Karst Rice–Tomato Cascade System: A Sustainable Agricultural Strategy for Groundwater Protection

Karst aquifers, despite their importance as drinking water sources, are highly susceptible to contamination due to rapid surface-groundwater interactions. This study investigates how crop spatial zoning—specifically planting rice in upper elevations and tomatoes in lower elevations—acts as a Best Management Practice (BMP) to enhance groundwater quality in the Chenqi karst watershed, Guizhou Province, China. Over a 7-year period (2016–2022), 19 water quality parameters were monitored at five hydro-logical sites, including wells located in distinct cropping zones. Results revealed that groundwater in the downstream tomato field exhibited significantly higher Water Quality Index (WQI) values (up to 72.2) compared to both upstream rice paddies and outlet sites, suggesting that the rice-tomato zoning strategy supports nutrient attenuation and pollutant buffering. Although effluent water quality at the outlet remained de-graded, likely due to non-agricultural inputs, our findings provide empirical evidence that crop zoning based on elevation and hydrology can effectively mitigate diffuse pollution in karst systems. This research contributes to the development of BMP frameworks tailored for karst agroecosystems.