Estimating hysteresis in the soil water retention function from cone permeameter experiments

Data obtained from modified cone penetrometer experiments were used to esti mate the hysteretic soil hydraulic properties with a parameter estimation t echnique which combined a numerical solution of the Richards equation with Marquardt-Levenberg optimization. The modified cone penetrometer was design ed to inject water into a soil through a cylindrical screen, measure the in filtration rate with time, and track the movement of the wetting front usin g two tensiometer rings positioned above the screen. After reaching relativ ely stable tensiometer readings during the experiments, the source of water was cut off and pressure head readings measured while water in the soil pr ofile redistributed. Cumulative inflow and pressure head readings for two e xperiments with different supply pressures were analyzed to obtain estimate s of the soil water retention and hydraulic conductivity functions. Analysi s of flow responses obtained during the infiltration period, and of those o btained during the combined infiltration and redistribution phases, demonst rated the importance of hysteresis of the soil hydraulic functions. We foun d that the redistribution phase could not be described accurately when hyst eresis was neglected. Hysteresis in the soil hydraulic functions was modele d using a relatively simple empirical model in which wetting scanning curve s are scaled from the main wetting curve and drying scanning curves are sca led from the main drying curve. This model was deemed adequate for our exam ples. Optimization results for various combinations of unknown soil hydraul ic parameters were compared to results of standard laboratory and in situ m ethods. Estimates of the saturated hydraulic conductivity were well within the range of in situ measurements. The estimated main hysteretic loops of t he soil water retention curve were for the most part situated between the w etting and drying curves obtained with standard methods.