漓江流域氮磷排放对水肥管理和下垫面属性变化的响应

为分析漓江流域农业面源污染氮磷排放及其主要影响因素,该研究在多年野外监测试验的基础上,运用SWAT(soil and water assessment tool)模型模拟水肥管理和下垫面属性对青狮潭灌区会仙试区氮磷排放的影响。研究中根据会仙试区径流量、总氮及总磷月排放监测数据校准和验证模型,进而采用情景模拟方法,分析稻田化肥施用量(氮肥和磷肥)、灌溉水量、湿地面积和蓄水容积以及岩溶发育程度等变化对试区径流及氮磷排放的影响。结果表明:1)校准期和验证期径流、总氮和总磷排放模拟值与实测值的决定系数和纳什系数分别在0.70和0.63以上,说明SWAT模型在会仙试区径流、总氮和总磷排放模拟上具有较好的适用性。2)针对不同水肥管理水平,化肥施入量和灌溉水量减少30%时,试区出水口多年平均总氮、总磷排放量分别下降11.45%和8.98%、7.79%和5.81%,化肥施用量减少对总氮、总磷排放的削减效果优于降低灌溉水量的削减效果。3)湿地面积或蓄水容量的增加和减少会相应降低和增大试区总氮、总磷排放量,若将湿地面积和蓄水容量均扩大50%,出水口总氮、总磷排放量可削减12.40%和10.44%;同时改变湿地面积和蓄水容量对试区总氮、总磷排放量的影响,超过单独改变其中一个属性影响的叠加。4)岩溶发育程度参数对子流域出水口氮磷排放影响各异,土壤厚度和土壤容重降低可减少试区径流量和氮磷排放量,而土壤有效含水量和饱和渗透系数则表现出相反的作用。该研究显示合理控制化肥施用量,防止湿地面积萎缩和蓄水能力退化,改善和保持岩溶区土壤结构,有助于削减漓江流域岩溶灌区氮磷污染的排放,为当地农业措施优化和水土管理方法提供指导。

Responses of nitrogen and phosphorus emissions to water and fertilizer management and underlying surface property changes in Lijiang River Basin

Abstract: This study aimed to simulate the processes of non-point source pollution transport and to identify the corresponding influencing factors using the SWAT model (soil and water assessment tool) in the Qingshitan Irrigation District (QID) of Lijiang River Basin. Field measurements were conducted in Huixian experiment area (HEA) of QID to monitor runoff and the concentration of total nitrogen (TN) and total phosphorus (TP) at the outlet of HEA. In the study measured monthly data were used to calibrate and validate the SWAT model, in which the thickness, bulk density, saturated hydraulic conductivity and available water content of soil were selected to quantify the karst development in HEA. Then scenario analysis was carried out to study the impacts of the irrigation and fertilization in paddy fields, the attributes (area and water storage capacity) of wetlands, and karst development degree on runoff, TN and TP emissions from HEA. The results showed that the SWAT model performed well in simulating runoff, TN and TP emission in the study area with all the relationship coefficient (R2) and Nash-Suttcliffe efficiency (NSE) between the simulated and measured data higher than 0.70 and 0.63, respectively. The scenario simulation showed that lower fertilization and irrigation could reduce TN and TP emissions by 2.46%-19.18% and 1.86%-14.21%, respectively. When fertilization was decreased by 30%, TN and TP emissions from HEA declined by 11.45% and 8.98%, respectively; and the reductions were 7.79% and 5.81%, respectively, when irrigation water was reduced by 30%, indicating that it was more efficient to decrease TN and TP emissions by reducing fertilization than irrigation and that TN emission was more prone to be reduced by optimized fertilization and irrigation than TP emission. However, the effects of reducing fertilization and irrigation in paddy fields on improving the water quality of outflow were relatively limited. It was needed to excavate the potential of pollutant purification of wetlands in HEA to meet the water quality standard of water function division in Guilin. The scenario simulation showed that variations of area and/or water storage capacity of wetland also affected TN and TP emissions from HEA. Runoff was hardly affected by changing area and water storage capacity of wetlands, however, 12.40% of TN and 10.44% of TP emission were decreased with the area and water storage capacity of wetlands rising by 50%, while TN and TP emission increased by 11.75% and 10.97%, respectively, with decreasing 50% of area and water storage capacity of wetlands. And the efficiency of reducing TN and TP emission of wetland area was higher than water storage capacity. Moreover, it had synergistic effects on TN and TP emissions by simultaneously increasing the area and water storage of wetlands compared with only changing one attribute. Soil parameters describing karst development degree had diverse influences on runoff, TN and TP emissions from subbasins 2, 4, 5. Thinner soil thickness and smaller bulk density decreased runoff, TN and TP emissions, while available water content and saturated hydraulic conductivity demonstrated the opposite effects. The conclusion indicated that more developed karst landscape exacerbated runoff, TN and TP emission. Besides, the runoff was more sensitive to soil thickness and available water content than bulk density and saturated hydraulic conductivity, while TN and TP emissions were more sensitive to soil thickness and bulk density. This study showed that controlling fertilization reasonably, preventing wetland shrinking and water capacity degrading, and maintaining and improving soil structure in the karst area was helping to alleviate nitrogen and phosphorus emissions in the karst irrigation area of Lijiang River Basin, which guided optimizing local agricultural measures and soil and water management.