Assessment of the important Factors on Gully Erosion in Arid and Semi-arid Region in Ilam Province

Infiltration is the process of water penetration from the ground surface into the soil and is an important process in the hydrological cycle by which surface runoff and groundwater recharge can be linked. Over the years, the importance of the infiltration process resulted in the development of several simplified analytical models for predicting infiltration. These infiltration models range from entirely empirical to physically based models. The most serious problem associated with infiltration modelling of a catchment is how to express the spatial soil variability. In the present study, various infiltration models were fitted to the observed infiltration data of 27 double ring infiltrometer tests and the best-fit infiltration model for Darabkola watershed was identified and evaluated. In addition, the spatial variability of the selected infiltration model parameters was analyzed using the geostatistical techniques. Results showed that among of four models, the Green - Ampt model could determine the infiltration rate with smallest values of RMSE. Hence, saturated hydraulic conductivity parameter (KS) and suction head at the wetting front (Sw) were estimated for all the test points. Evaluation of spatial variability of these parameters indicated that parameters KS and Sw had the spatial dependencies of 0.49 and 0.25 respectively, showing medium spatial dependencies of both parameters. Also, investigation of interpolation parameter maps showed that in the upland with forest land use, relative to other areas in the watershed, the saturated hydraulic conductivity (KS) and suction head at the wetting front (Sw) have larger (1.57-2.69 cm/hr) and smaller values (12.12-1737 cm), respectively.