Estimating saturated and unsaturated hydraulic conductivity and sorptivity coefficient in transient state in sloping lands

Double ring and tension infiltrometer are simple and suitable instruments for determining soil hydraulic conductivity and soil sorptivity coefficient. The effect of land slope on soil properties, such as saturated and unsaturated hydraulic conductivity and soil sorptivity coefficient, has been reported by various researchers. The aim of this study was to estimate soil hydraulic conductivity and soil sorptivity coefficient values in lands with different slope gradients, under transient flow conditions. Field experiments were conducted in a loamy soil with different slope gradients in Gonbad Research Station, Hamadan, Iran. Soil surface slope gradients 0 (level), 10, 20, 30 and 40 degrees were selected in this study. For each slope gradient, water infiltration experiments were carried out using a double ring and a tension infiltrometer at tensions of 0, 6, 9 and 15 cm in three replications. Totally 60 water infiltration experiments were carried out. In transient state, Philip's two-term infiltration equation was applied to determine soil sorptivity coefficient and soil hydraulic conductivity using Cumulative infiltration data obtained from the double ring and tension infiltrometer. In this state, the sorptivity coefficient and the hydraulic conductivity for different land slopes and water pressure heads were calculated from Philip's two-term infiltration equation, which can be obtained by fitting the Philip's equation to cumulative infiltration data. Results indicated that both the sorptivity coefficient and the hydraulic conductivity values were decreased with increase in tension values. Also the sorptivity coefficient value was increased and the hydraulic conductivity value was decreased with increase in slope gradient. The higher value for decreasing rate of hydraulic conductivity was obtained in lower tensions. With increase of slope gradient from 0 to 40 degrees, decreasing rate of hydraulic conductivity in 0 tension was 3.7 times higher than that in 15 cm tension. With increase in slope gradient the higher value for increasing rate of sorptivity coefficient was obtained in lower tensions.