Living with uncertainty: climate change, river flows and water resource management in Scotland.

The recent increased variability of Scotland's hydroclimate poses major problems for water resource managers charged with making informed investment decisions given the likely impact of future climate change. Two strategies are developed in this paper to assist managers faced with this environmental uncertainty. The first involves trend analysis of precipitation and runoff since the 1960s and 1970s viewed against longer-term variability reported from instrumental records. The second strategy is based upon current climate change scenarios coupled with GCMs, and downscaling of precipitation and temperature to provide inputs to rainfall-runoff models. The long-term records of precipitation (back to the 1860s) and runoff (back to the 1930s) reveal the late 1980s and early 1990s as the wettest period on record for the west but not for the east. Over the period 1961-1996 the precipitation gradient has intensified across Scotland: wetter west; relatively dry east. Changes in runoff over the period 1970-1996 are also reported with increases in annual flows at 33 out of 38 stations (significantly at 12 stations) and decreases in low flows at 21 out of 38 stations (significantly at one station). The bulk of these flow increases occurred in the south and west especially in the autumn and spring. In terms of high flows over the period 1970-1996, four out of 44 stations reported a change in magnitude and 15 reported an increase in the frequency of POT events. In terms of future climate change, Hulme and Jenkins (1998) predict annual precipitation increases of 6-16% (Scotland) and 6-14% (Scottish Borders) from the 2020s to the 2080s based on the Hadley Centre model (HadCM2) medium-high scenario. Seasonal changes are concentrated in the autumn (SON) and winter (DJF) with increases as high as 24 and 29% for the autumn by the 2080s. (Arnell NW, et al. Institute of Hydrology Report No. 107, Wallingford, 1996), using an earlier transient Hadley experiment (IS92a), predict a 5-15% increase in annual runoff across Scotland by the 2050s, locally rising to 25%. Simulation flow duration curves for the 2050s generate Q95 values up by 5% or less (Rivers Don, Almond and Nith) and Q5 up by 10-24% (Rivers Don, Almond, Nith and Lyne Water). In terms of water resource planning, these predicted changes should be regarded as first order approximations, as they take no account of natural climatic variability, and could generate different absolute values if other scenarios were used. The predictions are, however, broadly consistent with trends in precipitation and runoff for Scotland since the 1970s. Major issues of concern to water resource managers are identified and commented upon in the light of these predictions.