Modeling Infiltration Capacity Of Major Soils: The Case Of Upper Awash Sub-Basin

Abstract

Infiltration is complex process, challenging and time-consuming process to quantify it, because the parameters are very difficult to measure directly. It is also vary with respect to time, soil type and land uses. But identifying infiltration rate of certain soil and specific site is very important for different water resource managements. This study was needed to to determine infiltration characteristic of Cambisols and Vertisol, which are most dominant soils of Upper Awash Basin. Double Ring Infiltrometer of 60cm and 30cm diameter for outer and inner diameter was used to measure infiltration capacity of these soils. Field data collection was carried out at ASTU and Bishoftu Research Center, where proposed soil types exist during dry season. Horton, Philip, Kostiakov and Green Ampt infiltration models were used for this study. The values of various parameters of these infiltration models were calculated by graphical approach. Using these determined infiltration parameters, infiltration models for the site was developed. The observed infiltration rates of those soils from DRI was compared with the infiltration rates calculated by Kostiakov, Philip?�?s, and Horton?�?s and Green-Ampt infiltration models and the best fitting model was tested using RMSE, NS and R2. The R2 and NS values of Horton, Kostiakov, Philip and Green Ampt infiltration models for Cambisols were 0.991 & 0.967, 0.869 & 0.798, 0.990 & 0.990 and 0.995 & 0.996 respectively. In similar way, the values of RMSE were 6.660, 16.520, 3.632 and 2.332 for Horton, Kostiakov, Philip and Green Ampt infiltration models respectively. For vertisols, the result of the R2 and NS values were 0.992 & 0.963, 0.885 & 0.805, 0.980 & 0.980 and 0.987 &0.987 for Horton, Kostiakov, Philip and Green Ampt infiltration models respectively. RMSE values of Horton, Kostiakov and Green Ampt infiltration models for Vertisolsalso showed 2.059, 5.142, 1.628 and 1.32 respectively. From this comparison, Green Ampt infiltration model is best fit model for both sites. The values of field infiltration capacity and the result of the model were compared to set the valid model for the site. Model Equation were tested for accuracy by comparing their cumulative infiltration capacity with field cumulative infiltration capacity of the soil. R2, NS and Percentage of Error were used to test the accuracy of these models. Accordingly, Green Ampt infiltration model predicted the most accurate cumulative infiltration capacity of Cambisols showing the values R2, NS and RMSE of 0.999, 0.999 and 0.298 respectively. For Vertisols, Green Ampt and Kostiakov infiltration models showed the nearest value of cumulative infiltration capacity. The Values of R2 and NS of Kostiakov and Green Ampt were 0.998 & 0.996, 0.984 & 0.984 respectively. Therefore, Green Ampt infiltration models was selected as accurate model to estimate both Cambisols and Vertisols for dry season.