Uncertainty-based analysis of water balance components: a semi-arid groundwater-dependent and data-scarce area, Iran

In regions where gauging is lacking or limited, the development of a methodology for water balance assessment is a key challenge. Water balance assessment is a significant task in managing the sustainable use of water resources. This study aims to improve the accuracy of water balance components including actual evapotranspiration and groundwater recharge rate in the Hashtgerd study area, Iran. Groundwater extraction volumes are determined based on two-period data collection; while precipitation, evapotranspiration, and groundwater storage change are calculated from annual datasets. To address the data-scarce problem, as an additional measure of actual evapotranspiration, satellite measurements are used to improve recharge rate accuracy. To understand the impact of satellite data uncertainties on water resources studies, each estimated water balance component is compared to observation data, and the uncertainty of these components is quantified using statistical methods, variability, and standard error. The results show that the Hashtgerd aquifer receives 199 MCM from the main drains which is a major part of the water inflow. The extraction of wells is one of the most significant outflow components of the aquifer (i.e., 284 MCM); while water loss by evaporation is estimated to be 207 MCM of the outflow. The finding shows that satellite-based evapotranspiration can reduce recharge uncertainty which can improve the resolution of groundwater balance. This study supports that the aquifer is under severe environmental pressure. One of those concerns is that groundwater levels decreased by 0.92 m per year between 2000 and 2019.