Investigation of surface and subsurface runoff of catchment's hillslopes with loamy sand soil, case study: Estahban Watershed

Catchments consist of a series of sloping pervious overland whose surface and subsurface runoffs are transmitted to their outlet through stream networks. In the catchments with high perviousness and good vegetation cover, the amount of subsurface gains more significance and it might have a considerable share in the direct runoff. In this study, a hydrologic rainfall simulator model with the length of 1.92 meter and width of one meter and depth of 0.35 meter has been used which its surface and subsurface amount of flow have been measured by means of two different weirs. The texture of soil in Estahban Watershed was loamy sand. The experiments were conducted under three slope angles of 0.1, 3, 6 and 9 degrees and under rainfall intensity of 31.73, 47.6 and 63.46 millimeter per hours. Based on the results, the slope changes from 0.1 to 3 degrees resulted in 50 percent decrease in the subsurface flow and 10 to 15 percent increase in the surface flow in different rainfalls, but, from the slope of 3 to 9 degrees, no significant change was observed in the two flows and in the slope change from 6 to 9 percent of subsurface flow and 2 percent of surface flow, there was surface flow observed. The increase in rainfall intensity causes rise in hydrograph amount of surface and subsurface flow. The proportion of surface to subsurface flow changed on average between 7.5 and 14.5 times the subsurface flow under three rainfalls for the loamy sand. With the increase in slope, the surface flow amount increases and infiltration decreases. In this study, two non-linear regression equations were presented for measuring surface and subsurface peaks which is a function of length, slope and rainfall intensity of the slope.