Dynamic Characteristics Of Land Exploitation And The Assessment Of Its Pressure On Water Quality Security In The Three Gorges Reservoir Area

The Three Gorges reservoir (TGR), a super large reservoir in China, is regarded asthestrategic reserve of freshwater resources with extremely importantecological functions.However, the contradiction between human activities and land resources in the reservoir area is prominent, and the water environment security is confronted with the menaceofgenerally normal stressorssuch as land development.Taking the TGRarea as the case study area, multiple methods were used in this research including spatial analysis based on GIS, statistical analysis, field survey and sampling data analysis, and mathematical model for assessment. Spatial and temporal changes of land cover in different spatial scales were investigated. Geographical distribution of land use and its influence on water environment were analyzed. On this basis, the land exploitation stressor (early warning oriented) assessment methodfor water quality security was finally established and applied to the case study of the Three Gorges Reservoir area. The aim of research is to quantitatively estimate the pressure of land development on water security and to warn its adverse status and changes, and to provide technical support for water quality risk management in the process of land development in the Three Gorges Reservoir area and other similar areas.The results obtained are listed as followings.According to the analysis of spatial and temporal changes of land cover in the TGR area during 2000-2010, land usein this period was dominated by woodland(61.63%-62.14%)followed by cultivated land(28.34%-24.70%). The woodland was dominated by high density forest, and the cultivated land is mainly dry farmland.Area of built-up land (artificial surface) and water bodywere relatively small.The main direction ofland use transfer was fromcultivated land to woodland and built-up land.The source of new woodland was mainly dry farmland, and that of new built-up landwas mainly dry farmland and paddy field.The center of the development of the TGR area was still the main city of Chongqing, among which the Yubei District and the Changshou Districtwere the regions with most rapid urban expansion.The water area showed an obvious upward trend during 2000-2010 with the increased area of 626.07 km2, which was probably influenced by the impoundment of the TGR.But the proportion of water areawas still low (2.49% in 2010).The geographical distribution of land usein the TGR area showed that the land cover distribution was greatlyaffected by the regional topography (elevation and slope) and location differences (distance to the river). The elevation and the slope had the similar influence rule.That is, with theincrease of elevation and the slope, the built-up land decreased. In addition, the distribution of built-up land and cultivated land were more sensitive to the location differences.84.38% of the total built-up land and 75.19% of the cultivated land were concentrated in the area close to river (distance<1000 m). Thus, from the direct spatial relationship,the built-upland and the cultivated land were more likely to influence the water quality in the adjacent area. Based on the data analysis of 4 land use types (cultivated land, forest land, grassland, and built-up land) and water quality of typical subbasins, total phosphorus (TP)concentration was found to be suitable for linear correlation analysis with respect to the causal factors of area, slope,and distance. The area of built-up land and grassland showed a greater impact on TP concentration more than that of cultivated land and forest land.Compared with other land use, the forestland with high slope could reduce the TP concentration effectively, and the cultivated land along the river could significantly increase TP concentration.Based on the above analysis, the land exploitation stressor assessment method for water quality security was put forward. Firstly, sensitive land use types and negative land use transitions were determined using pollutant output parameters of different land use types. Then, sensitive districts or area units were selected taking into account both its land use status and trends. Finally, the early warning indicators were ascertained to valuate and to analyse the stressor itself. In addition, with the selected protection object for early warning, namely water quality target sections, the integrated evaluation and warning level division of the land development stressorswere ultimately finished after coupling the contaminants trasport distance index.The assessment of land explotitation stressor in the TGR areaindicated that 9 districts ofDadukou, Jiulongpo, Nan’an, Jiangbei, Shapingba, Yubei, Beibei, Fuling, Changshou reached red alert level (severe warning), while Banan District and ZhongxianCountyshowed an orange alert level (moderate warning). Jiangjin City and Fengdu Countybelonged to yellow alert level (mild warning).Under the condition of coupling the index of transport distance,11 districts such as Banan District and Dadukou District showed red alert level (severe warning). Early warning level and Ranking of some distrcits and counties has changed. The case study proved the operability ofproposed assessment method, and the results could provide the decision support for the water environment risk management in the Three Gorges Reservoir area.