Abstract
Abstract. Modelling and monitoring of hydrological processes in the unsaturated zone of chalk, a porous medium with fractures, is important to optimize water resource assessment and management practices in the United Kingdom (UK). However, incorporating the processes governing water movement through a chalk unsaturated zone in a numerical model is complicated mainly due to the fractured nature of chalk that creates high-velocity preferential flow paths in the subsurface. In general, flow through a chalk unsaturated zone is simulated using the dual-porosity concept, which often involves calibration of a relatively large number of model parameters, potentially undermining applications to large regions. In this study, a simplified parameterization, namely the Bulk Conductivity (BC) model, is proposed for simulating hydrology in a chalk unsaturated zone. This new parameterization introduces only two additional parameters (namely the macroporosity factor and the soil wetness threshold parameter for fracture flow activation) and uses the saturated hydraulic conductivity from the chalk matrix. The BC model is implemented in the Joint UK Land Environment Simulator (JULES) and applied to a study area encompassing the Kennet catchment in the southern UK. This parameterization is further calibrated at the point scale using soil moisture profile observations. The performance of the calibrated BC model in JULES is assessed and compared against the performance of both the default JULES parameterization and the uncalibrated version of the BC model implemented in JULES. Finally, the model performance at the catchment scale is evaluated against independent data sets (e.g. runoff and latent heat flux). The results demonstrate that the inclusion of the BC model in JULES improves simulated land surface mass and energy fluxes over the chalk-dominated Kennet catchment. Therefore, the simple approach described in this study may be used to incorporate the flow processes through a chalk unsaturated zone in large-scale land surface modelling applications.
Highlights
Chalk can be described as a fine-grained porous medium traversed by fractures (Price et al, 1993)
Previous studies showed that the unsaturated zone of the chalk aquifers plays an important role in groundwater recharge in the United Kingdom (UK) (e.g. Lee et al, 2006; Ireson et al, 2009)
We propose a new parameterization, namely the Bulk Conductivity (BC) model, as a first step towards a simple chalk representation suitable for land surface modelling
Summary
Chalk can be described as a fine-grained porous medium traversed by fractures (Price et al, 1993). Lee et al, 2006; Ireson et al, 2009). The matrix provides porosity and storage capacity, while the fractures greatly enhance permeability (Van den Daele et al, 2007). Water movement through the chalk matrix is slow due to its relatively high porosity (0.3–0.4) and low permeability (10−9–10−8 m s−1). A fractured chalk system, in contrast, conducts water at a considerably higher velocity because of the relatively high permeability (10−5– 10−3 m s−1) and low porosity (of the order 10−4) of fractures (Price et al, 1993)
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