Towards a simple representation of chalk hydrology in land surface modelling

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.

Using observed river flow data to improve the hydrological functioning of the JULES land surface model (vn4.3) used for regional coupled modelling in Great Britain (UKC2)

“…To our knowledge, this is the first study that analyses river flow model outputs from a LSM over a wide enough area (the 13 selected catchments) driven by the CHESS-met dataset (Robinson et al, 2017a, b). The availability of this dataset opens new possibilities to study land surface hydrology and interactions with the atmosphere using LSMs (that typically require gridded forcing datasets) on the kilometre scale driven by gridded rainfall derived from gauge stations.…”

“…The meteorological driving dataset used to run the model is CHESS-met (Robinson et al, 2017a). This database provides all the required surface variables to run JULES (precipitation, input long-wave and short-wave radiation, air temperature, specific humidity, wind speed and pressure) at 1 km 2 spatial resolution for Great Britain and daily time resolution.…”

Hillslope Hydrology in Global Change Research and Earth System Modeling

“…From the perspective of hillslope and catchment hydrologists and CZ scientists, there are many aspects of ESM hydrology that are considered unrealistic and in need of major revisions or expansions (as reviewed in Clark et al, ). CZ science is revealing increasing complexities and evermore nuanced insights, particularly regarding moisture availability in the weathered bedrock beneath soil (Rempe & Dietrich, ; Salve et al, ), subsurface preferential flow paths with multiple perched lateral flows (the fill and spill concept; Tromp‐van Meerveld & McDonnell, ), and the intermittent hydrologic connectivity that drives plant water use and geochemical releases from catchments (e.g., Brantley, Lebedeva, et al, ; Godsey et al, ; Hopp & McDonnell, ; Lanni et al, ; McDonnell, ; Rahman & Rosolem, ). Indeed, attempts have recently been made to represent the rapid recharge to the deep store via rock fractures in global models (Hartmann et al, , ; Vrettas & Fung, , ).…”

“…Groundwater basins are often incongruent with river basins (e.g., Schaller & Fan, ), and topography alone offers incomplete guidance, particularly in karst terrain (Ford & Williams, ; Worthington et al, ) where the cave and conduit network can reach >500 km (e.g., the Mammoth Cave System in Kentucky; Groves & Meiman, ) through which groundwater moves as fast as rivers. Recent work including karstic structures significantly altered the modeled hydrologic partitioning (Hartmann et al, , ; Longenecker et al, ; Rahman & Rosolem, ). The karst example illustrates that many of the improvements in large‐scale models would be futile without acknowledging such first‐order geologic controls on groundwater flow.…”

A soil moisture monitoring network to characterize karstic recharge and evapotranspiration at five representative sites across the globe

“…The white Chalk formation is the principal aquifer of the region. It is highly permeable and karstified with dry valleys that run perpendicular to the Lambourn River, and 10 has a groundwater table at tens of meters depth (Wheater et al 2007;Rahman and Rosolem 2017).…”