A new physics-based rainfall–runoff method of the Soil and Water Assessment Tool (SWAT) was developed, which integrates a water balance (WB) approach with the variable source area (WB-VSA). This approach was further compared with four methods—soil-water-dependent curve number (CN-Soil), evaporation-dependent curve number (CN-ET), Green and Ampt equation (G&A) and WB—in a monsoonal watershed, Eastern China. The regional sensitivity analysis shows that volumetric efficiency coefficient (VE) with river discharges is sensitive to the most parameters of all approaches. The results of model calibration against VE demonstrate that WB-VSA is the most accurate owing to its reflection of the spatial variation of runoff generation as affected by topography and soil properties. Other methods can also mimic baseflow well, but the G&A and CN-ET simulate floods much worse than the saturation excess runoff approaches (WB-VSA, WB and CN-Soil). Meanwhile, CN-Soil as an empirical method fails to simulate groundwater levels. By contrast, WB-VSA captures them best.

Global warming has led to accelerated transitions between dry and wet periods, causing plants to experience more water stress and adapt water use strategies. This could have a significant impact on plants in karst regions due to the substantial spatial and temporal variability of soil and rock moisture. In this study, we monitored and compared transpiration (sap flow) responses to meteorological variables, soil moisture content and rock moisture content at five sites with a variety of plant‐soil‐rock compositions in the karst region of southwest China. Results show that the soil‐rock composition generally controlled plant growth and transpiration amount, and over 80% transpiration was concentrated in wet growing period. The thin soils provide limited soil moisture, with rock moisture primarily driving transpiration variability and influencing the physiological strategies of plant water use. High and steady rock moisture in soil‐filled fractures helps plants buffer seasonal variability in transpiration. Conversely, limited rock moisture storage capacity prompted plants to enhance transpiration during the wet period, thereby maximizing growth and carbon gain to improve drought resistance in the subsequent dry period. Plants adapt to varying environmental conditions by modulating their water use strategies. This shift can decrease the sensitivity of plant transpiration to meteorological factors, such as vapor pressure deficit. Since plant physiological behavior is extremely sensitive to climate variations and soil‐rock compositions, the acceleration of dry‐wet transitions will likely increase ecosystem vulnerability.