Investigating the Effects of Lithological Units on Runoff Coefficient (A case study of 18 watersheds in three climatic regions of the Iran)

Geology is one of the most important factors in the production of runoff and sediment. Geological formations, on the one hand, control the topographic features of a domain, and on the other hand, control the soil and surface features of the earth which are considered to be the function of this organization. The purpose of this study was to compare the effects of Lithological units as the main factor in determining the volume of runoff production in watersheds. For this purpose, in the present study, 18 small watershed (the smallest area with an area of ​​17.2 km2 and the largest one with ​​134.1 km2) with a predominant lithology (as much as 70 percent of the study area is covered by the similar lithostratigraphic units) in three different climatic regions were analyzed based on the cumulative rainfall-runoff curves. The results of the study clearly indicate the significant effect of the Lithological units on the runoff coefficient, so that the watersheds with identical Lithological units have similar curves, and the watersheds with different Lithological units have different curves. Therefore, lithostratigraphic units with high porosity have high penetration and less runoff potential. For example, the Phyllithic areas in Khorasan Razavi have higher runoff potential than others, in fact, their porosities are less than those of andesitic and volcanic tuffs. The results also show a seasonal variation of runoff coefficient in the studied watersheds.