ENVIRONMENTAL RISK OF CLIMATE CHANGE AND GROUNDWATER ABSTRACTION ON ECOLOGICAL CONDITIONS IN A DANISH CATCHMENT

Abstract:

The Danish drinking water supply is sourced almost entirely from groundwater. Balancing water abstraction demands and the ecological conditions in streams is one of the major challenges for water resource managers. With projected climate change, characterised by increased annual temperature, precipitation, and evapotranspiration rates for Denmark, the impact to low flows and groundwater levels are especially of interest, as they relate to aquatic habitat and nitrate leaching, respectively. On the island Sjælland, which includes urban and agricultural regions, a doubling of groundwater abstraction rates has been proposed in selected areas to meet water resource demands. This study evaluates the risk to stream ecological conditions for a lowland Danish catchment under multiple scenarios of climate change and groundwater abstraction. Projections of future climate (i.e. precipitation, temperature, reference evapotranspiration) come from the ENSEMBLES climate modelling project. Climate variables from 11 climate models are first bias corrected with a distribution based scaling (DBS) method (Seaby et al., 2013) and then used to force hydrological simulations of stream discharge, groundwater recharge, and nitrate leaching from the root zone under present (1991-2010) and future (2071-2100) climate conditions. Hydrological modelling utilises a sequential coupling methodology with DAISY, a one dimensional crop model describing soil water dynamics in the root zone, and MIKE SHE, a distributed groundwater-surface water model which the National Water Resources Model (DK-model) is set up in (Henriksen et al., 2003). We find low flow and annual discharge to be most impacted by scenarios of climate change, with high variation across climate models (+/- 40% change). Doubling of current groundwater abstraction rates reduces annual discharge by approximately 20%, with higher reductions to low flows seen around 40%. The combined effects of climate change and increased groundwater abstraction result in the largest impact to low flow and annual discharge (+/-50% change). The relative and combined impacts on groundwater level and nitrate leaching from the root zone are also quantified and compared to assess the water resource sensitivity and risk to stream ecological conditions.