Predictions of soil and nutrient losses using a modified SWAT model in a large hilly-gully watershed of the Chinese Loess Plateau

Hydrological models are effective tools for assessing the effects of soil and nutrient losses on land degradation. SWAT (Soil and Water Assessment Tool) model is widely used to simulate soil and nutrient losses caused by various management regimes. However, its performance of predicting nutrient loss has not been assessed adequately on the Loess Plateau. This study proposed a modified SWAT model by incorporating the modified Soil Conservation Service curve number method, the storm-based Chinese soil loss equation and the nutrient loss model. The observed daily data of runoff and sediment over 16 years and the monthly soluble phosphorus (P) and nitrate losses over 9 years and 4 years, respectively at the outlet of the upper Beiluo river (UBR) basin were used to assess the model performances. Global sensitivity and uncertainty analyses of parameters to runoff, sediment, soluble P and nitrate in the modified SWAT were conducted. The findings during calibration and validation showed that the modified SWAT was highly accurate in terms of model efficiency (calibration: 0.83, 0.83, 0.48, and 0.49; validation: 0.58, 0.57, 0.53, and 0.65) for runoff, sediment, soluble P loss and nitrate loss, respectively. High model efficiency indicated that the modified SWAT could accurately predict soil and nutrient losses at the river basin scale for the Loess Plateau. Moreover, the temporal variations from month to year and the spatial variations at the sub-basin scale for soil loss and the total N and P losses were analysed using the data simulated by the modified SWAT. The results indicated that the critical loss period occurred in July and August, and the Grain for Green project significantly affected the hydrological behaviour and reduced the soil and nutrient losses in the UBR basin.