Investigation of the Effect of Wetting Depth on Estimation of Runoff and Sediment Production Capacity Using the Modified Williams Equation (Study Area: Qalat Shahrekord Range)

Depth to wet front is generally considered as the amount of water which penetrates into soil and wets the internal soil layer. This is an important variable especially in applications such as runoff generation and sediment yield estimation. This variable in some cases is used as a hydrological science in the form of a proxy for infiltration as an important factor in soil erosion processes. The importance and necessity of this research is estimating the performance of suspended sediment by adding the wetting depth variable to the regional model. The aim of this study was to investigate the effect of wetting depth on runoff and sediment production capacity by using the modified Williams equation.

In this field study, a rain simulator equipped with drip systems installed on a test site on a mountain slope to produce rainfall with intensities of 45 and 60 mm per hour on three slopes of 10, 20 and 30% was used with three replications. Adjacent to the experimental plots, soil surface depth was used to determine the physical and chemical properties of the soil, including soil texture, lime percentage, organic matter, initial moisture and aggregate stability. After setting the artificial sprinkler on the experimental plot, runoff and sediment obtained from each plot were collected in special bottles at 10-minute intervals and transferred to the laboratory. Then the amount of sediment produced at the end of each precipitation was weighed and calculated for each plot. Then, to measure the wetting depth using thin tissue rods, the wetting depth was measured every 10 minutes in diameter with 3 repetitions inside the plot. Explanation coefficient (R2) and Nash-Sutcliffe return (Nse) were used as sediment yield criteria for the model.

According to the results obtained in the study area and the efficiency coefficient, before adding the wetting depth parameter, the effect of precipitation intensity on sediment indicates a significant difference. The Nash coefficients at intensities of 45 and 60 mm/h are 0.84 and 0.63%, respectively, while the Nash coefficient for the combined intensities (total) of 45- and 60 mm/h is 0.55. Due to the low Nash coefficient in the Williams equation, for investigating the wetting depth on the amount of sediment, the Williams equation was modified. Based on the results in the study area, the relationship between wetting depth and the amount of suspended sediment in the mentioned intensities and slopes through the equation of Qs=(qe×qpe)0/32×S0/28×1/17dp×DA2/89 was obtained. Sediment performance has a non-linear and inverse relationship with wetting depth at the desired intensities and slopes, and also with the parameters of slope, runoff volume, runoff peak discharge and area. The results showed that the extent that increasing the slope, runoff, runoff peak and area leads to increasing the amount of sediment, and increasing the wetting depth reduces the amount of suspended sediment. The calibration and validation results of estimating the amount of suspended sediment in the developed model showed the value of Nse= 0.81; R2 = 0.81.

This study investigated the effect of wetting depth on runoff and sediment production capacity in the field at slopes of 10, 20 and 30% with rainfall intensities of 45 and 60 mm/h using the modified Williams equation. In the present case without considering the wetting depth variable, the results showed that at intensities of 45 and 60 mm/h, the amount of sediment and runoff increases by increasing the slope. This is because the shear energy poured on the soil by raindrops has produced runoff and sediment. But by combining rainfall intensities of 45 and 60 mm per hour, the amount of Nash coefficient decreased. One of the reasons for this is the increase in the diameter of raindrops and also the increase in the number of droplets that hit the soil surface. Although increasing the intensity of rainfall in most soils increases sediment by increasing runoff, it can sometimes reduce sediment efficiency by reducing sediment due to the spatial homogeneity of the slope of the soil surface layer. Addition of slope and wetting depth parameter, which is due to the amount of infiltration that moistens the soil around the bottom layers, is one of the factors that affect the production of sediment. The results of evaluating the efficiency of the regional model showed that the equation obtained for the region fits well with the observational data to the extent that the coefficient of explanation of fitted lines on the data increased by adding the wetting depth variable. There is an inverse exponential relationship between the amount of leached sediment and the wetting depth. Performance coefficient indices such as sedimentation coefficient and explanation coefficient, as two of the most widely used indicators to evaluate the results in the simulation of the amount of sediment, showed the value of Nse = 0.88 and R2 = 0.88. The results of estimating the amount of suspended sediment in the calibration (Nse = 0.81; R2 = 0.81) and validation (Nse = 0.81; R2 = 0.81) stages were very suitable for the study area.