Spatial variations of runoff generation at watershed scale

Designating contributory areas in runoff generation at watershed scale is a vital need for efficient and appropriate water management. The spatial variability of runoff generation and identifying critical areas have been therefore addressed in the Sarab Watershed comprising an area of 3668ha in western Iran. To this end, the infiltration measurements were taken through applying rainfall simulation in 47 sites within 18 work units resulted from overlaying land use and soil hydrologic groups maps. The Horton infiltration model was parameterized at each study site by entering the infiltration data and soil texture data into Infilt. Software package. The study watershed was then divided into 500x500m grid cells. The rainfall excess was ultimately extracted at each cell for eight natural rainfall storm events. The spatial dependency of runoff generation was then demonstrated via variography, and the spatial distribution of runoff was mapped using ordinary kriging method. The results showed that the initial infiltration rate (f(0)), the constant infiltration rate (f(c)) and the decay constant (k) varied as follows: 17.74-21.45mmh(-1), 1.16-4.86mmh(-1) and 1.80-5.01min(-1), respectively. The results further showed a strong spatial structure and a very diversified potential of runoff generation in different parts of the study watershed. It could be concluded that the upstream steep lands with low-density forest land use and weak soil hydrologic group had the highest runoff generation in the study watershed. The results of the study can be therefore practically applied by local watershed managers to adopt appropriate and space-specific water conservation measures.