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. The proportion of sites showing a significant increase in flood occurrence in recent years fell as the threshold for the selection of events increased. This implies that changes in the occurrence of the larger flood events have been less marked than changes in lesser floods: this is considered further in the next section.

Changes in the occurrence of extreme floods

The occurrence of extreme floods in recent years was examined by looking at the timing of the largest events in the data series. One approach is to apply the same technique as used in the previous section, but with a very high threshold giving a low rate of occurrence. When just the largest 5 events are considered, for example (giving a rate over 30 years of 0.167 per year), only one site - in northern Norway - shows a significantly higher rate of occurrence in the period 1975 to 1984. However, when attention is directed to the few largest events in a record, an alternative approach based on simple sampling theory can be adopted. If a sample consists of N items, of which k are "successes", the probability of obtaining n successes in M attempts can be determined with the hypergeometric distribution:

p(n successes in M trials) - fkj [N - kj / |N| ...(2)

In the present problem the sample size N is the record length, a "success" is defined to be one of the k top rank events and M is the test period. For example, the probability of experiencing 2 of the top 5 events in the last (or indeed any specified) 7 years of a 30 year record is 0.261 (i.e. N-30, M-7, k=5 and n=2). The observed numbers of "successes" in particular periods can be compared with the expected number. 241

Table 1: The variation in the frequency of flood occurrence over time, in the period 1955 to 1984. See Gustard et al (1989) for details on the location of the gauging stations.

S009 Duinain at Salnaan Bridge British Isles 15013 Almond at Alwondbank 28008 Oove at Rocester Weir 52002 Willow Brook at Fotheringhay 32003 Harpers 9rook at Old Hill Br 32006 Hena/Kltt Ingbury at Dodford 37001 T@r at Crabbi Bridge 3900a Windruth at Newbridge 39011 Wey at Ttiford 1*1001 Hunninghaa Screen a Til ley Br 41002 Ath Bourns at Ha««*r Wood Brf 41013 Kuggietts Strean at Henley Br 142001 Wellington at North ferebaa 53002 Seafngton Brook at Sowington 73010 Leven at Hewby Bridge «401002 Llndenborg at Llndenborg Sro 401004 Arhus at Ski by 401006 Cudena at Astedbro 402002 Spang at Bredstrup 1*02003 Lindved at Hyborg LandeveJ 402004 Odonse at Hr BfOby 402006 6rede at 6redefro 4Q3001 Graese at Horup 403003 Tryggevaelde at LI Liode «03004 Salto at Cronbro 403005 Harrested at Kraesvad 403006 Auto to at Bronolte 423001 Wintertal at Oberharz 423002 Lange Breaks at Oberharz 501005 ftasss«uor Ache at I I sank Oanube/C. Bav. 502025 'sen ait Éntw.Craben at Siegrau. 503001 ti.tr at Hittertwald Karwendeist. 503012 Lout»ten at Mittenwaid 503020 Loisacn at Caraiieh u.d. Partn. 503030 A«*er ic Oberanmergau 503046 Crone Vil* at Vilsbiburg 507005 Grosser Regen at Zwiesel 1002076 Grande Cau at Aigle 1409006 Ouniere at Vauvfirlet 14 10010 Chapuauroux at Au roux 1502013 Urft at Ceouend 1502037 Scnvaln at Rannenatume 1605005 Ulster at Cuenthers Rn.ne/F.R. Germany 1605056 Oioiaot at Heiminghausen 1606002 Werre at Ehrentrup 1612023 Lenne at Oberkirchen 1612029 Lenne at Kickenbach 1613010 Frendorf at Krcuitaf 1613026 Wupper at WuppertaI-Elberfeid 1615038 EUave »t fiuock 1616009 Rotor Ha in at Bay rout h 1616014 Redact* at Strei tnushle 1616022 Itz at Coburg Guetortoahnhof 1616024 Baunsch et Leucherhof 1629009 Rotbach et FriesneJn 1620009 Pleaaur at Chur Rhine/Switzerland 1621001 Sitter at Appenzelt 1621003 Murg at Wangj 1621011 Tot* at Keftenbach 1622001 Ergo I z at Lle*tal 1622002 Bfrte at Moutior 1622004 Arouse at Cheep du Houlin 1622006 CBSO at ERtaenaatt 1622011 Allenbach at Adelboden 1622012 Sinae Oberried at Lenk 1622013 Sinae at Obarwii 1622014 Curbe at Betp 1622016 Sense at Thorisheus 1622018 Broye at Payeroe 1716002 Tannseiv at Tannsvetn 1716007 Crosetbekken at Grosettjem 1720002 Rett* og Dige at Oggo 1727003 Hetlefandtelv at Cya 1750001 Kioto at Ho ten 1755001 Oselv at Roykenes 1768001 Klovtvelttelv at tClovtvei'tvat 1781001 Hageeiv at Hertvikvatn 1833001 Hordelva at Krunsvatn 1838001 Argerdseiv at Oyongen 1850001 Sorelv at Sorra 1851001 Svenningdaiselv at Xapskarmo 1S53O01 Leireiv at Storvatn 1867001 sorfjordetv at Sorfjordvetn 1868001 Lo»«erelv at Storvatn 1908001 Oktfjardelv at Oksfjordvatn 2040001 Rutalven at Ackiingen 2046002 Tannan at Litlolan 2048003 Ljutnan at Ljusnedal 2086701 vetanan at Kalaback 2096001 83 1 jane* at Kltppan 2108001 Osan at Tornestorp 2108004 Siuntpan at Vrangebacken = 2 or 3 events - 4 to 7 events - 8 or more events » Missing year Table 2 shows the observed frequency of extreme events in the decade 1975-1984, with the period 1955-1984 as the baseline. There are rather more instances of 3 or more of the largest 5 events occurring between 1975 and 1984 than would be expected if the events were completely randomly distributed through time, indicating that over part of the study area at least extreme floods have become more frequent.

Table 2: The observed and expected numbers of gauging stations with different numbers of extreme floods in the 10 year period 1975-1984. An extreme flood is defined as one of the largest 5 events in the 30-year period 1955-1984.

Number of extreme events 1 or more 2 or more 3 or more 4 or more Expected proportionl : 0i.89 1 0.551 0.191 0.031 Observed and expected numbers of sites: British Isles obs. 13 8 5 3 (14 sites) exp. 12 8 3 0

Denmark obs. 9 5 2 {13 sites) exp. 12 7 2

Danube (E. Bavaria) o. 7 .5 (8 sites) exp. 7 4 1 France/Belgium obs. 3 2 1 (4 sites) exp. 3 2 1

Rhine (Germany) obs. 12 10 4 (12 sites) exp. 11 7 2 Rhine (Switzerland) o. 14 8 3 (14 sites) exp. 12 8 3

Nordic countries obs. 18 11 4 3 (19 sites) exp. 17 11 4 1

The expected numbers of stations assume that the sites in region are all Independent.

There are some areas within western Europe where a large number of the largest events have occurred in the last few years. In isolated cases this can be attributed to local catchment or channel changes, but in the headwaters of the Danube in western Bavaria it is difficult to conceive of any mechanism other than regional climatic variations underlying the increase in flooding in recent years. There are no long records from this area in the FREND archive, but virtually all of the records in the Danube upstream of Neu-Ulm and its tributaries published in yearbooks (Deutsches Gewasserkundliches Jahrbuch, 1987) show that many of the largest 243

floods in up to 50 years have occurred in the six years between 1980 and 1985: Table 3 shows the stations and gives the probability of experiencing the observed number of extreme events given a stationary time series. Whilst it is conceivable that catchment changes have affected all the rivers in the upper Danube, a climatic change is more plausible. Floods in the upper Danube occur in winter or spring, whereas further east in the Danube catchment floods are more likely to occur in summer: these catchments show no significant increase in the frequency of extreme floods in recent years.

Table 3: Observed number of the largest five events in the upper Danube catchment which have occurred in the period 1980-85. (k=5, M-6)

Gauging station Record length observed1 prob. (to 1985) events of years '80-'85 n events N n p(n)

Danube at Kirchen-Hausen 54 2 0.082 Danube at Moehrhigen 60 2 0.068 Danube at Beuron-u-Wert 59 4 0.0002 Danube at Hundersingen 55 3 0.007 Danube at Berg 55 1 0.365

Breg at Hammereisenbach 54 2 0.082 Urach at Urach 24 3 0.072 Brigach at Villingen 25 3 0.064 Krahenbach at Moehringen 25 3 0.064 Ablach at Menningen 28 2 0.235 Kanzach at Umlingen 51 2 0.091 Grosse Lauter at Lauterach 54 2 0.082 Westernach at Achstetten 52 2 0.088 Blautupf at Blauberen 33 4 0.002

Data fronts Deutsches Gewasserkundliches Jahrbuch, Donaugebeit (1987).

VARIATIONS OVER TIME IN LOW FLOW CHARACTERISTICS

Over most of the FREND project area the lowest flows in the year are in late summer or autumn, although in parts of the Nordic countries and in some Alpine rivers flows are at a minimum just before the onset of winter snow melt. With this exception, low annual minimum flows tend therefore to be associated with drier, warmer summers, and several such summers occurred during the early and mid 1970s when cyclonic activity was low (Jones and Kelly 1982; Jones and Wigley 1988î Brazdil et al 1985). The summer of 1976, for example, was warm and dry over much of northwestern Europe, and 1984 too was characterised, at least over north and western Britain, by a high frequency of dry anticyclonic conditions. Jones and Wigley (1988) have shown that there has been a significant increase in the frequency of extreme dry summers over Britain in the 20 years 244

between 1968 and 1987, compared with the period since 1873, and Table 4 shows that a high proportion of the driest and warmest summers over much of western Europe have occurred in the period 1975 to 1984. The table uses data from the Climatic Research Unit gridded data base of temperature and precipitation (Jones et al 1986; Bradley et al 1987). It can therefore be hypothesised that recent years will have seen an Increased frequency of low flows.

Table 4: The number of the five driest or warmest summers in the period 1955 to 1984 that have occurred in the 10 years from 1975 to 1984.

Dry summers: Number of the 5 most extreme summers in the period 1975 to 1985 France and Southern Britain 5°W to 5°E, 45°N to 55°N Northern Britain and North Sea 5°W to 5°E, 55°N to 55°N F.R.Germany and the Alps 5°E to 15°E, 45°N to 55°N Denmark, Southern Norway and Sweden 5°E to 15°E, 55°N to 65°N warm summers: Southwest France 0°, 45°N English Channel 0°, 50°N N.E.Britain 0°, 55°N Central Germany 10°E, 50°N Denmark 10°E, 55°N Southern Norway 10°E, 60°N

Probability in a 10 year period of experiencing n or more of the 5 largest events in 30 years: p(l or more) - 0.891 p(2 or more) - 0.551 p(3 or more) - 0.191 p(4 or more) 0.031 Temperature and precipitation data taken from the Climatic Research Unit (CRU) gridded data bases.

However, it is more difficult to evaluate changes in the occurrence of low flows over time than changes in floods, because "low flows" are harder to define. Event duration is important as well as event magnitude, and many indices have been proposed. In the current study attention was focussed on two measures: (1) the number of days in a year with daily mean flow less than the value exceeded over the entire period of record by 90% of all daily mean flows, and 245

(2) a simple index of runoff deficit. This index is calculated by summing the deficit incurred when flow falls below the daily mean flow exceeded on 95% of days over the long term, and setting the index to back to zero once the deficit has been replenished by flows greater than the 95% flow. The largest value for each year is then abstracted: the index basically reflects the storage that would be required to maintain flows at the 95% level. The index is crude, but gives an indication of the severity of a low flow period. Further problems in studying the variation in low flows over time are introduced by the great variability in storage between and within catchments. The greater the natural storage available the slower will be the response of the catchment to climatic variations. Marsh and Lees (1985), for example, show that flows on the Mimram in southern England during the severe 1975/1976 drought were maintained by groundwater sources in the underlying chalk filled in the preceding wet winter. Flows in other catchments without the benefit of groundwater storage fell much more quickly during the dry period.

Table 5: The observed and expected numbers of gauging stations with different numbers of extreme runoff deficits in the 10 year period 1975-1984. An extreme runoff deficit is defined as one of the largest 5 events in the 30-year period 1955-1984.

Number of extreme events 1 or more 2 or more 3 or mor< Expected proportion: 0.891 0.551 0.191 0.031

Observed and expected numbers of sites: British Isles obs. 15 8 5 (16 sites) exp. 14 9 3 Denmark obs. 14 12 7 (14 sites) exp. 12 8 3

Danube (E. Bavaria) 1 (8 sites) exp. 7

France/Belgium obs. 5 2 1 (5 sites) exp. 4 3 1

Rhine (Germany)obs. 10 5 1 (13 sites) exp. 12 7 2 Rhine (Switzerland) 9 2 (14 sites) exp. 13 8 Nordic countries 23 17 7 3 (24 sites) exp. 21 13 5 1 The expected number of stations assume that the sites in a region are all independent. 246

The number of days with low flows

The number of days in each year with flows less than the long-term daily mean flow exceeded on 90% of days was determined for all the stations with data spanning the period 1955 to 1984. There were more "low flow days" per year in the period 1975 to 1984 than the previous 20 year period at 40 of the 89 sites considered, and these sites were concentrated in Denmark, Belgium and northern Germany, southern Norway, Sweden and parts of the UK. However, the increases were small, and in the Danube catchment there were fewer low flow days in the decade 1975 to 1984. It was not possible to use the same statistical test for the occurrence of low flow days as used for floods because the rates of occurrence were very clearly not consistent with a Poisson assumption. It can therefore be inferred that, whilst the period 1955 to 1984 has seen some increase in the occurrence of low flows at many sites, the increase is not large even in areas which suffered large runoff deficits in the early and aid 1970s. When just the period 1970 to 1976 - a period with low summer rainfalls associated with strong anticyclonic circulation patterns - is considered the number of sites showing an increase In the number of low flow days per year increases to 57 (64%) with only France (represented by only 3 stations) and northern Norway showing fewer low flow days in this period. However, only 7 of the 89 catchments showed a significant increase in low flow days with the Mann-Whitney U statistic. There is little evidence, therefore, to suggest that low flows have become generally sore frequent in western Europe in recent years.

The occurrence of extreme low flows

The frequency of occurrence of extreme low flows - as reflected in the runoff deficit index - In recent years was assessed using the same methods as in the flood investigations. Table 5 shows the proportion of catchments in each region which have experienced different numbers of the largest five runoff deficits in the 10 years from 1975 to 1984. Denmark, in particular, has experienced many of Its largest runoff deficits in the last decade, as have southern Norway, Sweden and parts of the UK. but it is notable that the Danube catchment In eastern Bavaria has had very few low flows In recent years and few of the lowest flows have been experienced between 1975 and 1984 in parts of the Rhine catchment. Overall, whilst the low flows over large areas of western Europe were extremely low in the mid-1970s, few of the most extreme low flow events have occurred since. It is only in a relatively restricted area in Denmark and western and northern UK where extreme low flows have been experienced in the 1980s as well as the 1970s.

CONCLUSIONS AND IMPLICATIONS

This paper has presented the results of a preliminary investigation into changes in the frequency of flood and low flow events in western and northern Europe, and has shown that there have been important changes in recent years. In general, it has been found that there has been an increase in 247

flood frequency related to changes in climatic inputs. The 1980s, for example, have seen large floods associated with high winter and spring rainfalls, but although the number of "floods" shows a large increase there is no evidence to support (or Indeed refute) the suggestion that very large extreme floods have become more frequent over extensive areas. A slightly increased frequency of low flows over the study area in recent years is primarily due to large events in the mid and late 1970s, and in most of Europe the 1980s have had relatively high low flows. The increase in the number of dry and warm summers in the 1980s does not seem to be strongly reflected in the low flow data, and this is perhaps because summer flows have been maintained to a certain extent by the high winter and spring rainfalls. Indeed, whilst there is little correlation between winter and summer rainfall totals (based on the CRU data archive), there are positive correlations between maximum and minimum low flows at virtually all of the sites considered which have winter or spring maxima and summer minima (correlations are lower in northern and alpine areas where maxima occur after minimum flows). Further work is necessary to determine the effect of catchment characteristics on the carry-over effect of winter rainfalls. It is important to recognise that temporal characteristics in a particular basin will be influenced not just by climatic variability but also by local channel, catchment and human influence changes superimposed onto the underlying climatic signal. The strength of this signal may make it difficult to identify the effects of particular catchment or channel changes in some areas: in others the extent of the human influence may mask the climatic effect. Whilst the study has provided some interesting insights into the spatial consistency of recent temporal variations, the analyses have been constrained by the data available. Records from small basins in western and northern Europe are generally short, and the longest records tend to be concentrated in particular areas such as central Germany and Switzerland. A better spatial coverage may only be achieved if data from larger basins are considered. The use of data extending only to 1984 also means that the results are already "out of date": there have been large floods, for example, in Germany, Austria and Norway in 1987 and 1988. There are two particular implications of the results of these preliminary analyses which merit further consideration. Firstly, the observed changes seem to be consistent with - but do not necessarily confirm - the possible effects of global warming associated with the greenhouse effect. Although regional scale predictions of the effects of climatic change are currently very imprecise, there are indications that global warming will lead to increased winter precipitation in northern Europe (north of approximately 50°N) and a decrease in summer precipitation over all except northern and western parts of Scandinavia and the British Isles (see Wilson & Mitchell 1987, for example). Further studies, using as recent data as possible, are needed both to monitor regional scale trends and to indicate the susceptibility of different catchments to changed climatic inputs. The results of the current study imply, for example, that higher winter and spring rainfalls will often compensate to a certain extent for reduced summer rainfalls (Bultot et al (1988) drew similar conclusions after using a rainfall-runoff model to estimate the effects of climatic change). The second important implication of the study relates to the 248

estimation for practical purposes of extreme flood and low flow characteristics. Conventional approaches assume that the past is a reasonable model for the future, but this is increasingly seen to be unrealistic. Determining the effects of temporal change on estimates of events with high return periods is difficult because such estimates have a high sampling variability even under stationary conditions. Whilst some studies have considered the effects of certain types of temporal pattern on, for example, the estimation of high flood quanti les (Landwehr et al 1979), further work is necessary using more realistic models of temporal change.

ACKNOWLEDGMENTS The research presented in this paper was conducted under the framework of the FREND (Flow Regimes from Experimental and Network Data) project, with funding from the British Natural Environment Research Council, the Department of the Environment and the Overseas Development Administration.

REFERENCES Bradley, R.S., Diaz, H.F., Eischeid, J.K., Jones, P.D., Kelly, P.M. and Goodess, CM. (1987) Precipitation fluctuations over northern hemisphere land areas since the mid-19th century. Science 237, 171-175. Brazdil, R., Samaj, F. and Valovic, S. (1985) Variation of spatial precipitation sums in central Europe in the period 1881-1980. J. Climatology 5, 617-631. Bultot, F., Coppens, A., Dupriez, G.L., Gel lens, D. and Meulenberghs, F. (1988) Repercussions of a C02 doubling on the water cycle and on the water balance - a case study for Belgium. J. Hydrology 99, 319-347. Cox, D.R. and Lewis, P.A.M. (1966) The Statistical Analysis of Series of Events Methuen: London. Cunnane, C. (1973) A particular comparison of annual maxima and partial duration series methods of flood frequency prediction. J. Hydro1. 18, 257-272. Deutsches Gewasserkundliches Jahrbuch, Donaugebeit (1987) Abflussjahr 1985. Bayerischen Landesamt fur Wasserwirtschaft, Munchen. Gustard, A., Roald, L.À., Demuth, S., Lumadjeng, H.S. and Gross, R. (1989) Flow Regimes from Experimental and Network Data (FREND). Volume 1 Hydrological Studies. Institute of Hydrology, Wallingford, Oxon, UK. Jones, P.D. and Kelly, P.M. (1982) Principal components analysis of the Lamb catalogue of daily weather types: Part 1, annual frequencies. J. Climatology 2, 147-157. Jones, P.D. and Higley, T.M.L. (1988) Recent changes in precipitation and precipitation variability _in England and Wales: an update to 1987. CRU, University of East Anglia, Norwich, UK. 249

Jones, P.D., Wigley, T.M.L. and Wright, P.B. (1986) Global temperature variations between 1861 and 1984. Nature 322, 430-434. Landwehr, J.M., Mataias, N.C. and Wall is, J.R. (1979) Probability weighted moments compared with some traditional techniques in estimating Gumbel parameters and quanti les. Water Resources Research 15, 1055-1064. Marsh, T.J. and Lees, M.L. (1985) The 1984 Drought. Hydrological Data UK Institute of Hydrology, Wallingford, Oxon, UK. Wilson, C.A. and Mitchell, J.F.B. (1987) Simulated climate and C02-induced climate change over western Europe. Climatic Change 10, 11-42.