08 Sep 2022
08 Sep 2022
Status: this preprint is open for discussion.

Evidence-based requirements for perceptualising intercatchment groundwater flow in hydrological models

Louisa D. Oldham1, Jim Freer1,2,3, Gemma Coxon1,2, Nicholas Howden4, John P. Bloomfield5, and Christopher Jackson6 Louisa D. Oldham et al.
  • 1School of Geographical Sciences, University of Bristol, Bristol, BS8 1SS, United Kingdom
  • 2Cabot Institute, University of Bristol, Bristol, BS5 9LT, United Kingdom
  • 3Centre for Hydrology, University of Saskatchewan, Canmore, Alberta, T1W 3G1, Canada
  • 4Department of Civil Engineering, University of Bristol, Bristol, BS8 1TR, United Kingdom
  • 5British Geological Survey, Wallingford, OX10 8BB, United Kingdom
  • 6British Geological Survey, Keyworth, NG12 5GG, United Kingdom

Abstract. Groundwater-dominated catchments are often critical for nationally-important water resources. Many conceptual rainfall-runoff models tend to degrade in their model performance in groundwater-dominated catchments as they are rarely designed to simulate spatial groundwater behaviours or interactions with surface waters. Intercatchment groundwater flow is one such neglected variable. Efforts have been made to incorporate this process into existing models, but there is a need for improving our perceptual models of groundwater-surface water interactions prior to any model modifications.

In this study, national meteorological, hydrological, hydrogeological, geological and artificial influence (characterising abstractions and return flows) datasets are used to develop a perceptual model of intercatchment groundwater flow (IGF) and how it varies spatially and temporally across the River Thames, United Kingdom (UK). We characterise the water balance, presence of gaining/losing river reaches and intra-annual dynamics in 80 subcatchments of the River Thames, taking advantage of its wealth of data, densely gauged river network, and geological variability.

We show the prevalence of non-conservative river reaches across the study area, with heterogeneity both between, and within, geological units giving rise to a complex distribution of recharge and discharge points along the river network. We identify where non-conservative reaches can be attributed to IGF, and where other processes (e.g. surface water abstractions) are the likely cause. Through analysis of recorded water balance data and hydrogeological perceptualisation, we conclude that outcrops of carbonate fractured aquifers (Chalk and Jurassic Limestone) show evidence of IGF both from headwater to downstream reaches, and out-of-catchment via spring lines. We found temporal as well as spatial variability across the study area, with more seasonality and variability in river catchments on Jurassic Limestone outcrops compared to Chalk and Lower Greensand outcrops. Our results demonstrate the need for local investigation and hydrogeological perceptualisation within regional analysis, which we show to be achievable given relatively simple geological interpretation and data requirements. We support the inclusion of IGF fluxes within existing models to enable calibration improvements in groundwater-dominated catchments, but with geologically-specific temporal and spatial characteristics, and (when perceptually appropriate) connectivity between catchments.

Louisa D. Oldham et al.

Status: open (until 03 Nov 2022)

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  • RC1: 'Comment on egusphere-2022-529', Anonymous Referee #1, 02 Oct 2022 reply

Louisa D. Oldham et al.

Louisa D. Oldham et al.


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Short summary
Water can move between river catchments via the subsurface, termed Intercatchment Groundwater Flow (IGF). We show how a perceptual model of IGF can be developed with relatively simple geological interpretation and data requirements. We find that IGF dynamics vary in space and time, correlated to the dominant underlying geology. We recommend that IGF “loss functions” may be used in conceptual rainfall-runoff models but should be supported by perceptualisation of IGF processes and connectivities.