climate change, land use change and runoff prediction in the rhine–meuse basins

RIVER RESEARCH AND APPLICATIONS

River Res. Applic. 20: 229–241 (2004)

Published online in Wiley InterScience(www.interscience.wiley.com). DOI: 10.1002/rra.775

CLIMATE CHANGE, LAND USE CHANGE AND RUNOFF PREDICTION INTHE RHINE–MEUSE BASINS

LAURENT PFISTER,a* JAAP KWADIJK,b ANDRE MUSY,c

AXEL BRONSTERTd and LUCIEN HOFFMANNa

a Centre de Recherche Public—Gabriel Lippmann, Cellule de Recherche en Environnement et Biotechnologies, 162a,

Avenue de la Faıencerie, L-1511 Luxembourg, Grand-Duchy of Luxembourgb WL|Delft Hydraulics, Rotterdamseweg 185, NL-2629 HD Delft, The Netherlands

c Ecole Polytechnique Federale de Lausanne, Laboratoire d’Hydrologie et d’Amenagements Ecublens, Batiment GR,

CH-1015 Lausanne, Switzerlandd Potsdam University, Faculty for Mathematics and Natural Sciences, Department for Geo-Ecology, D-14415 Potsdam, Germany

ABSTRACT

As a consequence of increasing winter rainfall totals and intensities over the second half of the 20th century, signs of increasedflooding probability in many areas of the Rhine and Meuse basins have been documented. These changes affecting rainfallcharacteristics are most evidently due to an increase in westerly atmospheric circulation types. Land use changes, particularlyurbanization, can have significant local effects in small basins (headwaters) with respect to flooding, especially during heavylocal rainstorms, but no evidence exists that land use change has had significant effects on peak flows in the rivers Rhine andMeuse.

For the 21st century, most global circulation models suggest higher winter rainfall totals. Most hydrological simulations ofthe Rhine–Meuse river basins suggest an increased flooding probability, with a progressive shift of the Rhine from a ‘rain-fed/meltwater’ river into a mainly ‘rain-fed’ river. A very limited effect of changes in land use on the discharge regime seems to existfor the main branches of the Meuse and Rhine rivers. For mesoscale basins, future changes in peak flows depend on the changesin the variability of extreme precipitations in combination with land use changes. Copyright # 2004 John Wiley & Sons, Ltd.

key words: Rhine–Meuse basins; climate change; land use change; hydrological modelling

INTRODUCTION

Recent flood events and socio-economic developments have increased awareness of the need for improved flood

risk management along the Rhine and Meuse rivers. While in the past the building and implementation of river

flow controlling structures and operating policies were mainly guided by the quest for further economic develop-

ment (Loucks and Avakyan, 1998), in the last few years a more sustainable use of the river is advocated, in view of

(1) the expected hydrological consequences of climate change and land use developments, (2) the economic and

ecological developments in the river floodplains, and (3) the changing views in society on issues like safety and

ecological functions.

Today, the climate system is clearly recognized as being subject to natural, as well as man-made changes

(Kondratyev and Cracknell, 1998). The observed changes in the climatological variables could in many cases

be identified as the cause of detected trends in hydrological time series (Mansell, 1997; McCabe, 1996; Pfister

et al., 2000; Hisdal et al., 2001; de Wit et al., 2001; Burn and Elnur, 2002). Furthermore, land use has also under-

gone significant changes over the last few centuries in all European countries, especially in the highly industria-

lized Rhine and Meuse basins. Land use changes relevant to hydrology include forest clear-cutting, agricultural

drainage and urbanization (Robinson et al., 2000; Cosandey and Robinson, 2000). The complexity of the climate

system and of its interactions with other sub-systems of the geosphere, in particular with the hydrological cycle,

Received 2 May 2003

Revised 9 May 2003

Copyright # 2004 John Wiley & Sons, Ltd. Accepted 30 May 2003

*Correspondence to: L. Pfister, Centre de Recherche Public—Gabriel Lippmann, Cellule de Recherche en Environnement et Biotechnologies,162a, Avenue de la Faıencerie, L-1511 Luxembourg, Grand-Duchy of Luxembourg. E-mail: [email protected]

makes it very difficult to detect the causes (climate and/or land use change) that are responsible for changes in the

rainfall–runoff relationship. Flood management in a changing environment thus clearly needs a multidisciplinary

approach, since it simultaneously has to address the issues of flood-generating processes, including their scale-

dependency, the detection of changes in the occurrence and magnitude of floods, the origins of these changes,

the trends that can be expected in the future, and the development of the most effective flood prediction and flood

protection techniques.

Unfortunately, no river systems have been monitored with sufficient spatio-temporal resolution over long

periods to deal with the above-mentioned issues through field observations alone. The extrapolation from those

measurements in space and time to ungauged catchments and/or into the future in view of the assessment of

expected climate and land use changes on the rainfall–runoff relationship can thus only be achieved by hydrolo-

gical models (Beven, 2001), which can be used for prediction or forecast purposes and thus enter various decision-

making processes.

In the present paper, changes in the magnitude and occurrence of floods in the Rhine and Meuse rivers during the

20th century will be viewed and discussed in the light of changes in the environment, especially related to climate

and land use before addressing the impact of future predicted environmental changes on flood probability. The

main conclusions are a result of three IRMA-SPONGE projects: (1) DEFLOOD, ‘Development of methodologies

for the analysis of the efficiency of flood reduction measures in the Rhine basin on the basis of reference floods’

(Krahe et al., 2002); (2) ‘Development of flood management strategies for the Rhine and Meuse basins in the con-

text of integrated river management’ (Van Deursen and Middelkoop, 2002); and (3) FRHYMAP ‘Flood risk and

hydrological mapping’ (Hoffmann and Pfister, 2002).

THE HYDROCLIMATOLOGY OF THE RHINE AND MEUSE BASINS

In western central Europe, annual rainfall is mainly dependent on eastward moving Atlantic depressions. The

Rhine and Meuse basins are thus located in a zone of temperate climatic conditions, characterized by frequent

weather variations.

The Rhine basin has great contrasts in physiography and its 185 300 km2 basin extends from the Swiss Alps to

the North Sea, with altitudes ranging from 0 (Rotterdam) to 4275 m a.s.l. (Finsteraarhorn/Swiss Alps). In its

upstream Alpine parts, the Rhine basin receives up to 2000 mm of precipitation per year, with precipitation falling

as snow on average above 3050 m a.s.l. The German and French parts of the Rhine basin further downstream are

characterized by a temperate oceanic climate that is gradually changing into a more continental climate from

northwest to the east and southeast. Annual rainfall ranges from 1100 to 570 mm in the German part of the Rhine

basin. The Netherlands, formed mainly by a large delta composed of silt from the Rhine, Waal, Meuse, Ijssel and

Schelde rivers, have a temperate oceanic climate, influenced by the North Sea and the Atlantic Ocean. Rainfall is

evenly distributed and reaches an annual average of 800 mm. The hydrological regime of the Rhine river is largely

influenced by the spatio-temporal distribution of rainfall, snow storage and snow melt in the Alps and the German

and French Middle Mountain ranges further north (Middelkoop et al., 2000). The regime of the Rhine in

Switzerland is of pluvio-nival type. Maximum runoff of the Alpine rivers is observed during summer, with the

melting of the stored winter snow cover. Water is temporarily stored in the Alpine border lakes that have a smooth-

ing effect on the Rhine discharge. Downstream of Basel, the pluvial regime of the Rhine gradually increases in

domination. High discharges shift to the winter season downstream of the Moselle confluence.

With a mean annual discharge of 2490 m3/s at its outflow into the North Sea (Herschy and Fairbridge, 1998), the

river Rhine floods can constitute a major hazard in different reaches of the stream (Smith and Ward, 1998). Due to

the heterogeneity of the meteorological conditions and the heterogeneous physiographical conditions, different

rainfall patterns can result in different flood hydrographs, even when total basin rainfall is very similar. Flooding

in the Rhine floodplain is thus most of the time of regional character.

Floods in the Upper Rhine are most frequent during spring (Rheinfelden; Figure 1), since they are generated by

snowmelt in the Alpine parts of the basin. In the Middle Rhine, floods are in the majority of cases observed during

the winter semester, like for example in Koblenz (Figure 1). In the Lower Rhine, most floods are entirely restricted

to the winter semester (Emmerich; Figure 1). High-magnitude floods in the main channel were observed in the

last two decades in the High Rhine region near Basel (May 1994) and in the Middle Rhine at the Nahe and Lahn

230 L. PFISTER ET AL.

Copyright # 2004 John Wiley & Sons, Ltd. River Res. Applic. 20: 229–241 (2004)

confluences (March 1988), between Koblenz and Cologne (December 1993 and January 1995), as well as down-

stream of the Sieg confluence (January 1995). The so-called Christmas floods of 1993/94 had been caused by high

antecedent soil humidity due to a first sequence of rainfall events on the one hand, abundant rainfall events that

followed in a second sequence and the melting of previously fallen snow on the other hand. Exceptional rainfall

totals had been recorded in the Moselle basin and it was the combination of flood waves from the rivers Neckar,

Main, Nahe and the Moselle that ultimately brought the Rhine to record flood levels (Smith and Ward, 1998).

The magnitudes of extreme flood events depend, apart from the incoming rainfall amounts, on the contributing

basin area. In order to counteract the difficulties in comparing floods of various amplitudes that occur in basins of

different sizes, extreme flood events are considered hereafter as events that equal or exceed a return period of 50

years. In this context, Krahe et al. (2002) have determined that extreme flood events in river basins with a catch-

ment area of approximately 30 000 km2 are caused by precipitation events of duration longer than 30 days. It

appears that extreme flood events, especially during winter, are often caused by rather moderate events in terms

of rainfall intensity in the various tributaries, but which rather unfavourably coincide.

The two major rainfall-generating processes are either of convective (heavy rainfall at local scale, e.g. thunder-

storms) or cyclonic (large rainfall events, both in time and space) type. Convective rainfall events, although gen-

erally of short duration, can cause extreme flood events and important damage at local scale. Cyclonic rainfall

events are induced by low-pressure cells and are characterized by lower intensities (�20% compared to convective

storms). These large rainfall events can cause soil saturation and ultimately generate large floods in the main

branches of the river system. Helbig (2001) has also outlined the importance of the spatial configuration of pre-

cipitation over the Moselle basin. High rainfall totals cause high discharges in the Upper Moselle, with a progres-

sive reduction of the flood waves through the retention areas of Lorraine. Extreme flood events in the Moselle,

downstream of Trier (Germany), are caused by extreme precipitation fields in the north half of the basin, as well

as by shorter flow distances of the Sauer and Saar and fewer retention areas. Extreme flood events can be expected

by intensive rain fronts, moving over the whole basin from southwest to northeast. In most cases, rainfall events

Figure 1. Months of occurrence of the 10 highest water levels registered on the Rhine at Rheinfelden, Koblenz, and Emmerich (1882–2001;LWRP, 2002) and of the 18 heaviest documented floods registered in the Ardennes part of the Meuse (1906–2000; IRM, 2001)

CLIMATE CHANGE, LAND USE CHANGE AND RUNOFF PREDICTION 231


with daily rainfall totals exceeding 10 mm are strongly linked to westerly atmospheric fluxes during winter months

(relative frequency of 70% for all winter days).

The Meuse river basin extends over parts of France, Luxembourg, Belgium, Germany and the Netherlands and

covers approximately 33 000 km2 with a maximum altitude of 500 m a.s.l. in the Ardennes. The spatial distribution

patterns of rainfall in the Meuse basin clearly reflect the differences in altitude, with annual rainfall totals ranging

from up to 1500 mm in the Ardennes massif to 750 mm in the lowlands, close to the Netherlands. Precipitation is

evenly distributed throughout the year, with low flow occurring during summer, when evaporation rates are high,

while the highest runoff values occur during winter, when evaporation rates are at their lowest level (de Wit et al.,

2001). The average annual discharge of the Meuse reaches 269 m3/s at its outflow into the North Sea (Herschy and

Fairbridge, 1998). The Meuse river reacts rapidly to precipitation, so it is relatively sensitive to both flooding and

drought. The Meuse river flood waves are mainly generated by heavy rainfall events that cause, in parallel, snow

melt in the Vosges mountains, and thus mainly occur during the winter semester. The natural bottleneck in the

schistose Ardennes massif is an additional flood-generating source, in the sense that the restricted dimension of

the floodplain offers little room for natural flood retention areas and moreover hampers the progression of the flood

waves. Floods in the Meuse basin were observed quite regularly in the Ardennes region (Belgium), an area where

rainfall is highest. Most floods are observed during the winter semester (October to March), but convective storm

events are likely to cause floods, especially from July to September (Figure 1).

CURRENT SIGNALS OF AN ALTERED RAINFALL–RUNOFF RELATIONSHIP IN

THE RHINE–MEUSE BASINS

Observed changes in discharge

With several extreme flood events having taken place during the last decade, an increase in flood frequency in

the Rhine–Meuse basins is often suggested. Only the analysis of long hydroclimatological observation series can

help to separate a significant change from the noise of a natural variation. Knowledge about average precipitation

and runoff conditions in a river basin is thus of major importance when it comes to detecting any significant

changes in the rainfall–runoff relationship. Detection of trends in occurrences or magnitudes of extreme flood

events is even more difficult, as they require very long discharge series. It is furthermore important to realize that

there is a fundamental uncertainty in any prediction of changes in flood frequency due to environmental changes as

the statistical extrapolation of the frequency of floods from observed time series for discharges or water levels

requires that the factors causing floods have not changed during the observation period (stationarity in land use,

climate, river morphology, etc.).

As shown in Table I, an increased flooding probability has been suggested for the middle and lower Rhine and

parts of the Meuse basin.

In the Swiss upper Rhine basin, the high relief of the Alps can alter the progression of the large atmospheric

circulation types and thus cause extremely complicated spatial and temporal rainfall patterns. While average win-

ter rainfall has increased by up to 30% in some areas (Widmann and Schar, 1997) and while high rainfall intensities

are more frequent in autumn and winter, particularly on the northern side of the Alps (Blochliger and Neidhofer,

1998), the floods observed during the last century seem to have remained within the limits of natural variability

(Schadler, 2001), an observation also corroborated by sediment analysis in Swiss lakes (Blochliger and Neidhofer,

1998). Extreme flood events have indeed been observed throughout the centuries in the Swiss Alps (Pfister, 1999).

Causes of changed discharge: evidence from historical time series research

Can we attribute changes in occurrence and magnitude of floods to changes in the environment, especially to

land use or climatic changes? To answer this question historical research is needed, which is not restricted to dis-

charge records only, but also includes climate time series, such as precipitation, as well as studies of changes in

land use, land management and hydraulic conditions in the river channels. In larger basins different changes have

occurred in the past. This makes the attribution of any discharge change to a specific environmental change a very

difficult task, particularly as various changes are expected to give comparable responses, while others might lead to

opposite hydrological responses. High spatio-temporal resolution data on land use and climate variables are only



available since 1945 in most regions of the Rhine and Meuse basins, hydrological observation data being even

scarcer. Thus, studies on any long-term changes that might have affected land use, climatological and hydrological

variables are difficult to conduct.

Changed discharges linked to climatic changes. Different types of documentary data (Pfister, 1999), as well as

various classifications of atmospheric circulation patterns (Lamb, 1972; Jenkinson and Collison, 1977; Hess and

Brezowski, 1977) can be used prior to the period of instrumental measurements to link frequency and severity of

natural hazards to climatic variations in the past.

As shown in Table I, significant increases in winter rainfall totals and intensity over the last century have been

reported for most parts of the Rhine and Meuse basins, and are often correlated to an increased flooding probability.

The flood discharges in the downstream parts of the Rhine and Meuse rivers in 1993 and 1995 are not considered

to be statistically highly exceptional according to Konnen (1999), since discharges of similar magnitudes have

already been observed in the 1920s. However, the average discharge rates and the frequency of flood discharges

in the Rhine have increased during the last century (Engel, 1997), which was attributed to the increase of winter

precipitation. Thus, in the middle and lower stretches of the Rhine basin in Germany, strong links between changes

in the atmospheric circulation patterns and flood occurrence have been detected. The frequency of high intensity

rainfall events has very strongly increased during winter, whereas their volume has only been subject to minor

changes (Schonwiese et al., 1997). The resulting floods are in many cases preceded and accompanied by

Grosswetterlagen with a strong westerly component (Uhlenbrook et al., 2001). A significant increase in the

occurrence and persistence of these westerly atmospheric fluxes over the last 25 years has been detected in some

parts of the Black Forest Mountain which is correlated with a significantly increased flooding probability (Caspary

and Bardossy, 1995). Pfister et al. (2000) and Pfister and Hoffmann (2001) have shown for the Alzette river

(located in the Luxembourg part of the Moselle basin) an increase in winter maximum daily streamflow correlated

to an increase in rainfall totals, duration and intensity due to westerly atmospheric fluxes. Rainfall due to westerly

atmospheric fluxes has thus increased from 20% of total winter rainfall in the 1950s to more than 50% during the

1990s. The duration and intensity of extreme rainfall events during winter (Figure 2) has also significantly

increased over the same period. The spatial variability of the trends affecting winter rainfall has led to spatially

varying positive trends in maximum daily water levels (Pfister et al., in press). The observed trends in rainfall

characteristics and maximum daily water levels during winter show closely linked spatial patterns that are strongly

related to topography.

Table I. Observed changes in winter rainfall characteristics, land use and flooding probability evolution in the Rhine–Meusebasins during the 20th century

Rhine Rhine Rhine Rhine, Meuse Meuse(CH) (D) (L) (NL) (B)

Significant Significant Significant Significant Slightincrease increase increase increase increase

Winter rainfall (Widmann and (Caspary and (Pfister et al., 2000) (Konnen, 1999) (de Wit et al.,Schar, 1997; Bardossy, 1995; 2001)Blochliger and UhlenbrookNeidhofer, 1998) et al.,2001)

Significant Significant Significantincrease increase increase

Winter rainfall intensity (Blochliger and (Schonwiese (Pfister et al., 2000)Neidhofer, 1998) et al., 1997)

Land use Increase in forested area in the River Meuse basin, as well as in the Rhine basin upstream ofBasel; general increase in urbanization, of artificially drained agricultural lands since 1945,of river canalization and regulation.

No trend Increase Increase Increase No trendFlooding probability (Schadler, 2001) (Caspary and (Pfister et al., 2000) (Engel, 1997) (de Wit

Bardossy, 1995) et al., 2001)



For the Alzette basin (Grand-Duchy of Luxembourg), Pfister (2000) has shown that under increasing winter

rainfall totals, the occurrence of days with groundwater resurgence has risen over the last two decades. As a con-

sequence, the possibility of significant rainfall amounts falling on largely saturated river systems has equally

increased, thus increasing the possibility of high stormflow coefficients, leading in the end to flooding events.

For the Netherlands, Konnen (1999) has reported a warmer and wetter climate, observed during the second half

of the 20th century. All winters with rainfall exceeding 500 mm were recorded after 1960. The changes in both

temperature and precipitation, strongest during winter, are supposed to have been generated by significant changes

in atmospheric circulation patterns.

For the Meuse basin, de Wit et al. (2001) report a small but perceptible increase in precipitation, especially

during winter months (Table I). This increase is nonetheless considered as still being within the limits of natural

variability of rainfall. While average annual and seasonal discharge of the Meuse has hardly changed over the last

century, the maximum daily winter discharge seems to have increased, but no significant trend in flooding prob-

ability has been detected so far (de Wit et al., 2001).

Changed discharges linked to land use changes. The highly industrialized and populated Rhine and Meuse

basins have undergone many land use changes during the last century. Those land use patterns that are important

for flood occurrence include more features than just the common classifications of forested, agricultural or urban

lands. Aspects such as drainage networks, dams, or highways must also be considered.

With respect to land use changes the following general observations can be made for the Rhine, as well as for the

Meuse basin (Table I).

� Contrary to current opinion, the forested area has increased in most river basins in western Europe (Cosandey

and Robinson, 2000).

� Urbanization and impermeabilization of surfaces has increased in both the Rhine and the Meuse basins

(Liebscher et al., 1995).

� Agricultural lands are increasingly being artificially drained since 1945 (Robinson et al., 2000).

Figure 2. Duration and daily intensity of winter rainfall events with the highest seasonal rainfall totals (Belvaux station, Grand-Duchyof Luxembourg, Pfister, 2000)



� Re-allocation of lands has induced a lot of modifications in land management and consequently on water man-

agement (Loucks and Avakyan, 1998).

� Both the rivers Rhine and Meuse and many of their tributaries have been straightened during the last century. In

many parts, the active floodplains have been reduced through the construction of levees, dykes and other struc-

tures, changing the discharge regime. Ebel and Engel (1994) have evaluated the loss in floodplain areas to as

much as 70% of the initially existing 1400 km2.

Within a whole basin, the land use change pattern may vary geographically. Thus, for example, in the Meuse

basin afforestation has been dominating in the Ardennes region, whereas in the Dutch and Flemish parts the

increase of drained agricultural land has been particularly important (de Wit et al., 2001).

Runoff is generated within a catchment through infiltration excess, subsurface stormflow, as well as saturation-

excess overland flow or riverbed exfiltration. Potential discharge capacity is strongly dependent on the hydraulic

conditions within the river system. As outlined by Bronstert et al. (2002), both runoff generation and discharge

conditions can be altered by human activities. Agricultural practices and urbanization that take place within the

catchment area mainly influence runoff generation, whereas river engineering and management measures influ-

ence discharge conditions. In general, field drainage, wetland loss and urbanization result in increased ‘flashiness’

of runoff, more rapid downstream transmission of flood waves, and less floodplain storage (Newson et al., 2000).

No clear evidence exists from historical time series for the impact of land use changes on flood frequency and

magnitude in the main channels of the Meuse and Rhine rivers. The canalization of the Meuse that started during

the 17th century has, however, led to accelerated discharge during periods of peak flows and a delayed discharge,

due to the operation of weirs and locks during low flow periods (de Wit et al., 2001).

Land use changes may nevertheless have a significant impact on the hydrological behaviour in micro

(<100 km2) or mesoscale (100–1000 km2) river basins. Thus for example, the Dudelingerbach (part of Moselle

basin in Luxembourg), heavily influenced by massive infiltrations of water in collapsed mines, has a storm runoff

coefficient of 0.10, whereas in neighbouring basins with similar physiographic characteristics the storm runoff

coefficient is 0.38 (Pfister et al., 2002). In the same way, mining activities have severely influenced the hydro-

logical regime of many rivers in northeastern France (Kang et al., 1992). In contrast, urbanization leads to

increased storm runoff coefficients. Thus, the urbanized Petrusse basin, which includes the city of Luxembourg,

has a storm runoff coefficient of 0.66, while agricultural neighbouring basins have a storm runoff coefficient of

0.40 (Pfister et al., 2002). For the Geer river (Belgium), the frequency of events with bank overtopping reached

0.25 events per year before 1965 and four events per year after 1980 (Mabille and Petit, 1987). This increase in

flood frequency has mainly been attributed to river straightening, leading to an increase of the flood wave propa-

gation speed on the modified upstream river segments.

Causes of changed discharge: results from hydrological models

As the attribution of signals cannot be derived unambiguously from studying time series only, other ways should

be used to shed light on this issue. Experiments with changing land use cannot provide the answer to what response

can be expected from changes in land use, management or instream type of measures in such large basins as the

Rhine and Meuse. This means that these effects cannot be assessed experimentally on the scale of the basins of the

Rhine and Meuse by changing the land use, widening the flood plains, etc. The main tools to assess the various

contributions of environmental conditions or changes in these conditions to changes in river regime and floods are

hydrological and/or hydraulic models. The application of these models leads to the following results.

� Spatial variations in land use may affect the response of small basins, particularly during convective rainfall

events (thunderstorms). This response, however, is very dependent on antecedent conditions (wet or dry soils)

and may differ from basin to basin (Van Deursen and Middelkoop, 2002).

� Extreme flood events can be reduced by changes in land use, if runoff formation can be influenced on extended

areas. However, land use changes result only in a reduction of the flow when the dominant runoff process on an

area can be changed from a fast to a more delayed reaction (Krahe et al., 2002).

� Land use is important for flood generation during heavy local rainstorms, but has lesser effects during large

scale, low intensity (frontal rainfall) rainfall events (Van Deursen and Middelkoop, 2002).



� Model results suggest that land use change had only little effect on the 10-daily discharges at the Lobith gauging

station (Van Deursen and Middelkoop, 2002).

� Groundwater depth may in some regions have a great impact on the hydrological hazard, as illustrated by the

simulation of a 30-year daily rainfall event falling on the Alzette river floodplain with and without groundwater

resurgence (Hoffmann and Pfister, 2002).

� With increasing duration of a flood, the efficiency of retention decreases dramatically and the coincidence of the

flood peaks from the different tributaries becomes more important than the influence of the retention (Krahe

et al., 2002).

� The drainage and irrigation practices in agricultural lands lead to an increase in winter discharge (accelerated

runoff) and a decrease in summer discharge (decrease of groundwater flow) (de Wit et al., 2001).

Conclusions

Land use changes, particulary urbanization, can have significant local effects in small basins (headwaters)

with respect to flooding, especially during heavy local rainstorms, but no evidence exists that land use change

had significant effects on peak flows in the rivers Rhine and Meuse. At the scale of the entire Rhine and Meuse

basins, rainfall characteristics linked to global circulation patterns have a much stronger influence on extreme

flood events. For many areas an increase in westerly atmospheric fluxes correlated to higher rainfall duration

and intensity has been observed during winter months, and this may be responsible for the observed increased

flood probability.

EXPECTED FUTURE CHANGES IN THE PRECIPITATION–RUNOFF RELATIONSHIP IN

THE RHINE–MEUSE BASINS

Changes in frequency and magnitude of floods in the future are related to environmental changes in the river

basins. These changes include climatic changes associated with the increases of greenhouse gas concentrations

in the atmosphere, land use changes as well as changes in land management practices, instream changes, changes

in floodplain development, changes in water demand, etc. (Smith and Ward, 1998). Many of these changes are

dependent on political, technological and socio-economical developments and population growth, all of which

are by definition unknown. In this respect, the water management practices that will be applied in the future will

have an impact on flood probability. Public safety can be obtained by rising dykes and embankments, by widening

the floodplains of the rivers, or by increasing retention in the catchments in all its different forms. This means that

such changes can only be assessed by scenario studies. Different global circulation models (GCMs) will envisage

different climatic changes, based on the same boundary conditions. These last variations are related to a second

type of uncertainty. To study hydrological effects, the climate models are run to provide future changes, e.g. in

precipitation, typically on a monthly time scale and a spatial scale of several hundred kilometres. For hydrological

studies these changes need to be downscaled to the size of a (small) river basin. Also this downscaling is far from

trivial and different methods can lead to very different precipitation inputs in the hydrological models (Bardossy

and van Mierlo, 2000).

Overall scenarios for environmental changes in the Rhine and Meuse basins

According to the IPCC Third Assessment Report (Cubasch and Meehl, 2001), the following projected changes

in extreme weather and climate events are considered as being very likely:

� higher maximum temperatures and more hot days (>32�C);

� higher minimum temperatures, fewer cold and frost days;

� increase in intense precipitation events and more rainfall from individual rainfall events.

According to the UKHI GCM experiment (Middelkoop et al., 2000), the lower and upper estimates of tempera-

ture and precipitation changes during the 21st century for the Upper Rhine, the Middle Rhine and the Lower Rhine

will be as shown in Table II.



Winter and annual rainfall totals are expected to rise in considerable proportions over the entire Rhine basin.

Moreover, temperatures could rise by as much as 4.5�C in the Alpine region, which would considerably reduce the

snow storage reservoir during winter and thus largely contribute to a shift of flood events in the Upper Rhine from

spring and summer to winter.

No detailed spatial scenario for land use change is available for the Rhine and Meuse basins, but it can be expected

that the existing trend of increasing area used for housing, trade and traffic will continue and lead to an expansion of

impervious areas in the landscape. Greatest increases are expected in the coming decades for the backcountry of

agglomerations and in concentrated as well as rural areas within urbanized regions, because building land is available

at low prices. A moderate growth is expected for major cities in urbanized regions and for the highly concentrated

surrounding areas of agglomerations. The smallest growth is expected for major cities in agglomeration areas, because

the prices for built-up land are very high here (Van Deursen and Middelkoop, 2002). Changes in agricultural policies

may also have significant effects on land use with a possible decrease in cultivated lands/changes in crops and an

increased afforestation, with impacts on water cycle and hydrological behaviour of river basins.

Modelling hydrological responses to predicted environmental changes

Hydrological responses on the basis of predicted climate changes. For the Alpine and pre-Alpine catchments

of Switzerland, important changes in the discharge regime of the Rhine are to be expected (Grabs et al., 1997).

Thus winter and spring peak flows are expected to rise by 9 to 16% in the alpine and pre-alpine catchments

(Table III). This is due to a shift in the rainfall regime from early winter to spring, which increases the probability

Table II. Lower and upper estimates of changes in annual, winter and summer rainfall and temperature for the Upper Rhine, theMiddle Rhine and the Lower Rhine (until 2100), according to the UKHI GCM experiment (Middelkoop et al., 2000)

Upper Rhine Middle Rhine Lower Rhine

Year Winter Summer Year Winter Summer Year Winter Summer

Precipitation, lower estimate (%) þ0.8 þ3.9 �2.3 þ2.4 þ5.7 �0.9 þ5.1 þ7.6 þ2.5Temperature, lower estimate (�C) þ1.0 þ1.1 þ0.9 þ1.0 þ1.1 þ0.8 þ0.9 þ1.0 þ0.8Precipitation, upper estimate (%) þ3.4 þ16.6 �9.8 þ10.3 þ24.3 �3.7 þ21.8 þ32.7 þ10.9Temperature, higher estimate (�C) þ4.2 þ4.5 þ3.9 þ4.1 þ4.5 þ3.6 þ3.8 þ4.3 þ3.2

Table III. Most likely projected changes in winter rainfall, winter snow cover and winter flooding probability in the Rhine–Meuse basins during the 21st century

Rhine Rhine Rhine Meuse Rhine, Meuse(CH) (D) (L) (B) (NL)

Increase Increase Increase Increase Increase

Winter (Kwadijk and (Kwadijk and (Kwadijk and (Kwadijk and (Kwadijk and

rainfall Rotmans, 1995; Rotmans, 1995; Rotmans, 1995; Rotmans, 1995; Rotmans, 1995;

Grabs et al., 1997; Van Deursen, Van Deursen, Van Deursen, Van Deursen,

Van Deursen, 2000) 2000) 2000) 2000) 2000)

Decrease

Snow (Kwadijk and

cover Rotmans, 1995;

Grabs et al., 1997)

Increase Increase Increase Increase Increase

Flooding (Grabs et al., 1997; (Van Deursen, 2000; (Van Deursen, (Van Deursen, (Van Deursen,

probability Van Deursen, 2000; Van Deursen and 2000;Van Deursen 2000; Van Deursen 2000; Middelkoop

Van Deursen and Middelkoop, and Middelkoop, and Middelkoop, et al., 2000;

Middelkoop, 2002) 2002) 2002) 2002; de Wit Van Deursen and

et al., 2001) Middelkoop, 2002)



of a coincidence of extensive rainfall and snow melt runoff. As a consequence of increased winter precipitation,

increased winter temperatures resulting in a decrease of snow storage and snow melt, as well as increased tem-

peratures resulting in increased summer evapotranspiration, the Rhine river would change from a combined ‘rain-

fed/meltwater’ river into a mainly ‘rain-fed’ river (Kwadijk and Rotmans, 1995; Van Deursen, 2000) with an

increase in winter runoff and a decrease in summer runoff.

For the Meuse river, climate change scenarios based on outputs from several GCMs showed an increase in tem-

perature and winter precipitation, as well as a decrease in summer precipitation, despite large differences in the

GCM predictions (Table III). The overall trend that emerged from simulations with different models showed that

climate change may lead to an increase in average discharge in late winter and early spring, whereas during autumn

there will be a decrease in average discharge (de Wit et al., 2001).

According to the results obtained for different scenarios with the GIS-based water balance models Rhineflow

and Meuseflow for the entire Rhine and Meuse basins respectively, three clusters of discharge regime emerge

which are partly sensitive to the future water management policies (Van Deursen and Middelkoop, 2002).

� Regime 1 has characteristics similar to the current situation for rainfall and temperature regime and so the input

into the hydrological system remains unchanged. Slightly higher maximum and Q95 values can be expected due

to a large increase in urban areas, which decreases infiltration and increases direct runoff.

� Regime 2 is characterized by a very significant increase in high discharges. The maximum discharge, Q95 and

mean discharge all show a remarkable increase, mainly caused by the changes in precipitation.

� Regime 3 is characterized by a significant decrease in discharge volumes associated with a significant decrease

in precipitation, linked to an increase in temperature. The total input of water into the catchment is in this situa-

tion significantly less than in the current situation, resulting in decreased mean and minimum flows.

Hydrological responses on the basis of predicted land use changes. In comparison to the impact of climatic

change, models show in general a very limited effect of future changes in land use on the discharge regime in the

main branches of the lower Rhine and Meuse basins.

The impacts of land use changes on peak discharges in small catchments are more pronounced but strongly

depend on the type of precipitation and antecedent conditions. Bronstert et al. (2002) have shown that for the

115 km2 Lein catchment (southwestern Germany), an increase in flood volume and peak runoff due to urbanization

is much more distinct for a convective storm event than for an advective one, even for similar precipitation volume

and peak flow (Figure 3). The markedly slighter effect of the advective event is the result of (1) higher antecedent

soil moisture which levels differences in soil characteristics as well as (2) lower precipitation intensities which

prevent an overflow of the sewer system.

Figure 3. Simulation of two flood events in the Lein catchment (115 km2) as a response to (a) a convective storm event and (b) an advectivestorm event for present conditions and two urbanization scenarios



Conclusions

The predicted temperature increase will probably lead to a change in the hydrological regime of the Rhine from

a pluvio-nival to a pluvio-evaporal type river. An increase in winter precipitation, as well as a shift in the rainfall

regime from early winter to spring, which increases the probability of a coincidence of extensive rainfall and snow

melt runoff, may lead to a rising flood risk towards the end of this century. A very limited effect of changes in land

use on the discharge regime seems to exist for the main channels of the Meuse and Rhine rivers.

For mesoscale basins (100–1000 km2), future changes in peak flow depend on changes in variability in extreme

precipitation events, in combination with land use changes. Indeed, storm runoff generation is stronger for con-

vective storm events with high precipitation intensities than for long advective storm events with low precipitation

intensities. Precipitation volume, as well as antecedent soil moisture conditions and groundwater depths are of

major importance for the degree up to which land use can influence storm runoff generation. The convective storm

events are, however, of very minor relevance for the formation of floods in the large river basins of western Europe,

since they are usually restricted to local scales during summer months. Extreme flood events in the large river

basins in western Europe can however result from a coincidence of flood events in a great number of subcatch-

ments, even if the floods are not necessarily extreme in the upstream areas.

GENERAL CONCLUSIONS

Current knowledge on past and present hydrological extremes in the Rhine and Meuse basins clearly indicates that

both land use and climate change do have an impact on flood generation. Although the analysis of lake sediments

in the Alps, as well as the search for statistical trends in historical streamflow and rainfall observation series have

shown contrasting results, for most of the areas throughout the Rhine and Meuse basins an increase in flooding

probability has been observed during the 20th century. This increase in the main channels is correlated to an

observed increase in westerly atmospheric fluxes leading to an increase in winter rainfall duration and intensity

and not to observed land use changes, which however can have significant effects in headwaters, especially during

heavy local rainstorms. The interpretation of trends, whenever they are detected, is, however, rendered very diffi-

cult by the natural variability of the selected hydrological and climatological variables. The frequency of floods

(indicated by return periods) is generally determined from statistical extrapolation of observed time series for dis-

charges or water levels. Extreme flood events are by definition very rare and thus their statistical properties are

extremely difficult to determine via the existing observation records. This extrapolation is possible if the factors

causing floods have not changed during the observation period (e.g. stationarity in land use, climate, river mor-

phology). It is therefore important to realize that there is a fundamental uncertainty in any prediction of changes in

flood frequency due to land use change and climate change. Some of the observed climatological trends may still

remain within the limits of natural variability, but in many cases their impact on the hydrological behaviour of the

river systems leaves little doubt. Many studies on the hydrological impacts of climate change, however, refer to

average patterns, since climate scenarios are based upon average changes too. The impact of global warming on the

occurrence of extreme meteorological events is still very difficult. Local topographical patterns are likely to

strongly influence the impact of changed occurrences in atmospheric circulation types on the spatial and temporal

characteristics of precipitation fields. As a consequence, the evaluation of the changes in hydrological conditions

due to climatic change is equally difficult.

The identification of the consequences of future changes in land use and climate on the hydrology of the Rhine

and Meuse basins is even more complicated by the impact that climate change could have on land use. Under a

changed climate, the vegetation cover is indeed also likely to be strongly altered, which in return can have a sig-

nificant impact on major hydroclimatological processes, such as surface runoff, infiltration or evapotranspiration.

Given the large complexity of the interactions and the spatial variability existing between flood-generating pro-

cesses, as well as the disturbances due to land use and climate changes, the development of robust water manage-

ment strategies for the Rhine and Meuse basins is rendered extremely complicated. The water management

strategies that are to be formulated have to take into account, among others, the uncertainties related to future land

use and climate changes, as well as the uncertainties that are inherent to the models and concepts used. Given the

greater agreement between outputs of most GCM simulations, there must now, however, be serious concern that a



further increase of the westerly atmospheric circulation types will increase the flooding probability in many parts

of the Rhine and Meuse basins. The effect of land use change will also play some role in the future evolution of the

rainfall–runoff relationship, but given the very large uncertainties that are still affecting the spatial rainfall distri-

bution patterns, as well as rainfall intensity, much research is still required in this regard.

Some currently available distributed and process-based hydrological models seem to be the most adequate for

assessing the impact of land use change and/or climate change on the hydrological behaviour of the Rhine and

Meuse basins. Further research is nonetheless needed in order to improve the parameterization of land-surface

hydrological processes, as well as to reduce the uncertainty related to climate scenarios, namely by enhancing

the combination of climate physics and statistics.

ACKNOWLEDGEMENTS

Part of the results presented in this paper were obtained in the framework of the IRMA-SPONGE programme. The

two anonymous reviewers are gratefully acknowledged for their very helpful comments to improve the manuscript.

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