Modelling the drought response of adited Chalk groundwater sources

Abstract

Effective water resources management requires a good understanding of how groundwater supply yields vary with climatic conditions. Specifically, it is vitally important to know how groundwater abstraction sources respond during drought. Being based on historical data, the current approach adopted by water companies to assess drought response provides limited information, and does not consider interactions between abstraction sources and with connected rivers. This problem is addressed for complex adited Chalk groundwater sources along a river valley, which are challenging to simulate and have rarely been investigated, by developing a continuous simulation methodology. A modelling methodology is developed to simulate the dynamic response of water levels in adited Chalk abstraction boreholes of the upper Colne valley during the 2010-2012 drought. This makes use of the unstructured-grid version of the MODFLOW groundwater flow model, and the Connected Linear Network package to represent adits. The issue of the dependence of aquifer-adit interaction on mesh spacing is solved by developing a new conductance term that corrects for this. Head losses due to turbulence within adits are found not to be significant. To model the response of the abstraction sources during the drought accurately, a novel flow-dependent riverbed conductance is included. A new conceptual model of the system is therefore developed, as it is shown that river flow variability is a major control on groundwater levels. A series of drought scenarios are simulated, also considering changes in abstraction and discharges to the river. Anthropogenic discharge supports both river flow and abstraction, but its influence is controlled by the variation in river-aquifer connectivity over time. The model of the upper Colne aquifer provides a valuable tool that can be applied to improve integrated surface water-groundwater management. The broader methodology will be of benefit to those developing models to assess sustainable yields of complex adited groundwater sources.