Small water bodies influence river water quality in agricultural watersheds

Small water bodies, such as ponds, are widely distributed in agricultural watersheds. They not only serve agricultural irrigation and drainage but also play a crucial role in reducing nitrogen and phosphorus in agricultural non-point source pollution and improving river water quality. However, the linkage between river water quality and the spatial distribution pattern of small water bodies is still unclear. In this study, an agricultural watershed, consisting of nine sub-watersheds and over 1800 ponds was selected, to evaluate the impact of small water body distribution on river water quality. Despite ponds account for less than 0.6 % of the sub-watershed area, they largely determine water quality variations. Specifically, sub-watersheds with lower connectivity among ponds and shorter distances between ponds and farmland exhibited better water quality. These two indicators alone explain more than 87 % variations of nutrient concentrations among the sub-watersheds. This indicates that dispersed ponds that are located close to farmlands serve as important sinks for pollutants in agricultural watersheds. In this study, we emphasize the importance of location of small water bodies in maintaining river water quality, which can provide crucial information for future agricultural watershed land use planning and the development of optimized strategies for water quality improvement.