Spatially distributed recharge in groundwater models:
bridging the gap between the soil profile and the catchment scale
Flow and transport in aquifers of lowland areas are strongly influenced by recharge processes, that is, the downward fluxes of water from surface to groundwater. Precipitation is transferred to aquifers through a complex and interconnected set of processes, including the interactions between vegetation, surface water, vadose zone (soil) and shallow groundwater, which altogether form the so-called critical zone. Groundwater recharge can thus vary significantly over short distances due to spatial variability in soil (type and structure) and land cover (vegetation, buildings…). However, this spatial variability is in general addressed either by applying over-simplistic GIS-based models, or by using more complex, process-based models that are hardly calibrated/validated at the scale of application. The validity of these approaches is highly questionable given the critical zone interaction processes mentioned above. There is thus clearly a need to better characterize and model recharge processes, in order to take into account this spatial variability in an appropriate way in catchment-scale ground water models.
Recharge processes can be monitored and modelled at the soil profile scale, by detailed measurements of important hydrological processes and variables. The research gap identified for the present proposal is to determine how to upscale such point data to the plot and catchment scales at which groundwater models are usually run. In addition, an important question related to the modelling aspect of the research is to assess how the hydrological processes described above are best integrated into one model, coupling all compartments of the critical zone.
A coupled and spatially distributed hydrological model needs to be calibrated and validated for a specific catchment. The aim is to use the Kleine Nete catchment as a case study, since there is quite some hydrological information available. We propose to install a detailed experimental setup, equipped with state-of-the-art instrumentation for monitoring variably-saturated flow processes in selected prototype combinations of land cover, soil type and shallow aquifer (critical zone prototypes).
The overall aim of this PhD proposal is to develop, calibrate and validate a spatially-distributed integrated model for groundwater recharge, based on flow and transport data obtained from an experimental setup. This will allow to bridge the gap between the soil profile and catchment scales using appropriate upscaling techniques. To achieve this, the study is subdivided into several milestones:
To set up a long-lasting observational network for the critical zone in the Kleine Nete catchment. This setup consists of the detailed monitoring of variably-saturated flow dynamics at selected prototypes of vegetation-soil-phreatic zone combinations. More specifically, the following processes and variables will be monitored: meteorological variables, rainfall interception by vegetation, root water uptake, soil water matric potential, volumetric soil water content, soil water quality (hydrochemistry, stable isotopes), water table level.
To use these experimental data for guiding the development of a conceptual and numerical flow model that couples surface, vadose zone and groundwater processes at the scale of the Kleine Nete catchment (~800 km²). This coupled model will then be inverted for each of the selected experimental sites, in order to find (a representative ensemble of) appropriate values of all relevant model parameters.
To identify and apply a suitable upscaling procedure in order to transfer the knowledge gathered for the individual experimental (prototype) sites to the catchment-scale model parameterization. The main underlying questions being how to best account for the within grid element variability in the catchment-scale model, and how to interpolate between the critical-zone prototypes based on spatially exhaustive information.
To validate the catchment-scale coupled model using appropriate validation data such as soil water matric potential and volumetric water content measurements, piezometric levels, isotope and age tracer concentrations in the phreatic zone, river discharge, etc.
The minimum diploma level of the candidate needs to be:
Master of sciences , Master of sciences in engineering
The candidate needs to have a background in
Bio-engineering , Geology