| As the characteristics of the nonpoint source pollution exist, such as uncertainty, complexity, and the broad sphere of influence, it is hard to monitor effectively. A large number of studies have shown that land use or landscape pattern have a certain effect on nonpoint source pollution, and the watershed model method was an effective method to research the nonpoint source pollution. However, in China, because of flat topography, river development, complex river network in the wetlands agricultural watershed, there were some errors in the digital river network extracted based on the DEM data compared with the river network at watershed. Because the river direction was controlled by many water conservancy systems, the water quality model was not steady. The study area in this paper——Sihu basin, Hubei province was a typical wetlands agricultural watershed. So we used DEM data and digital river network modification technique provided by Turcotte, which the altitude of man-made ditches, canals, rivers and lakes were decreased, so that the modified DEM data can reflect the direction of surface runoff correctly. Model validation was executed by comparing calculated results and observed water quality data from17hydrological units. The average load of nonpoint source nitrogen and phosphorus at each exit of the units were calculated based on suppose that pollution load was inverse to the distance of water flow. Using the model results quantified described the nonponit source nitrogen and phosphorus under different land uses. Based on the simulation of non point source pollution, we used GIS technology to model land use change and analyzed the impact on nonpoint source nitrogen and phosphorus after land use land use change in order to determine the key land use that affect nonpoint source pollution at watershed. This could provide a reference for the landscape ways to improve nonpoint source pollution at watershed in further explore. The works in this paper were as follows:1. Digital river network extraction in Sihu basinWe extracted the water resources information in Sihu basin based on the ETM satellite remote sensing data using the LOC-Water (Lines of Communication) technology provided by ENVI software. The digital river network was extracted using the DEM data and digital river network modification technique provided by Turcotte, which the altitude of man-made ditches, canals, rivers and lakes were decreased, so that the modified DEM data can reflect the direction of surface runoff correctly.2. The process and validation of L-THIA modelThe input parameters of L-THIA (Long-Term Hydrologic Impact Assessment) model were land use data, soil data and rainfall data. The runoff was calculated by the SCS runoff curve method, and finally the nonpoint source nitrogen and phosphorus under different land use was exported.Since we only observed the water quality at the outlet of the17hydrological units, the model results must be calculated in order to validate the reliability of model results in order to test results in the same location. So the average load of nonpoint source nitrogen and phosphorus at each exit of the units were calculated based on suppose that pollution load was inverse to the distance of water flow. The results demonstrated that there were highly significant linear correlation between the calculated average load of nonpoint source nitrogen and phosphorus and the measured concentrations of TN and TP, and F test results showed that there were highly significant correlation between them R2TN=0.6859; R2TP=0.9159. F(1,17)TN=32.8>F0.001; F(1,17)TP=172.2>F0.001. The results showed that the model results were more reliable to describe the nonpoint source pollution in Sihu basin accurately.3. The distribution and concentration rate of nonpoint source nitrogen and phosphorus under different land uses in Sihu basin(1) Nonpoint source nitrogen and phosphorus under different land uses in Sihu basinThe results from L-THIA model demonstrated that dry farm lands generated the largest nonpoint source nitrogen, the load was731.68kg·a-1·km-2; followed by paddy fields, residential areas, fish ponds and ditches, the load were692.20kg·a-1·km-2,584.80kg·a-1·km-2,161.71kg·a-1·km-2, respectively; grasslands and woodlands generated the minimum load of nitrogen, the load were less than60kg·a-1·km-2. Overall, the average annual nonpoint source phosphorus load in Sihu basin was smaller than the average annual nonpoint source nitrogen load. Dry farm lands generated216.18kg·a-1·km-2nonpoint source phosphorus; followed by residential areas, paddy fields, fish ponds and ditches, the load were183.15kg·a-1·km-2,165.30kg·a-1·km-2,30.84kg·a-1·km-2, respectively; grasslands and woodlands generated the minimum load of nitrogen, the load were less than1kg·a-1·km-2.(2) Distribution of the nonpoint source nitrogen and phosphorus in Sihu basinAccording to the land use data of Sihu basin in2000, the annual nonpoint source nitrogen and phosphorus load in Sihu basin were605.63kg·a-1·km-2,156.65kg·a·km-2respectively. The annual nonpoint source nitrogen and phosphorus load in the upper area were617.47kg·a-1·km-2,153.23kg·a-·km-2, the annual nonpoint source nitrogen and phosphorus load in the middle area were622.05kg·a-1·km-2,163.36kg·a-1·km-2, the annual nonpoint source nitrogen and phosphorus load in the down area were544.74kg·a-1·km-2,143.49kg·a-1·km-2.(3) The contribution rate of nonpoint source nitrogen and phosphorus under different land uses in Sihu basinThe proportion of the nonpoint source nitrogen and phosphorus under different land uses and the nonpoint source nitrogen and phosphorus in Sihu basin was used to represent the contribution rate. The order of the contribution rate of the nonpoint sources nitrogen and phosphorus under different land uses were basically the same, that paddy fields> dry farm lands> residential areas> fish ponds and ditches> woodlands> grasslands. The contribution rate index was the ratio of the contribution rate under different land uses and the proportion of the area of different land uses and area of Sihu basin. The contribution rate index (UCI) of nonpoint source nitrogen load in descending order was:dry farm lands> paddy fields> residential areas> fish ponds> woodlands> grasslands; the UCI values of nonpoint source phosphorus load in descending order was: dry farm lands> residential areas> paddy fields> fish ponds> woodlands> grasslands.4. The effect of modeled land use change on the nonpoint source nitrogen and phosphorus in Sihu basinL-THIA model calculation results showed that the dry lands and paddy fields are the main sources of nonpoint source nitrogen and phosphorus, and grasslands, woodlands, fish ponds and ditches may play a role part in reducing the nonpoint source nitrogen and phosphorus in Sihu basin. To further analyze the impact on nonpoint source nitrogen and phosphorus brought by the land use change, we set six types of land use change:dry farm lands-grasslands, dry farm lands-woodlands, dry farm lands-fish ponds and ditches, paddy fields-grasslands, paddy field-woodlands, paddy fields-fish ponds and ditches. We reduced the area ratio of dry farm lands and paddy fields by5%,10%,15%,20%,25%, and correspondingly increased in grasslands, woodlands, fish ponds and ditches to analyze the impact on nonpoint source nitrogen and phosphorus brought by the land use change.(1) The effect on nonpoint source nitrogen and phosphorus after dry farm lands reduced in Sihu basinThe model results showed that in the way of dry farm lands-grasslands, dry farm lands-woodlands, dry farm lands-fish ponds and ditches, the nonpoint source nitrogen and phosphorus in Sihu basin showed an increasing trend of increase in the range of3.86×103kg·a-1-818.22×103kg·a-1and6.15×103kg·a-1-268.96×103kg·a-1.(2) The effect on nonpoint source nitrogen and phosphorus after paddy fields reduced in Sihu basinThe model results showed that in the way of paddy fields-grasslands, paddy fields-woodlands, nonpoint source nitrogen and phosphorus in Sihu basin increased by an range of23.58×103kg·a-1-53.87×103kg·a-1and5.39×103kg·a-1-12.84×103kg·a-1. In the way of paddy fields-fish ponds and ditches, the nonpoint source nitrogen and phosphorus in Sihu basin decreased. And when the paddy fields area decreased in ratio of20%, the nonpoint source nitrogen and phosphorus reduced by73.933×103kg·a-1,24.043×103kg·a-1, respectively. |
PDF Full Text Request