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The sediment yield potential estimation of Fariabad and Kordian watersheds using MPSIAC model in the GIS framework
Surface geology
To calculate this factor, at first geology map was digitized and then based on the rocks sensivity to erosion?this map was encoded and a new data field in the geology map database ( based on X1 factor ) was created .Using the facilities of Spatial Analyst module in the Arc View software , the average of this factor in these basins(Fariabad and Kordian) was obtained 3.26 , 1.89 .
Soil
With due attention to the soil studies and soil experiments,the effective factors on the K( erodibility factor in the USLE method), namely, silt + very fine sand percent,sand percent,organic matter percent, soil structure and permeability were determined and then, using related nomograph, K value and at last X2 value in any land unit components was estimated.Then, land units map was digitized and was encoded based on the soil erodibility factor and a new data field in the land units map database (based on X2 factor) was created. The average of this factor in the Fariabad and Kordian basins was obtained 3.41 , 2.24.
Climate
To obtain 6-hour rainfall with 2-year return period, IDF (Intensity Duration Frequency) curves data were used and this factor (X3) for these basins was obtained 3.84. Then, basin border map was digitized and this map was encoded based on the climate factor value and at last a new data field in the basin map database ( based on X3 factor ) was created .
Runoff
To providing this layer , two maps were provided , runoff height map (R) and specific peack discharge map (Qp) . At first, the DEM layer was prepared , then with applying the precipitation gradient equation on the DEM layer ,the precipitation map was obtained .
{2} P=64.28 + 0.1335 H n=19 , r=0.88987
Where :
P:annual precipitation (mm)
H: elevation (m)
Then,sub-basins map (or hydrologic units map) were encoded based on the runoff coefficients of rational method(Cr) and a new data field in the sub-basins map (based on Cr) was created . At the next step ,the structure of this map was changed to the raster structure , based on the run off coefficients , and then this map and rain map were multiplied together and the runoff height in each pixel was obtained . To providing Qp layer, the rational method was used. In this method ,at first with due attention to IDF curves and cocentration times, precipitation intensity of these basins was calculated and then using runoff coefficients in the rational fomula and the other parameters, the basins flood with different return periods were obtained.
{3} Qp= 0.278 CIA
r= -0.98
where :
Qp = peak discharge (m3 / sec )
A = basin area ( km2 )
I = rainfall intensity(mm/hr)
C=runoff coefficient
And then, the hydrologic units map was encoded based on Qp and at last by using this equation ,
{4} X4 = 0.006R + 10 Qp
runoff factor value was obtained in each pixel . The average of X4 factor in these basins was obtained 9.33 , 9.12 .
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