Effects of global warming on floods and droughts and related water quality of rivers

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(1)BTO 2006.029 (s) June, 2006. Effects of Global Warming on Floods and Droughts and related Water Quality of Rivers A review.

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(3) June, 2006. Effects of Global Warming on Floods and Droughts and related Water Quality of Rivers A review. © 2006 Kiwa N.V. All rights reserved. No part of this book may be reproduced, stored in a database or retrieval system, or published, in any form or in any way, electronically, mechanically, by print, photoprint, microfilm or any other means without prior written permission from the publisher.. Kiwa N.V. Water Research Groningenhaven 7 Postbus 1072 3430 BB Nieuwegein The Netherlands Telephone +31 30 60 69 511 Telefax +31 30 60 61 165 Internet www.kiwa.nl.

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(5) Colofon Title Effects of Global Warming on Floods and Droughts and related Water Quality of Rivers A review Projectnumber 111.1583.300 Project manager T.A.B. Ramaker Quality Assurance J.J.G. Zwolsman Author B. de Jong.

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(7) Preface Writing this review was part op my traineeship at Kiwa N.V., supervised by Gertjan Zwolsman (Kiwa N.V.) and Edwin Peters (Wageningen UR). I got time to study the very interesting and highly important relations between global warming and water quality. In the beginning, I did know less about global warming. For me it was a vague term surrounded by much uncertainty and bickering between scientists, ‘believers’ and ‘unbelievers’. This made it hard to form an opinion on the subject. However, now I am a critical believer. Our climate is changing, also on local scale. One of the most impressing examples of that is the melting of the polar and alpine glaciers. That we are mainly responsible for it is harder to establish, but looking at the results of climate change research it is very plausible. The words of Spencer Weart (2003) are apt here: “Of course climate science is full of uncertainties, and nobody claims to know exactly what the climate will do. That very uncertainty is part of what, I am confident, is known beyond doubt: our planet’s climate can change, tremendously and unpredictably. Beyond that we can conclude (with the IPCC) that it is very likely that significant global warming is coming in our lifetimes. This surely brings a likelihood of harm, widespread and grave. The few who contest these facts are either ignorant or so committed to their viewpoint that they will seize on any excuse to deny the danger.” And: “Yet the biggest source of uncertainty now is not in the science. To predict climate change, you would first have to predict changes in CO2, methane, and other greenhouse gases, plus emissions of smoke and other aerosols, not to mention changes in crops and forests. These changes depend less on geochemistry and biology than on human actions. Whether the world will experience a mild or a drastic warming depends above all on future social and economic trends - population growth, the regulation of soot from smoke stacks, and so forth. In the third report of the IPCC, scientists have given their best answer. Now the main question is what people will choose to do.” Time was flying this period. Only the deadline could stop me and pressed me to write functionally and just that which is of importance for Kiwa N.V.. In a short time I gathered many journal articles, books and symposium reports. This was only possible due to the online support of the library of Wageningen UR. Comparing with past times, this is a time winning innovation. So are the online databases on www.waterbase.nl and www.aqualarm.nl. With a couple of mouse clicks years of data of different physical and chemical variables of surface waters can be downloaded for free.. Climate Change © Kiwa N.V.. -1-. February 9, 2006.

(8) My thanks go to Gertjan for the pleasant overall support, for reading the report and giving constructive criticism and for the non-subject related talks about Kiwa, policy, management, jobs and so on. Thanks for accepting my RSI and giving the time to deal with it. Also thanks to Edwin Peters for his help on distance. The comments given on the review were very useful. Furthermore, I will thank Hans Middelkoop, Tom Booggaard, Daniel Mourad, Rens van Beek, Sibren Loos and Marcel van de Perk from the University of Utrecht, Adri Buishand and Albert Klein Tank from the KNMI, Johan Kieft from the Drinking Water Production Company Groningen, and Toine Ramaker from Kiwa N.V. for their discussion time. It helped me to not losing myself in the subject of global warming and focused my eyes on the relations between global warming and water quality. Also thanks to Ad van Bokhoven and Antoanella Domnişoru. Without you, it would be very boring to work the whole day in our lonely situated ‘Kennishok’ in the Kiwa Waterhouse. Last but not least, thanks to Ineke and other friends for their home support. Without that I could not work as fine as I did.. Enjoy the world of global warming research,. Barend de Jong Nieuwegein, The Netherlands May, 2006 S.D.G. Climate Change © Kiwa N.V.. -2-. February 9, 2006.

(9) Summary Water quality is under pressure during hydrological extremes. Problems with water quantity and quality during extremes like the 2003 drought and the Rhine and Meuse floods in 1993 and 1995 will happen on a more regular base under global warming, causing problems for e.g. drinking water production. Besides, the aims of water programmes like the European Water Framework Directive will be under pressure. However, in contradiction to water quantity, water quality is a supposititious child in impact studies of climate change. Therefore, the effect of global warming on extremes and related water quality of rivers deserves attention. This review focuses on the effect of global warming on droughts, rainstorms and floods and related water quality of rivers. Relations of temperature, rainstorms and river discharges with water quality variables like water temperature, chemical concentrations and microbiological activity are discussed. Examples out of literature are given, mainly for Europe and The Netherlands. Due to the background of Kiwa, the focus has been on water quality variables that are interesting for drinking water companies. Water quantity has gotten most attention in impact studies of climate change. One of the impacts is an intensifying hydrological cycle. Temperature rise, changing precipitation regimes, melting of the alpine glaciers, sea level rise and non-climatic variables have been increasing the risk on droughts, rainstorms and floods. ‘Melt-water’ rivers like the Rhine are projected to change into ‘rain-fed’ rivers like the Meuse, with higher winter (increasing flood risks) and lower summer flows (increasing drought risks). Rain-fed rivers like the Meuse will become more extreme than nowadays. The possibility and intensity of such extremes is increasing. During rainstorms, floods and droughts water temperature standards can be crossed. There is a strong relation between air temperature and water temperature. Therefore, warming of water bodies may be expected under global warming. During meteorological and hydrological extremes the air- water temperature gradient will change, resulting in relatively high water temperatures during droughts and relatively low temperatures during floods. Due to evaporative cooling, rivers may have an upper bound of water temperature. Nevertheless, water temperature thresholds (25°C) may be crossed as happened in the Meuse and the Rhine rivers during the 2003 drought. Increasing water temperatures of drinking water accelerate microbiological growth, possibly causing Legionella problems in residential buildings. Drinking water treatment processes could be affected as well by increasing temperatures; natural filtration processes (river bank filtration, artificial recharge) might be less effective due to increasing biomass growth and fouling. During floods, droughts and rainstorms concentrations of several chemicals may rise. Chemicals from point loadings are important during droughts and may pass raw drinking water thresholds, as for chloride in the Rhine and fluoride in the Meuse during the 2003 drought. Also new chemicals may become problematic, as mentioned for a micropollutant in the Meuse in 2003. During rainstorms and floods, non-point sources and first flush effects will increase chemical loadings of pesticides, nutrients and potassium into streams. During floods, suspended sediment concentrations and concentrations of adsorbed chemicals will increase. High concentrations may force drinking water companies to close river water intake stations more often during hydrological extremes under global warming.. Climate Change © Kiwa N.V.. -3-. February 9, 2006.

(10) Salinization and salt water intrusion need special attention. Nowadays, these are already problematic. For example, several drinking water production sites throughout The Netherlands had to close down due to salinization and another 29 sites are confronted with upconing brackish groundwater. Sea level rise and the increase in drought frequency will worsen this situation due to extra upconing of brackish groundwater and due to salt water intrusion during low river discharges. This will also be problematic for agri- and horticulture in the low lying delta of the Netherlands. Finally, biological water quality will be under pressure during hydrological extremes. Problems with cyanobacteria, amoeba and botulism may increase during droughts. On the other hand, for pathogens (viruses, parasites and fungi), higher water temperatures and residence times may decrease pathogen concentrations. However, the pathogen loading during rainstorms may increase problems with biological water quality, increasing health risks due to recreation in contaminated water and due to contamination of raw drinking water. The best advice to water resource managers regarding climate change is to start addressing current stresses on water supplies and build flexibility and robustness into utilities. Flexibility helps to ensure a quick response to changing conditions, while robustness helps people prepare for and survive the worst conditions. With this approach to planning, water system managers will be better able to adapt to the impacts of climate change, whatever they may be, and will also be better equipped for the climate variability we have now. A good idea about water quality under global warming can be obtained by easy correlating water quality variables with temperature, precipitation or discharge and by combining these correlations with projected changes of these variables. If research indicates that water quality thresholds will be crossed or water quantity shortage will occur in the warmed future, adaptive strategies must be developed. Although the drinking water supply in The Netherlands is very well organized, climate change urges us to carry out research on new concepts for drinking water production. New concepts compromise new sources for drinking water production, storage concepts and flexible treatment techniques in the short term and a redesign of large centralized systems in the long term. Also flexible legislation and regulation can help preventing water quantity and quality problems during extremes.. Climate Change © Kiwa N.V.. -4-. February 9, 2006.

(11) Contents. Preface. 1. Summary. 3. Contents. 5. 1. Introduction. 7. 1.1. The Globe is warming. 7. 1.2. Impacts of global warming on floods and droughts. 7. 1.3. Impact of global warming on water quality. 7. 1.4. Objective of this study. 8. 2. Global warming - discovery and research. 11. 2.1. The discovery of antropogenic global warming. 11. 2.2. Impacts on the global climate system. 13. 2.3. Modeling of climate change. 15. 2.4. Projections of climate change. 17. 2.5. Water quality in global warming research. 21. 3. Hydrological extremes under a warming climate. 3.1. Droughts, floods and rainstorms. 25. 3.2. Temperature and precipitation extremes under global warming. 27. 3.3. Droughts and floods under global warming. 29. 3.4. Glacier melting and sea level rise affect flood and drought risks. 35. 3.5. Non-climatic processes affecting droughts, water shortages and floods. 38. 4. Water quality during droughts. 4.1. Important variables for water quality during droughts. 41. 4.2 4.2.1 4.2.2. Water temperature General relations between air temperature and water temperature Maximum water temperature during droughts. 42 42 44. 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5. Chemical water quality Carbon dioxide and oxygen Chemical loading and dillution Major ions and nutrients Heavy metals Organic micropollutants. 46 46 47 49 51 52. 4.4. Biological water quality. 53. Climate Change © Kiwa N.V.. -5-. 25. 41. February 9, 2006.

(12) 4.4.1 4.4.2. Cyanobacteria Pathogens. 53 57. 4.5. Salinization. 57. 5. Water quality during floods and rainstorms. 5.1. Important variables for water quality during floods. 59. 5.2 5.2.1 5.2.2. First flush Phenomenon Oxygen. 59 59 61. 5.3 5.3.1 5.3.2 5.3.3. Floods Suspended solids Heavy Metals Nutrients. 62 62 64 65. 5.4. Biological water quality - Pathogens. 67. 6. Conclusions and recommendations. 71. 6.1. Global warming - discovery and research. 71. 6.2. Hydrological extremes under a warming climate. 71. 6.3. Droughts under global warming. 72. 6.4. Floods under global warming. 72. 6.5. Implications for drinking water production and water management. 73. 6.6. Recommendations. 74. Literature. 77. Climate Change © Kiwa N.V.. -6-. 59. February 9, 2006.

(13) 1 Introduction 1.1. The Globe is warming That our globe is warming is a fact and it is generally accepted that human activities are mostly responsible for the rapid accumulation of carbon dioxide and other greenhouse gases in the atmosphere (IPCC, 2001b; EEA, 2004; Barnett et al., 2005). Some scientist refer to global warming as a huge human experiment on the Earth, for which we have little idea of the ultimate outcome, limited ways of finding out a priori, and perhaps no way of reversing (Sutherst 2004). Global warming makes clear that we live in a global village. National boundaries are becoming less and less important; pollution in one country can now affect the whole world. Further, it is increasingly realized that problems of the environment are linked to other global problems such as population growth, poverty, water use and so on. All these pose global challenges that must be met by global solutions (Houghton, 2004). However, the complexity of the world system makes it hard to see the direct and indirect impacts of global warming. Nowadays, many aspects are wrapped in darkness. To support politicians in making decisions for reducing greenhouse gases or in anticipation on the impacts of global warming, reliable research is needed to give insight in the complex interaction between greenhouse gas emission, global warming and impacts. International agreements, like the Kyoto protocol, are helpful tools to press governments to think about global warming and anticipate on its impacts.. 1.2. Impacts of global warming on floods and droughts A hot item in global warming research is the effect of global warming on the hydrological cycle. The focus is mainly on how water resource related variables such as precipitation, evaporation, sea level, glacier melt, river discharge, groundwater and drinking water will change on the long term (e.g. Frederick and Major, 1997; IPCC, 2001b; EEA, 2004; Varis et al, 2004; Hock, 2005). However, compared to the existing information on past changes in the mean, far less is known about changes in extremes, such as the change in the number of intense rainfall events, floods and droughts (Klein Tank, 2004; Tank et al., 2005). Nevertheless, the projection is that extremes of temperature and precipitation will occur more often, resulting in an increase of flood and drought risks (IPCC, 2001b; EEA, 2004). Thus, impacts of global warming can be met both in long term and short term periods. This is alarming, because nature and society are more vulnerable for changes in severe weather and climate events than for long term changes in the mean state of the climate (IPCC, 2001a). As well known, extreme flood events and prolonged droughts can cause tremendous damage to economy and ecology and in the worst case, bear enormous risks for life. In Europe, droughts and floods are the most common natural disasters and in terms of economic and insured losses, the most costly.. 1.3. Impact of global warming on water quality Water quality can be affected by global warming on long term as well on short term periods. The long term is related to gradual changes in for example climate, land use and urbanization which result in a changing water use and chemical loading of surface waters (Domnişoru, 2006). The short term is related to hydrological extremes like rain storms,. Climate Change © Kiwa N.V.. -7-. February 9, 2006.

(14) floods and periods of heat waves or droughts. These can cause conditions that exceed water quality thresholds, which may increase problems for ecology, industrial cooling water, recreation, drinking water companies and human health. Therefore, the relation between global warming, hydrological extremes and water quality deserves attention. However, beside hydrological extremes, also less attention is given to the effect of global warming on water quality (Murdoch et al., 2000; Krysanova et al., 2005; Mooij et al., 2005). The Intergovernmental Panel on Climate Change (IPCC; the most authoritative organization in the field of climate change) included only a small chapter in their report of 1998 giving an overview of publications related to climate change and water quality (IPCC, 1998). This overview is focused mostly on lake and river temperatures and the corresponding effects. The latest IPCC report about this topic is very concise as well (IPCC, 2001b). However, recent water quality problems during the 2003 drought in Europe and new (inter)national water quality standards like the European Water Framework Directive will make water quality in relation to hydrological extremes and global warming a growing field of attention.. 1.4. Objective of this study The above paragraphs make clear that the impact of global warming on hydrological extremes and water quality deserves more attention. Therefore, the question that will be answered in this review is: What is the effect of global warming on floods and droughts and related water quality of rivers. Sub-questions are: • What effects has global warming on the hydrological cycle? • What effects has global warming on flood and drought risks? • How can global warming be related to water quality during floods and droughts? • Which relations between global warming and water quality pose potential problems for physical, chemical or biological water quality in rivers during extremes? • What are the implications of a changing water quality for drinking water production? Questions not included are: • How is water quality affected on the long term? See for answers Domnişoru (2006); • What is the effect of global warming on aquatic ecology? See e.g. Mulholland et al. (1997), Murdoch et al. (2000) and Mooij et al. (2005); • How can we adapt to global warming induced changes in water quantity or quality? See e.g. Parry (2000), NWP (2003) and Kabat et al. (2005). The review is mainly focused on Europe and The Netherlands. Relations between global warming, hydrological extremes and water quality of rivers will be mentioned and illustrated by examples out of literature. It gives an indication of the importance of some relations for physical, chemical and biological water quality. Due to the background of Kiwa N.V., this review mainly focused on water quality variables important for the drinking water sector and is written within the BTO project Risico-analyse effecten van klimaatverandering. It has been tried to be complete, but that is simply impossible for such a complex subject. If a subject takes interest the cited literature is a good start for further deepening.. Climate Change © Kiwa N.V.. -8-. February 9, 2006.

(15) Before the questions will be answered, Chapter 2 starts with a brief introduction on global warming: what is it, what causes it, what impacts does it have, how can future climate be modeled, are the models reliable, and so on. This chapter can be skipped by them who are familiar with the phenomenon of global warming. Water quality is directly related to air temperature and water quantity. Therefore, Chapter 3 zooms in on extremes of temperature, precipitation and river discharge. Global warming has been changing the risk on rainstorms, floods and droughts. Chapter 4 and Chapter 5 respectively zoom in on water quality during droughts and rainstorms and floods. Conclusions are given in Chapter 6 together with implications and recommendations for drinking water production and water management.. Climate Change © Kiwa N.V.. -9-. February 9, 2006.

(16) Climate Change © Kiwa N.V.. - 10 -. February 9, 2006.

(17) 2 Global warming - discovery and research 2.1. The discovery of antropogenic global warming It is now generally accepted that our climate is changing and that human activities are mostly responsible for the rapid accumulation of carbon dioxide and other greenhouse gases in the atmosphere (IPCC, 2001b; EEA, 2004; Barnett et al., 2005). However the discovery of global warming enclosed a long period. A nice book that reveals the history of global warming is written by Weart (2003). In short, two facts had to be discovered. The first was that carbon dioxide and other gases (e.g. methane, nitrous oxide, halogenated gases and ozone) cause a greenhouse effect. It prevents the earth from cooling by absorbing longwave radiation from the earth surface and by back radiation to the surface. This was done by the French scientist Jean-Baptiste Fourier in 1827 and by the Britisch scientist John Tyndall in 1859. Fourier, best known for his contributions to mathematics, introduced the comparison between the atmosphere and a greenhouse glass. Tyndall took the next step and measured the absorption of infrared radiation by carbon dioxide and water vapor. The second fact that had to be discovered was that beside emission of carbon dioxide by volcanic eruptions also human emission by burning fuels (coal, oil, gas) can affect the global climate. This was discovered by Svante Arrhenius. He developed the first global climate model and calculated, by hand, that a world wide doubling of the carbon dioxide concentration in the atmosphere would rise the Earth’s temperature by about 5 or 6ºC. In 1896, Arrhenius published his calculation, but scientist thought his calculation was altogether wrong; the climate was stable, by definition. However, paleo-proxy data from research on ice-cores, tree-rings, pollen and lake sediments made clear that carbon dioxide concentration and temperature are not as constant as had been thought. For example, in the Cretaceous, carbon dioxide levels were 3-12 times higher than they are currently (Beardall & Raven, 2004). So, climate change is normal for the Earth. However, Siegenthaler et al. (2005) and Spahni et al. (2005) showed that since 650,000 years the concentration of carbon dioxide (Figure 1) and methane has not been higher than the concentration nowadays (365 ppm in 1998; EEA, 2004). Furthermore, the present warming (Figure 2) is unique in the last millennium both for its size and rapidity (Mann et al. 1999; Barnett et al., 2005).. Figure 1. A composite CO2 record over six and a half ice age cycles, back to 650,000 years B.P. The record results from the combination of CO2 data from three Antarctic ice cores (Siegenthaler, 2005). The CO2 concentration was about 365 ppm in 1998.. Climate Change © Kiwa N.V.. - 11 -. February 9, 2006.

(18) Figure 2. Four reconstructions of the mean temperature on the Northern Hemisphere for the past 1000 years. The grey area indicates the uncertainty in the most advanced graph, the black one of Mann et al. (1999), the so called ‘hockeystick’. The horizontal zero line denotes the 1961 to 1990 reference period mean temperature. (IPCC, 2001a) This is remarkable, but not directly proof of anthropogenic influence. Climate change depends also on other variables like solar radiation, distance earth-sun and greenhouse gas emission by volcanism. However, it is statistically possible to split the separate influences of these forces on the climate. In this way it was possible to detect and distinguish the anthropogenic signal in all paleoreconstructions toward the end of the twentieth century and in many reconstructions by the middle of the twentieth century (Hegerl et al., 2003; Barnett et al., 2005). The anthropogenic climate change signal has also been detected in many other variables as for example ocean heat content (Barnett et al., 2001; Levitus, 2001), global sea level pressure data (Munk, 2002) and Arctic sea ice extent (Gregory et al., 2002). The observed trends do not seem explainable via natural forcing. Thus, there is enough evidence that the climate is changing due to anthropogenic activities. Nevertheless, since the publication of Arrhenius, many years of research in different fields of science and different signals from nature itself have been needed to convince scientists and politicians that human induced global warming is already a fact on both global and local scales. Nowadays, there are just a few skeptical scientists. For example, Lomborg (2001) and De Freitas (2002) do not believe that global warming is such a serious problem for politicians and environmental managers as has been stated by the IPCC (2001a 2001b). And McIntyre & McKitrick (2005a, 2005b, 2005c) are convinced that nowadays temperature rise is not a unique event in recent history. They calculated that the temperature was even higher in the 15th century. Although it is an interesting result, it does not deny an anthropogenic global warming. Yet, there is just uncertainty in estimates of the extent of changing (Allen, 2000; Stott & Kettleborough, 2002; Stainforth et al., 2005) and about the contribution of factors other than greenhouse gases e.g. feedbacks within the climate system or non-linear or so-called. Climate Change © Kiwa N.V.. - 12 -. February 9, 2006.

(19) singular events (e.g. slowdown of the thermohaline circulation in the North Atlantic; melting of the polar ice sheets; emission of large amounts of methane from natural gas hydrates in fresh and marine waters) which could be induced by further global warming (EEA, 2004; Houghton, 2005). The probability that a singular event will happen within the next hundred years is relatively low, but if it does occur, the impacts will be extremely high. For example, runs with coupled climate models that had the Atlantic overturning circulation shut off exhibited a cooling over northwest Europe with temperatures 4ºC lower than at present (Vellinga & Wood, 2002). Recent, Bryden et al. (2005) indicated already a slowdown of the Atlantic heat conveyor, which may probably be caused by global warming. However, further research is needed to test whether this is the start of a real trend. The implications of these observations are considerable. Palaeoclimate records show that northern air temperatures can drop by up to 10°C within decades (Dansgaard, 1993), and that these abrupt changes are intimately linked to switches in the ocean circulation (Ganapolski, 2001). If it occurs, it would have devastating effects on nature and socioeconomic conditions in the countries bordering the eastern North Atlantic. Adaptation by both nature and mankind to such a big and quick coming impact will be very difficult.. 2.2. Impacts on the global climate system In the past decades many research has already been done on effects of greenhouse gas increases. A main part of this research on global scale is summarized by the Intergovernmental Panel on Climate Change (IPCC, 2001b) and by the European Environment Agency (EEA, 2004) for Europe. The IPCC is the most authoritative organization in the field of climate change. In 1988, it has been established by the Word Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess scientific, technical and socio-economic information relevant for the understanding of climate change, its potential impacts and options for adaptation and mitigation. The IPCC does not carry out research itself, but bases its assessment mainly on peer reviewed and published scientific/technical literature. Its main publications are the Assessments Reports, which come out about once per five years. The most recent one is the Third Assessment Report (TAR) and came out in 2001. The Fourth Assessment Report (AR4) is in preparation and will be completed in 2007. A glimpse in the TAR, thousands of pages thick, makes clear that the increase of greenhouse gases in the atmosphere affects a wide scale of processes in all components of the earth system (Figure 3). The climate system, as defined by the IPCC (IPCC, 2001a), is an interactive system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, forced or influenced by various external forcing mechanisms, the most important of which is the Sun. Also the direct effect of human activities on the climate system is considered as an external forcing.. Climate Change © Kiwa N.V.. - 13 -. February 9, 2006.

(20) Figure 3 Schematic view of the components of the global climate system (bold), their processes and interactions (thin arrows) and some aspects that may change (bold arrows)(IPCC, 2001a). Many physical, chemical and biological interactions occur among the various components of the climate system on a wide range of space and time scales, making the system extremely complex. Although the components of the climate system are very different in their composition, physical and chemical properties, structure and behavior, they are all linked by fluxes of mass, heat and momentum: all subsystems are open and interrelated. As an example the effect of climate change on the eco-hydrological system is given (Figure 4). The increase in greenhouse gas concentrations results in an increase in net radiation at the earth surface. This leads to an increase in temperature, which results in changes in atmospheric moisture contents and circulation patterns. These in turn produce (regionally variable) changes in rainfall and evaporation regimes and hence soil moisture regimes. Temperature, rainfall, evaporation and soil moisture all affect plant growth and therefore the ecosystem, as do changes in radiation and the atmospheric carbon dioxide concentration. Changes in rainfall and evaporation (compounded by changes in vegetation) result in changes in groundwater recharge and river flow which, together with changes in temperature, impact upon stream water chemistry and biology. Finally, a rise in sea level will affect hydrological characteristics in low lying areas.. Climate Change © Kiwa N.V.. - 14 -. February 9, 2006.

(21) CLIMATE CHANGE Greenhouse gas increases. CLIMATE CHANGE IMPACT. Increase in nett radiation Change in ecosystem. Change in catchment land use. Increase in temperature. Change in sea level. Change in water quality. Change in river flow and groundwater. Change in rainfall and evaporation. Change in soil moisture. Figure 4. Impact of climate change on the eco-hydrological system (Arnell, 1994). Note that feedbacks to the climate and anthropogenic interventions are not included. The marine and terrestrial biospheres have a major impact on the atmosphere’s composition (IPCC, 2001a). The biota influence the uptake and release of greenhouse gases (e.g. Raghoebarsing, 2006). Through the photosynthetic process, both marine and terrestrial plants (especially forests) store significant amounts of carbon from carbon dioxide. Thus, the biosphere plays a central role in the carbon cycle, as well as in the budgets of many other gases, such as methane and nitrous oxide (IPCC, 2001a). Because the storage of carbon and the exchange of trace gases are influenced by climate, feedbacks between climate change and atmospheric concentrations of trace gases can occur (IPCC, 2001a). The influence of climate on the biosphere is preserved as fossils, tree rings, pollen and other records, so that much of what is known of past climates comes from such biotic indicators. It has to be realized that global warming is just one of several changes that can affect our environment. For example for Cluis & Laberge (2001) it was hard to find regionally consistent evidence supporting global climate change for several rivers in the Asia-Pacific region. This was due to completion of a number of large dams and reservoirs. Moreover, for some regions human induced climate change is undetectable relative to natural climate variability (Hulme et al., 1999).. 2.3. Modeling of climate change Since Arrhenius published the results of his hand calculated model in 1896, more advanced computer models have been developed. Although they are estimates of reality, computer models are considered as the most effective tools to deal with the complex global climate system and to estimate further climate change and it impacts (IPCC, 2001b; Houghton, 2004; Varis et al., 2004). Global climate models, often called General Circulation Models. Climate Change © Kiwa N.V.. - 15 -. February 9, 2006.

(22) (GCMs), are based on mathematical descriptions of physical processes and interactions between components of the global climate system like atmosphere, ocean and land surface. Since these equations are non-linear, they need to be solved numerically by means of wellestablished mathematical techniques. Current atmosphere models are solved spatially on a three-dimensional grid of points on the globe with a horizontal resolution typically of 250 km and some 10 to 30 levels in the vertical (IPCC, 2001a). A typical ocean model has a horizontal resolution of 125 to 250 km and a resolution of 200 to 400 m in the vertical. Their time-dependent behavior is computed by taking time steps typically of 30 minutes. A realistic representation of the coupling between the various components of the climate system is essential. In particular, the coupling between the atmosphere and the oceans is of central importance (IPCC, 2001a). The oceans have a huge heat capacity and a decisive influence on the hydrological cycle of the climate system, and store and exchange large quantities of carbon dioxide. To a large degree the coupling between oceans and atmosphere determines the energy budget of the climate system. There have been difficulties modeling this coupling with enough accuracy to prevent the modeled climate unrealistically drifting away from the observed climate. Such climate drift may be avoided by adding an artificial correction to the coupling, the so-called “flux adjustment”. Coupled atmosphere-ocean models are called Atmosphere-Ocean General Circulation Models (AOGCMs). They are combined with mathematical representations of other components of the climate system, sometimes based on empirical relations, such as the land surface and the cryosphere. The most recent models may include representations of aerosol processes and the carbon cycle, and in the near future perhaps also the atmospheric chemistry (IPCC, 2001a). Over the last decade, GCMs have grown in complexity at a fast pace (Selten & Kliphuis, 2005). One reason is the inclusion of an increasing number of physical processes that have been found to be relevant. Another reason is the increased numerical resolution in order to capture an extended range of spatial scales. Both factors increase the computational load of climate model simulations. Climate simulations require the largest, most capable computers available. That is why the development of these very complex coupled models goes hand in hand with the availability of ever larger and faster computers to run the models (IPCC, 2001a). The IPCC (2001b) used seven GCMs in its data sets, which, due to limiting computational capacities, worked with a horizontal resolution of around 300 km. Weather and climate on scales large in comparison to this grid size can be described reasonably well. However, at scales comparable to or smaller than the grid size, results from GCMs possess serious limitations. Therefore, downscaling techniques and Regional Climate Models (RCMs) have been developed to get more reliable estimations at smaller spatial and temporal scales (Prudhomme et al., 2002; Houghton, 2004; Varis et al. 2004; Lenderink et al., 2005). Downscaling methods regionalize the output of GCMs. RCMs represent the atmosphere and the land surface at high resolution (typically 20-50 km) on a limited domain (typically 5000 x 5000 km2). The higher resolution better resolves fine-scale dynamics, land-sea interaction, interaction with the topography and small-scale physical processes like cloudradiation interaction and boundary-layer processes. RCMs are fed at their lateral boundaries by atmospheric fields from GCMs or by data from so called re-analyses (see ECMWF, 2006). In Europe there are around 10 RCMs in use all with their own properties and peculiarities (e.g. RACMO of the Dutch weather service (KNMI), HRM of the German weather service (DWD), CHRM of the Institute for Atmospheric and Climate Science of the ETH Zürich and different HadRM versions from the Hadley Centre for Climate Prediction and Research in the UK).. Climate Change © Kiwa N.V.. - 16 -. February 9, 2006.

(23) Beside GCMs and RCMs, another methodology used in impact studies is looking for analogues in past climate changes to explore the likely consequences of for example an increase in temperature or rain intensity (Glantz, 1991). The most popular analogue to study the impact of global warming is the El Niño phase of the El Niño Southern Oscillation (ENSO) Cycle (Sutherst, 2004). ENSO describes the ocean (El Niño) – atmosphere (SO) interactions throughout the tropical Pacific which cause a large scale climate pattern. Several other large scale climate patterns exist. For example, the North Atlantic Oscillation (NAO) represents the dominant climate pattern in the North Atlantic region (Hurrell et al., 2001; Stenseth et al., 2003). These patterns are driven by planetary waves in the extratropical atmosphere, which displace air north and south around our planet. The transient behavior of these waves generates anomalies in climate on seasonal and longer timescales over large geographical regions. In consequence, some regions may be cooler or drier than average, while thousands of kilometers away warmer and wetter conditions may prevail. The climate patterns are described by indices. These can be used to find relations between climate changes and impacts (e.g. Rodo et al., 2002; Sandvik, et al., 2005; Blenckner, 2005). Difficulties with this approach are related to the not always straightforward relationship between climate indices and local weather patterns. At last, several types of human perturbations can be interpreted as climatic warming ‘experiments’, such as the effects of thermal effluents, the dewatering of streams, changes in the levels and thermal structure of lakes, river regulation by dams, land use changes and so on (Schindler, 1997).. 2.4. Projections of climate change Climate models are used to simulate and quantify the climate response to present and future human activities. Most response studies follow a conventional methodology which consists of two steps (Hulme, 1999; Mimikou et al., 2000). First run the simulation under current climate and then under a scenario of external climate forcing. The difference between the two simulations is the ’impact signal’. The external climate forcing can be simply represented by for example a doubling of greenhouse gases in a certain year, but also by more complex, time-dependent and model based ‘scenarios’. A scenario is a set of descriptions of a possible future state of the world (IPCC, 2000, 2001a) and can be about variables like atmospheric carbon dioxide, greenhouse gas emissions, aerosol emissions, land-use, socio-economic development and so on. Scenarios provide the ’context’, a description of a future world with which the climate interacts. The climate and impact scenarios by their very nature should not be used and regarded as ‘predictions’; the term ‘projection’ is used (IPCC, 2001a). ‘Projection’ becomes ‘forecast’ or ‘prediction’ when the projection is branded ‘most likely’ to happen. The outcome of a climate model can in turn be used as projection for impact studies of climate change on for example river discharge, crop yield, drinking water demand or water quality (e.g. Hitz and Smith, 2004; Elliot et al., 2005; Shabalova & Van Deursen, 2003). Some climate projections are given in Figure 5. It gives directions of changes in the temperature and hydrological indicators and the uncertainty of the projections on global scale. More quantitative projections are given in Table 1 for Europe and The Netherlands together with trends in instrumental observations of climate change indicators. In May 2006, the Royal Netherlands Meteorological Institute (KNMI) presented new climate scenarios for The Netherlands (Van den Hurk et al., 2006). GCM simulations which have become available during the preparation for the upcoming Fourth Assessment report (AR4) of IPCC have been used. Innovative is that these scenarios include, beside changes in temperature and precipitation, changes in atmospheric circulation (wind direction), which on its turn will affect temperature and precipitation.. Climate Change © Kiwa N.V.. - 17 -. February 9, 2006.

(24) This chapter makes clear that our climate is changing. However, a question that naturally arises is whether the evolution of the state of the climate system is predictable? In other words, are the projections worth considering? It is well known that complex non-linear systems, like the climate system, have limited predictability, even though the mathematical equations describing the time evolution of the system are perfectly deterministic. However, there is evidence that internal and external forced climate variations may be predictable to some extent (IPCC, 2001a). Experience has shown that events dominated by the long oceanic time-scales, such as the NAO and the ENSO, may possess a fair degree of predictability for several months or even a year ahead. Other examples are the mean annual cycle and short-term climate variations from individual volcanic eruptions, which models simulate well. Furthermore, regularities in past climates, in particular the cyclic succession of warm and glacial periods forced by geometrical changes in the Sun-Earth orbit, are simulated by simple models with a certain degree of success. At last, the global and continental scale aspects of human-induced climate change, as simulated by the climate models forced by increasing greenhouse gas concentration, are largely reproducible. Although these examples are not an absolute proof, it provides evidence that climate change may be predictable, if their forcing mechanisms are known or can be predicted. Thus, the projections are worth considering. The accelerating rate of global climate change and its impacts on both global and local scales call for policy responses. Even if society reduces its emissions of greenhouse gases over the coming decades, global warming is projected to continue to change over the coming centuries (IPCC, 2001a). Thus, we have to prepare for and adapt to the consequences of some inevitable climate change, in addition to taking action to mitigate it (EEA, 2004). One of the first conventions dates from 1992. In this year the United Nations opened the Framework Convention on Climate Change (UNFCCC) for signature. The convention came into force in 1994 and its ultimate objective was, in short, to achieve stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system (UNFCCC, 1992). Many countries joined this treaty. Recently, a number of nations have approved an addition to the treaty: the Kyoto Protocol with emission targets for 20082012, which has more powerful (and legally binding) measures (UNFCCC, 2003). Beside threats, climate change can also bring opportunities, which can be realized or increased by appropriate adaptation and awareness (Kabat et al., 2005). However, the higher the rate of climate change, the more difficult it will be to realize such benefits or to adapt to its impacts. To deal with the threats and opportunities of climate change in the future, further integration and harmonization of policy measures are needed. To support this process, to develop emission reducing techniques and to give insight in the coming impacts, further research is needed in an interdisciplinary way and on a world wide scale. With the coming of refined analyses the probability on liability claims for costs incurred by climatic shifts may increase. This may profoundly affect the course of international negotiations on ways to mitigate, adapt to and ultimately pay for the consequences of climate change (Allen & Lord, 2004).. Climate Change © Kiwa N.V.. - 18 -. February 9, 2006.

(25) Figure 5. Summary of some projections of future climate change for the end of the 21st century (IPCC, 2001b).. Climate Change © Kiwa N.V.. - 19 -. February 9, 2006.

(26) Table 1. Climate change indicators and projections for Europe and The Netherlands Quantity carbon dioxide concentration 1750 concentration 1998 present rate of concentration change projection for 2100 Temperature trends. historical extremes. Europe1. Netherlands2 about 280 ppm 365 ppm 1.5 ppm/yr concentration increases to 540-970 ppm. 1900-2004: average surface temperature increased with 0.95 °C. 1900-2006: +1.6 C°. in past 100 years: number of cold and frost days decreased in most parts. the 1990s was the warmest decade in the instrumental record since 1901. in past 100 years: number of days with temperatures above 25°C (summer days) and of heatwaves has increased. 2000, 1999, 1990 are the warmest years in the instrumental record since 1901. 1998 was the hottest year in historical records and 2005 will end up just above or below it (Henson, 2005).. projections (without policy measures). 1990-2100: increase of 2.0-6.3°C cold winters disappear almost entirely hot summers become much more frequent. the top 10 of warmest years consists of years between 1989-2005; the top 10 of coldest years consists of years between 1902-1963 both for instrumental records since 1901 (KNMI, 2006a) 1990-2050: mean summer: +0.9 - 2.8 K mean winter: +0.9 - 2.3 K yearly warmest day: +1.0 - 3.8 K yearly coldest day: +1.0 - 2.9 K. Precipitation. trends 1900-2000. historical extremes. projections. northern part: 10-40% wetter. mean precipitation: increase of 20%. southern part: up to 20% drier. winter precipitation increased. changes have been greater in winter in most parts. summer precipitation no change. northern and mid part: frequency of very wet days increased in recent decades. >50% more days with > 15, 20 or 25 mm. southern part: frequency of very wet days decreased in recent decades. more dry years with 2003 the driest summer since 1901 (not the driest year). northern part: 1-2% increase/decade in annual precipitation. 1990-2050:. southern part: up to 1% decrease/decade in annual precipitation. 2100: more intense precipitation events more droughts during summer times. Climate Change © Kiwa N.V.. - 20 -. summer: mean precipitation: -19.0 - +5.5 % wet day frequency: -19.3 - -1.6 % precipitation on wet day: +0.1 - 9.1 % 10 yr return level daily precipitation sum: +5 - 27 % potential evaporation: +3.4 - +15.2 % winter: mean precipitation: +3.6 - 14.2 % wet day frequency: +0.1 - 1.9 % precipitation on wet day: +3.6 - 12.1 % 10 yr return level 10-day precipitation sum: +4 - 12 %. February 9, 2006.

(27) Table 1. Continuation Quantity. Europe1. Netherlands2. Sea level. trend 1900-2000. projections Glaciers trends 1900-2000. projection for 2050. 1975-2005: global mean sea level rose with 75 mm with an increase of 2.5 mm·y-1 (Holgate & Woodworth, 2004; Church & White, 2006) sea level around Europe increased by within several decades, level increased between 0.8 mm·y-1 (Brest and Newln) and 2.5 ± 0.6 mm·y-1 3.0 mm·y-1 (Narvik) 2100: increase until about 88 cm 1990-2100: 35 - 85 cm eight out of nine glaciers are in retreat. -. glaciers in the Alps lost more than 1/3 of area and more than 1/2 of mass. -. about 75% of the glaciers in the Swiss Alps are likely to have disappeared. -. River discharge eastern part: annual discharge increased trend 1900-2000. projections for 2100. number of flood events increased. mean winter discharge: increase normative discharge: increase (Rhine: 15.000 m3 to 16.000 m3 mean summer discharge: decrease. northern and Northeastern part: increase in annual discharge. winter discharge: increase (Rhine 3-10%; Meuse 5-20%). south and southeastern part: decrease in annual discharge. mean monthly summer discharge: up to 50% decrease. southern part: annual discharge decreased. floods and droughts: more frequent floods and droughts: more frequent 1. EEA, 2004 2. Projections are from the newest KNMI climate scenarios (Van den Hurk et al., 2006) other info from Verbeek, 2003; Beersma et al., 2004 and Bresser et al., 2005. 2.5. Water quality in global warming research Water quantity is under research since centuries (Archimedes in 250 BC, Stevin in 1600, Pascal in 1650, Bernoulli in 1738, Navier Stokes in 1882, Reynolds in 1900, Eckman in 1900 and so forth). However, water quality is just in scope since the past century. In 1925 Streeter and Phelps described the oxygen concentration downstream a drain off in Ohia (Streeter, 1925). It looks like that this delay of attention repeats itself in the impact studies of global warming. A recent literature search with Web of Science (Anonymous, 2005) resulted in Table 2. Apparently, the more complex the subject and the less general the search term the lower the search hits are. However, then for water quality, a relatively general term, a higher hit rate than 1.0% should be expected. This may indicate that water quality is a supposititious child in climate change research. Several reasons can be adduced for this phenomenon. The great number of processes and interactions involved makes it a complex research area and the side effects of climate adaptations in water quantity management on water quality make this not easier (NWP, 2003). Furthermore, extremes like floods and droughts which cost lives and damage houses and harvests, get first attention. Finally, a scarcity or inconsistent quality of databases over long periods makes it difficult to find significant trends or correlations in water quality variables (IPCC, 2001b; Patz, 2002). Many organizations have been taken water samples with too low frequencies to include water quality data during extremes.. Climate Change © Kiwa N.V.. - 21 -. February 9, 2006.

(28) Table 2. Literature search with Web of Science in titles, abstracts and keywords on climate change and related variables (Anonymous, 2005) Search term. Hits. climate change OR global warming. 21052. 100,0%. AND temperature(s). 7075. 33.6%. AND carbon dioxide OR carbon OR methane. 6464. 30.7%. AND (rainfall OR precipitation). 3358. 16.0%. AND ecosystem(s). 2768. 13.1%. AND catchment(s) OR basin(s). 2045. 9.7%. AND lake(s) OR reservoir(s). 1711. 8.1%. AND river(s) OR stream(s). 1586. 7.5%. AND flood(s)(ing(s)) OR drought(s). 1409. 6.7%. AND sea level. 1160. 5.5%. AND radiation. 1086. 5.2%. AND hydrolog(y)(ical). 996. 4.7%. AND nutrient(s). 947. 4.5%. AND land use. 936. 4.4%. AND evaporation OR transpiration OR evapotranspiration. 810. 3.8%. AND pollution(s). 776. 3.7%. AND adaptation(s). 699. 3.3%. AND discharge OR river flow OR stream flow. 465. 2.2%. AND nitrate OR phosphorus. 370. 1.8%. AND extremes. 288. 1.4%. AND phytoplankton. 276. 1.3%. AND water resource(s) OR water supply. 256. 1.2%. AND water quality. 205. 1.0%. AND eutrophication. 193. 0.9%. AND drinking water. 40. 0.2%. Water quality can be defined by different variables within three characteristics which are strongly interrelated (Figure 6): physical, chemical and biological. The little attention that water quality gets is due to health and environment risks, but also due to national or international legislation that gives standards for different water quality variables. In this way, water ‘quality’ is a subjective term because it implies quality in relation to some standard (Crane et al., 2005). An example of an international legislation is the Water Framework Directive (WFD), adopted by European Parliament on 22 December 2000 (EU, 2005). The general objective of the WFD is to achieve a good ecological and chemical status for European surface and groundwater by the year 2015. Member states should achieve this objective by defining and implementing integrated programmes of measures, taking into account existing Community requirements. The objective of achieving good water status should be pursued for each river basin, so that measures with respect to surface water and groundwater belonging to the same hydrological system are coordinated. It is obvious that the WFD will have a tremendous impact on today’s water management in the EU. And due to global warming, the achievement of the WFD objectives may be harder to reach.. Climate Change © Kiwa N.V.. - 22 -. February 9, 2006.

(29) Change in water quality. standards. WATER QUALITY. physical. chemical. biological. flow. gases. phytoplankton. temperature. nutrients. viruses. sediment load. heavy metals. bacteria. ……. ……. ……. Figure 6. Three characteristics of water quality with some examples. Water quality covers many aspects and is always related to certain standards. Studying water quality in relation to global warming can be done on different time scales. Long term water quality changes are related to changes in the mean of climate variables (temperature, precipitation excess), hydrological variables (sea level, soil moisture, groundwater levels), (aquatic) ecosystems and to changes in urbanization, industrialization and agriculture. Studying the long term is complex and gives more uncertain results, because of the uncertainties of occurrence and magnitude of different bidirectional relations (Domnişoru, 2006). Research has mainly been done via computer models and scenario studies. This review focuses on the short term changes. These are related to changes in extremes of climate variables (temperature, precipitation) which cause extremes in river discharge (floods and droughts). These pose potential threats to water quality (Miller and Yates, 2005). Insights and ideas about the effects on water quality can be gotten by recombination of past research. There is, for example, much literature about the relation between river discharge and water quality. Combining this with the projections for frequency, duration and intensity of precipitation and drought events, gives some idea of what water quality can be expected due to global warming. Furthermore, beside global warming, economical, social and political forces have not to be undervalued for their effect on both the long and short term effects on water quantity and quality. (Tol & Langen, 2000; Dolman et al. 2003; Domnişoru, 2006).. Climate Change © Kiwa N.V.. - 23 -. February 9, 2006.

(30) Climate Change © Kiwa N.V.. - 24 -. February 9, 2006.

(31) 3 Hydrological extremes under a warming climate 3.1. Droughts, floods and rainstorms Following (Table 1), trends occurred in instrumental measurements of extremes and the projection is that droughts, floods and rainstorms will occur more often and with a greater intensity in the future. These extremes are the most problematic issues when talking about climate change and water quantity, because nature and society are more vulnerable for changes in severe weather and climate events than for changes in the mean state of the climate (IPCC, 2001a). Floods and droughts are phenomena that are not constrained by watershed or international boundaries, and they can grow to afflict large areas and many countries simultaneously. Examples are the severe 2002 floods, which were induced by the same meteorological event and affected a region reaching from Germany and Austria over the Czech Republic to Romania and Russia; or the European drought of 1976 which stretched from Spain over France, Germany, and Britain to Scandinavia (Bradford, 2000). Zooming in on drought, it is a global phenomenon that occurs virtually in all landscapes causing significant damage both in natural environment and in human lives. While much of the extreme weather that we experience is brief and short-lived, drought is a more gradual phenomenon, slowly affecting an area and tightening its grip with time. Although drought is slow, its costs and indirect effects add up to devastation that rivals that of hurricanes or floods. For example, Ross & Lott (2003) pointed out that the ten major droughts that occurred in the United States between 1980 and 2003 accounted for the largest percentage (42%) of weather-related monetary losses. The second largest percentage (28%) was due to hurricanes and tropical storms. And for Europe, the hot 2003 summer (after the cold and wet summer of 2002) caused crop losses of around US$ 12.3 billion, while forest fires in Portugal were responsible for an additional US$ 1.6 billion in damage. The electricity markets reacted erratically to increases in demands, as power plants had to curtail production owing to the lack of cooling water, and electricity spot prices soared beyond €100 per MWh. In the Alps, many glaciers underwent unprecedented melting, and the thawing of permafrost led to a series of severe rock falls (Schär & Jendritzky, 2004). Besides global warming, the vulnerability to drought has increased steadily over the world, because of an ever-increasing population that puts heavy demand on water and natural resources (Ross & Lott, 2003) and because of the increasing ageing which causes more deaths (Schär & Jendritzky, 2004). Research has shown that the lack of a precise and objective definition of drought in specific situations has been an obstacle in understanding drought (Mishra & Desai, 2005). However, there is no universally accepted definition of drought. There are hundreds of definitions, viewed out of different fields (Lehner & Döll, 2001) e.g. climatological drought (deficit in precipitation), hydrological drought (deficit in discharge), agro-meteorological drought (deficit in soil water), groundwater drought (deficit in groundwater storage) and operational drought (conflict of water shortage and water management demands) (Figure 7). Moreover, making it more complex, a climatological drought does not directly result in a hydrological drought. On the other side, a hydrological drought is a result of a meteorological and agricultural drought. A general definition used by the National Drought Mitigation Center (Nebraska, US; NDMC, 2005) can be: “Drought is when a. Climate Change © Kiwa N.V.. - 25 -. February 9, 2006.

(32) shortfall in precipitation creates a shortage of water, whether it is for crops, utilities, shipping, municipal water supplies, recreation, wildlife, or other purposes”. The most important for water quality is the hydrological drought, characterized by low river discharges and low water levels. In this review, drought is taken for hydrological drought.. Figure 7. There are different drought definitions. The main three are about meteorology, agriculture and hydrology (NDMC, 2005). Floods can have important beneficial effect for river ecosystems, groundwater recharge and soil fertility. However, flooding, i.e., the destructive abundance of water (freshwater or sea water), has been a major concern of people populating the vicinity of rivers and water bodies since pre-historic times (Kundzewicz & Schellnhuber, 2004). As well known, extreme flood events can cause tremendous damage to economy and ecology and in the worst case, bear enormous risks for life. In Europe, storms and floods are the most common natural disasters and in terms of economic and insured losses, the most costly (EEA, 2004; Zimmerli, 2003). For example two winter floods along the Rhine in December 1993 and January 1995 have caused 10 fatalities and a total damage of about US$ 0.9 billion in Germany alone. In August 2002, the flooding along the Elbe and Danube rivers caused damage of over US$ 15 billion in Germany and Eastern European countries (Kleinn et al., 2005). Tol & Langen (2000) tone down these costs because they are low relative to Gross Domestic Production. They state that even when climate change doubles or triples, the costs are still low and no reason to worry. Nevertheless, in the opinion of many researchers, looking to the large amount of articles about this subject, the costs in the broadest way of the word are high enough to give it research-time and money. So, it is not amazing that the flood phenomenon gets much attention in the Third Assessment Report of the IPCC (Kundzewicz & Schellnhuber, 2004). In short, recent floods seem to be more abundant and destructive in many regions of the globe. The number of flood disasters in the nine years 1990-1998 was higher than in the three-and-half decades 1950-1985 together. This increasing trend continued to the 21st century. For example for. Climate Change © Kiwa N.V.. - 26 -. February 9, 2006.

(33) Europe, the flood damage recorded in 2002 is higher than in any single year before (Kundzewicz and Schellnhuber, 2004). A special type of flood is the flash flood caused by heavy rainstorms. A rainstorm is an intense storm which drops large amounts of rain within a brief period. Flash floods can occur as a result, with little or no warning, and can reach a full peak in only a few minutes. Stormwater runoff has been identified as one of the leading causes of degradation in the quality of receiving waters, especially during the first flush. Rainstorms can happen the whole year round, but storms are more intensive during summer times. This will cause extra loading due to the wash-off of chemicals which were accumulated during dry periods. An example of a flash flood is that one that occurred in river Aude in France. On the 12th and the 13th November 1999, the Aude region received amounts of rainfall that locally exceeded 550 mm in 24 h and 620 mm in 48 h (Bechtold & Bazile, 2001; Gaume et al., 2004). This extreme rainfall event resulted in one of the century’s most significant floods in the Aude river catchment area and produced remarkable flash floods in some of its tributaries. Peak flood discharges from many upstream watersheds (area smaller than 100 km2) exceeded 10 m3·s-1·km-2. This event caused 35 fatalities and severe damage in properties. In comparison to The Netherlands, such Mediterranean basin as the Aude one is more frequently subjected to severe weather. Thereby, the steep orography around the Mediterranean basin increases the risk on flash floods. Nevertheless, rainstorms have also been causing inundations in The Netherlands (Neuvel, 2004), mainly in low laying polder areas in the West part of The Netherlands. Examples are the inundations in Delfland in The Netherlands in the successive years 1998, 1999 and 2001. The cause was a combination of rainstorms with quick drain offs from a great amount of greenhouses and problems with pumping of rainwater out of the polders. Intensities that were measured were 130mm in 24h in 1998, 80mm in 24h in 1999, 80-90mm in 24h on 5 August 2001 and 106mm in 13h on 19 September 2001 (Anonymous, 2005b). In conclusion, under global warming more frequent extreme events such as droughts, floods and rainstorms could end up being more cause for concern than the long-term change in temperature and precipitation averages (NDMC, 2005).. 3.2. Temperature and precipitation extremes under global warming Changes in mean and extreme values of temperature and precipitation will affect the occurrence and intensity of droughts, floods and rainstorms. However, compared to the existing information on past changes in the mean, far less is known about seasonal changes and changes in extremes of temperature and precipitation (Reynard & Prudhomme, 2001; Lehner et al., 2005; Tank et al., 2005). The coarse spatial and temporal resolution of the climate change scenarios available from the output of General Circulation Models (GCMs) is debit to this. Extremes of temperature and especially of precipitation are of local character and ask for high resolution RCMs (Christensen & Christensen, 2003, 2004; Déqué et al., 2005). Another reason is a lack of usable data. Problems with the data are about nonhomogeneity, missing values and gaps. Furthermore, the data may be unavailable to the scientific community or scattered over many data holders (Tank et al., 2005). Finally, the probability for trend detection of rare weather events decreases strongly with the record length (Frei & Schär, 2001). To give an idea, it is suggested that at least 50 years of records are necessary for climate change detection (Kundzewicz & Schellnhuber, 2004). However, even if the data is perfect, extreme events with a repeat frequency of for example 50 years occur theoretically once in this record length. This makes that the needed 50 years of records are not sufficient for trend detection.. Climate Change © Kiwa N.V.. - 27 -. February 9, 2006.

(34) Nowadays, extremes are a key aspect of climate change and have received increased attention in the last few years (IPCC, 2001a). Europe is traditionally one of the regions of the world lacking a dataset of high-resolution observational series with sufficient density and quality that is readily available and accessible. Recently, different projects were established to get more insight in the development of extremes. One example is the European Climate Assessment & Dataset project (ECA&D) directed by the Royal Dutch Meteorological Institute (KNMI, 2002). The key questions addressed are: how did the extremes of daily surface air temperature and precipitation change in Europe’s climate of the 20th century, and what can we learn from this? Another example is the Dutch Challenge project directed by the Centre for Climate Research (CKO) (Selten & Kliphuis, 2005; KNMI, 2006b). In this project the following questions are dealt with: are the warm extremes of 1947 and 2003 just random fluctuations of nature or are they proof of a changing climate, what is the probability of occurrence of a period of extreme temperatures, and how does this probability change in a warming climate?. Temperature [°C]. Some first results of the Challenge project are in Figure 8. Not going into detail, the simulated temperatures covered the observations very well and warming due to the increase of greenhouse gases may be expected. Interestingly, the probability distribution of extreme hot summers increased stronger than might be expected on the basis of the mean warming (Selten & Kliphuis, 2005). It suggests that beside the mean also the variance is changing (Figure 9). Additional analyses have shown that the depletion of soil moisture is one contributing factor. This limits the cooling effect of evaporation and thus increases the temperature during hot summers. Another contribution is a change in the mean summer circulation with a stronger southeasterly flow over Europe bringing warm, dry air into the region (Selten & Kliphuis, 2005).. Figure 8. Global annual mean temperature of all 62 simulations (red crosses), the ensemble mean (black line) and observed temperatures (blue dots) over the calendar years 1940-2080 (KNMI, 2006b).. Climate Change © Kiwa N.V.. - 28 -. February 9, 2006.

(35) Figure 9. Schematic picture showing the effect on extreme temperatures when (a) the mean temperature increases, (b) the variance increases, and (c) when both the mean and variance increase for a normal distribution of temperature (IPCC, 2001a). The result of the changing probability distribution is in agreement with the IPCC (2001a) and Schär & Jendritzky (2004). The same is in account for precipitation (IPCC, 2001a; Christensen & Christensen, 2004, Déqué et al., 2005). In general, the increase in precipitation extremes is significantly larger than the increase in the mean. Moreover, increases in heavy precipitation were also documented even when mean total precipitation decreases. There is a solid physical mechanism which can support this finding (Frei et al., 1998; Christensen & Christensen, 2003, 2004). In a warmer climate, more moisture will evaporate particularly over sea than at present due to higher sea surface temperature, and hence result in a higher saturation mixing ratio (described by the Clausius–Clapeyron relation). This will facilitate latent heat release during the buildup of weather systems, thereby possibly both intensifying the systems and making more water available for precipitation. The phenomenon of the changing probability distribution makes the understanding of changes in climate variability and extremes more difficult. Even when changes in extremes can be documented, unless a specific analysis has been completed, it is often uncertain whether the changes are caused by a change in the mean, variance, or both. In addition, uncertainties in the rate of change in the mean confound interpretation of changes in variance since all variance statistics are dependent on a reference level, i.e. the mean results. For variables that are not well approximated by normal distributions, like precipitation, the situation is even more complex (IPCC, 2001a).. 3.3. Droughts and floods under global warming As mentioned above, global warming has been intensifying the hydrological cycle. This will result in a changing flow regime (magnitude, frequency, duration, timing and rate of change). Following the projections (Table 1), trends in mean discharge will continue, affecting water quantity and quality of surface waters. Together with the projection of more extremes in precipitation and temperature (Table 1) it can be concluded that the increase or. Climate Change © Kiwa N.V.. - 29 -. February 9, 2006.

(36) decrease of annual discharge will be unequally distributed over the year which results in more summer and winter floods and hydrological droughts. Also urban storm drainage systems will more often have to deal with rainstorms, increasing the risk of urban floods and increasing the occurrence of first flush effects. For example, Ashley et al. (2005) calculated that the flood risks may increase by a factor of almost 30 times for the UK, which will worsen the water quality of UK rivers.. discharge [V·t-1]. The hydrological cycle is responsible for the water supply of rivers and lakes. Geology, orography, land use and human activity translate precipitation excess into a certain river type with a more or less unique discharge regime. There are several ways in which precipitation (rain or snow) is transported towards a river. The main paths are the short term surface runoff and subsurface runoff and the long term groundwater flow or glacier melt (Webb & Walling, 1992). The occurrence of these flows is responsible for the precipitation- discharge transformation of a river catchment, as can be presented by a hydrograph (Figure 10; Figure 11). The short term processes, surface runoff and subsurface runoff, are important for floods and depend mainly on precipitation characteristics (intensity, distribution), topography, infiltration rate, soil moisture content and groundwater level (Burt, 1992). The baseflow is important during droughts and consists of groundwater and/or glacier melt. The various interacting processes that involve the transformation of precipitation excess into discharge are complex and spatially as well as temporally variable. The rain to discharge transformation is non-linear and subject to hysteresis effects. This makes the processes sensitive to their initial and boundary conditions (Beven, 2001). Adding to this that projections of temperature, precipitation and floods and droughts are surrounded by uncertainties (Figure 10), makes the predictions for water quantity and thus for water quality of river systems for the future clearly a far from trivial task. 3 2. surface runoff rainfall. subsurface runoff. 1. 4. 5. baseflow. time [t]. Figure 10. A hydrograph and its main parts. Red arrows indicate uncertainties in: (1) initial condition; (2) occurrence, intensity and duration of rainfall; (3) peak height and moment; (4) increase in baseflow; (5) flood duration.. Climate Change © Kiwa N.V.. - 30 -. February 9, 2006.

(37) Figure 11. Water balance of the Girou watershed (France) during the hydrological year 1980 where P=precipitation; ETR=evapotranspiration; I=infiltration (Probst, 1985). Probst (1985) investigated a water balance for the Garonne Basin (52,000 km2) and the Girou Basin (520 km2) in the southwest of France for the year 1980. With easy mathematical methods they subdivided the contribution of the three flows to river discharge (Figure 11). On a relatively long time scale, streamflow (162mm) was the result of the difference between rainfall (763mm) and evapotranspiration (601mm). However, it is known that during extreme rainfall a relatively great amount of precipitation goes to the river via surface runoff. This will change the relative proportion between the surface runoff, subsurface runoff and groundwater flow, which is important for the flux of chemical substances and sediment to a river. A changing evapotranspiration will also affect this proportion. Hereby, one point of discussion is the transpiration of vegetation under a warmer climate and the effect of it on soil moisture and total river inflow (Mulholland et al., 1997). Higher temperatures, particularly in winter, would extend the growing season. If water levels were maintained at reasonable levels, net plant productivity should increase markedly as a result of the extended growing season. Moreover, in view of the well known evapotranspiration laws (Penman-Monteith, Priestly-Taylor, Makkink) an increase of evapotranspiration can be expected under a warmer climate. However, the increase of atmospheric CO2 may create a more favorable situation for the uptake of CO2 by vegetation whereby the stomata will be less open or closed for longer periods. Complex bidirectional relations make this a complex subject and nowadays the final effect on transpiration is uncertain (Mulholland et al., 1997; Witte et al., 2005). Furthermore, although potential evaporation will increase with temperature, depletion of soil moisture during dry periods may decrease or even stop real evaporation. The combination of changing precipitation and changing flows could affect ground water tables and thus could have environmental impact. Decreasing ground water tables cause settlement (and oxidation) in peat and clay soils, which has been resulted in interruptions in drinking water supply due to breaches in mains (Ramaker et al., 2005). Focusing on droughts, the immediate cause of droughts is the predominant sinking motion of air (subsidence) that results in compressional warming or high pressure, which inhibits cloud formation and results in lower relative humidity and less precipitation. Regions under the influence of semi-permanent high pressure during all or a major portion of the. Climate Change © Kiwa N.V.. - 31 -. February 9, 2006.