Catch one and lose another: Water quality risks arising from policy-driven cropland displacement

Safeguarding water quality is essential for achieving sustainable development goals. Although land-use change is widely recognized as a key contributor to water quality degradation, the impacts of policy-driven cropland displacement on watershed water quality remain insufficiently understood. Using Zhejiang Province as a representative case, this study developed a two-step Patch-generating Land Use Simulation (PLUS) model to construct three land-use scenarios: the baseline scenario (BS), the area balance scenario (AB), and the quality balance scenario (QB), representing different intensities of cropland protection policies. These scenarios were subsequently coupled with the Nutrient Delivery Ratio (NDR) model to quantify total nitrogen (TN) and total phosphorus (TP) loads at the subwatershed scale. The results indicate a clear trade-off between cropland protection and water quality. Compared with the BS scenario, TP and TN loads increased by 23.39% and 29.29% under the AB scenario, and by 30.21% and 37.78%, respectively, under the QB scenario. Spatially, increases in TP and TN were observed across most watersheds, with upstream regions experiencing the most severe water quality deterioration. Contribution analysis further indicates that cropland compensation, rather than urban expansion, became the dominant contributor to water quality deterioration under the AB and QB scenarios. These findings demonstrate that cropland protection policies implemented under rapid urbanization may unintentionally intensify nutrient pollution, which reflects a situation of gaining one objective but losing another. Future cropland protection strategies should incorporate stronger water quality safeguards to better balance agricultural production goals with watershed environmental protection.