Soil erosion mapping with Remote Sensing method
on The Loess Plateau
....The example of Mizhi county, Shaanxi province
Zhang Fengrong, Lin Pei, Xia Fuchuan and Wang Defu
Beijing Agricultural University, Ministry of Hydrology, China
Abstract
Soil erosion is a very difficult job with normal method in the hilly and gully Loess Plateau. In this research the color infrared aerial photo was selected as the remote sensing data of soil erosion survey. Soil erosion factors were identified from the color infrared aerial photo. According to the different combination of soil erosion factors, the legend of soil erosion intensity was established for the Loess Plateau hilly and gully region. The whole county was mapped by using that legend of soil erosion intensity. In doing so a new method of mapping soil erosion intensity with remote sensing data in the hilly and gully region of the Loess Plateau was developed. It also resolves the soil erosion modulus to reveals the different erosion degree of different geomorphological locations in a basin and the mechanism of erosion. It provides the basis of designing soil erosion control planning for different situations.
The objective of this study is to probe the method and procedure of soil erosion mapping with remote sensing techniques in hilly and gully area of the Loess Plateau. The test region is Mizhi County of Shaanxi Province.
Selecting Remote Sensing data for mapping soil erosion
Due to longterm's erosion and mankind's activities, the topography is rather scrap. So, it is very difficult to apply remote sensing data to soil erosion survey in this region. Selecting of suitable remote sensing data is the key to the survey of soil erosion intensity. The suitability of remote sensing data to soil erosion survey as judged. Two false color TM images (May 17 and Sep. 23, 1987) two false color SPOT images (May 5, 1987 and Nov. 28, 1988) and the color infrared aerial photo (Sept. 21, 1987) were used, All these data are applicable to mapping of various soil erosion types such as water erosion, water-gravity erosion, and wind erosion. Since the metric resolution of TM and SPOT images is relatively low, terraced field, shallow ditches and gully can not be recognized by visual interpretation. The color infrared aerial photo (at scale 1:50,000), because of its higher metric resolution, was chosen as the data for soil erosion intensity survey in this region.
Quantitative analysis of soil erosion intensity in Mizhi County
The soil erosion intensity remote sensing mapping is done is two steps:
- Analysis of Soil Erosion Intensity Data with Hydrologic Data in Mizhi County.
- The average of soil erosion modulus is about 20,000 ton/km2/year in the county.
- It's believed that the sediment-delivery ratio is 1:1 in the Yellow River basin. This ratio was proposed by checking the runoff and sediment data acquired only from the hydrologic gauging stations, which the amount of sediment deposited in silt arresters have not been considered. By the end of 1987, there were totally 1067 silt arresters in the county.
- Determining Soil Erosion Intensity with Stereoscopic Photo Pairs of the Silt Arresters
By measuring the stereoscopic aerial photo pairs with stereoplotter, the sediment yield in silt arresters can be calculated. In this way the erosion area relating the silt arrester can be determined. The number of years of the silt arresters, since it was built, can be investigated. Finally, the modulus of soil erosion in the basin controlled by the silt arrester can be determined. The steps of this stereoplotting of silt arresters are as follows.
- Information Acquisition
- In order to acquire and representative values, the distribution of silt arresters in the whole county was mapped.
- The number of the years of the silt arresters since they were built was determined.
- Topographic maps (at the scale 1:10,000) were used to obtin values of elevation and slop etc.
- Stereoplotting.
Silt yields in the silt arresters were measured using HPZ-2 stereotopograph.
- Calculating the Sediment Yeild in Silt Arresters
Many studies showed that the volume of silt arrester correlates highly with the height of silt arresters (H), the average gradient of gully (J) and the average width of the silting surface (B). According to field survey data, correlation coefficients of V-H/2 or V-H/J can be 0.99 and V-B 0.95. To calculate sediment yield in silt arrester the regression equation was used, which was developed by Mr. Fan Rueiyu in 1985.
V = 3.648*10-5 H2.172 * B0.897/ J1.081
Field survey data for 50 silt arresters were compared with the corresponding calculated values, which reveled that for 57% of the 50 arresters, the error was within + 3%, and for 98% of them, the error was within + 5%. Considering the hydrologic gauging data are referred to suspended silt, and do not include trapped amount by silt arresters and the sediment yield of silt arrester includes bedload as well as suspended silt, it is reasonable to conclude that the measured value of sediment yield with stereo plotting method is very close to the corresponding hydrologic data.
