Development of a water resource sustainability index based on actual water consumption and environmental constraints applied to Aji Chai River Basin, Iran.

This study develops a novel water resources sustainability index based on actual water consumption (AWC, i.e., total evaporation and actual evapotranspiration) and environmental flow requirements in the Aji Chai Watershed, a sub-basin of Lake Urmia Basin in Iran. This index is defined as the ratio of AWC in a basin to its renewable water resources (RWR). The SWAT model was calibrated and validated from 1987 to 2021 to estimate average AWC and RWR. This study's novelty lies in developing an AWC-based sustainability index that integrates environmental flows, enabling basin-scale estimates of the withdrawal reductions needed to achieve sustainable water use. The results showed that during the validation period, correlation coefficient (R) and Nash-Sutcliffe Efficiency (NSE) values ranged from 0.4 to 0.93 and 0.01 to 0.74, respectively. Based on environmental flow allocations and groundwater depletion rates, the required decreases in surface water and groundwater withdrawals were estimated. Results show that the mean RWR and AWC are 582 MCM and 398 MCM, respectively, resulting in a sustainability index of 0.68, which exceeds the critical threshold of 0.40. To contribute the Aji Chai watershed's share toward meeting the environmental water requirement of Lake Urmia and sustain groundwater resources, annual surface and groundwater withdrawals should be reduced by 32% and 38%, respectively, mainly in the southern and western parts of the Aji Chai watershed. Additionally, when AWC surpasses its allowable limit, improving irrigation efficiency alone is not enough. Sustainable management instead requires changes in crop patterns, partial shift to rain-fed systems, deficit irrigation, or a combination of these strategies. A 32% reduction in surface water withdrawals would increase the average annual inflow to Lake Urmia by 19.5%, fully satisfying the lake's environmental flow requirements for this river. The methodology proposed has relevance for other stressed watersheds and drying lakes in arid and semi-arid regions.