures are essential for ensuring that artificially recharged water remains available for use when needed. Once water is recharged into the aquifer, it becomes subject to natural groundwater flow regimes, which may lead to its movement away from the desired area. Conservation techniques aim to slow or control this flow, enhancing sustainable management of groundwater resources and improving long-term water availability [54].

Subsurface Dams/Underground Barriers are structures built across streams, or other flow paths to slow the natural flow of groundwater and create subsurface storage. By impeding the movement of groundwater, these dams increase the storage capacity within the aquifer and allow water to be retained during peak usage periods. The primary objective of a subsurface dam is to prevent the outflow of groundwater from the sub-basin while simultaneously enhancing the aquifer’s storage potential. This method is particularly effective in areas where aquifers experience rapid depletion or where seasonal water demand is high [52].

The Fracture-Sealing Cementation Technique is a specialized groundwater conservation method used in arid and semi-arid regions. In these areas, boreholes may initially provide good yields but often dry up by the end of the winter or summer due to limited groundwater storage along preferential flow paths. This method involves sealing fractures in the rock with specially formulated cement to reduce water loss through these pathways. In addition to conserving groundwater, this technique is highly effective in preventing the intrusion of saline or polluted water from known sources, thereby protecting the quality of the aquifer. It is particularly useful in regions with challenging topography or aquifer conditions where water management is critical [52].

5.4. Groundwater Recharge (GWR) Estimation Techniques

GWR estimation involves quantifying the amount of water that infiltrates into the groundwater system from various sources. This is a critical process for understanding and managing groundwater resources. A variety of methods for estimating both natural and artificial recharge are documented in the literature [34,35,36,37,39]. The selection of an appropriate method depends on several factors, including (1) data availability, (2) local geographic and topographic conditions, (3) spatial and temporal scales required for the analysis, and (4) reliability of results for specific context.

According to Singh et al. [34] and Islam et al. [37], the following are the main techniques used to estimate GWR: (1) water table fluctuation (WTF), (2) water budget, (3) Darcy’s law, (4) empirical relationships, (5) tracer techniques, and (6) groundwater models.

Each method has its own strengths and limitations (see

Table 2. Summary of GWR estimation techniques: assumptions, applicability, and limitations (from the reviewed papers).

GWR Estimation Methods	Assumptions	Applicability	Limitations
Water Table Fluctuation (WTF)	* fluctuations of the water table are entirely due to recharge or discharge, * specific yield of the aquifer remains constant over time	* restricted in regions, where water table scrutiny is frequently performed	* does not consider lateral flow from high to low water head
Water Budget Method (WBM) (e.g., Soil Moisture Balance (SMB), Base flow separation (BFS))	* accurate data collection and homogenous medium * accounts for all inflow and outflow constituents	* accuracy is heavily reliant on the precision of the individual components	* require analysis of a large volume of hydrological data
Darcy’s Law	* groundwater flow is laminar * homogeneous and isotropic medium	* employed to estimate the seepage velocity of lateral flow	* applicable only for laminar flow, homogeneous and isotropic media
Empirical Methods	* consider the (annual) rainfall as a function of groundwater recharge	* watersheds that do not have observed recharge measurements	* require extensive historical data * Site specific equation needed
Tracer Techniques	* shorter-term and straightforwardly available data	* effective in recharge estimation regardless of the fact whether the recharge is diffused or focused	* used in water-scarce areas * costly in applying and the time required between applications and sampling
Groundwater Models	* long term and complete data availability	* identify recharge hotspots and areas of low recharge potential, * can evaluate how precipitation and temperature patterns shifts	* accuracy of model prediction depends on successful calibration and verification * lack of on-site recharge measurements for validation

), and the choice of the technique is often influenced by the specific hydrogeological conditions of the area under study. Combining multiple methods can also improve the accuracy and reliability of recharge estimates. Brief descriptions of each method are presented below based on the current literature (e.g., [34,37,56]).

5.5. Water Table Fluctuation (WTF) Method

The WTF method is a widely used approach for estimating GWR by analyzing changes in the water table. Recharge causes the water table to rise, which increases the volume of water stored in the aquifer. This method assumes that the rise in water table is directly related to the amount of recharge entering the aquifer. However, it also accounts lateral drainage that occurs during the recharge process, providing a more accurate assessment of how much water is stored and how it interacts with surrounding areas.

To apply the WTF method, a hydrograph of the water table is analyzed to observe fluctuations over time. The method is particularly effective in areas where aquifers are unconfined and where water table and specific yield data are readily available. By examining the modified rise in the water table, the method estimates both the water stored in the aquifer and the portion that contributes to discharge. This technique is commonly used because of its simplicity and reliance on readily available data, such as water table measurements and specific yield values. However, it is most effective in regions with minimal external influences, such as pumping or evapotranspiration, which can complicate the interpretation of WTF [60].

A comprehensive and detailed account of the WTF method is presented in [61]. The WTF method is widely applied in various hydrogeological settings, including humid regions and fractured aquifers. However, it is often criticized for its limitations, such as its inability to account for spatial heterogeneity in aquifers or to provide accurate recharge estimates in areas with complex groundwater flow systems [38].

Despite these challenges, it remains a valuable tool for local-scale recharge estimation and short time span and is often used in combination with other methods to improve accuracy [60]. On the other hand, a summary of different studies indicates that the WTF method often overestimates recharge compared to other field-based approaches. Unfortunately, the significant uncertainty of other field-based approaches to recharge estimation precludes a clear interpretation of the reliability of the WTF method [61]. The applicability and limitations of the WTF method are thoroughly documented and tested by [38,61]. In regions where factors such as pumping, tidal effects, and lateral flow lead to significant fluctuations in water levels, the effectiveness of this method was reported as questionable.

5.6. Water Budget Method

The Water Budget Method (WBM) is a widely used approach for estimating GWR by balancing the various components of the hydrological cycle. This method evaluates all inflows and outflows of water within a specific area over a defined time period. It involves quantifying water inputs (recharge) and outputs (discharge) in a watershed or aquifer system to determine the net change in water storage.

According to Islam et al. [37], the parameters required for recharge estimation include precipitation, water flow into and out of the area, evapotranspiration, changes in subsurface storage, and base flow. While most of these parameters can be measured or estimated, the accuracy of the recharge calculation is highly dependent on the precision of the other components.

The infiltration of rainwater into the groundwater is influenced by factors such as soil hydraulic properties, topography, vegetation, and land use [62]. Infiltration coefficients are used to estimate the amount of water that percolates into the ground. These coefficients vary based on surface conditions, such as built environments, vegetation, or bare soil. For example, built areas without RWH systems typically have a very low infiltration coefficient compared to natural landscapes like meadows or forests [62,63]. The effective infiltration coefficient for an area can be calculated by considering the contributions of different land cover types and their respective infiltration rates. This value helps determine the volume of water that infiltrates into the groundwater system from precipitation across various surfaces.

Surface runoff is another critical component of GWR calculations. Runoff is affected by factors such as rainfall intensity and duration, land use/cover, soil type, and topography. RWH and GWR systems can significantly reduce surface runoff by increasing the infiltration of water into the subsurface, thereby enhancing recharge potential [23,63]. A commonly used model for estimating surface runoff was developed by the United States Soil Conservation Service (SCS) [64,65]. This method is particularly effective for assessing runoff depth across watersheds by using runoff curve numbers (CNs). The CN reflects the combined effects of soil type, vegetation, and land use on surface runoff. Higher CN values indicate greater runoff potential, while lower values suggest more infiltration and less runoff [64,65]. The potential maximum retention of water in a region is closely tied to the CN value, which serves as a critical parameter for estimating how different landscapes respond to precipitation. This relationship allows for the calculation of storm runoff depth and helps quantify the portion of precipitation that contributes to surface runoff versus infiltration [66].

While the WBM is comprehensive, its accuracy is heavily reliant on the precision of the individual components, such as precipitation, evapotranspiration, and surface runoff estimates. Small errors in these parameters can lead to significant inaccuracies in recharge calculations. Additionally, variations in soil conditions, land use, and topography can further complicate the estimation process. Therefore, careful consideration and accurate data collection are essential for reliable results [34].

5.7. Darcy’s Law

Darcy’s law serves as a foundational principle in hydrogeology, providing critical insight into the flow of groundwater through porous media. This principle, established by Henri Darcy in the 19th century, is vital for understanding dynamics of groundwater movement and particularly significant in estimating GWR—the process by which water infiltrates from the surface into the groundwater system [31,38].

Darcy’s Law explains how groundwater flows in response to differences in hydraulic head and the properties of the geological materials through which it moves. By analyzing the hydraulic gradient (the change in water pressure over a distance) and the hydraulic conductivity (which reflects the permeability of the soil or rock), the rate at which water infiltrates into aquifers can be estimated [38].

5.8. Empirical Methods

Empirical relationships between groundwater recharge and rainfall are often established through seasonal groundwater balance studies. By analyzing data such as precipitation, evapotranspiration, and changes in groundwater levels across different seasons, recharge rates can be identified linking with rainfall amounts. These studies help to better understand how variations in rainfall contribute to groundwater replenishment, providing a foundation for predicting GWR under different climatic and hydrological conditions.

Such an approach enables the development of predictive models that estimate GWR based on seasonal rainfall variability. These models are particularly valuable for water resource management, as they can guide decision-making in regions where rainfall patterns directly influence aquifer sustainability. For example, [37] explored the relationship between rainfall and recharge using empirical methods, highlighting how recharge rates vary with seasonal precipitation changes in different hydrogeological settings.

Establishing empirical relationships between rainfall and recharge is critical in regions affected by climate variability or water scarcity. By identifying and quantifying these relationships, water managers can

• Predict the impacts of drought or changing rainfall patterns on groundwater resources.
• Design effective groundwater management and recharge enhancement strategies.
• Support sustainable groundwater extraction practices in agricultural, urban, and industrial sectors.

5.9. Tracer Techniques

Tracer techniques are widely employed in arid and semi-arid regions to estimate GWR from sources such as irrigation and rainfall. These methods rely on the use of environmental tracers to track water movement and quantify recharge rates. For instance, the chloride mass balance (CMB) method [34,36] is commonly applied to assess recharge by analyzing chloride concentrations in soil and groundwater, providing insights into recharge processes under dry conditions.

Additionally, radiocarbon dating using Carbon-14 is employed to determine the age of groundwater [36], which helps us to understand long-term recharge dynamics. Other isotope-based approaches, such as the use of stable isotopes (e.g., δ²H and δ¹⁸O) [67,68], radioactive isotopes (e.g., Radon (²²²Rn) and Tritium (³H)), and noble gasses (e.g., Argon and Neon) are used to define and interpret groundwater ages, flow paths, recharge areas, leakage, and interactions with surface water [37,69]. These tracer studies are particularly valuable in regions where direct measurement of recharge is challenging due to limited water availability or complex hydrogeological settings [69].

Wilske et al. [70] applied a multi-environmental tracer study in a structurally complex multi-aquifer system. The calculated recharge rates estimate were found to be comparable well with CMB and numerical flow modeling. The multi-tracer methodology presented in [70] are applicable in other data-sparse areas with complex hydrogeology (karst or fractured) with or without anthropogenic influence.

5.10. Groundwater Models

Field measurements of GWR often carry a significant degree of uncertainty, primarily due to the complexity of natural systems and the limitations of measurement techniques [54]. Recharge estimation is influenced by numerous factors, including variability in precipitation, soil properties, vegetation cover, and hydrogeological conditions, all of which contribute to the challenges of obtaining precise measurements. Despite ongoing advancements in methods and technologies, reducing this uncertainty remains an active area of research in many regions [52,54].

In addition to direct field measurements, groundwater models have become essential tools in estimating recharge. These models incorporate a range of hydrological, geological, and climatic data to simulate recharge processes under various conditions. By integrating spatial and temporal variability, groundwater models can provide valuable insights into the distribution and dynamics of recharge over large areas and extended periods. For example, models can account for differences in recharge rates across diverse geological formations, as well as the impacts of seasonal or long-term climatic variations.

Groundwater models also serve as predictive tools, enabling researchers and water resource managers to forecast recharge under changing environmental conditions, such as land use changes or climate variability. This predictive capability is crucial for sustainable water resource management, as it helps assess the resilience of aquifer systems and informs decisions on groundwater extraction and recharge enhancement strategies.

In summary, utilization of WH and artificial GWR techniques varies significantly. Similarly, recharge estimation techniques differ across the literature [71], with their application varying based on location and data availability. For instance, [72,73] employed the SMB and CMB methods in coastal aquifers of the Gaza strip, whereas [74,75] applied SMB, BFS, and WTF methods in volcanic aquifers with diverse topography in Ethiopia. Additionally, the WTF and BFS methods were used in the volcanic aquifer of Mount Meru in Tanzania by [76].

Terminology for WH techniques also varies across the literature and regions. Standardizing these technical terms improves communication among professionals and decision makers, facilitating the effective dissemination of experiences and knowledge both globally and regionally.

6. Conclusions

This paper systematically reviewed 76 studies published in international peer-reviewed journals to inform future WH and GWR interventions. The findings strongly advocate WH and GWR as essential strategies to combat the growing challenges of water scarcity and promote environmental sustainability, particularly in arid and semi-arid regions. Groundwater remains a vital resource for drinking water, agriculture, and ecosystem health. However, increasing pressures from urbanization and climate change underscore the urgent need for effective management practices, such as WH and artificial GWR, to ensure sustainable water resource utilization. Few papers highlighted the scarcity of source water for WH and GWR in arid areas, lack of awareness, and research related to these interventions. Financial limitation is another constraint for their limited distribution and application in developing countries.

This review further emphasizes the successful application of WH and artificial GWR in various regions, with suitability mapping identified as an essential tool for locating favorable areas/sites. Such tools are particularly valuable under changing rainfall regimes driven by climate change, which further highlight the significance of WH and Artificial GWR. The integration of multiple WH techniques, including rainwater, floodwater, and groundwater harvesting, offers practical solutions to enhance water availability and quality while contributing to the long-term sustainability of groundwater systems.

Tailored approaches that consider local hydrogeological conditions are fundamental for effective GWR implementation. The accurate estimation of GWR remains a scientific challenge, but employing multiple methods offers a more reliable framework for improving estimation accuracy. Despite inherent difficulties in reconciling results from different techniques, their combined use is recommended to guide water resource management strategies.

Finally, advancing WH and artificial GWR practices is essential for sustainable water resource management amid growing demand and climate variability. Collaborative efforts among policymakers, researchers, and practitioners are necessary to implement these strategies effectively. By prioritizing these actions, the resilience of groundwater systems could be improved, safeguard critical resources for future generations, and support ecosystems reliant on groundwater.