Browsing by Author "Kuhlmann, Anna"

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Open Access
Influence of soil structure and root water uptake on flow in the unsaturated zone
(2012) Kuhlmann, Anna; Neuweiler, Insa (Prof. Dr. rer. nat.)
The unsaturated zone is the part of the soil between the aquifer and the atmosphere. Unsaturated flow processes are highly dynamic and control e.g. the growth of plants or groundwater recharge. Environmental problems such as the agricultural use of arid regions and groundwater contamination call for sustainable solutions, which can only be achieved with model predictions. To improve the quality of models, a sound understanding of unsaturated flow processes and the used model approaches is necessary. The present work is intended to contribute to the understanding of modeling unsaturated flow with focus on the influence of water extraction by plant roots (root water uptake) and soil structure. The model for root water uptake, in the following called standard or basic approach, is determined by the atmospheric demand, the distribution of roots in the soil and the soil water status. Soil properties are described by autocorrelated random fields with layered structure (1D) and multi-Gaussian or non multi-Gaussian distribution (2D). For steady state flow in layered media, a semi-analytical first-order second-moment solution for mean and variance of pressure head is presented. Flow in 2D heterogeneous media is analyzed using numerical simulations where steady state and dynamic scenarios with one or several drying-rewetting phases are carried out. The results show that only under very wet conditions, the mean pressure head in the differently structured fields is well predicted by the analytical solutions while variances of pressure head are overestimated if the variance of the loghydraulic conductivity is large. Under drier conditions, root water uptake and soil structure have combined effects on unsaturated flow, which are not observed if one of these two factors is neglected, and which cannot be predicted by first-order second-moment effective models. In particular, distinct regions with pressure head values at the wilting point, where root water uptake is zero (local wilting), occur in lenses of coarse material. Furthermore, root water uptake affects the variance of pressure head and saturation during drying and rewetting phases in comparison to an equally dry state where root water uptake is not accounted for. Other effects introduced by root water uptake arise from a decreasing local net infiltration rate with depth, caused by the continuous extraction of water by roots within the root-zone. With decreasing local net infiltration rate, which leads to drier states, the impact of soil structure increases. For water flow, this leads e.g. to a depth dependency of the width of the infiltration front during rewetting. For solute transport, earlier arrival, smaller tailing and less impact of the considered structures of soil properties are observed due to root water uptake, when scenarios with and without root water uptake, which have the same groundwater recharge rate, are compared. To test modeling approaches for root water uptake, alternative uptake strategies that allow for compensation of stressed (uptake-reduced) locations by enhanced uptake at other, more favorable locations are considered. These strategies affect the distribution of pressure head and saturation, leading to smaller variances in the root-zone and attenuated local wilting, but do not prevent local wilting. The difficulty to evaluate the effect of local wilting as realistic physical phenomenon or unrealistic model artifact, the fact that the trend of the impact of root water uptake on the variability of flow depends on the uptake strategy and the lack of knowledge of how roots really extract water in heterogeneous soils emphasize the need for a deeper understanding of root functioning at smaller scales before macroscopic models for root water uptake can be used for reliable predictions of flow in heterogeneous media.