Gully Change Of Lanni River Watershed During The Past 20 Years Based On RS At Meso-scale

The Loess Plateau is the most serious soil erosion area in China, and the hilly-gullied Loess Plateau is the most serious soil erosion area in the Loess Plateau and is the major source of Yellow River silt. Therefore, the soil erosion rule study on the Loess Plateau is one of the most important scientific problems of the world environment research, and gully erosion is an important component of surface erosion processes. Currently, the use of 3S and traditional technology has become a hot topic at home and abroad in study of gully erosion changes. In this study, we analyzed the gully erosion change of Lanni River Watershed during the past 20 years, and take TM images in 1990 and 2005 and 1:50000 topographic map to based analysis data, and take Lanni River Watershed in Xiji County in Ningxia Hui Autonomous Region as the study area. The study supported by GIS,RS and other analysis software: firstly, on the basis of digital elevation model (DEM) directly extracted technology research of gully network, selected the high-precision and high level detailed gully network to extract algorithm, at the same time through contrast, superposition and error analysis with 2 TM images identified different periods corresponding to the threshold (the minimum cumulative amount of convergence): 1990 to 400 and 2005 to 300, achieved the automatic extracted 2 gully network from DEM. Secondly, utilized DEM to extract factors related channel: slope, aspect, gully length, gully depth, ravines density, vertical slope, and gully width through 2 TM images. Thirdly, according to the research needs, selected gully length, gully width, gully density and vertical slope as the evaluation index. Finally, analyzed gully change of Lanni River Watershed during the past 20 years through the four selected evaluation indexes. The results showed:(1)Gully network of Lanni River watershed is developing and changing year after year, gully of all watersheds is increasing 195719m during the past 20 years and average 12232m annual, the growth rate reached 17.10%. Every gully prolonged average 7.9m annual in all watershed. Meanwhile, low grade gully developed to a higher one with the new gully produced.(2)Rate of gully width change of all watersheds reached 8.42%. The fourth, five grade channels expanded to gully bank at an average annual rate of nearly 1m, the rate of expand of other grade slowed down, this shows that the gully be controlled effectively and at the stage of stability, the development stage of expand to gully bank already elapsed.(3)Ravines density has stated changes in the rate of 17.07%, from 1.64km/km~2 to 1.92km/km~2.(4)Main gully vertical drop reduced from 0.6418% to 0.6354% during the past 20 years, variation is only 0.0064% and small, and this shows that the gully down-cutting stopped and have certain sedimentation in all watersheds. Resulting in the vertical slope of main gully smaller, but there is still changes on every channel levels: the most change of increasing of vertical slope is first-grade gully, while the fourth and fifth grade gully vertical slope decreases.