Effect of rainfall intensity and slope gradient on infiltration and surface runoff in rainfed lands of Kalaleh region, Golestan Province

Various factors such as rainfall intensity and slope affect hydrological processes such as infiltration and runoff. Study on rainfed soils is very important because of intense degradation and sensitive ecosystem. The present research was conducted to investigate the effect of rainfall intensity and slope gradient on infiltration and surface runoff using rainfall simulator in rainfed lands of Kalaleh region, Golestan Province. To achieve the study purposes, four rainfall events with the intensities of 33, 64, 80 and 110 mm hr-1 and duration of 15 min were simulated. Three slope gradients of 6, 12 and 25% were selected and infiltration and surface runoff were measured in four 25×25 cm2 plots. The results showed that the infiltration average rate and runoff average coefficient were increased with increasing rainfall intensity and slope gradient. The main and interaction effects of rainfall intensity and slope gradient on infiltration and surface runoff were significant (p≤0.01). The results of subgroups of rainfall intensity and slope gradient using Duncan test showed that two turning points in hydrological response of plot are distinguishable, one between rainfall intensities of 64 and 80 mm hr-1 and another between slope gradients of 12 and 25%. In other words, at the rainfall intensity more than about 70 mm hr-1 and the slope gradient more than about 15%, average infiltration rate and average runoff coefficient were significantly decreased and increased, respectively. These results emphasized on land use management via avoiding change from rangeland to rainfed land especially when the slope is more than 15%. Agricultural and land cover management was also emphasized especially in high rainfall intensities (more than 70 mm h-1).