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Estimation of groundwater recharge in semiarid regions under variable land use and rainfall conditions: A case study of Rajasthan, India [1]
['Basant Yadav', 'Department Of Water Resources Development', 'Management', 'Indian Institute Of Technology Roorkee', 'Uttarakhand', 'Alison Parker', 'School Of Water', 'Energy', 'Environment', 'Cranfield University']
Date: 2023-03
In the semiarid regions of India, the annual rainfall is very low (~650 mm) and erratic; hence groundwater recharge is vital to support crops, especially in the winter season. For groundwater budgeting it is essential to consider how groundwater recharge is affected by both land-use and rainfall distribution. This study used a soil water balance approach, considering hydrological, meteorological, hydrogeological and crop information to understand the recharge process in semiarid regions. The approach was used at a sub-watershed scale where farmers grow rainfed and irrigated crops. Delayed recharge response on the water table was considered to estimate actual recharge, which closely matches the observed water levels in the field. The recharge estimated in rainfed agricultural lands, rainfed-irrigated agricultural lands, and barren lands was 29%, 17%, and 31% of the total inflow.
Copyright: © 2023 Yadav et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Several studies have considered the impact of land-use change during recharge estimation using various direct or indirect approaches [ 18 , 26 – 28 ]. Similarly, the impact of rainfall seasonality, intensity, and distribution has also been studied independently under various climatic conditions [ 29 – 31 ]. However to the best of our knowledge, the combined impact of land-use changes and rainfall variability on the groundwater recharge in semiarid conditions has not been studied before. Therefore, this study aims to examine the effect of land-use change and rainfall variability on groundwater recharge and evaluate the impact on crop conditions. This was achieved by quantifying the soil water balance components on various land use patterns using observed data in the study site. Further, the soil water balance approach’s estimated recharge in the soil zone was converted into actual recharge reflected on the water table using a delayed recharge process. Obtained actual recharge was validated using water level data obtained from large diameter open wells. This approach provides flexibility to consider natural processes, rainfall variability, and land-use change while estimating actual groundwater recharge and understanding its role in the sustainability of rainfed and irrigation-based crops by providing routine recharge estimation at the local scale.
There is no single method universally applicable for the accurate estimation of the groundwater recharge rates [ 18 , 19 ]. It is important to distinguish ’actual recharge’ reflected on the water table and ’potential recharge’, which is merely water that infiltrates below the soil layer [ 18 ]. Actual recharge has been estimated under various field conditions using methods that include empirical formulas, hydrograph analysis, water budget, water table fluctuation, tracer methods, calculations using Darcy’s law in the unsaturated zone, and numerical methods [ 18 ]. A soil water balance or budgeting technique can be used for routine potential recharge estimation in many situations, provided that physical processes are represented adequately [ 15 ]. For example, conceptual understanding and impact of geological formation on the recharge mechanism and recharge rates were highlighted in Fitzsimons and Misstear [ 20 ] and Chung et al. [ 21 ]. A soil water balance approach, along with detailed information from the FAO report including crop evapotranspiration [ 3 ], can be used to estimate recharge. Eilers et al. [ 22 ] used this approach to estimate recharge in semiarid regions of Nigeria. Further, de Silva and Rushton [ 16 ] applied this approach when studying rice fields in the tropical climate of Sri Lanka. Rushton et al. [ 23 ] recently used this method to estimate recharge in a multi-aquifer system of north-west Bangladesh. However, the applicability of soil moisture balance techniques in semiarid regions has been questioned as often the magnitude of recharge is often comparatively less than the other variables such as evapotranspiration [ 5 , 24 , 25 ]. This can be overcome if the recharge is estimated using daily time steps coupled with an understanding of near-surface processes in the soil zone and subsequent water movement through underlying strata [ 18 , 23 ].
Similarly, land-use changes directly impact the groundwater recharge rate, mechanism, and spatio-temporal variability [ 10 – 13 ]. Potential recharge in a specific land use condition depends on the physical state of the soil surface as it controls soil hydraulic properties (water retention and hydraulic conductivity) and hence the infiltration process [ 14 ]. Crops can collect sub-surface moisture and transpire water to the atmosphere [ 15 ]. Where there is no or partial vegetation, bare soil evaporation is significant [ 16 ], which decreases as the water table falls [ 17 ]. A non-varying crop coefficient (Ke = 1.05) can be used to estimate bare soil potential evaporation from potential evapotranspiration [ 3 ].
Variable rainfall should undoubtedly be considered during recharge estimation, as the recharge in semiarid areas is erratic and may only occur on a few occasions per year [ 6 ]. Therefore, considering recharge as a proportion of mean annual rainfall is not practical, as the recharge rate is determined by the distribution of extreme events over threshold levels [ 7 , 8 ]. In India and other East Asian countries, more than 75% of the annual rainfall occurs during a 4-month monsoon season. The rainfall variability is perceived as the greatest threat to agricultural production in arid and semiarid regions, especially where rainfed agriculture is prevailing [ 9 ].
In arid and semiarid areas where irrigated agriculture prevails, accurate groundwater recharge estimation is crucial for assessing scarce water resources and their sustainable management [ 1 , 2 ]. In India, as in many other developing nations with agriculture-based economies, water resources are critical for economic development, and agriculture accounts for approximately 85% of the total annual abstraction [ 3 , 4 ]. Therefore, accurate estimation of the current groundwater recharge rate is essential for efficient and sustainable groundwater management in these regions. However, groundwater recharge estimation in semiarid regions has been a challenging task due to temporal variability of precipitation in semiarid climates, spatial variability in soil characteristics, topography, vegetation, and land use, and uncertainty in hydrogeological variables [ 4 , 5 ].
2. Materials and methods
2.1 Study area Lapodiya watershed of Jaipur district was selected as a study area. This watershed has a semiarid climate, a variety of different land-use patterns, and rainfall conditions where rainfed and irrigation-based agriculture are practiced. Because of the scarcity of surface water resources, inhabitants depend mostly on groundwater for both domestic and agricultural water supply [32]. Fig 1 shows the Lapodiya watershed (23.3 km2) located around 90 km west of the city of Jaipur in Rajasthan, the driest state of India where 90% of rural and 50% of urban water supply is met by groundwater [33]. The average annual rainfall in the study area for the last 34 years (1971–2014) is 575.7 mm, out of which over 90% is distributed in the monsoon season (June–October) and is subject to a lot of inter-annual variations, with a standard deviation of 205 mm [34, 35]. The area has been classified as semiarid as, over a year, rainfall represents less than 50% of potential evapotranspiration [36]. The mean maximum temperatures in this region can be bas high as 48°C in June, while in January the temperatures drop to between 7.7°C and 21°C [37]. PPT PowerPoint slide
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TIFF original image Download: Fig 1. Lapodiya watershed with the locations of 36 large diameter open wells which are used to observe water levels from March 2019 to May 2020.
https://doi.org/10.1371/journal.pwat.0000061.g001 The Lapodiya watershed has a small population of 1764 inhabitants, and agriculture is a primary source of livelihood for 87% of the workforce [38]. The farming system is complex in the study area as some farmers grow two to three crops in a year. The cropping seasons are divided into three periods, Kharif (monsoon season), Rabi (winter), and Zaid (summer). In the Kharif season (July-October), generally, maize, millet, sorghum, groundnut, black gram, mung-bean, and vegetables are grown, which are entirely dependent on rainfall. In the rabi season (November-February), crops like wheat, barley, mustard, and gram are grown, while in Zaid (March- June), fodder crops are grown in the areas where irrigation is available. Groundwater-based irrigation using large-diameter open wells abstracts water from shallow aquifers. Deep tube wells are not used due to the deep aquifer’s high salinity. The shallow aquifers has moderate salinity and high fluoride concentration, but it is deemed fit for irrigation. Aquifers in this region comprise hard rocks of the Bhilwara Super Group, comprising granulitic gneisses, quartz mica schist, phyllite, and granite and pegmatite intrusive [39]. In these aquifers, the movement of groundwater is controlled by the size, continuity, and interconnectivity of weathered and fractured parts and other secondary porosities. A report from Rajasthan Ground Water Department [40] suggests that the topsoil (<1 m) is dominated by sandy loam and loam soils. Further, weathered gneiss is from 1 to 20 m, and the deeper region (>20 m) is dominated by schist mixed with mica, quartz, and feldspar pieces (see Table 1). Bedrock depth is about 40 to 80 m deep, and most of the aquifers are unconfined in this region [35]. Groundwater in this area occurs both in unconsolidated Quaternary formations and consolidated formations at shallow depth under water table conditions and semi-confined conditions at depth. PPT PowerPoint slide
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TIFF original image Download: Table 1. Geological profile with a depth of Lapodiya catchment.
https://doi.org/10.1371/journal.pwat.0000061.t001
2.2 Field data collection The watershed selected in this study has never been studied scientifically, so the historic information was very minimal. A local observatory was established in the watershed to collect daily rainfall, temperature, and evaporation data. Historic daily rainfall data for the period of 2010–2019 was also gathered from the department of irrigation, government of Rajasthan (
http://water.rajasthan.gov.in/content/water/en/) Further, a monitoring network of 36 large diameter open wells was established to record the water table depth at weekly intervals from March 2019 to June 2020. Before selecting these open wells, a survey was conducted of all the wells (open wells and tube wells) in the study area, and water samples were collected. These 36 open wells were selected as they represent the varying quantity and quality of groundwater conditions in this watershed. Out of 36 open wells, 4 have a depth less than 10 m; 23 are between 10 to 20 m deep and, 9 are more than 20 m deep. Most of these wells are being used for irrigation and domestic purposes; however, nine wells are inactive or abandoned. The local non-governmental organization (NGO) staff were trained to collect meteorological and groundwater table depth data. During the field visits, farmers were interviewed to obtain historical information about the water conservation structures, large diameter open wells, drilling methods, irrigation systems, and crop patterns. Further, these farmers also provided crop and irrigation information (i.e. photos and videos) for the study. To understand the hydrogeology, two 12.7 cm or 5-inch diameter boreholes, as shown in Fig 2, were drilled in the study area using a down-the-hole drill (DTH rig), and sediment samples were collected at every one-metre interval. Analysis of collected samples and pumping tests in the study area suggests that the average hydraulic conductivity of unconfined aquifers in the study area consistently varied from 1 to 6 m/day, and the average specific yield varied between 1 and 7%. The data for crop patterns and land use were collected through a village-level survey in 2019–20. The land use data is divided into three categories, i.e., rainfed agriculture, irrigated agriculture, and barren land. Fig 2 shows that in this watershed, out of 23. 3 km2 area, 65% is rainfed, 11% is irrigated, 19% is barren land, and the rest is covered by ponds, farm ponds, and rural settlements. PPT PowerPoint slide
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TIFF original image Download: Fig 2. Land use map of Lapodiya watershed, showing the agricultural and barren land-use area used for the recharge estimation using the soil water balance approach.
https://doi.org/10.1371/journal.pwat.0000061.g002
2.4 Soil water balance in rainfed, irrigated and barren lands In the study area, monsoon season crops are entirely dependent on rainfall; therefore, the conceptual model for this period includes inflows due to rains and outflows due to bare soil evaporation, crop evapotranspiration, and excess runoff. At the onset of monsoon, when the millet crop is planted, the soil moisture deficit in the beginning (SMD b ) is higher than TAW. As the excess rainfall infiltrates the soil zone, SMD b is reduced where RAW<SMD 1 <TAW, however, the millet crop remains stressed. Further rainfall is infiltratedand this brings SMD 1 to SMD 2 <RAW, and at this stage, PE = AE and the millet crop has sufficient water for its growth and development (Fig 3A). Further percolations bring SMD 2 to SMD 3 , which means the soil zone has reached its field capacity, and excess water is being percolated to below the soil zone as potential groundwater recharge (Re). PPT PowerPoint slide
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TIFF original image Download: Fig 3. Conceptual model of soil water balance for a) rainfed cropland, b) irrigated cropland and c) barren land.
https://doi.org/10.1371/journal.pwat.0000061.g003 Irrigation is primarily used in the rabi season, starting in early November and ending in February. This season’s crops include chickpea, wheat, barley, and green peas, which generally require irrigation during the early, growing, and mature stages. Crops in this season are grown using groundwater-based irrigation, and hence, in this case, the inflows also include irrigation. The first irrigation brings SMD b to SMD 1 <RAW, which allows wheat planting. After 21 days of sowing (DAS), second irrigation (45 DAS) is provided, which reduces SMD 1 to SMD 2 <RAW, as shown in Fig 3B. At this stage, the wheat crop has enough water available for evapotranspiration and crop growth. Further irrigations at 65, 90,105, and 125 (DAS) keep SMD less than RAW without providing any excess water for potential groundwater recharge. Therefore, in this case, irrigation-based winter wheat does not contribute to the potential groundwater recharge. In this study area, barren lands cover a significant proportion of the area. They include both managed grazing grounds locally known as chaukas (small infiltration pits, [44, 45]) and unmanaged grazing areas covered in shrubs. The barren lands are used as grazing grounds for cows and goats. Fig 3C depicts a conceptual model for barren lands which is similar to the one presented in Fig 3A for rainfed crops. However, in this case, the millet crop is replaced by natural grass. Evaporation from bare soil depends on the atmospheric conditions and moisture content in the soil profile. The bare soil evaporation happens in three stages, namely: weather-controlled stage, soil profile control stage, and residual slow rate stage. In the weather-controlled stage, evaporation occurs at a constant rate as the soil is moist and able to supply water for evaporation. In the soil profile stage, evaporation rate is equal to the rate at which the gradually drying soil profile can provide water for evaporation. Lastly, at the residual slow rate stage, evaporation has virtually ceased as the surface soil is dry, and water is held only in the disconnected pores that are immobile. The limited bare soil evaporation and smaller root growth of grass in barren lands result in less evapotranspiration thus making more water available for potential groundwater recharge.
2.5 Delay between potential and actual recharge The estimated potential recharge leaving the soil zone will eventually join the water table. However, it has been observed in many studies that there can be some delay between the potential recharge leaving the soil zone and the response at the water table [42]. Time is required for recharged water in the top layer of the unsaturated zone to move downward. This delay in the downward movement of recharged flux can be related to recharge rate, soil-water content, and water table depth. This delay is required for the pressure front from increased deep drainage to move downward through the unsaturated zone [46, 47]. Such delays can also be attributed to the matrix storage in the deep vadose zone [48, 49]. This delay can further increase with increased aquifer depth [50]. Lee et al. [51] suggested that if the depth of the unsaturated zone is more than 18 m, the time lag between potential recharge and water table response increases rapidly. Moreover, complex geology including vertical fissures, or high permeability zones make this delay estimation even more challenging. Therefore, such a delay can be simulated using coefficients based on field observations [42]. In this study, the selected site’s unsaturated zone thickness varies between 5 and 10 m. The water levels of a piezometer (BH1) were collected daily and studied using hydrographs to identify the delay between rainfall and response on the water levels. The estimated daily potential recharge from SWB was matched with a daily incremental water level increase in BH1 to identify the delay factors, which later were used to transform the potential recharge into actual recharge. Daily water level rise and estimated incremental potential recharge depth of current day (t) and previous days (-t) were used to identify the delay factors as presented in Eq (8). The two coefficients (0.1 and 0.05) are used to explore the methodology, but when it is applied in practice, a smoother distribution is used. The approximated coefficients are site-specific and may change for other field conditions. (8) Where R t represents the recharge in day t.
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