Changing Frequency of Extreme Events in Northern
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237 CHANGING FREQUENCY OF EXTREME HYDROLOGICAL EVENTS IN NORTHERN AND WESTERN EUROPE Nigel Arnell, Institute of Hydrology, Wallingford, Oxon, 0X10 8BB, UK ABSTRACT The paper uses the FREND regional flow data base to examine the variation over time in floods and low flows in western and northern Europe. Changes over time show a very strong spatial coherence across the study area, and reflect variations in climatic inputs. Recent years have seen more and bigger floods than earlier periods, related to an increase in wet winters and springs, but there is no evidence to suggest that very extreme floods are becoming more frequent. An increase in dry and warm summers does not appear to have resulted in lower low flows: low flows were lowest over much of Europe during the mid-1970s, and only in parts of northern Britain and Denmark were extreme low flows experienced in the 1980s. INTRODUCTION In recent years there has been an apparent increase in the occurrence of extreme hydrological events in many areas of the world. This may in part reflect increasing media coverage of disasters and environmental issues, but possibly indicates a real increase in risk. In common with many other aspects of the environment, however, the behaviour of rivers varies considerably from year to year, and it is difficult to distinguish significant changes over time from the short-term variability expected from an underlying stationary process. The problems of detecting a possibly weak "signal" from a "noisy" process are compounded by the short periods of record generally available for analysis. One approach is to use data from many measuring sites over a large area, and hence to draw inferences about temporal trends using the spatial characteristics of change. This paper adopts such a regional perspective to address the question of whether flood and low flow events are becoming more frequent in western Europe, using data collected for the FREND (Flow Regimes from Experimental and Network Data) project. Whilst a regional approach can assist with the understanding and identification of change in flow regimes, it is important to recognise that several changes at a range of spatial and temporal scales may be operating together. The factors which may lead to changes over time in flow regimes can conveniently be grouped into five classes! 1) hydrometric changes: an apparent change in frequency in extreme events may be due simply to changes in hydrometric practices or stage-discharge rating curves. Such errors would be very localised; 2) changes in channel characteristics: it is well known that channel improvement can lead to increased flood peaks downstream. Again, such effects would be very localised in the small basins in 238 the FREND data base; 3) changes in the characteristics of the basin: many studies have shown that changes in land cover, for example, in a basin may produce changes in flood and low flow behaviour downstream. Whilst effects may be seen over a large area, such changes are more likely to have local consequences. 4) changes in upstream water use: over a period of time both the use of water and the return of treated effluents may change considerably. This would primarily affect low flows. 5) changes in climatic inputs: year-to-year variability in hydrological characteristics are clearly related to variability in climatic inputs. A trend in climate characteristics is likely to be manifest over a large area and thus similar changes in flow behaviour will be evident at many sites, in contrast to the effects of the other changes. Variations in climate between years may themselves reflect variability about an underlying "constant" process, or may be a response to changing atmospheric conditions such as those associated with the greenhouse effect. The paper considers separately the variations in flood and low flow characteristics over time in western Europe, and in the final section some Imp11cations of the results are outlined. First, however, it is necessary to describe briefly the data base employed. THE FLOW DATA SET The FREND data base is described in detail in Gustard et al (1989), and consists of data from nearly 2000 basins in western Europe (the UK, Ireland, France, F.R. Germany, Belgium, Netherlands, Luxembourg, Denmark, Austria and Switzerland) and the Nordic countries (Norway, Sweden and Finland). Flow data have been collected from basins with areas less than 500 km2, but most of the analyses reported in this paper use a subset of approximately 90 basins with data spanning the period 1955 to 1984. This subset was chosen to maximise the number of "long" records used whilst minimising the spatial bias in station coverage: even so, some areas such as central F.R. Germany and Switzerland are well covered whilst others - in particular France and eastern Austria - are less well represented. Most of the analyses compare the last decade (1975 to 1984) with the rest of the record. À weakness of any study which compares the characteristics of different time periods is that the results depend to a degree on the periods chosen, and there is a temptation to chose periods which emphasise differences: in the current study this temptation was resisted. VARIATION IN FLOOD OCCURRENCE This section explores some aspects of recent flood history in western Europe, beginning with the magnitudes of annual flood maxima and subsequently considering whether the number of events has increased. Variations in flood magnitude The broad characteristics of the variability over time in flood occurrence in western Europe are outlined in the FREND report 239 (Gustard et al 1989). In summary, the annual floods in the 1980s over much of western and northern Europe have been larger than in previous years - particularly the early 1970s - although large floods were also experienced in the late 1960s. This general pattern is consistent across most of the FREND project area, and is linked to inter-annual variations in atmospheric circulation. The only exceptions to this pattern are in eastern Austria, where the maritime influence is minimal, and northern Norway and Finland, where polar influences become important. Years with strong cyclonic patterns tend to give large floods across much of Europe whilst, in contrast, annual maximum floods are smaller in years with strong anticyclonic patterns. More particularly, variations in time in flood magnitude appear to respond to variations in winter and spring precipitation totals. Variations in the number of floods The number of floods per year was calculated for all the gauging stations in FREND archive with daily flow data spanning the period 1955-1984. "Flood peaks" occurred on days when the daily mean flow was higher than the flow on both the preceding and following days, and the "peaks" above various thresholds were abstracted. Three thresholds, giving an average of one, two and three peaks per year, were used. It is recognised that some multi-peaked events may contribute more than one flood, but the definition adopted was easy to apply automatically over the large FREND data base. If the rate of occurrence of a process can be described by a Poisson distribution, variations in the rate of occurrence in different time periods can be evaluated using the test statistic {Cox & Lewis 1966): z - lM - N91 - °-5 ...(1) y (Ne (1-e)) M is the number of events in the "test" time period; N is the total number of events in the complete period; e is the duration of the test period as a proportion of the complete period. The statistic z is normally distributed, and can therefore easily be used to test the significance of the difference in frequency of events between the test and background periods. A Poisson model for the occurrence of flood peaks is not unrealistic (see Cunnane, 1973, for example). Of the 88 records used to compare the period 1975-1984 with the "background" period 1955-1984, 18 (20%) showed a rate of occurrence significantly in excess of the long term average rate (at the 1% significance level), when an average of three peaks per year were abstracted. Basins showing such an increase were mostly located in Denmark and the upper Rhine (particularly in Switzerland), with some basins in Britain and the Main catchment in central F.R. Germany also showing an increased frequency. However, due to a lack of long records, few basins in France and eastern Austria and, to a lesser extent, the Low Countries, the upper Danube, and Scotland were included in the data set, so it is difficult to ascertain trends in these areas. 240 Table 1 summarises the variation in the frequency of flood occurrence over time, assigning years to one of four classes. This pattern reflects very closely the variations over time in annual maximum flood magnitudes, and, as shown in the FREND report (Gustard et al, 1989), is related to variations in seasonal rainfalls: it is also apparent that over much of the study area the increase in flood occurrence is greatest in the 1980s. However, it is not just the recent period which has seen an increase in flood occurrence. Annual maximum floods were also high in the late 1960s, and a comparison of the decade 1961-1970 with the "background" period 1955-1984 shows a significantly increased frequency of events in 5.7% of the basins studied. The basins with the higher frequency of flooding in this decade (most actually occurred in the late 1960s), however, tend to be in eastern Bavaria (the Danube catchment) and central F.R. Germany (the Rhine), with a few in the upper Rhine in Switzerland. Of course, if a basin has many floods in both the late 1960s and the 1980s it is possible that the statistic used will imply that neither period has an increased flood frequency, and indeed it is clear from Table 1 that floods have occurred more frequently in the 1980s - and the 1960s - over most of the study area of western and northern Europe.