PSIAC模型.doc

Determining factors in PSIAC model
The PSIAC Model (Pacific Southwest Inter-Agency Committee, 1968) was developed to allow the estimation of sediment yield for a large variety of factors within a watershed. The PSIAC model assists nine factors were recommended for erodibility and sediment yield as follows: surface geology (X1), soils (X2), climate (X3), runoff (X4), topography (X5), land cover (X6), land use (X7), upland erosion (X8) and channel erosion (X9). In this model, the values of nine factors ranked based on corresponding tables which compacted as shown in Table 1. Each of these factors is to be ranked based on a visual appraisal of the watershed. The total ranking values ranged from 0 to 150 categorized into five sediment yield classes (Table 2).
Surface Geology (X1)
Knowing the surface geology features of a watershed is important for estimating the erosion and sediment yield processes. Based on PSIAC model, the ranking values of surface geology factor ranged from "0 to 10" in which soft stones such as quaternary terraces are eroded easily and play an important role in sedimentation at the study area. The surface geological formations were classified into nine features from granite and locogranite rocks at the north to phylitic dark gray and shale stones at the south of the basin (Fig.2b). Field observations revealed that debris slides and erodible materials were found mainly at the contact of shale and sandstone formations which correspond to fault lines of the study area according to geological surveying map (Sc, 1:250000).
Soil (X2)
In PSIAC model, a range of "0 to 10" was given to soil factors. This range is based on soil texture, the resistance of particles, lime stone, clay disperse and primary humidity of soil; normally, this range is determined by soil texture. The soil and land units of the study area were categorized into six classes from mountains at the west to hills at the east of the basin, where the most sheet erosion forms were observed on hilly areas (Fig.2c).
Climate (X3)
The sedimentation depends on climate (precipitation and temperature) since climate not only affects soils but also vegetation cover. Precipitation and especially intensive rainfall are even more effective; rainfall has great effect on both runoff and erosion. Temperature is important when the difference between night and day is great. Based on PSIAC model, the ranking value of climate factor ranged from "0 to 10". The produced isohypse (Fig.2d) and isotherm (Fig.3a) of the study area were categorized into four and six classes, respectively, based on interpolation analysis of meteorological data (1996-2007).
Runoff (X4)
In PSIAC model, the runoff range varies between "0 to 10". Zero is for watersheds with high permeability and very low runoff and 10 is for a watershed that converts most part of precipitation in to runoff. Based on soil permeability the runoff potential map of the study area was categorized into four classes (Fig.3b). Field observations revealed that the intensity of runoff potential at the westward of the basin due to steep slopes of mountains is much higher than the other parts of the basin, although thin soil depth at mountainous areas resulted in lower sediment transport.
Topography (X5)
One of the important factors in erosion is topography which is normally estimated by slope. The importance of slope and topography in erosion has a range of "0 to 20" in the PSIAC model. The produced elevation (Fig.3c) and slope map (Fig.3d) of the study area were classified into nine and four classes, respectively, based on interpolation analysis of topographical data of the study area (Scale, 1:25000). The elevation ranges were classified into nine classes from 880 to 2860 meter a.s.l., where the most erodible areas with the slope average of 30%, are located at the center and westward of the basin over than 1300 meter a.s.l..
Land Cover (X6)
Land cover means every type of land cover that protects soil against erosion factors such as rain drop hits, runoff and wind. The land covers such as dry farming and scattered pasture lands could have accelerating effect on erosion processes. The land cover map of the study area was categorized into seven classes, where the plugged lands of dry farming exhibited the most soil losses (Fig.4a).
Land Use (X7)
Human activities or land use is an important factor in increasing erosion, whereby inappropriate land use causes erosion. Among common activities that erode land are: tillage, over-grazing by animals and other inappropriate land uses. Over-grazing can be considered as the most destructive factor. The art of land use at the study area consists mainly of cultivated lands, gardens, roads and villages (Fig.4b).
Upland Erosion (X8)
The upland erosion includes several kinds of erosion such as rainfall erosion, sheet erosion, rill erosion and gully erosion (except erosion of channels). Based on different land types of the study area the upland erosion potential was mapped and categorized into three levels (Fig.4c). Field observations revealed that rill and sheet erosions were formed mainly at elevated areas and hilly lands, respectively.
Channel Erosion (X9)
In this factor both river-bank erosion and sediment transport by flood are important. This erosion is caused by the abrasion of channel walls and mostly occurs in times of flood and full-water seasons. Hence, the main streams and erosive channels at the study area were determined based on streams flooding rates (Fig.4d).