Impacts of change in multiple cropping index of rice on hydrological components and grain production in the Zishui River Basin, Southern China

Recent declines in the rice Multiple Cropping Index (MCI) have reshaped grain production and water dynamics across Southern China, yet their effects on production stability and watershed hydrology, particularly in subtropical hilly regions, remain insufficiently studied. To address this, we extracted the current rice cropping structure in the Zishui River Basin (ZRB), Hunan Province, leveraging high-resolution Remote Sensing (RS) data. A planting suitability evaluation system for double cropping rice was developed by integrating climatic, soil, and site conditions through an Analytic Hierarchy Process (AHP) and GIS overlay. Based on these inputs, alternative rice cropping scenarios were simulated using the Soil and Water Assessment Tool (SWAT) to evaluate changes in hydrology and yield. The current rice planting scenario (S0) included 27.4 % single and 72.6 % double cropping areas, while 81.5 % of paddy fields were suitable for double cropping. The SWAT model, parameterized with RS-derived structures, achieved excellent streamflow simulation, with a Nash–Sutcliffe efficiency (NSE) of 0.86 and 0.88 during calibration and validation periods, and percent bias (PBIAS) of 4.5 % and 3.1 %, respectively. Simulation results indicated that the optimized rice planting structure (S3) enhanced rice yield with minimal hydrological impacts. Compared to S0, S3 increased irrigation, evapotranspiration, percolation, and rice yield by 4.8 %, 1.4 %, 5.5 %, and 4.0 %, respectively, while full double cropping scenario (S2) achieved an 11.0 % yield increase but raised irrigation demand by 11.2 %. The opposite results were found for full single cropping rice scenario (S1). This study demonstrates RS-coupled watershed modeling as a robust framework for optimizing rice cropping systems and promoting sustainable agriculture in subtropical hilly regions.