Evaluation of double-ring infiltrometers method for measuring the vertical infiltration in different soil textures using HYDRUS

Infiltration is one of the important parameters of the soil, which affects many hydrological processes in the watersheds. The importance of the Infiltration has led to various laboratory methods used to measure this process. Various methods have been developed for measuring the infiltration, which are based on the measurement of the vertical flow of water to the soil. One of the standard methods for measuring infiltration, is double-ring infiltrometers method. In this method, it is assumed that the external cylinder prevents lateral flow and creates a completely vertical flow in the soil. Therefore, the present study was conducted to evaluate the double-ring infiltrometers method for measuring the vertical infiltration compared to actual vertical infiltration data simulated using HYDRUS-1D software. In this study, in order to evaluate the double- ring infiltration data, infiltration was measured in several regions with different soil textures by double- ring infiltrometers. Then, HYDRUS-1D numerical model was used to simulate infiltration and the vertical infiltration data were obtained through forward solution of the Richards equation. van Genuchten-Mualem model was used to quantitatively soil hydraulic properties of the Richards equation. In order to the hydraulic parameters of van Genuchten-Mualem model were optimized using inverse modeling in the HYDRUS, for each region's soil Assessment of measured data was performed using mean error (ME), root mean square error (RMSE), coefficient of determination (R2) and normalized mean square root (NRMSE). Findings: Comparison of simulated and measured infiltration data showed that double ring infiltration data is much higher than the simulated vertical infiltration data. The measured infiltration data in all soil texture was much different from that of simulated data. The lowest error in the measurement of vertical infiltration was observed in sandy loam soil that is a light texture. The values of coefficient of determination, root mean square error, mean error and normalized mean square root (R2, RMSE, ME and NRMSE) were 0.87, 8.51, -4.45, and 0.18 respectively in this texture. In measuring the infiltration of a double-rings, the size of the cylinders used, as well as the buffer index, is of great importance. Therefore, the use of cylinders of different sizes influences on the final infiltration values. The double-ring infiltrometers method has a lower error in the Sandy texture than heavy texture and the contribution of lateral infiltration is less in this texture. Therefore, the double-ring infiltrometers has a higher accuracy in light texture.