Subsurface storage of previous year’s precipitation contributes to mitigating water deficit and surplus in dryland woody basin

Understanding the intricacies of the relationships between subsurface water and vegetation in drylands is of paramount importance for preserving the ecological well-being, especially in arid Central Asian endorheic basins. However, the mitigation mechanism of subsurface water in the basin when vegetation encounters water deficit or surplus remains unclear, which limits our understanding of dryland function in maintaining ecosystem stability. This study aimed to bridge this knowledge gap within the sub-basins along Amu Darya (AD), one of the largest inland rivers in Central Asia. The extent of stored subsurface water (S) mitigation for the growing season (GS) water deficit and surplus from 1982 to 2018 was evaluated using the meteoro-hydrological data and woody vegetation Gross Primary Productivity (GPP) data. The nonparametric Spearman correlation coefficient, partial correlation analysis, and multiple linear regression method were employed to calculate the tendency of precipitation during pre-growing season (PGS) for subsurface storage, the response of vegetation to varying water conditions, and the mitigating capacity of subsurface water storage under water deficit and surplus situations, respectively. Our results showed that the downstream sub-basins tended to store PGS precipitation, and both S and GPP exhibited strong responses to the Standardized Precipitation Evapotranspiration Index (SPEI) on both 6- and 12-month timescales. The S from the previous year’s random precipitation effectively explained the mitigating effect of water carryover between the PGS and GS under the impact of water deficit or surplus on vegetation. In the AD sub-basins where the meteorological conditions are gradually becoming more arid, the S will become an important water source to mitigate water stress and ecological crises, despite its decreasing volume year by year. The study provides deeper insight into maintaining ecological stability in the endorheic basin, which will help provide solutions for disaster prevention under the conditions of climate change.