Establishment of Instream and Off-Stream Ecological Water Requirements and Its Climate Impact at a Semi-Arid Watershed

The determination of ecological water requirements (EWRs) is of critical significance for maintaining watershed sustainable development and river health. However, the estimation of instream and off-stream EWRs remains uncertain due to the complicated and competitive interaction between off-stream EWR resources (mainly vegetation water requirements in low-intensity human-use basins) and instream EWR resources (runoff), especially in arid watersheds. In this study, instream and off-stream EWRs are determined by considering the interaction between vegetation variations and hydrological processes, as well as their climate impact, using a two-way ecohydrological model in a representative semi-arid basin. The increased infiltration capacity of the substrate, resulting from continuous vegetation growth without mortality, enhances deep soil water return flow, thereby boosting baseflow to streams. Lateral flow is shown to contribute up to 39.50% of the instream runoff. While downstream grassland growth is dependent on vertical water input, upstream forests experience energy-limited transpiration despite increased water storage, regardless of lateral flow distribution. Changes in precipitation (either an increase or decrease) simultaneously affect (i.e., increase or decrease) both basin instream and off-stream EWRs. In contrast, temperature increases of up to 3 °C generally enhance instream EWRs by raising evapotranspiration (ET). However, this effect may be diminished or even reversed when plants become water-stressed under higher temperatures, resulting in a reduction of off-stream EWRs. The findings of this research provide a scientific foundation for water resource management in semi-arid basins.