Browsing by Author "Neuweiler, Insa (Prof. Dr. rer. nat.)"

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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.
Open Access
Influence of the soil structure and property contrast on flow and transport in the unsaturated zone
(2010) Vasin, Milos; Neuweiler, Insa (Prof. Dr. rer. nat.)
The unsaturated zone represents a transition zone for contaminants spilled on the ground surface, which by reaching an underlying aquifer cause groundwater pollution. The complexity of processes occurring in the unsaturated zone and highly heterogeneous structures, which are never known in detail are the main reasons why it is challenging to make predictions of processes under unsaturated conditions. Instead of resolving the exact distribution of heterogeneities,which would cause enormous computational and field effort, flow and transport in the unsaturated zone are very often modeled in an average sense, where the input parameters of the model are spatially averaged (effective parameters). Derivation of those simplified models is called upscaling. In this study, modeling of flow and transport in the unsaturated zone, when upscaled models are used has been investigated. This study focuses on upscaled models for flow and transport in the unsaturated zone derived by means of homogenization theory. Those two derived models can not be used in general since they are derived under certain assumptions, which are necessary when homogenization is used. Therefore, many questions rise, when those models are used considering their limitations. One of the major assumption of upscaled models derived by means of homogenization is domain periodicity. In this case the effective parameters could be derived explicitly as the structure is known in detail. However, in nature the structure of domain is usually unknown and effective parameters have to be estimated. The derived upscaled models could be only considered as reliable when the effective parameters are estimated in an adequate and effective way, capturing the influence of heterogeneities on the smaller scale. Additional to the periodicity and difficulty with estimation of effective parameters, mentioned models could be derived either for equilibrium or non-equilibrium conditions, dependent from the parameter contrasts between materials. However, in order to distinguish if equilibrium or nonequilibrium model is more suitable for modeling of flow and transport processes, typical time scales have to be estimated. In order to investigate above mentioned challenges with regard to effective parameter estimation, assumption of upscaled models and time scale analysis, three lab experiments have been performed. The experimental data have been compared with numerical simulations or analytical solutions. The experiments done here have been performed under well controlled conditions with artificial heterogeneous structures. As a result, the conclusions of the experiments are specific for these typical conditions. The first part of this study has been focused on flow in the unsaturated zone under equilibrium conditions, meaning that the upscaled model has been derived using a small soil parameter contrast. Different structures, with significantly different connectivity (periodic and random structure) have been investigated in order to gain a better knowledge of the structural influence on the estimation of effective parameters. Additionally, the applicability of the mentioned upscaled flow model under ideal and non-ideal conditions has been assessed such that the domain does not fulfil the assumption of periodicity, but also of small parameter contrasts needed in the case of equilibrium model. It has been shown that the estimated parameters used in the upscaled 1D model performed well. Estimated parameters based on only rather limited information were sufficient to predict the drainage process very well. The flow in the unsaturated zone under non-equilibrium has been investigated in the second part of the study. This implies that the parameter contrast between soil materials used in the experimental study was large. Different options for estimation of typical time scales have been presented and discusses as they are decisive in order to chose, which upscaled model (equilibrium or non-equilibrium) is more appropriate to be used. The obtained time estimates have been further compared with the experimental and numerical findings. It has been shown that the water capacity was the crucial parameter in order to make good drainage time predictions. The system in this example has reacted with the fastest predicted time scale. During the third part of this research, solute transport under equilibrium and non-equilibrium has been investigated. The goal was to observe if equilibrium or non-equilibrium of solute transport could be predicted by using time scale analysis. The estimated time scales have been compared with experimental results. The equilibrium and non-equilibrium have been obtained during the experiments leading to tailing and retardation of tracer. Both equilibrium and non-equilibrium conditions could be predicted by the time analysis. Model assuming equilibrium would give bad predictions of solute transport in case of experiment, where nonequilibrium occurred.