Soil erosion mapping with Remote Sensing method
on The Loess Plateau
....The example of Mizhi county, Shaanxi province
Methodology of mapping soil erosin intensity with Remote Sensing
Several considerations must be taken when mapping system of soil erosion intensity is set up: (Table 1 and 2).
Boundary between rill-interrill erosion section and gully-gravity erosion section must be portrayed according to the erosion types and characteristics in different geomorphologic locations (parts).
The grades of soil erosion intensity was set up, based on erosion types and characteristics in different sections.
Table 1. Mapping System of Soil Erosion Intensity
in the Rill-Interrill Erosion Section
| Category I |
Category II |
Category III |
| 1. Water Erosion |
11. delicate erosion
(<1000T/km2. Year) |
111. hilltop, terraced field
112. gentle surface of hills
113. vegetation cover >90% |
12. slight erosion
(1000-2500T/km2. year) |
121.hillside land 5° - 8°
122. vegetation cover 70-80% |
13. medial erosion
(2500-5000t/km2. year) |
131. hillside land 8° - 15°
132. vegetation cover 50-70% |
14. intensity erosion
(5000-8000T/km2. year) |
141. hillside land 15° - 25°
142. vegetation coxer 30-50% |
15. very intensity erosion
(8000-15000T/km2. year) |
151. hillside land 25° - 35°
152. vegetation cover 10-30% |
16. severe erosion
(15000-25000T/km2. year) |
161. hillside land > 35°
162. vegetation cover < 10% |
Table 2. Mapping System of Soil Erosion Intensity
In the Gully-Gravity Erosion Section
| Category 1 |
Category II |
Category III |
| 2. Water -Gravity | 21. delicate erosion
(<1000t/km2, year) | 211. channel, silt arrester land
212. vegetation cover >90% |
22. slight erosion
(1000-2500T/km2, year) |
221. gullyside land 5°-8°
222. vegetation cover 70-80% |
23. medial erosion
(2500-5000T/km, year) |
231. gullyside land 8°-15°
232. vegetation cover 50-70% |
24. intersity erosion
(5000-8000t/km2, year) |
241. gullyside land 15°-25°
242. vegetation cover 30-50% |
25. very intensity erosion
(8000-15000T/km2, year) |
251. gullyside land 25°-35°
252. vegetation cover 10-30% |
26. severe erosion
15000-25000T/km2, year) |
261. gullyside land>35°
262. vegetation cover<10% |
27. very severe erosion
(>25,000T/km, year) |
271. gully density >20/cm
|
In the rill-interrill erosion section, terraced field slop and length of hillside field, vegetation cover or land use type are the main factors. in terms of different combination of these factors, the grades of soil erosion intensity were established for that. Terraced field are usually build on gentle hillside, showing distinct texture of spiral shell on the aerial photoes. However, they are very rarely seen on the hillside field when slop is steeper than 20 degrees.
Applcation and test of the mapping system of soil erossion internsity
In order to test the mapping system of soil erosion intensity (tables0 1 and 20, application was made in trial area. firstly, six small watersheds controlled by silt arresters respectively were chosen for mapping soil erosion. Then, the sediment yields of different areas which have different erosion internsity were measured and calculated. Fanally, the calculated values of the sedment yields were compared with the data from field survey. the result was shown as follows:
the boundary between rill-interrill erosion section and guly-gravity erosion section was precisely portrayed. The average area percentage of the rill-interrill erosion section to the total area is 40.105; in gully-gravity erosion secton, the percentage is 59.90%. This value is very close to that one acquired from detailed land resources survey.
Texture of gully is seen distinctly on the aerial photoes. The boundary between the rill-interriall erosion section and gully-gravity erosion section was portrayed along the end of v-shap gullies. So, the precision of the boundary was higher than the boundary portrayed on the topographic map at the same scale.
The erosion intensity in gully-gravity erosion is higher than that in rill-interrill erosion section. Trial data from neighbour counties has also confirmed the result.
The interpreting result of soil erosion intensity from color infrared aerial photoes corresponded to the field survey data of silt arresters. so, the above results show that the mapping system of soil erosion intensity reflects the physical reality, and therefore can be applicable in this region. And it can also be used widely in the other regions under the same geomorphologic conditions as those in Mizhi County.
