An autoregressive procedure to predict the hydraulic conductivity - Comparison of measured and predicted results

An instantaneous profile method was used to measure the unsaturated hydraulic conductivity. Relatively new techniques involving undisturbed soil samples instrumented with minitensiometers and Time-Domain-Reflectometry (TDR) mini-probes were used for the experiments. The laboratory method allows a high spatial and temporal resolution. Laboratory measurements were carried out for 40 soil horizons with a wide spectrum of texture and bulk density. In addition, retention curves were measured using the standard pressure plate apparatus. Using this homogeneous set of data, an autoregressive model was developed which allows a stepwise calculation of the hydraulic conductivity for a water potential range of -30 up to -600 hPa. This model was developed for loamy sands, sandy, silty and clayey soils in conjunction with data from the retention curves. The calculation procedure starts with the determination of an initial unsaturated conductivity (k) close to field capacity, i.e., for water potential from -60 hPa up to -100 hPa. This first value is then used to predict other conductivity values using appropriate changes in soil water content corresponding to a defined range of the soil water potential. Subsequently, the hydraulic conductivities for higher and lower potentials were estimated considering the k Value of the previous step in combination with the data of the retention curve of the next water potential range. The advantage of this empirical model is the indirect consideration of soil structure, in contrast to the closed-form van Genuchten-Mualem (VGM) model. To demonstrate these effects on different fitting procedures, the vGM model was also used to describe soil hydraulic functions. The accuracy of both, the VGM model and the autoregressive one. were compared for various fitting procedures and soils.