Study On The Rules And Controlling Method Of Nitrogen And Phosphorus Loss In Plain Watershed By SWAT Model

Eutrophication, engendered by agricultural nonpoint source (NPS) pollution, mainly nitrogen (N) and phosphorus (P), is one of the largest threats to aquatic ecosystems. In this paper, SWAT model is a tool to study the spatial and temporal distribution of the total nitrogen and total phosphorus in the agricultural area, which is based on the Jiaxing plain watershed and the Upper of East Tiaoxi watershed. To analyze the impact of different tillage methods on the loss load of TN and TP by changing the tillage method in farm management module of SWAT model that changing conventional tillage into no-tillage. And investigating the loss changes of TN and TP originated from different proportion paddy fields. To provide scientific basis for the effective management and control based on agricultural non-point source pollution in watershed. The main results were shown as follows:1) The research shows that the SWAT model can be applied to assess the spatial and temporal distribution of TN and TP in Jiaxing watershed due to R2>0.65, ENS>0.60 in calibration and validation time. Simulation results showed that there was large differnce in the loss load distribution of TN and TP in Jiaxing watershed, ranging from 6.9-17.4 kg/ha and 1.2-2.9 kg/ha, respectively.2) Seclecting the Jiaxing watershed and the Upper of East Tiaoxi watershed as the study areas, no tillage (NT) replaced the conventional tillage (CT) without other changes in paddy fileds management operations. The results showed that no tillage in paddy fields can reduce the non-point source pollution in watershed. The data suggested that runoff, TN, TP export decline rates of Jiaxing watershed were 14.9%-41.5%,4.0%-10.9%,5.6%-9.6%, respectively. While in Upper of East Tiaoxi watershed, the exports of TN and TP decreased by 4.1%-16.9% and 5.6%-10.3%, and the runoff was reduced about 16.0%-37.3%. In addition, there were some differences in the exports of NO3--N and gaseous nitrogen, but the changing trend was similar. At the same time, the effect of NT on rice yield was observed:the rice yield was slightly reduced and was flat with the yield in CT in the Jiaxing watershed in 5 years. However, in the Upper of East Tiaoxi watershed, rice yield in NT was less than the yield in CT in the first 4 years, then slightly increased in the 5th year. The results suggested that no tillage was beneficial to the reduction of the TN and TP exports of nutrient in agricultural fields. But implementing NT practice alone was limited and other practices for water and nutrient management should be combined with NT practice to obtain better effect.3) The research suggested that the export loads of TN and TP were going to reduce when the rice ratio of agriculture land between 30% to 71% in watershed. Through the curve fitting analysis of TN and TP loss loads in different paddy field proportion of 75 small watersheds in Jiaxing watershed and the Upper of East Tiaoxi watershed, the results showed when the percentage of paddy field was 30% and 71%, the change trend of TN and TP output loads changed. The output loads of TN and TP increased with the increaseing of the paddy field proportion, when the ratio of paddy field was less than 30% and higher than 71% in small watersheds. The output loads of TN and TP decreased with the increaseing of the paddy field proportion when the paddy field ratio between 30%-71%. This study showed that in a certain proportion, paddy fields has the potential to work as nutrient sink in small watersheds.