Climate Change Impact on Catchment Hydrology & Water Resources for Selected Catchments in Ireland

This paper analyses the likely impacts of changes in climate for nine hydrologically diverse catchments throughout Ireland. When assessing the impacts of climate change on water resources there is a cascade of uncertainty that begins with the establishment of future pathways of development and ends with impact assessment (Wilby, 2005). In order to represent uncertainty in future simulations, statistically downscaled output from three Global Climate Models (GCMs), forced using two emission scenarios is used to force a lumped, conceptual rainfall-runoff model for three future time periods; the 2020s, the 2050s and 2080s. Changes in catchment storage, streamflow and extreme events are assessed through comparison with the GCM modelled control period 1961-1990. Future simulations suggest that reductions in soil moisture storage throughout the summer and autumn months are likely for each catchment. The extent of decreases are largely dependent on the storage potential of individual catchments; the lower the capacity of catchments to store water, the greater the sensitivity to climate change. Reductions in groundwater storage during the recharge period will increase the risk of severe drought, as the failure of winter or spring precipitation may result in prolonged drought periods where the groundwater system is unable to recover. Greatest reductions in streamflow are likely for the autumn months in the majority of catchments, while greatest increases are suggested for the month of February. The magnitude and frequency of flood events are shown to increase, with the greatest increases associated with floods of a higher return period. Uncertainty in future simulations derived from HYSIM parameter uncertainty is found to be more important than uncertainty due to emission scenario. INTRODUCTION There is broad agreement that anthropogenic climate change is likely to have a large impact on water resources with the availability of water to meet future demands and the magnitude and frequency of future extreme events being uncertain. Increases in temperature associated with an enhanced greenhouse effect are likely to result in an increase of atmospheric water content, due to increases in surface evaporation and the water holding capacity of the atmosphere. Such a response is liable to lead to an increase of precipitable water in the atmosphere (Douville et al., 2002). Given the importance of precipitation and evaporation in driving the hydrological cycle, any changes in these primary processes may have considerable knock on effects for the rest of the system; such as changes in the volume and timing of runoff and streamflow, changes in soil water storage, groundwater-surface water interactions as well as in the variability of hydrological processes, with consequences for extremes of flooding and low flows. This research investigates the likely impacts of climate change on Irish hydrology and highlights the key vulnerabilities of Irish water resources. Uncertainty in future simulations is also accounted for. OVERVIEW OF METHODOLOGY In order to obtain the objectives outlined a conceptual rainfall-runoff model is applied to a number of catchments throughout Ireland. Statistically downscaled data from a suite of GCMs, run using a range of emissions scenarios is incorporated to force the rainfall-runoff model for three future time periods, the 2020s, 2050s and 2080s. Changes in catchment hydrology as a result of climate change are assessed for each catchment, with the uncertainty in future impacts derived from different GCMs, emission scenarios and the rainfall-runoff model employed highlighted. The following sections will contend the methodology adopted while section three will highlight the key vulnerabilities to emerge. CATCHMENTS ANALYSED Individual catchments were selected to encompass as wide a range of hydrological conditions as possible so as differences in the hydrological response to climate change can be assessed for each. In total nine National Hydrology Seminar 2006 Conor Murphy & Ro Charlton 39 catchments were included as outlined in Table 1. For ease of presentation the results from two catchments, the Suir and the Boyne are provided here. Detailed results for each of the remaining catchments will be published shortly in Murphy and Charlton (2006). Catchment Area (Km) Gauge data (days) Mean Rainfall