Study On Relationship Between Watershed Topography And Sediment Yield By Rainfall Erosion In Loess Area

Based on indoor simulating experiment, combinating data of actual watershed and appling statistics, fractal theory, high precise photogrammetry and CIS technology, this paper studied topography changes, sediment-yield characteristic and their relationship. Relationship model among topography fractal dimension, runoff eroding power and sediment transport modulus was established after proposing topography fractal dimension as comprehensive quantitative index for topography by replacing rainfall erosivity with runoff erosivity. Based on this model, relationship between watershed topography parameter and sediment yield were setup using the observed data from Cha Bagou watershed, together with parameter calibration. The following conclusions were reached:i) In different watershed, runoff depth and sediment transport modulus have power function (y=axb) relationship and the non-line regression equation can well simulate their relationship. Parameter a and b can reflect difference of watershed harness degree. In harnessed watershed, a is smaller and b is bigger, but they are contrary in no-harnessed watershed. The flood peak volume modulus and sediment transport modulus is also power function relationship. But parameter a and b is different largely in different watershed and the relativity is not well than relationship between runoff depth and sediment transport modulus.ii) Runoff erosivity reflects relationship of water erosion forces and sediment yield more directly than rainfall erosivity. It intergrated ability of runoff volume and flood peak on detaching soil and transporting sediment and is more reasonable than single runoff depth or flood peak volume. Analyzed result on observed data shows that runoff erosivity and sediment transport modulus have a good power function (y=mxn) relationship. All the correlation coefficients of regression equation are bigger than 0.9 in different watershed and power exponent b is 0.4-0.65, which average is 0.52. And n is bigger as harness degree high. However, a is related with watershed area and harness condition, m decreased with increase of watershed area and harness degree.iii) The result from simulating rainfall on watershed model indicats that changes of projective area, surface area and volume represented development of watershed physiognomy. Volume of model body decreased and projective area and surface area increased with developing process of watershed model. When development of gully trending to stablization, projective area become stable and surface area changed fluctuantly. The relationship between volume of each part of model and its surface area can be described by Gaussian model. Because change of volume reflected eroding process of watershed and surface change reflected change of topography surface, this model described relationship between watershed topograph and erosion.iv) As a quantitative index of wateshed topography, topograph fractal demension can represent surface shape information of watershed topography in the round. Reseach find that watershed model and actual watershed show good fractal character. Topography fractal dimension related with process of development of watershed physiognomy and represented degree of watershed development, v) Sediment yield intensity of watershed model reflected erosion characteristic of watershed model in different development phase. Simulating experiment indicated that sediment yield intensity took onthe trend that they changed from small to big then decreased gradually. The relationship of average sediment transport rate and sediment yield rate per mm rainfall with topography fractal demension is expressed by Gaussian model and this model is similar with relationship between volume and surface area. It shows that topograhy fractal demension is reseanable in describing change of topography.Based on this model and after introducing runoff erosivity factor, sediment yield model of watershed model was setup by using multi-regression analysis method.vi) The relationship model between watershed topography par...