Process-based modelling to evaluate simulated groundwater levels and frequencies in a Chalk catchment in south-western England

Abstract

Abstract. Chalk aquifers are an important source of drinking water in the UK. Due to their properties, they are particularly vulnerable to groundwater-related hazards like floods and droughts. Understanding and predicting groundwater levels is therefore important for effective and safe water management. Chalk is known for its high porosity and, due to its dissolvability, exposed to karstification and strong subsurface heterogeneity. To cope with the karstic heterogeneity and limited data availability, specialised modelling approaches are required that balance model complexity and data availability. In this study, we present a novel approach to evaluate simulated groundwater level frequencies derived from a semi-distributed karst model that represents subsurface heterogeneity by distribution functions. Simulated groundwater storages are transferred into groundwater levels using evidence from different observations wells. Using a percentile approach we can assess the number of days exceeding or falling below selected groundwater level percentiles. Firstly, we evaluate the performance of the model when simulating groundwater level time series using a spilt sample test and parameter identifiability analysis. Secondly, we apply a split sample test to the simulated groundwater level percentiles to explore the performance in predicting groundwater level exceedances. We show that the model provides robust simulations of discharge and groundwater levels at three observation wells at a test site in a chalk-dominated catchment in south-western England. The second split sample test also indicates that the percentile approach is able to reliably predict groundwater level exceedances across all considered timescales up to their 75th percentile. However, when looking at the 90th percentile, it only provides acceptable predictions for long time periods and it fails when the 95th percentile of groundwater exceedance levels is considered. By modifying the historic forcings of our model according to expected future climate changes, we create simple climate scenarios and we show that the projected climate changes may lead to generally lower groundwater levels and a reduction of exceedances of high groundwater level percentiles.

Validating the KAGIS black‐box GIS‐based model in a Mediterranean karst aquifer: Case of study of Mela aquifer (SE Spain)

“…Piezometric levels obtained by KAGIS reported good agreement between the observed and simulated values. Similar trustworthy results were also obtained in the simulation of the karst groundwater levels in previous studies (e.g., Adinehvand, Raeisi, & Hartmann, 2017;Brenner, Coxon, Howden, Freer, & Hartmann, 2018). In the present study, main differences between the observed and simulated values were identified between September 2009 and September 2011, probably related to the abovementioned problems in the measurement of the experimental values.…”

Stratigraphical influence on chalk cave development in Upper Normandy, France: implications for chalk hydrogeology

“…The study reinforces the concept that the chalk aquifer is heterogeneous and should be treated as a karst aquifer (e.g., El Janyani et al, 2014;Valdes et al, 2014;Chédeville et al, 2015;Delbart et al, 2016;Grube et al, 2017 andBrenner et al, 2018). The evidence from our study indicates that conduit development in the Chalk is often guided by hardgrounds, sheet/semi-tabular flint bands and marl seams (section 5.1).…”

“…In practice, in many areas, the functioning of the Chalk groundwater system is rather more similar to a karst aquifer than a granular or single porosity media (Bloomfield et al, 2013;Barhoum et al, 2014). Flow velocities measured from tracer tests can be >300-400 m h -1 (Maurice et al, 2006;Keim et al, 2012) and spring discharges in excess of 5 m 3 s −1 (Brenner et al, 2018). For these reasons, an increasing number of hydrogeological studies consider the Chalk as a triple porosity aquifer, with a combination of fracture and matrix porosity, and localized karst permeability (Mangin, 1975;Pennequin et al, 2017), or just as a karst aquifer in which karstification is less well developed than in other more massive and indurated limestones (Maurice et al, 2006;El Janyani et al, 2014).…”

“…Notwithstanding the above, the Chalk aquifer is still far too often visualized as a homogeneous porous medium (Brenner et al, 2018), rather than a karstic aquifer. Only a few studies have considered the role of lithological heterogeneity and stratigraphical discontinuities on groundwater flow (Descostes et al, 2012;Gallagher et al, 2012;Barhoum et al, 2014).…”

Impacts of climate change on groundwater flooding and ecohydrology in lowland karst

“…Jackson et al (2015) utilised a distributed ZOOMQ3D groundwater model of the chalk aquifer, with various emission scenario input data, to investigate the predicted changes in groundwater levels. Brenner et al (2018) conducted a further study of this chalk catchment and showed that projected climate changes may lead to generally lower groundwater levels and a reduction in exceedances of high groundwater level percentiles in the future. Chen et al (2018) conducted a study into the effects of climate change on alpine karst using GCM data.…”