Risk Assessment Of Nitrogen And Phosphorus Loads In Hainan Island Based On InVEST Model

With the pollution control technology increasingly maturing, point source pollution were controlled preferably in recent years, and non-point source pollution (NPS) has been the major factor of water pollution because of its elusiveness, dispersion, uncertainty and undetectable. For this reason, an all-round understanding of distribution and the risk of NPS pollution in Hainan Island based on the research of NPS pollution were quite necessary. And the research method can also be used as a source of reference for researching the NPS in other region.The research studied the simulation of NPS in Hainan Island with the data of observed climate, soil, landform, hydrology and water quality. The characteristic of temporal and spatial distribution of NPS pollution in Hainan Island were analyzed, and the risk zonation of nitrogen and phosphorus load were identified with ArcGIS. This paper analyzed the advantage of the InVEST compared with Phosphorus Index and the response of the single land use and land cover transformation to hydrology, water quality, and risk areas by setting3scenarios based on the existing conditions of land use and land cover and applied the newly developed model InVEST, which is an innovation on analysis method for risk assessment of nitrogen and phosphorus loads, to provide qualitative, quantitative, and visual result and contribute suggestions to risk assessment of region scale. In this paper, following contents are researched:(1) There was a high water yield but of unbalanced distribution in Hainan Island. The annual water yield depth was979.04mm, the central region of Hainan Island had higher water yield reached12239.7m3·ha-1, and western area had lower water yield volumes per hectare which was5021.18m3·ha-1.(2) The highest total nitrogen output load was in Haikou, reaching5.8818kg·ha-1and the lowest load was in Baishao, reaching1.6353kg·ha-1. Wenchang had the highest phosphorus output load, which was0.5093kg·ha-1, and the lowest load was still in Baishao, reaching0.1216kg·ha-1.Northern area (Lingao, Haikou, Chenmai, Dingan, Wenchang) had higher nitrogen and phosphorus output load than other region, and the central mountain region (Baisha, Qiongzhong, Wuzhishan) had lower output load.(3) Olipotrophication areas of total nitrogen load covered21.42%area of Hainan Island, including Baoting, Wuzhishan, Qiongzhong and Baisha. Olipotrophication-mesotrophication areas, covered49.98%area of Hainan Island, including Ledong, Linshui, Dingan. Chengmai, Danzhou, Qionghai, Tunchang. Wanning and Sanya. Mesotrophication areas, which covered11.58%area, including Wenchang and Changjiang. Mesotrophication-eutrophication areas based on the total nitrogen content, including Lingao, Haikou and Dongfang, covered17.02%area of Hainan Island(4) Olipotrophication-mesotrophication areas, distributed sporadically in Qiongzhong and upstate, and were1.48%on the total area. Mesotrophication areas, including Baisha, Qiongzhong, Wuzhishan, Baoting, Wanning, Qionghai and Tunchang, covered35.4%area of Hainan Island. Mesotrophication-eutrophication areas based on the total nitrogen content covered47.49%area, including Changjiang, Ledong, Wenchang, Sanya, Linshui, Danzhou, Chengmai and Dingan. The risk area with high total phosphorus load were mainly located in Haikou, Dongfang and Lingao.(5) The simulation result of In VEST and Phosphorus Index are consistent, but In VEST model is more applicability.(6) With the urban area extending, the space distribution of water yield and nitrogen and phosphorus load will be changed. When the forest area was gradually reduced and the urban built-up area was doubled, water yield had the greatest increase, reaching1004.06mm, and nitrogen and phosphorus load compared with sceneriol separately increased by16.05%and14.09%. When the cultivated area was decreased gradually and the urban built-up area was doubled, water yield reached1002.22mm and nitrogen and phosphorus load separately increased by12.4%and10.14%. With the urban area extending, the space distribution of risk zonation would be changed too. Mesotrophication areas and mesotrophication-eutrophication areas in scenario2compared with scenario1increased by10.66%and6.82%respectively, and mesotrophication areas in scenario3increased by17.92%according to risk zonation area percent of nitrogen. Based on risk zonation area percent of phosphorus, mesotrophication-eutrophication areas and risk areas in scenario2compared with scenario1increased by3.19%and8.32%respectively, and mesotrophication-eutrophication areas and risk areas in scenario3increased by2.58%and8.32%.(7) To reduce the nitrogen and phosphorus inputs and to erect wetland buffer strips is the effective measure to control NPS.