Characterization of soil infiltration by stable water isotopes and an improved lumped-parameter model approach

In order identify the impact of chemicals on water quality and related risks, an understanding of soil infiltration processes in the unsaturated zone is required. In this work, two lysimeters installed at a test field south of Munich, Germany, were investigated. Maize was cultivated at the test field, and lysimeter soil cores are characterized by sandy gravels (lysimeter 1) and sandy-clayey silt (lysimeter 2). For three years, stable water isotopes in precipitation and seepage water were measured in 1-2 week intervals. Observations were interpreted by modeling in order to identify mean transit times of water and dispersion properties. A lumped-parameter model (LPM) implementing an analytical solution was applied. By subdividing the whole observation period into seasonal and vegetative periods with quasi steady-state flow the LPM was improved. Mean transit time of water, dispersion parameters and the contribution of preferential flow paths for all sub-periods with constant conditions were estimated. The improved LPM allows to mimic transient flow conditions and was able to describe the stable water isotope observations more accurately. Hence, the improved LPM approach could reduce model uncertainties as compared to the consideration of steady-state flow. In order to validate the findings from the improved LPM and enhance process understanding, unsaturated flow was also modeled numerically using Hydrus 1D. Soil hydraulic parameters were deducted from laboratory experiments and further adjusted by inverse modeling. Findings from applying the improved LPM could generally be confirmed by numerical modeling. Advantages of the improved LPM over numerical models include the need of a lower number of fitting parameters, which are often associated with higher uncertainties and required efforts concerning model input data. Combing the measurement of stable water isotopes in precipitation and seepage water with the improved LPM revealed a promising approach that could also be applied to support decision making, such as for agricultural practices that aim at minimizing chemical impacts to soil and groundwater quality. Investigations are currently continued for improving simulations by the consideration of mobile and immobile water and root water uptake.