川中丘陵区不同下垫面集水区氮磷流失特征

通过对2019~2020年不同下垫面集水区(农田集水区与复合集水区)径流及氮磷浓度的连续逐日定位监测,研究川中丘陵区不同下垫面集水区氮磷径流流失过程与强度,探讨下垫面对集水区氮磷径流流失特征的影响.结果表明:不同集水区的径流过程因下垫面不同而存在明显差异,农田集水区内的水田和坑塘的拦蓄作用滞缓了汇流过程,而复合集水区中居民点、公路等不透水下垫面缩短了汇流时间,使得复合集水区的降雨径流量峰值更高,响应速度较农田集水区快12~25min,年径流深较农田集水区多28.1%;次降雨径流过程中磷浓度变化较氮更剧烈,浓度峰值出现时间较氮早约1.2h,在降雨后期磷浓度下降速度更快,降幅更大;复合集水区的氮磷平均事件浓度(EMC)、峰值浓度均高于农田集水区,且两集水区氮流失形态均以硝酸盐氮为主,占总氮的65.9%;磷流失以颗粒态为主,占总磷的67.5%;复合集水区的氮磷流失负荷分别是农田集水区的3.01和4.03倍,氮磷流失强度分别是农田集水区的1.88和2.51倍.因此,复合集水区内氮磷随径流流失的防控可能是未来川中丘陵区面源污染治理的重点.

Patterns of nitrogen and phosphorus losses in two catchments with contrasting underlying surfaces

To investigate the effects of different underlying surfaces on nitrogen (N) and phosphorus (P) losses via surface flow at a catchment scale, continuous daily monitoring of the surface flow and the concentrations of N and P was conducted in an agricultural catchment (AC) and a compound catchment (CC) in a hilly area of central Sichuan from 2019 to 2020. Results demonstrated a significant difference in the runoff process between the two catchments. The peak value of the surface flow was larger, the response speed was12 to 25min faster, and the annual runoff depth was 28.1% greater in CC than in AC. These results highlighted the effects of the contrasting underlying surfaces on the speed of runoff process, with the process being slowed down by the paddy fields and ponds in AC but speeded up by the impervious surfaces of the residential areas and roads in CC. During each rainfall event, the P concentration in runoff dropped more drastically, particularly in the late stage of the event, and the timing of peak P concentration was about 1.2hours earlier than those of N, respectively. Both the event mean concentration (EMC) and peak concentration of P or N were higher in CC than in AC. Nitrate-N was the main form of N losses in both catchments, accounting for 65.9% of the total N loss; while particulate P was the primary P form in the runoff, contributing to 67.5% of the total P loss. The N and P loads in CC were 3.01 and 4.03 times larger than those in AC, respectively, and the loss intensity of N and P in CC were 1.88 and 2.51times greater than those in AC, respectively. Therefore, the control of N and P losses from the compound catchments could be critical for the non-point source pollution control in hilly areas of central Sichuan in the future.