Modelling of water flow through typical Bavarian soils: 1. Estimation of hydraulic characteristics of the unsaturated zone

Over an eight-year period (1984–1991), seven experimental lysimeters filled with different soil materials from the State of Bavaria (quartz and tertiary sands, quartz and fluvioglacial gravels) were used to study water flow and tracer transport through the unsaturated zone under natural atmospheric conditions. The experiments were performed in the input—output mode using weekly water flow and tracer contents (environmental deuterium) in the precipitation and in the outflow in order to estimate the unsaturated hydraulic and transport parameters of the soils with limited measured data. This paper (Part 1) presents the evaluation of soil hydraulic parameters for tested soils, while the second paper (Part 2) is concerned with the estimation of transport parameters (dispersion, water content and possible zones with stagnant water). The estimation of the soil physical characteristics, i.e. the relationship between water pressure head and water content, performed by applying the van Genuchten model, showed a satisfactory accuracy against the measured data. Determination of the hydraulic conductivity as a function of water content using the van Genuchten-Mualem conductivity model did not yield good results, due to the fact that the theoretical water flow model based on such soil parameters was not able to fit the measured values of water content at the upper 20 cm and the cumulative water outflow simultaneously. To estimate the conductivity—water content characteristics, the inverse modelling technique was used for each lysimeter. The cumulative outflow curves were fitted to the observed outflow data by a trial-and-error procedure using the conductivity—water content relationship as the fitting function. It was shown that this method of estimating the soil characteristics yields parameters which are closer to the true values compared to those found in small-scale column experiments. The soil characteristics obtained in this part of the study were further used for modelling environmental deuterium transport through the soils (Part 2).