Water conservation in arid and semi-arid basins exhibits a nonlinear effect on water quality improvement

Study region The Weihe River Basin (WRB), a typical arid and semi-arid basin in China. Study focus Water conservation regulates hydrological processes and pollutant transport, yet the response mechanisms of water quality to water conservation remain an interdisciplinary challenge. This study assessed the spatiotemporal dynamics of water conservation and water quality in the WRB, and developed a distributed time-variant gain water conservation model (DTVGM-WC) to quantify the nonlinear responses, identify critical threshold zones, and elucidate the underlying mechanisms linking water conservation and water quality. New hydrological insights for the region The daily-scale hydrological model achieved acceptable performance, with mean NSE values of 0.74 and 0.73 and mean KGE values of 0.77 and 0.70during calibration and validation periods, respectively. The annual mean water conservation during 2008–2019 was 47.6 mm, with a non-significant increasing trend of 1.67 mm·a⁻¹ and a spatial pattern of higher values in the south. Correlation analysis identified total nitrogen (TN) as the monitored water-quality constituent most clearly associated with water conservation at the monthly scale, with pronounced nonlinear sensitivity during seasonal transition periods. Water conservation exhibited a nonlinear effect on TN dynamics. A quantitative response function was developed to characterize this relationship, revealing threshold zones for water conservation regulating TN concentrations at [5 mm, 15 mm] in April and [4 mm, 8 mm] in October. These nonlinear patterns reflect complex ecohydrological regulation and provide insights into water management in arid and semi-arid basins.