Present and future groundwater depletion rates in Wadi Zabid, Tihama Coastal Plain, Yemen

This paper presents a simple water budget model that can be used to quantify present and to predict future groundwater depletion rates in areas where there is a lack of the comprehensive long-term data needed to develop sophisticated numerical groundwater models. This study applied the water budget model in Wadi Zabid, Yemen, a region where groundwater withdrawals have far exceeded replenishment rates for 50 years, resulting in falling groundwater levels. The findings indicate that the present groundwater use in the wadi is unsustainable, mainly due to the expansion of agricultural lands. The current average groundwater depletion rate was calculated as - 0.93 m/yr, which is in line with the observed average of - 1.11 m/yr (1972-2016). Scenario analysis shows that reducing the groundwater depletion rate by two-thirds of the current rate (from 0.93 to 0.32 m/yr) would require a one-third reduction in agricultural lands (from 435 to 305 km2) in the study region, combined with a one-third increase in surface water inflow from upstream (from 132 to 172 mm/yr, also necessitating a reduction of agriculture in the upstream region). Economic incentives to support alternative livelihoods with lower water requirements, alongside utilization of non-conventional water sources (e.g., exploring the feasibility of seawater desalination) could reduce groundwater depletion. The simple water budget approach proved to be a useful means for this type of analysis in data-scarce regions.