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<br />L- <br />. <br /> <br />. <br /> <br />A Study on Mathematical M ode] for Sediment Yield <br />in Mountain Regions with Large Hillside Slopes <br /> <br />Duo Fangl <br /> <br />GuoliaDg Yul <br /> <br />Abstract <br /> <br />Based on tbe characteristics of water aod soil toss in mountain region with larac <br />hillside slopes in lOuthwest of China. the equations of runoff formation and sediment <br />yield are obtained. In a wet area, when precipitation intensity is large enough, runoff <br />can be formed even before soil moisture being less than soil absorption capacity. The <br />sediment discharge and the soil er( "lion rate per unit area can be presented as the sinti. <br />lar form of Meyer-Peter equation. The incipient shear stress should inlude the <br />parameter of soil cohesion. The watcnhcd model based on these characteristics is used <br />in a small basin, and a satisfactory result has been obtained. <br /> <br />Introduction <br /> <br />The water and IOUlolS varies intensively with time and places. The world wide <br />land erosion has been concernod .s one of the mosl important ecological problems. <br />The phenomenon orland erosion and sediment yield was rll'Stly studied in the late 19th <br />century. With the development of agriculture and river eopneering projects as wen IS <br />intensirmg of water and soil 10" many mathematical modelt of forecasting water and <br />lIOilloss and sediment yield were proposed. In southwest of Chin., the main type or <br />land erosion is hydraulic erosion. For example, the hydraulic erosion capacity in the <br />upper Yanatzl River watenhed at Cunlan Hydrotogic Gaainl Station iI <br />IS.1 * 101lkilopam. in an averaae annual, whereas the gravitational erosion capacity <br />it about 3 * lol'kilogram.. The hydraulic erosion includes sheet erosion and rill ero- <br />sion. The sheet erosion widely occurs on the upland, cultivated land and badly <br />vegetated places. For instance., 70% of cultivated land are on hillsides in the upstream <br />of the Jia1injang River. ne hillside slopes are 20't0 40'. When occuring rainstorms <br />IOU Ion arc rather serious, the erosion modulus can reach 3.6 * IO'kilogram/ Km2.a. <br />In thi'l.! case, the sonlolS becomes the main sediment resource in rivers. <br /> <br />Prof.. the IMtitutc of Hydraulic 1leIean:b. ChcoJdu University of ScieDcz .nd TcchnololJ, <br />Chenadu. Chin.. 61006~ <br />2 Auociate Prof., the .me addrat...bove <br /> <br />752 <br /> <br />-,..... <br /> <br />. <br /> <br />. <br /> <br />MODEL--SEDlMENT YIELD <br /> <br />753 <br /> <br />The mountain region of southweste China will be one of the important resources of in. <br />now sediment of the beeing about to construct Three Gorges Reservoir. How to re- <br />duce soil loss in this region is a much conc:crned problem. In this paper, a <br />mathematical model of sediment yield for mountain region with large hillside slopes <br />will be built based on the basic physic mechanism of the production, movement aod <br />deposition of runoff and sediment. The model is composed of a hydrological model <br />and a sediment yield model. <br /> <br />The ca1c::ulation clements of watershed <br /> <br />As shown in Fig. I, a watershed can be divided into several calc::ulation clements. <br />Each element with its own watershed boundary is considered as a sub-watershed <br />(Simons, 1984). The watershed is divided into two hillsides ( area AI' area A2) and a <br />channel ( length Lt.). Each sub-watershed is numbered according to its geographic lo- <br />cation so as to calculate them one by one. <br /> <br /> <br /> <br />, <br /> <br />Fig. I The sketches of watershed division and one element <br /> <br />The hydroloJic model <br /> <br />There are two kinds of tipical runoff computation models ( Zhou, 1984). One <br />considers that soil docs not produce runoff when precipitation amount does not meet <br />ilS absorption capacity, and otherwise precipitation amount from which the evapora- <br />tion and seepage amount being minused produces runoff amount, i.e., <br />P - E = W, - W, if P < W. - W, (I) <br />P - E - R - W. - W, if P> W. - W, (2) <br />in wbieb, P is interval precipitation amount, E is interval evaporation amount, W land <br />W ~re soil mqistures at the beginning and the end respec!.ively. W.is soil absorption <br />capacity, R is interval runoff. <br />Another, then, considers runoff is determined by precipitation intensity I and <br />teCpagc intensity F, i.e., <br /> <br />R=l-F, if l>F (3) <br />R-O if l<F (4) <br />The former can be used in wet regions with small hillside slopes, the tatter is suit- <br />able for dry or semi-dry regions. <br /> <br />~ <br /> <br />