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Spatially distributed recharge and groundwater – surface water interactions in groundwater models: from the field to the catchment scale

Boek - Dissertatie

Flow and transport in lowland area aquifers are strongly influenced by recharge processes and interactions with the surface water network. In lowlands, the groundwater table is close to the topographic surface. Therefore, the interactions between groundwater, atmosphere, soil, vegetation and surface water take place over short distances, within the so-called critical zone. Because of these short distances and the non-linearity of flow processes in the critical zone, a correct representation of these processes in catchment/regional scale groundwater models remains a challenge. This thesis presents an attempt to improve the representation of critical zone water transfer processes in a temperate lowland area catchment scale groundwater model while maintaining computational costs reasonable. In particular, the representation of groundwater - surface water interactions and groundwater recharge were studied in depth. The general approach used in this work is a bottom-up upscaling. First, processes are mechanistically simulated at a smaller scale (point and field scales), considering the small scale spatio-temporal features of the system. These simulated processes are then averaged or aggregated to a larger scale (catchment scale) and functional relations between larger scale averaged processes and features of the smaller scale structure are derived. In this way, small scale process understanding is effectively used to constrain relations that emerge at larger scales. In regional models, groundwater - surface water exchange fluxes are usually represented with a Cauchy boundary condition governed by a conductance parameter. To improve the representation of groundwater - surface water interactions in regional hydrogeological models, a new expression of the groundwater - surface water conductance was derived. Parameterization of the conductance parameter is currently problematic because it relies on the assumption that conductance is exclusively controlled by streambed properties. However, depending on the specific system and the spatial discretization of the hydrogeological model, aquifer conductance can be a limiting factor for groundwater - surface water interactions. The resulting scale dependency of this parameter is not considered in the current conceptualization. The new expression, derived using analytical equations and 2D vertical field scale simulations of a stream-aquifer cross section, links the conductance to aquifer hydraulic properties and thickness as well as the discretization of the groundwater model and the surface water network density. The expression was evaluated using 3D hydrogeological simulation models at different spatial resolutions and compared against previously published parameterization approaches. The results show that the new expression captures accurately both the grid size and the surface water network density dependency of the conductance, without adding complexity to the numerical model. The third chapter of the thesis presents a field scale modeling study investigating the link between diurnal groundwater level fluctuations and root water uptake from the groundwater. A 3D variably saturated model was set-up to simulate the observed diurnal groundwater fluctuations in a deciduous tree plot and in the adjacent grass plot. The model simulated the main features of the observed diurnal groundwater level fluctuations properly, though a more advanced vegetation compartment would be needed to better represent the timing of the diurnal fluctuations. Significant diurnal patterns were simulated in the fluxes time series between the tree plot, the neighboring grass plot and the adjacent river. Furthermore, excluding root water uptake from groundwater led to larger (10 to 30 %) groundwater - surface water fluxes during dry periods. This suggests that groundwater uptake by plants is potentially an important process to represent in a catchment scale model. The last chapter of this thesis presents a steady-state groundwater model of the Kleine Nete catchment (northeastern Belgium). We considered different parameterizations of groundwater recharge and groundwater - surface water interactions and assessed their effects on simulated groundwater head and fluxes. Concerning groundwater - surface water interactions, we used the new groundwater - surface water conductance expression and compared this approach to the calibration of a uniform groundwater - surface water conductance value. Our groundwater recharge calculations relied on 1D variably saturated simulations, which were integrated to the groundwater model through a look-up table, summarizing the relationships between groundwater depth and groundwater recharge. The 1D variably saturated simulations considered variable groundwater depth, sub-grid variability and tree root water uptake from the saturated zone. The relative importance of considering these factors on simulated groundwater recharge and groundwater flow fields was evaluated. The results highlight the relevance of using our new groundwater - surface water conductance expression and the importance of considering the coupling between groundwater depth and groundwater recharge in shallow groundwater environments.
Jaar van publicatie:2020
Toegankelijkheid:Open