Introduction :
Nowadays the degradation of renewable natural resources is one of the most important problems of mankind . Soil is one of the most important natural resources and the degradation of that is caused , the decline of fertility , weakening of plant cover and at last aggravation of desertification specially in the arid and semiarid regions . In accordance with the estimation of FAO organization , in excess of 75 billion tons soil in the whole earth is eroded . In the neighborhood of 76 percent of Iran exposed to the erosion . In Iran,the climate variation and topographic conditions have important role on the increasing of erosion . This watershed has located on the north of Kashmar city (Khorasan province, Iran, between 58° 28 30 to 58° 34 00 E and 35° 17 25 to 35° 23 00 N). Torrential discharges of this watershed is entered from east into kashmar city .In the study area there is two independent hydrologic subbasins , kal-e-asgir and kal-e-gorg .This region is high mountainous and its area is 5153 hectare .This basin is located in kavir-e-namak basin .With due attention to the suitable compatibility of MPSIAC model to arid and semiarid conditions of Iran in the erosion and sediment studies of this watershed was applied .In this study we used newest GIS softwares to obtain a good and accurate result at the least time .
Materials and methods :
By reason of the great number of data, activity and changeability of these data in the natural resources , geographic information system , as a useful tool , is carried to solving many problems .In this study , effective factors in the erosion ( to MPSIAC model ) are gathered by using existing conditions maps , statistical information , field investigation , topographic maps and aerial photographs . Then by using geographic information system and Arc view , Arc/info , R2V and Auto CAD softwares ,the maps and tables was digitized and based on table 1 the erosion factors maps were encoded and with overlaying these maps ,the sediment yield and erosion maps in accordance with this equation were obtained
[1]QS = 38.77 e0.0353R
where:
Qs : total sediment yield ( m3. km-2 .y-1 )
R: sediment yield score .
Surface geology :
To calculate this factor, at first geology map was digitized and then based on the stones 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 the whole basin was obtained 2.2 .
Table1.effective factors on the erosion in the MPSIAC model
| Description | Equation | Effective factors | No |
| X1=Stones sensivity to erosion(0-10) | Y1=X1 | Surface geology | 1 |
| K=soil erodibility | X2=16.67K | Soil | 2 |
| P2=6-hour rainfall with 2-year return period | X3=0.2P2 | Climate | 3 |
| R=runoff height Qp=1-year specific pick discharge | X4=0.006R+10Qp | Runoff | 4 |
| S=slope(%) | X5=0.33s | Topography | 5 |
| Pb=bare ground percent | X6=0.2Pb | Land cover | 6 |
| Pc=crop canopy percent | X7=20-0.2Pc | Land use | 7 |
| SSF=the score of soil surface erosion in the BLM method | X8=0.25SSF | Surface erosion | 8 |
| SSFg=the score of gully erosion in the BLM method | X9=1.67SSFg | Gully erosion | 9 |
Soil :
With due attention to the soil studies and soil experiments , the effective factors on the K (k is erodibility factor in the USLE method ) , namely, silt + very find sand percent , sand percent , organic matter percent, soil structure and permeability were determined and then by using nomograph1, K value and at last X
2 value in any land unit components was estimated.Then land units map was digitized and this map was encoded based on the soil erodibility factor and a new data field in the land units map database (based on X
2 factor) was created. The average of this factor in the whole basin was obtained 2 .93.
Climate:
To obtain 6-hour rainfall with 2-year return period, we used IDF (Intensity During Frequency) curves data and this factor (X
3) for Kashmar watershed was obtained 2.52. 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 X
3 factor ) was created .
Run off :
To providing this layer , two maps were provided , run off height map (R) and specific pick 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=2.157 + 0.172 H
Where :
P:precipitation (mm)
H: height (m)
Then,subbasins map (or hydrologic units map) was encoded based on the runoff coefficients of rational method(Cr) and a new data field in the subbasins 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 run off height in each pixel was obtained . To Providing Qp layer, we used the regional analysis method . In this method , discharges around study area were studied and the relation between specific discharge and basin area was obtained at the various return periods .These equations are significant at one percent level .
The relation between 1- year specific pick discharge and basin area is as following :
{3} Q= 0.55 A
0.393
r= -0.98
where :
Q = 1- year specific pick discharge (m
3 / km
2/y )
A = basin area ( km
2 )
r = correlation coefficient
And then the hydrologic units map was encoded based on Q
p and at last by using this equation ,
{4} X
4 = 0.006R + 10 Q
p
runoff factor value was obtained in each pixel . The average of x4 factor in this basin was obtained 4.31 .