Role of Groundwater in the Dryland Ecohydrological System: A Case Study of the Heihe River Basin

Groundwater sustains food production and ecosystem health in drylands. Groundwater influences the interlinked hydrological processes and ecological state; however, the function of groundwater in the ecohydrological system is yet to be quantified clearly. In this study, we assess the role of groundwater in the ecohydrological system of the Heihe River Basin, using a regional-scale integrated model with multiple independent databases. Our results show that first, in over 40% of mid-lower plain, the groundwater table is less than 10m deep, and both recharge and evaporation of groundwater dominate seasonal water budget variations; recharge is more affected by land use than groundwater evaporation, and it has a higher variability ranging over 2 orders of magnitude within vegetated land use type. Second, mountain block recharge maintains the piedmont groundwater; the upper mountainous partition between baseflow and mountain block recharge impacts the evaporation and two-way exchange between groundwater and stream/lake in the downstream alluvial plain. Third, the groundwater system stores over 50% of streamflow in the dry season and about 10% in the wet season along leakage segments; groundwater dominates the streams by contributing about 40% of streamflow in the dry season along the seepage segments of the middle alluvial plain. Fourth, shallow groundwater sustains a vibrant ecosystem directly by providing a stable source of water for vegetation and indirectly by buffering lateral groundwater flow. Our study not only bridges the quantification gap for regional studies of drylands but also provides suggestions on water management and future water sustainability and food security.