Abstract
Abstract. Lowland karst aquifers can generate unique wetland ecosystems which are caused by groundwater fluctuations that result in extensive groundwater–surface water interactions (i.e. flooding). However, the complex hydrogeological attributes of these systems, linked to extremely fast aquifer recharge processes and flow through well-connected conduit networks, often present difficulty in predicting how they will respond to changing climatological conditions. This study investigates the predicted impacts of climate change on a lowland karst catchment by using a semi-distributed pipe network model of the karst aquifer populated with output from the high spatial resolution (4 km) Consortium for Small-scale Modelling Climate Lokalmodell (COSMO-CLM) regional climate model simulations for Ireland. An ensemble of projections for the future Irish climate were generated by downscaling from five different global climate models (GCMs), each based on four Representative Concentration Pathways (RCPs; RCP2.6, RCP4.5, RCP6.0 and RCP8.5) to account for the uncertainty in the estimation of future global emissions of greenhouse gases. The one-dimensional hydraulic/hydrologic karst model incorporates urban drainage software to simulate open channel and pressurised flow within the conduits, with flooding on the land surface represented by storage nodes with the same stage volume properties of the physical turlough basins. The lowland karst limestone catchment is located on the west coast of Ireland and is characterised by a well-developed conduit-dominated karst aquifer which discharges to the sea via intertidal and submarine springs. Annual above ground flooding associated with this complex karst system has led to the development of unique wetland ecosystems in the form of ephemeral lakes known as turloughs; however, extreme flooding of these features causes widespread damage and disruption in the catchment. This analysis has shown that mean, 95th and 99th percentile flood levels are expected to increase by significant proportions for all future emission scenarios. The frequency of events currently considered to be extreme is predicted to increase, indicating that more significant groundwater flooding events seem likely to become far more common. The depth and duration of flooding is of extreme importance, both from an ecological perspective in terms of wetland species distribution and for extreme flooding in terms of the disruption to homes, transport links and agricultural land inundated by flood waters. The seasonality of annual flooding is also predicted to shift later in the flooding season, which could have consequences in terms of ecology and land use in the catchment. The investigation of increasing mean sea levels, however, showed that anticipated rises would have very little impact on groundwater flooding due to the marginal impact on ebb tide outflow volumes. Overall, this study highlights the relative vulnerability of lowland karst systems to future changing climate conditions, mainly due to the extremely fast recharge which can occur in such systems. The study presents a novel and highly effective methodology for studying the impact of climate change in lowland karst systems by coupling karst hydrogeological models with the output from high-resolution climate simulations.
Highlights
Climate projections indicate that a shift in the magnitude and pattern of precipitation is likely to alter catchment runoff regimes in Ireland (Nolan et al, 2017; Blöschl et al, 2019; Murphy et al, 2019)
Many studies have previously attempted to postulate the likely impacts of climate change on groundwater resources without using a combination of numerical models driven by climate data derived from global climate models (GCMs; Dragoni and Sukhija, 2008; Howard and Griffith, 2009; Taylor et al, 2013; Meixner et al, 2016)
In order to assess the future risks relating to groundwater flooding and ecohydrology in lowland karst, it is imperative to understand the complex hydrological processes governing groundwater flow in karst bedrock and how it will likely be altered in the future (Morrissey et al, 2020)
Summary
Climate projections indicate that a shift in the magnitude and pattern of precipitation is likely to alter catchment runoff regimes in Ireland (Nolan et al, 2017; Blöschl et al, 2019; Murphy et al, 2019). Many studies have previously attempted to postulate the likely impacts of climate change on groundwater resources without using a combination of numerical models driven by climate data derived from global climate models (GCMs; Dragoni and Sukhija, 2008; Howard and Griffith, 2009; Taylor et al, 2013; Meixner et al, 2016) These studies tend to focus on groundwater resources in terms of the provision of a potable water supply or irrigation and so have not been considering groundwater flooding or ecohydrology in detail. Predicting extreme values with limited gauging data follows established, wellvalidated methodologies (Griffis and Stedinger, 2007; Shaw et al, 2011; Ahilan et al, 2012); no such established methods appear to be available currently for groundwater flooding in karst systems
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