Estimating soil hydraulic properties during constant flux infiltration: Inverse procedures

There is a need for accurate and cost-effective methods to estimate the hyd raulic properties of soils. past work indicated measurements of a single hy draulic response will not necessarily result in unique and stable estimates of hydraulic parameters when the number of unknowns is more than two. Prio r information regarding the parameters or additional measurements are neede d for the estimation problem to be well posed. However, accurate prior info rmation is seldom available due to variations of the hydraulic properties i n space and time. This paper presents a method for estimating hydraulic pro perties from simultaneous measurements of soil water storage to a fixed dep th as a function of time during constant flux infiltration, and steady-stat e pressure head readings using vertically installed multi-purpose time doma in reflectometry probes (MTDR), Multi-purpose TDR probes have a porous stee l cup at their ends allowing soil water storage and psi to be simultaneousl y measured at the same location. Our parameter estimation is formulated by an inverse procedure which combines a weighted nonlinear least square metho d with analytical solutions for soil water content and pressure head as fun ctions of depth and time during one dimensional infiltration. We analyze th e possibility of using water storage data combined with the initial and ste ady-state pressure head readings for the purpose of estimating soil hydraul ic properties. The uniqueness problem was analyzed by studying the behavior of response surfaces. The combination of water storage measurements during constant flux infiltration with an initial and a steady-state pressure hea d reading yielded unique and stable solutions of the inverse problem. The u tility of the parameter estimation procedure is demonstrated using experime ntal and theoretical data.