Impact of climatic variability and change on river flow regimes in the UK

The objectives of this report are to examine past variability in river flow regimes, concentrating in particular on recent years, and to consider the possible consequences of future climate change for river flow regimes in the United Kingdom. The geological and climatic characteristics of a catchment determine how variations in rainfall from year to year impact upon river flow variability. In general terms, the drier the catchment (as indexed by the proportion of rainfall which runs off from a basin) and the lower the base flow component, the greater the variation in flow regime between years. There is some evidence that years containing similar hydrological characteristics tend to cluster: wet winters tend to follow wet winters, for example. There is no conclusive evidence, however, that 'recent' years (excluding 1989 and 1990) have seen an unusually large number of extreme events, although the test used is rather conservative and the period defined as 'recent' influences the results. Annual and seasonal runoff totals during the 1980s were generally higher than in previous decades, and there are some indications that year-to-year variability was also higher. Data from 1989 and 1990 were not included in the analysis. Future changes in UK flow regimes depend very significantly on assumed changes in evapotranspiration and, particularly, precipitation, which are currently very difficult to predict. The study therefore examined the sensitivity of river flow regimes to a range of feasible climate change scenarios, biased towards generally wetter conditions but assuming both wetter and drier summers. Simple regression-type relationships were considered, but most of the analyses used monthly water balance models applied at a range of representative sites. Changes in average annual runoff under a given climate change scenario were found to depend strongly on catchment dryness. If potential evapotranspiration is assumed to remain constant, for example, a 10% increase in annual rainfall could produce up to 30% extra runoff in south east England, whilst in more humid western regions it would result in only an additional 12 to 15%. Increases in average annual rainfall of between 8 and 10% would be required to offset the effects of 15% higher potential evapotranspiration. The effect of drier summers on summer river flows depends upon the current summer water balance and catchment geology: the greatest relative reductions are expected in responsive catchments which currently have a close balance between rainfall and potential evapotranspiration. In catchments with a large groundwater storage, delayed drainage of additional winter rainfall may mitigate the effects of drier, warmer summers.