Sequential estimation of hydraulic parameters in layered soil using limited data

• Soil hydraulic parameters were estimated from the monitored soil water content.
• A method was developed for sequential parameter estimation in layered soil.
• The estimated parameters simulated the soil water change fairly well.

Field-scale estimation of soil hydraulic parameters is important for describing water movement in vadose zones. The importance of soil water measurements has been acknowledged with increasing soil water measurements becoming available; thus, the estimation of hydraulic parameters from observed soil water would be quite useful for hydrological modeling. This study estimated the hydraulic parameters of Brooks–Corey and Mualem model using the monitored soil water changes at two depths together with the rainfall intensity at two soil plots in a tropical rain forest in Indonesia. A one-dimensional multi-scale parameterization method was used for the analysis, beginning with homogeneous parameterization and identifying the depth of discontinuity using refinement indicators, thus increasing the number of zones. A method for sequential parameterization was developed in each step of zoning. The measured and simulated volumetric water contents with the optimized parameters showed good agreement for one plot (standard error is 0.0419) with 2-zone parameterization, and the effects of the initial parameters derived from different pedo-transfer functions on the optimized hydraulic functions were small, confirming the robustness of this method. However, at another field site, agreement between measured and simulated water contents was not very good (standard error is 0.0854), because the effect of the soil water repellency might have influenced the results, and the effects of the initial parameters were large. The algorithm proposed in this study systematically determines the hydraulic parameter set that describes field-scale water flow.