An Approach To Quantitative Assessment Of Gully Erosion Using GIS, GPS, And RS Technologies

Gully erosion, extensively distributed and intensively developed in the Upper Yangtze River Basin, is the major sediment source of the Yangtze River. However, the recent studies on soil erosion are mainly concerned on the gullies in the Loess Plateau and Black soil area in Northeast China in the small scale, there is a lack of study on gully erosion in the Upper Yangtze River Basin. Because of the extensively distribution of gully erosion in the world, especially in China, there is a urgent need to establish a quick and convenient method for assessing gully erosion in the big scale. With the development of science and technology, Geographical Information System (GIS), Global Positioning System (GPS), and Remote Sensing (RS) could be available for the assessment of gully erosion. Taking Changshanling region of Xichang as a case study, the objectives of this paper are: (1) trying to create a new approach to assess gully erosion based on the integration of GPS, GIS and RS technologies, (2) using the new method to study the spatial-temporal distributing patterns of gully erosion, the topographical threshold for gully erosion initiation, and effects of landuse on gully erosion. From this thesis, the following conclusions can be drawn,1. The new approach to gully erosion assessment based on GPS and DEM was presented. The results show that: (1) the RTK-GPS could be up to the needs for gully erosion monitoring with a high precision;(2) a GPS-survey scale of 5 m grid is suitable for assessment of gully erosion whereas a 3 or less than 3 m GPS-survey scale can meet with study needs of rills and shallow gullies in the upper Yangtze River basin;(3) Kriging interpolation method is a optimal method for establishing DEMs in catchment and region scale, and 1 m is a optimal DEM resolution in upper Yangtze River basin;(4) the methods for deriving gully system parameters from DEMs based on the slope gradient variance and profile curvature extremum is practical, and the results have little difference with gully system parameters directly from field surveying.2. Comparing with gully system parameters derived from DEMs based on RTK-GPS monitoring technology, the feasibilities of three methods for deriving gully system parameters using aerial photo interpretation, DEM created from aerial photo, and DEM creation by digitizing relief map, were assessed, and correction factors were introduced for determination of gully lengths and gully areas. The results showed that: (1) the method for deriving gully system parameters using aerial photo interpretation is feasible, and two correction factors such as Capi.gl =1.16 and Capi.ga= 1.42 were used to correct the gully lengths and gully areas derived from aerial photo interpretation;(2) the method for deriving gully system parameters based on DEM created from aerial photo is feasible, and twocorrection factors such as Capdem.gl= 1.34 and Capdem.ga= 1.24 were used to correct the gully lengths and gully areas derived from DEM created from aerial photo;(3) the method for deriving gully system parameters based on DEM creation by digitizing relief map is also feasible, and two correction factors such as Cmap.gl = 1.51 and Cmap.ga = 1.47 were introduced to conect the gully lengths and gully areas derived from DEM creation by digitizing relief map;(4) comparing three above methods, the method for deriving gully system parameters using aerial photo interpretation is the optimal method to assess gully erosion, as the gully density were taken as the key assessment factor.3. Using the method of gully erosion assessment based on RTK-GPS and DEM, and correction factors of aerial photo interpretation, gully system distribution patterns and relationship with landuse of Majiasongpou catchment and Changshanling region were assessed. The results showed that: (1) gully volume has an significant relationship with both gully lengths and gully areas, and has a moderated relationship with gully width and depths;(2) the gully density of Majiasongpou catchment is 6.69 km/km2, which belongs to serious soil erosion. During 1979 to 2000, the soil loss due to gully erosion in Majiasongpou catchment was about 16,500 t with the annual gully erosion rates of 25.59 t ha-1.(3) With the area of 799.67 ha, the gully density of Changshanling area is 8.58 km/km2, which belongs to intense soil erosion. From 1979 to 2000, 12 new gullies have been found and the annual gully erosion rate is 37.65 t/km2?a, yielding the total sediment about 632,300 t;(4) landuse types and changing have great effects on gully erosion. When the forestlands were converted to the farmlands, gully erosion rates of landscape were much bigger, indicating that human activities accelerated the development of gully erosion.4. Based on the RTK-GPS and DEM technology, the prediction model of topographical threshold for gully erosion initiation in the forestland was S = 0.1585 A-0.4145, where S is the critical slope gradient(m/m), a is the drainage area(ha). The prone areas for gully erosion initiation, predicted by above model, were fairly well consistent with the distribution of gully in the study field.