A modeling approach to evaluating the impacts of policy-induced land management practices on non-point source pollution: A case study of the Liuxi River watershed, China

- Simulate impact of conservation tillage and optimal fertilization on nutrient loss.
- Conservation tillage achieves effective nutrient loss reduction.
- No-tillage combined with straw mulch is more efficient than using no-tillage solely.
- Optimal fertilization greatly reduces nutrient loss with small impact on crop yield.

Conservation tillage and fertilization based on soil test have been promoted by the Chinese government in recent years. Their impacts on non-point source pollution were simulated using the Soil and Water Assessment Tool (SWAT) for the Liuxi River watershed, the only one local drinking-water-supply river basin of Guangzhou. The model was calibrated through comparing model outputs with observations to ensure reliable hydrologic, crop yield, and water quality simulations. The model results indicated that, 5% of total nitrogen (TN) load and 12% of total phosphorus (TP) load could be reduced at the watershed outlet by implementing the conservation tillage system of “rice parachute transplanting with no-tillage and straw mulching” for paddy rice fields which account for 9% of the watershed area. It is valuable to develop conservation tillage systems suitable for other types of agricultural land in southern China. When modeling the impacts of various fertilization levels derived from recommended fertilizer doses for medium fertility soil of the Pearl River Delta region where the watershed is located, the total crop yield was estimated to decrease by only 2% for up to 24% and 28% reduction in TN and TP loads at the watershed scale, respectively. The crop yields versus nutrient losses relationship simulated by SWAT indicated that great effects in TN and TP loads reduction could be achieved at the expense of minimal impacts on crop yield through optimal fertilization. The modeling approach presented in this study can be a useful tool for estimating the effects of policy instruments and pollution control measures.