Understanding surface water–groundwater interactions for managing large irrigation schemes in the multi-country Fergana valley, Central Asia
Introduction
Groundwater is a critically important global water resource and it is intensively extracted at the rate of 982 km3 yr−1 (Margat and van der Gun, 2013), of which around 60% is used for agriculture and the rest for domestic and industrial uses (Vrba and van der Gun, 2004). Around 38% of the irrigated areas worldwide have facilities for direct use of groundwater (Siebert et al., 2010). In other areas, capillary rise from shallow groundwater contributes around 20–25% to total crop water requirements (Awan et al., 2014; Ayars et al., 2006; Kahlown et al., 2005; Kazmi et al., 2012). Shallow groundwater within a few centimeters of the surface is, however, also a source of waterlogging and secondary soil salinization (Awan et al., 2011a; Kahlown et al., 2005). In many regions (e.g., North Africa, Arabian Peninsula and South Asia), groundwater is intensively extracted at rates that have resulted in declines in groundwater levels (Awan and Ismaeel, 2015; George et al., 2011; Venot et al., 2010). In the other areas, the contribution from shallow groundwater to crop water requirements is not accounted for (Awan et al., 2017), leading to freshwater over-supply and eventual waterlogging. Hence, appropriate groundwater management is a prerequisite for sustainable management of surface and groundwater resources in large irrigation schemes.
Water from the Amudarya and Syrdarya Rivers is a main source of irrigation in Central Asian countries, and drainage and groundwater serve as a safety net during severe droughts (FAO, 2001). In Central Asia, such as in the Fergana Valley (Karimov et al., 2014) and Khorezm (Awan et al., 2017) Provinces of Uzbekistan, groundwater serves as a storage of water that infiltrates due to excessive water applications in agricultural fields and seepage from earthen irrigation networks. The groundwater level reaches 1–3 m below the land surface during intensive irrigation and thus contributes to soil moisture enhancement through capillary rise. Due to uncertainty concerning timely surface water supplies, farmers rely on shallow groundwater of low salinity by blocking field drains. At the same time, the ratio between drainage discharge and irrigation water supplies from agricultural fields in Uzbekistan is extremely high. For example, Awan et al. (2011b) reported that in Khorezm Province, 55% of the surface water supply is drained out from the agricultural system – much above the target value of 10% for large irrigation schemes (Bastiaanssen and Bos, 1999). This drainage discharge during cropping seasons in the Central Asian lowlands is unsustainably excessive.
Simulation scenarios of future climate change using Global Circulation Models showed that the surface water supplies in Central Asia will be reduced (Immerzeel et al., 2012) due to reduced thickness of glaciers. This may lead to conflicts concerning water sharing due to the transboundary nature of the flow of the main Central Asian rivers, and the intensified competition for water among various sectors such as domestic water use, agriculture, industry and the environment. Therefore, changes in surface water supply will impact recharge and thus discharge rates. In turn, under maintained high drainage discharge rates, lower groundwater levels will contribute less to crop water requirements, forcing farmers to utilize more surface water for irrigation (Tischbein et al., 2013). It is thus a prerequisite to better understand the interaction between the surface and groundwater resources to develop a groundwater management policy that will not only consider the recharge into, but also the discharge from, the groundwater reservoir.
Various methods of estimating groundwater recharge and discharge have been developed. The groundwater recharge methods vary from the small-field scale to large irrigation schemes and from simple water balance estimations (Scanlon et al., 2002) to complex recharge models (Awan et al., 2013; Beekman et al., 1999; Lerner et al., 1990; Massuel et al., 2013; Sylvain et al., 2014). It is necessary to choose a groundwater recharge estimation method based on appropriate spatial and temporal scales, and to account for the range and reliability of the method (Scanlon et al., 2002). In the current study, the traditional root zone water balance method based on the concept of estimating the incoming and outgoing components and changes within the soil profile was chosen for its simplicity and ease of capturing real field conditions. This method in combination with groundwater modeling has been used in the irrigated areas of the Indus Basin to simulate groundwater flow and so predict changes in groundwater levels in response to intervention scenarios in irrigation practices (Sarwar and Eggers, 2006). Maréchal et al. (2006) used the water balance method to estimate specific yield and groundwater recharge in an unconfined aquifer with significant seasonal fluctuations in the water table. They noted that the method has the advantage of estimating these parameters at the scale of interest without the need for extensive data and measurements. Yin et al. (2011) applied three methods to estimate groundwater recharge, including the water balance method, and concluded that each method had inherent uncertainty. The water balance method is easy to apply, inexpensive and produces low estimation errors. Implementing it requires data that are easy to collect: discharge can be estimated by measuring field-level drainage discharge or groundwater abstraction from tube wells. Estimating upward flux is another way of measuring groundwater discharge. The groundwater abstraction can also be estimated using remote sensing techniques (Awan et al., 2016).
The objectives of this work were to quantify groundwater recharge, discharge and net groundwater recharge (recharge minus discharge) at a Water Consumers’ Association (WCA) level. The irrigated areas of the Fergana Valley in Uzbekistan were chosen for this study, and analyses were conducted on a monthly basis for the period 2004–2011. Policymakers in the region can use this knowledge to manage the surface and groundwater resources in a sustainable way under changing climate situations.
Section snippets
Materials and methods
The study area for this research is a typical environmental setting in Fergana Valley with low precipitation and, hence, frequent irrigation applications, shallow groundwater levels and massive irrigation and drainage networks. The activities included obtaining datasets of irrigation water supplies and drainage discharge, climate and cropping patterns to construct a water balance model. Components of the water balance such as actual crop-specific evapotranspiration, net groundwater inflow and
Dynamics of groundwater recharge
Over the study period, the average annual groundwater recharge constituted 780 ± 75.7 mm (or 62% of the average gross irrigation water supply). These groundwater recharge values are similar to the estimates of Abdullaev et al. (2009) for the environmental and management settings of the Fergana Valley. The highest groundwater recharge was during 2010 (930 mm) due mainly to excessive precipitation and gross irrigation; and the lowest recharge was in 2006–2008 (667–726 mm) and 2012 (685 mm),
Conclusion
Irrigated agriculture exerts high pressure on Central Asian surface water resources, which are virtually fully utilized. In the Fergana Valley, the shallow groundwater is hydrologically connected to surface water resources. High groundwater recharge during the irrigation season raises the groundwater level close to the drainage depth. The surplus groundwater recharge is drained into the extensive drainage system. The irrigation to drainage ratio is around 55%, whereas the groundwater recharge
Acknowledgements
The International Center for Agricultural Research receives financial support from the CGIAR Research Program on Water, Land and Ecosystems (WLE), which was used to support this study.
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