A critical assessment of simple recharge models: application to the UK Chalk

Abstract

Abstract. Quantification of the timing and magnitude of point-scale groundwater recharge is challenging, but possible at specific sites given sufficient high spatial and temporal resolution field observations, and a suitable physically based model. Such models are generally too computationally intensive and have too many unknown parameters to be practically applicable within distributed, larger-scale hydrological or groundwater models. This motivates the need for simpler recharge models, which are widely used within groundwater models. However, it is important that these models are able to capture adequately the unsaturated zone flow processes. We perform an inter-comparison of recharge simulated by a detailed physically based model and a simple recharge model, with both models applied to a field site in the fractured porous Chalk in the UK. Flow processes are simulated convincingly using a dual permeability, equivalent continuum, vertically heterogeneous, Richards' equation model, applied to a 2-D hillslope transect. A simple conventional recharge model was then calibrated to reproduce the water table response simulated by the physically based model. The performance in reproducing the water table was surprisingly good, given the known discrepancies between the actual processes and the model representation. However, comparisons of recharge fluxes simulated by each model highlighted problems with the process representations in the simple model. Specifically, bypass flow events during the summer were compensating for recharge that should have come from slow, continual drainage of the unsaturated zone. Such a model may still be useful for assessment of groundwater resources on a monthly basis, under non-extreme climatic conditions. However, under extreme wet or dry conditions, or under a changed climate the predictive capacity of such models is likely to be inadequate.

Diagnosing hydrological limitations of a land surface model: application of JULES to a deep-groundwater chalk basin

“…Diverse sources of information were used to guide the model assessment and include remotely sensed data (topography, land use), spatially extrapolated point data (soils, weather conditions), point measurements (soil moisture and matric potential, flow, groundwater level), regionalized hydrological information (BFIHOST), and states/fluxes extracted from an auxiliary physics based hillslope model (Ireson and Butler, 2013). Fewer data might result in a less detailed representation of the water cycle depending on the specifics the hydrological system being investigated.…”

“…The flow at the catchment outlet at Theale is monitored using a crump profile weir, where bypassing of the weir occurs above 29 m 3 s −1 . The unsaturated zone of the chalk has two characteristic behaviours: slow drainage over summer, and bypass flow during rainfall events (Ireson and Butler, 2013). Both behaviours are important under extreme conditions (i.e.…”

“…A physics-based model for 2-D flow in chalk (Ireson and Butler, 2013) represents a hillslope transect through unconfined chalk in the Pang catchment, located in close proximity to the river Kennet. While this model is an approximation itself and can only represent one set of hillslope properties, it is built upon the best current knowledge of the hydrology of chalk hillslopes and is the best available test bed for simpler approximations.…”

Towards a simple representation of chalk hydrology in land surface modelling

“…The flow regime shows a distinct characteristic of slow response to groundwater held within the chalk aquifer (Le Vine et al, 2016). According to Ireson and Butler (2013), the unsaturated zone of chalk shows slow drainage over summer and bypass flow during wet periods in this catchment. Table 1 summarizes the field measurements and remote sensing data used in this study.…”

Technical Note: Three-dimensional transient groundwater flow due to localized recharge with an arbitrary transient rate in unconfined aquifers

“…Groundwater flow behavior induced by recharge is therefore crucial in water resource management. The Boussinesq equation has been extensively used to describe horizontal flow without the vertical component in unconfined aquifers (e.g., Ireson and Butler, 2013;van der Spek et al, 2013;Yeh and Chang, 2013;Chor and Dias, 2015;Hsieh et al, 2015;. The equation can be linearized by assuming uniform saturated aquifer thickness for developing its analytical solution.…”