EFAS-Meteo: A European daily high-resolution gridded meteorological data set for 1990 - 2011

Data sets of spatially irregular meteorological observations interpolated to a regular grid are not only important for climate analyses but are also essential in order to derive climatologies for rainfall-runoff models which require meteorological data sets as input forcing. For example, in the European Flood Awareness System long term observed meteorological data are used to drive the hydrological model LISFLOOD to obtain long term time series of simulated discharges at a pan-European scale. Those long term time series of simulated “proxy” discharges can then be used for statistical analysis, e.g., to derive return periods or other time series derivatives. In this report, we present a comprehensive pan European high-resolution gridded daily data set (EFAS-Meteo) of precipitation, surface temperature (mean, minimum and maximum), wind speed, vapour pressure, calculated radiation and evapotranspiration (potential evapotranspiration, bare soil and open water evapotranspiration). The data set was created as part of the development of EFAS and has been continuously updated throughout the last years.JRC.H.7-Climate Risk Managemen

Assessment of the Observed and Future Climate Variability and Change in Hydroclimatic and Hydrological Extremes (Waargenomen en toekomstige klimaatvariabiliteit en -verandering in hydroklimatologische en hydrologische extremen)

For decades, scientists have copiously researched and ferreted out the unusual nature of the global significant climate changes. The preponderance of the climate change evidence is demonstrated from the level of agreement across diverse studies which indicate a high degree of robustness. Considering the last 1000 years, the recent warming is strikingly higher than previous warm periods. Other noticeable effects of global warming have been sea level rises, increased ice melting, increased floods anddroughts among others. There is ample evidence that this widespread warming has precipitated changes in hydrology and it is anticipated that the future changes will be enhanced. Anticipated climate change impacts onhydrology may lead to both opportunities and challenges. The frequency of floods and droughts could challenge existing water management measures, agricultural practices and the biodiversity in aquatic ecosystems. Increased flows are not necessarily negative; increased volumes of water could be beneficial for hydropower generation, groundwater recharge and agriculture. However, the confidence in the future changes is still a concern today because of the uncertainties in the climate change models. But climate models remain the only scientifically credible tools for investigating the dynamics of the global climate, and for investigating diverse scenarios. Moreover, the confidence in the climatemodels has been increasing over the last decades as modifications have been made to exploit the new advances in climate science and computer processing. Higher resolution Regional Climate Models (RCMs) have been nested within Global Climate Models (GCMs) to improve on the representationof the local climates which are less reliable in the coarse GCMs. With the improvements in the GCMs and RCMs, the increased climate change evidence and regional policy requirements, hydrological studies related to climate change have increased. The interfacing of climate modelling with hydrological modelling is one area that has received increased attentionbut challenging questions still exist. Thus, the principal objective of this research was to explore climate variability andfuture climate change in Belgium with the aim of understanding the possible consequences on hydrological regimes. The research benefited from the state of the art high resolution climate models and the availability of long term records. The first objective involved the diagnosis of the observed trends in extremes from long term observed records. Trend studies are important because they provide extra knowledgeon the possible climate change effects, and the nature of the natural climate variability in Belgium. The presence of cycles, a natural variability phenomenon, may debunk the argument of unprecedented climate changewhile the presence of significant trends may support it. This study provides a historical view that elucidates whether climate change in Belgium is plausible or implausible. The historical diagnosis showed some signals of anthropogenic influence. However, the evidence of the anthropogenic signals appeared to depend on the season with winter showing the clearest evidence. The major outcome of the trend study was that the change signal in time series was explained by either natural variability or thesuperposition of natural variability and climate change. Hence, before concluding that the recent extremes are due to anthropogenic climate change, other studies are necessary to establish whether natural variability can not explain the observed extremes. Since climate models contain the best current understanding for how the physical processes within the climate system interact, the second objective was to evaluate the physical soundness of the climate models. This was motivatedby the availability of many GCMs and RCMs. The fidelity of climate models was tested against the climate models and it was concluded that the models were still biased even at the higher resolutions. Hence, the outputs of the regional climate models require further refining or downscaling before direct use in hydrological studies. Climate changeimpact on hydrological regimes is conventionally investigated by comparing the outputs from the historical and perturbed inputs. Hence, it is necessary to apply a downscaling methodology that ensures that the climate change perturbations are applied to the hydrological variables in a more reasonable way; for instance to account for the changes in variability. Therefore, a method was formulated which ensured that the changes in extremes were realistically represented in the future climate series. This way, the future changes in extremes such as rainfall extremes (and flows) were better represented. The other research objective sought to analyse the impacts from the array of climate models and hydrological models to determine the direction and magnitude of climate change impact on hydrological regimes such as the peak flows and low flows. It was established that future trends suggest mixed consequences. Should the high flows be too high to be buffered, flood damages may increase above historic levels. However, the high flow projections also indicate a possibility of reduced high flows, which means that the future directionof change for high flows is not certain. In contrast, the impact of thefuture climate change impact on the low flows, as projected from the climate models, consistently exhibited reduced flows, which does not bode well for navigation and water quality during low flow periods. In addition, it was apparent that the hydrological model structure partly explained the magnitude of the changes with some models tending to have higher /lower extremes. This finding suggests that the effect of hydrological models, especially regarding extremes, should not be overlooked. Further, there is an increasing interest and demand for climate change scenarios for many local and regional impact studies in Belgium. Oneof the challenges such studies face is how to select a limited but useful set of climate scenarios. Hence, another goal for this research was to develop climate scenarios that would be surrogates for the examined large set of scenarios. Indeed, the spread of future climate model simulations is set to increase in the fifth IPCC assessment report making the challenge of impact analysis more intense. The demand for tailored scenarios can not be overstated. This research devised a methodology for transforming the simulations to a few scenarios that were tailored for impactanalysis. With such scenarios, it is possible to evaluate, in a tailored context, the range of impacts without simulating the entire set of scenarios. In conclusion, challenges still exist in the hydrological climate change impact investigations. Statistical downscaling, bias removal, and probabilistic scenarios are some of the areas that meritfurther research. But it is worthwhile to emphasize the necessity for hydrological impact analysts to interact with other experts such as climate scientists, statisticians, and policy makers for a better guidance on how to synthesize the dimensions of climate change. This will lead to more valuable studies of the hydrological changes linked to climate change.Acknowledgements i English summary iii Nederlandse samenvatting vii Chapter 1 Introduction 1 1.1 Objectives of the research 3 1.2 Outline of thesis 5 1.3 Overview of study areaAnthropogenic climate change 6 Chapter 2 An Overview of climate change 7 2.1 Introduction 7 2.2 Climate change factors 7 2.3 Potential hydrological impacts of climate change 8 2.4 Global trends in key hydrological variables 9 2.4.1 Global trends in temperature 10 2.4.2 Global trends in precipitation 11 2.4.3 Global trends in evaporation 11 2.4.4 Global trends in stream flows 12 2.4.5 Observed stream flow trends in Europe 13 2.5 The climate system and climate models 14 2.5.1 The climate system 14 2.5.1.1 Climate subsystems 15 2.5.2 Climate models 16 2.5.3 Climate and weather models 17 2.5.4 General Circulation Models/Global Climate Models 18 2.5.5 Overview of downscaling methods 19 2.5.6 Regional Climate Models 20 2.5.7 Bias correction methods 21 2.6 Uncertainty in hydrological climate change assessments 22 2.6.1 Emissions 23 2.6.2 Climate model uncertainty 24 2.6.3 Downscaling uncertainty 26 2.6.4 Hydrological model uncertainty 26 Chapter 3 Historical hydroclimatic variability 29 3.1 Trends and cycle analysis for historical precipitation 29 3.1.1 Extremes 31 3.1.2 Aggregation levels and time scales 32 3.1.3 Rainfall distributions 33 3.1.3.1 Fitting the rainfall distributions 35 3.1.4 Quantile-perturbation approach 37 3.1.5 Monte Carlo confidence intervals 43 3.1.6 Hypothesis testing on the clustering of rainfall extremes 45 3.1.6.1 Number of events 48 3.1.7 Discussion of perturbations 49 3.1.7.1 Hypothesis testing summer average perturbations 49 3.1.7.2 Hypothesis testing winter average perturbations 50 3.1.7.3 Synopsis hypothesis testing average perturbations 51 3.1.7.4 Autumn and spring perturbations 52 3.1.8 Concluding remarks on rainfall trends 53 3.2 Statistical trend analysis for evapotranspiration 55 3.2.1 Summer ETo 58 3.2.2 Autumn ETo 58 3.2.3 Winter ETo 59 3.2.4 Spring ETo 59 3.2.5 Concluding remarks on ETo trends 59 3.3 Trend analysis of historical river flow series 60 3.3.1 Flow perturbation analysis 61 3.3.2 Summer flow perturbations 62 3.3.3 Autumn flow perturbations 63 3.3.4 Winter flow perturbations 64 3.3.5 Spring flow perturbations 65 3.4 Concluding remarks for the flow perturbations 66 Chapter 4 Anthropogenic climate change 69 4.1 Introduction 69 4.2 Climate model evaluation 69 4.2.1 Measures of skill 70 4.2.2 Recent climate model evaluation metrics 72 4.3 PRUDENCE Climate Models 73 4.3.1 Interannual variability performance 75 4.3.2 Monthly performance 76 4.3.3 Event frequency performance 77 4.3.4 Frequency/Distribution performance for rainfall 79 4.3.5 Model performance over Belgium 81 4.4 AR4 GCM models 82 4.4.1 GCM performance 83 4.4.2 Conclusions regarding the climate model performance 86 4.5 Climate change perturbations 88 4.5.1.1 Quantile perturbation 90 4.5.1.2 Mean quantile perturbations 91 4.5.1.3 PRUDENCE quantile perturbation factors 92 4.5.1.4 Dependency of the perturbations on timescale 94 4.5.2 Changes in Intensity Duration Frequency (IDF) relations 98 4.5.3 Conclusions regarding the future projected changes 100 Chapter 5 Hydrological climate change impact 101 5.1 Introduction 101 5.2 The development of climate change scenarios 102 5.2.1 Rainfall perturbations 104 5.2.1.1 Rainfall wet-day frequency perturbation 105 5.2.1.2 Rainfall wet-day quantile perturbations 106 5.2.2 Evapotranspiration perturbations 108 5.2.3 Internal physical consistency 109 5.2.4 Non-parametric perturbation approach 111 5.2.5 Perturbation of rainfall series 111 5.2.5.1 Step 1: Wet-day frequency perturbation 112 5.2.5.2 Step 2: Wet-day intensity quantile perturbation 113 5.2.6 Perturbation of the ETo series 116 5.3 Hydrological models for climate change 116 5.4 Peak flow extraction 118 5.5 Climate change impacts on river flows in the Molenbeek Erpe-Mere catchment 120 5.5.1.1 High flow impacts in the Molenbeek Erpe-Mere catchment 123 5.5.1.2 Low flow impacts in the Molenbeek Erpe-Mere catchment 125 5.5.2 Identifying optimal climate change scenarios 126 5.5.3 Impacts from different conceptual hydrological models 128 5.5.3.1 High flows 129 5.5.3.2 Low flows 130 5.6 Tailoring climate change scenarios 131 5.6.1 Rainfall: High, mean and low perturbations 133 5.6.2 ETo: High, mean and low perturbations 134 5.6.3 Tailoring of impact scenarios 135 5.6.4 Tailoring the Winter High (WH) scenario 137 5.6.4.1 Tailoring the Summer High (SH) scenario 138 5.6.5 Tailoring the Winter Low scenario (WL) 139 5.6.5.1 The Ensemble Mean (EM) scenario 140 5.6.5.2 Evaluating the tailored scenarios 141 5.7 Broadening the uncertainty range 145 5.8 Conclusions regarding the hydrological climate change impact 149 Chapter 6 Conclusions and recommendations 153 6.1 General conclusions 153 6.2 Future directions of research 155 References 161 About the Author 183 Publications 183 Appendices Appendix A A.1 A.1 MIKE 11/NAM rainfall–runoff model A.1 A.2 The PDM model A.3 Appendix B B.1 B.1 Model parameters B.1nrpages: 219status: publishe

Tailored precipitation and evapotranspiration scenarios for hydrological climate change impact analysis

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Incorporating the correlation between upstream inland, downstream coastal and surface boundary conditions into climate scenarios for flood impact analysis along the river Scheldt

Climate change scenarios have been developed that are useful for flood impact studies along the river Scheldt in Belgium. They consist of scenarios for the downstream coastal boundary (sea level rise, storm surges), the upstream inland boundary (rainfall and related runoff discharges) and the surface boundary along the Scheldt reaches (wind speed and direction). Correlations between the climatic changes at the three boundaries have been considered. For each of the boundaries, the climate change scenarios are based on statistical analysis of an ensemble set of (at least 20) simulation results with regional climate models (RCMs). The RCM results are provided by several databases, including the CERA database (2 CLM runs), the EU-FP5 PRUDENCE database (31 runs with 12 RCMs) and the EU-FP6 ENSEMBLES database (18 RCM runs). RCM results of precipitation, temperature, potential evapotranspiration, wind speed, wind direction and sea level pressure (SLP) have been validated for historical periods (e.g. 1961-1990, 1981-2000) and analyzed for future changes till 2100 (e.g. for the near future, 2011-2040 or 2016-2035, the not so near future, 2041-2070 or 2046-2065, and the longer term future, 2071-2100 or 2076-2095). The validation is done for daily and monthly mean values, and daily extremes in different seasons. Future changes take the form of change factors, dependent on season, return period (for the extremes) and time scale. Based on the ensemble set of change factors, tailored climate scenarios (tailored for the specific application of flood impact analysis along the Scheldt) have been developed. After statistical analysis of the whole range of change factors, a reduced set of climate scenarios (“high”, “mean” and “low”) was derived for each boundary (upstream, downstream and surface). Smart combinations of these scenarios account for the correlations between the boundary changes. Changes in SLP were transferred to changes in storm surges at the Scheldt mouth (at Vlissingen) based on a correlation model between the SLP at the Baltic Sea and the storm surge level. This model was derived after analysis of SLP composite maps and SLP-surge correlation maps for days where the surge exceeds given thresholds (for different return periods). Changes in inland precipitation and evapotranspiration were transferred to corresponding changes in runoff discharges (upstream along the Scheldt) based on simulations in rainfall-runoff models of the upstream river basins. After calibration of the rainfall-runoff models to historical series, the precipitation and evapotranspiration input series were stochastically perturbed to account for the changes in the number of rain storms, the rain storm intensities and the correlated changes in evapotranspiration. This was done for each of the tailored climate scenarios. Extreme value analysis on the rainfall-runoff model results allowed assessment of the changes in runoff discharges at the upstream Scheldt boundaries, in relation to the return period of the runoff event.status: publishe

On the relative importance of the climate change factors along the river Scheldt considering climate scenarios for upstream inland and downstream coastal (mean sea level and surge) boundary conditions

To improve on the efficacy of flood risk mitigation measures, it is essential to investigate the relative importance of the future impact pressures. This is more so in areas which are found to be hot spots for flooding. One such area was identified in the Scheldt region located in Belgium. The Dendermonde area is a place where both the downstream coastal and the upstream river flow boundary conditions interact and jointly control the flood risk. Downstream of this area, the coastal level changes include both the sea level rise and storm surge changes due to climate change impacts on the wind climate over the North Atlantic and North Sea region. Upstream of the Dendermonde area lies the Dender river which introduces an extra pressure on the Dendermonde area. Against this back drop, impact analysis was performed using a hydrodynamic model that accounts for such changes. The climate data for future scenarios were extracted from the climate databases PRUDENCE (http://prudence.dmi.dk), ENSEMBLES (http://www.ensembles-eu.org/), IPCC AR4 (www-pcmdi.llnl.gov/ipcc/about_ipcc.php) and CERA (CLM from MPI-M/MaD). Future changes were derived from the large ensemble set of climate model runs and their effects simulated in the hydrodynamic model based on statistically processed climate change scenarios of sea level rise, SLP change and related storm surge changes and upstream runoff due to changes in rainfall and potential evapotranspiration. Changes in SLP were transferred to changes in storm surges at the Scheldt mouth (at Vlissingen) based on a correlation model between the SLP at the Baltic Sea and the storm surge level. This model was derived after analysis of SLP composite maps and SLP-surge correlation maps for days where the surge exceeds given thresholds (for different return periods). Correlations between the inland (rainfall, runoff) and coastal climatic changes were considered. The impact analysis to analyze the importance of the pressures for the Dendermonde area was based on hydrodynamic impact model results. From the water level impact results, it was deduced that the sea level rise and surges are by far the most important factors when evaluating the flood risk in the Dendermonde region. When the extreme changes for the three main boundary conditions (mean sea level, surge and upstream runoff) coincide, the impact is disastrous. For example, water levels at Dendermonde were simulated to change around +1.8m for return periods in the range between 100 and 10000 years and the scenario with +0.6m mean scenario for sea level rise ,+21% high scenario change in surge levels, and +30% high scenario change in upstream flow. For the high mean sea level rise scenario of +2m, the changes are even much higher. Water management plans, which are currently under way such as the Sigma Plan, can be informed from such studies so as to test the robustness of the proposed flood mitigation measures.status: publishe

The effects of climate change on hydrological extremes along rivers in Flanders’, In: “Bridges over Troubled Waters

For centuries, decision makers and water resources managers have used climate information to design systems that protect the livelihoods of the population. The recent resurgence of floods and droughts in Europe has, however, called into question the robustness of the existing infrastructure such as the sewer systems for urban drainage and the flood control systems. It is not obvious why this resurgence is occurring. The resurgence could be attributed to anthropogenic changes, climate change, climate variability or a combination of factors. It is certain that decision makers and water managers need to recognize the fact that relying entirely on past observations for design purposes is no longer advisable. It is important to look into the studies of climate variability and change for some clues on how to accommodate the alterations in the environment.status: publishe

Climate change and hydrological extremes in Belgian catchments

In this study we focus our attention on the climate change impacts on the hydrological balance in Belgium. There are two main rivers in the country, the Scheldt and the Meuse, supplied with water almost exclusively by precipitation. With the climate change projected by climate models for the end of the current century, one would expect that the hydrological regime of the rivers may be affected mainly through the changes in precipitation patterns and the increased potential evapotranspiration (PET) due to increased temperature throughout the year. We examine the hydrology of two important tributaries of the rivers Scheldt and Meuse, the Gete and the Ourthe, respectively. Our analysis is based on simulations with the SCHEME hydrological model and on climate change data from the European PRUDENCE project. Two emission scenarios are considered, the SRES A2 and B2 scenarios, and the perturbation (or delta) method is used in order to assess the climate change signal at monthly time scale and provide appropriate input time series for the hydrological simulations. The ensemble of climate change scenarios used allows us to estimate the combined model and scenario uncertainty in the streamflow calculations, inherent to this kind of analysis. In this context, we also analyze extreme river flows using two probability distribution families, allowing us to quantify the shift of the extremes under climate change conditions.status: publishe

Quantifying the impact of climate change from inland, coastal and surface conditions

The impact of climate change for the short, mid and long term horizons is investigated for an area along the Scheldt river, at the confluence with the Dender river. The downstream coastal boundary comprises of the sea level rise and storm surges from the North Sea while the upstream inland boundary comprises of rainfall and related runoff discharges. The third surface boundary comprises of wind speed and direction along the Scheldt. The climate change scenarios are based on statistical analysis of an ensemble of (at least 20) simulation results with Regional Climate Models (RCMs). The RCM results are provided by the CERA database, the EU-FP5 PRUDENCE and the EU-FP6 ENSEMBLES database. Outputs of precipitation, temperature, potential evapotranspiration, wind speed, wind direction and Sea Level Pressure (SLP) have been validated for historical periods and correlations are accounted for when quantifying future changes till 2100.status: publishe

Modelo de simulación hidráulica de drenaje dual en un área urbana para predicción de inundaciones en tiempo real

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