Mitigating agricultural non-point source pollution requires identifying when, where, and from which crop systems nutrient loads reach receiving waters. However, how nutrient loads from distinct crop systems respond to contrasting hydrological regimes remains poorly constrained. Here, we quantified spatiotemporal dynamics of total nitrogen (TN) and total phosphorus (TP) loads in China’s Luoyang River Watershed by coupling an Export Coefficient Model with the Soil and Water Assessment Tool. Annual TN and TP loads peaked during the wet year (77.2 and 3.0 t), substantially higher than those in the normal (56.1 and 1.0 t) and dry (55.9 and 0.8 t) years. Source apportionment revealed system-specific hydrological responses, with paddy fields and nurseries alternately dominating contributions. Paddy fields were the primary TN source in the dry (39.5%) and wet (47.8%) years, whereas nurseries led in the normal year (46.9%). Conversely, paddy fields drove TP contributions in the dry (42.9%) and normal (43.0%) years, while nurseries took over during the wet year (39.1%). Irrigated fields ranked third across all regimes. Temporally, nutrient loads were concentrated in “hot moments” (June–July, April/July, and July–August for dry, normal, and wet years, respectively). Spatially, critical source areas clustered midstream and downstream, reflecting high-risk crop distributions. Precipitation drives load magnitudes, whereas TN and TP exhibit distinct hydrological sensitivities associated with crop types, field structures, and runoff behavior. These findings highlight the need to move beyond uniform management toward crop-specific, hydrology-informed, and spatiotemporally targeted strategies for agricultural water management.
Li et al. (Sat,) studied this question.