Study On The Nitrogen & Phosphorus Loss By Runoff From Framland In HJH Plain Based On GIS

As the largest bread basket in Zhejiang Province, Hangjiahu Plain is facing more and more serious water pollution problem which is mainly caused by agricultural non-point source pollution. Research on N, P loss from farmland and its influent factors are very meaningful both for the increase of crop yield and improvement of water environmental quality. Considering the actual situation of Hangjiahu Plain, this paper focuses on runoff loss load rather than other loss ways of N, P, like leaching, lateral seepage, and so on.The paper estimates the N runoff loss load from submersed paddy field and dry cole field respectively, because the farmland in Hangjiahu Plain practices crop rotation of rice and cole. For submersed paddy field, field experiment, pointing observation, and regional survey were carried out to simulate the loss rule of N in runoff and collect useful N fertilizer materials. And for the dry cole field, available results in similar place to Hangjiahu Plain from other researchers were cited immediately to estimate the loss load, because of the lack of actual measurement. In the end, both paddy and cole fields loss load were calculated in GIS environment based on basic geographical data.In this paper, SCS model was used to calculate the runoff volume both in submersed and dry periods. For submersed paddy field, the parameters in the model are the same to He B G's research after considering the field overflow mouth. And for dry cole field, parameters were modified like this: Ia=12.8mm=0.2S', CN=S0. Pointing observation and rainfall simulation experiment showed the two new models basically accorded with fact with errors of -19.9⁺18.0% for paddy field and 5% for cole field.According to the field experiment, the concentration of TN, NH/-N, and NO₃^--N were all strongly related with rainfall intensity and applied N fertilizer amount. Thereby, the relation among TN, rainfall intensity and N amount could be expressed as y=0.483x₁+0.02979x₂2-10.433, which has a coefficient (R) of 0.948. In the formula, y, x₁, x₂ means concentration (mg/1), rainfall intensity (mm), and applied N amount respectively. This formula was used as the pollution (N) export model.Based on new SCS and N export models, and the collected materials, the N runoff loss load from submersed paddy field was estimated. The result showed that the average N loss loading reached 35.26kg N/hm², accounting for 12.69% of the applied N amount, and was also different from place to place. The loss load in west region was obviously higher than east and north region, and Anji county in west, which has a loss load of 48.41kg N/hm², is the most serious region in Hangjiahu Plain. The distribution of loss rate was similar to the loss load.The paper used two different methods to calculate the N, P runoff loss load from dry cole field;one was based on runoff volume and average concentration, and the other was based on USLE and nutrients in soil. Method one showed that the average loss load of N, P was 12.97kg N/hm2, 1.87 kg P/hm2 in Hangjiahu Plain and 12.48kg N/hm2, 1.80kg P/hm2 in Jiaxing region. Method two showed that the average loss load of soil, N and P was 836.22kg/hm2, 7.69kg N/hm2, 1.50kg P/hm2 in Jiaxing region. Comparing the results of these two methods, the loss load both of N and P by method two were lower than method one, and the gap of P between method one and two is small, which means USLE and be used in the research of P runoff loss load in Hangjiahu Plain dry farmland.The N runoff loss load from submersed paddy field was obviously higher than dry cole field, which was probably related with the strong rainfall and high water capacity in soil in rice season.