Large Scale Groundwater Modelling

The goal of this report is to evaluate large scale groundwater modelling techniques to be used at the European scale for flood and drought forecasting. In the current LISFLOOD model, the groundwater component is represented by two interconnected linear reservoirs, with the outflow in some unit of time being proportional to the volume of water stored in the reservoirs. Simulations with this setup have shown to yield acceptable reproductions of the observed hydrograph, hence the model can be considered to be sufficiently detailed to predict floods. However, a good reproduction of river discharges does not imply that other hydrological variables (soil moisture, groundwater levels) or processes (percolation, plant water uptake, and retention processes) are well reproduced. LISFLOOD is not able to simulate the spatial distribution of groundwater levels, hence a comparison with groundwater observations is not possible. This document proposes possible ways to adapt LISFLOOD such that it can provide an estimate of the groundwater elevation in space and time. This would also allow distributed groundwater measurements to be used in the calibration and validation of the model, and to gain insight in the local hydrological processes and their representation in the model.JRC.H.7-Land management and natural hazard

Development and Testing of Methods to Assess the Impact of Climate Change on Flood and Drought Risk at the European Scale

During the last 100 years global climate has warmed by an average of 0.6ºC, owing in part to human induced greenhouse gas emissions. Based on different scenarios of future greenhouse gas emissions projections of climate models indicate another 1.4 to 5.8 ºC of warming over the next century (IPCC, 2001a). The projected change in climate will significantly impact the hydrological cycle. A warmer climate will increase evaporation, the intensity of water cycling, and result in greater amounts of moisture in the air. It is expected that the magnitude and frequency of extreme weather events will increase, and that hydrological extremes such as floods and droughts will likely be more frequent and severe. The Joint Research Centre aims to develop knowledge and tools in support of the EU Climate Change Strategy that was recently put forward in the Commission’s Communication “Winning the Battle Against Global Climate Change” (COM(2005) 35). In view of this, an important research topic of the Land Management Unit of the IES is to assess the impact of climate change on the occurrence of hydrological extremes such as floods and droughts. This will be accomplished by developing an integrated modelling framework that combines regional climate predictions for Europe with the LISFLOOD model. LISFLOOD is a distributed, partially physically-based rainfall-runoff model that has been devised to simulate the hydrological behaviour in large European catchments (De Roo et al., 2000), with emphasis on predicting floods and droughts. Owing to its general nature, LISFLOOD is optimally suited for simulating the different hydrological regimes across Europe. Predicted climate for current conditions and for different scenarios of greenhouse gas emissions by the end of the 21st century will be used as input to LISFLOOD, after taking due account of any systematic bias in the climate forcing data obtained from climate models. Runoff statistics for the two periods will provide a means to estimate changes in the frequency and severity of hydrological extremes under different scenarios of future greenhouse gas emissions. Projections of future climate change are typically obtained from coupled Atmosphere-Ocean General Circulation Models (AOGCM). Because they require time steps of minutes but are used to predict climate change on time scales of months to centuries, their horizontal resolution is typically at least 100 km and hence their treatment of physical processes is approximate. Due to their coarse spatial resolution AOGCMs fail to explicitly capture fine-scale climatic structures needed for climate change impact studies and policy planning at the regional or sub-regional scale (e.g., catchment or basin scale). To resolve this problem, regionalization or downscaling methods have been developed that enhance regional detail and provide climatic information at smaller scales. The aim of this document is to provide an overview on existing methods for downscaling global climate information. Also, this document gives an overview of existing regional climate data sets for Europe, and details on how to use regional climate data for impact studies at the European and regional scale. The document is organised as follows. Section 2 presents a general overview of existing downscaling methods, with details of the underlying principles to generate regional climate information. In Section 3 an overview is given of regional climate data that are currently available to be used for impact studies at the European scale. In Section 4 some details are provided about the integrated modelling framework that couples the regional climate model data with the hydrological model LISFLOOD. Conclusions and an overview of current and further work are presented in Section 5.JRC.H.7-Land management and natural hazard

Will Europe See More Frequent and Severe River Flow Droughts in the Future?

For the coming decades climate change is projected not only to result in higher temperatures, hence higher evaporative demands, but also to induce changes in the seasonality of precipitation patterns, with wetter winters and dryer summers, as well as to increase the frequency and intensity of extreme climatic events. The combination of these patterns of change will likely result in more frequent, severe and persistent droughts in large parts of Europe, especially in the south. This study presents a pan-European assessment of the possible impacts of climate change on low flows in Europe by comparison of river flow drought characteristics for current and future climate. We employ high resolution (12 km spatial resolution) regional climate data from the HIRHAM model for the control period and the future period based on the SRES A2 greenhouse gas emission scenario to force a hydrological model. Low flow characteristics for current and future climate are derived from the simulated river flow series using extreme value analysis. More specifically, we employ the methods of block maxima and partial duration series to select minimum flows and maximum flow deficits and fit extreme value distributions by the maximum likelihood method. Regions most prone to an increase in river flow drought are southern and south-eastern Europe, but minimum flows will also decrease significantly in many other parts of Europe, especially in summer. In snow dominated regions winter droughts are projected to be less severe because a lower fraction of precipitation will fall as snow in warmer winters.JRC.H.7-Land management and natural hazard

Calibration of the LISFLOOD Model for Europe: Current Status and Way Forward

The aim of this document is to provide an overview and some results of the calibration work of the LISFLOOD model [De Roo et al., 2000; 2001] that has been carried out during 2005. LISFLOOD forms the core of the European Flood Alert System (EFAS) and is used for impact studies to evaluate the effect of land use and climate changes on the hydrological behaviour across Europe. It is a spatially distributed, partly physically-based hydrological model embedded within a PCRaster GIS environment. The model simulates river discharges in drainage basins as a function of spatial information on topography, soils and land cover. The accuracy of the model predictions depends on the ability of the model to capture the dominating hydrological processes that transfer precipitation into river runoff at the catchment scale, and on its ability to reproduce historical time series of observed river discharges. A crucial step which contributes significantly to the accuracy of the LISFLOOD forecasts and simulations is the calibration of the model for all European catchments. Owing to the general nature of LISFLOOD, its application to any given river basin requires that certain parameters of conceptual functions be identified for the particular basin. In the process of calibration, the values of unknown model parameters are tuned such that the model matches the observed predictions as closely as possible. During the early stages of the EFAS project, the LISFLOOD model has been crudely calibrated, without taking due account of the spatial variability of the parameters over the different hydrological regimes across Europe. A set of 240 parameters realizations was generated, and for large catchments the parameter set was chosen that best reproduced a time series of observed river discharges at the outlet. For ungauged catchments the parameter set that gave the best prediction in most other catchments was used. The underlying assumption was that the 240 parameter realizations were a representative sample of the feasible parameter space. More recently, several detached national experts have been working on a more detailed calibration of the LISFLOOD model for the Danube and Elbe catchments, typically by manually adjusting the parameters while visually inspecting the agreement between the observed and simulated discharges. However, the subjective and time-consuming nature of the trial-and-error method renders this method unappealing for use at a European scale. The large number of catchments for which the model needs to be calibrated calls for an automatic parameter estimation procedure. Besides shortening the implementation time this will also enhance the reliability of the calibrated parameters due to a more exhaustive exploration of the parameter space. An automatic calibration procedure has been developed for LISFLOOD, based on the Shuffled Complex Evolution Metropolis (SCEM-UA) global optimization algorithm [Vrugt et al., 2003]. The algorithm automatically searches through the space of feasible parameter values and finds the parameter values that produce the best model performance. It also yields a posterior parameter distribution, which reflects the residual uncertainty about the model parameters after taking into account the discharge observations. The posterior distribution forms the basis for making probabilistic flow predictions. To overcome the computational burden the optimization has been implemented using parallel computing. The work done on the calibration in 2005 resulted in one paper in the proceedings of the International Conference on Innovation, advances and implementation of flood forecasting technology in Tromsø [Feyen et al., 2005a], an oral presentation at the American Geophysical Union Fall Meeting in San Francisco [Feyen et al., 2005b], an article submitted to Journal of Hydrology [Feyen et al., 2006a], and a manuscript in preparation [Feyen et al., 2006b].JRC.H.7-Land management and natural hazard

Increasing flood risk under climate change: a pan-European assessment of the benefits of four adaptation strategies

Future flood risk in Europe is likely to increase due to a combination of climatic and socio-economic drivers. Effective adaptation strategies need to be implemented to limit the impact of river flooding on population and assets. This research builds upon a recently developed flood risk assessment framework at European scale to explore the benefits of adaptation against extreme floods. Four different adaptation measures are simulated in a physically based modeling framework, including the rise of flood protections, reduction of the peak flows through water retention, reduction of vulnerability and relocation to safer areas. Their sensitivity is assessed in several configurations under a high-end global warming scenario over the time range 1976-2100. Results suggest that the future increase in expected damage and population affected by river floods can be compensated by a combination of different adaptation measures. The adaptation efforts should favor measures targeted at reducing the impacts of floods, rather than trying to avoid them. Conversely, adaptation plans only based on rising flood protections have the effect of reducing the frequency of small floods and exposing the society to less-frequent but catastrophic floods and potentially long recovery processes.JRC.H.7-Climate Risk Managemen

Ensemble flood risk assessment in Europe under high end climate scenarios

AbstractAt the current rate of global warming, the target of limiting it within 2 degrees by the end of the century seems more and more unrealistic. Policymakers, businesses and organizations leading international negotiations urge the scientific community to provide realistic and accurate assessments of the possible consequences of so called “high end” climate scenarios.This study illustrates a novel procedure to assess the future flood risk in Europe under high levels of warming. It combines ensemble projections of extreme streamflow for the current century based on EURO-CORDEX RCP 8.5 climate scenarios with recent advances in European flood hazard mapping. Further novelties include a threshold-based evaluation of extreme event magnitude and frequency, an alternative method to removing bias in climate projections, the latest pan-European exposure maps, and an improved flood vulnerability estimation.Estimates of population affected and direct flood damages indicate that by the end of the century the socio-economic impact of river floods in Europe is projected to increase by an average 220% due to climate change only. When coherent socio-economic development pathways are included in the assessment, central estimates of population annually affected by floods range between 500,000 and 640,000 in 2050, and between 540,000 and 950,000 in 2080, as compared to 216,000 in the current climate. A larger range is foreseen in the annual flood damage, currently of 5.3 B€, which is projected to rise at 20–40 B€ in 2050 and 30–100 B€ in 2080, depending on the future economic growth

Global warming increases the frequency of river floods in Europe

EURO-CORDEX, a new generation of downscaled climate projections, has become available for climate change impact studies in Europe. New opportunities arise in the investigation of potential effects of a warmer world on meteorological and hydrological extremes at regional scales. In this work, an ensemble of EURO-CORDEX RCP 8.5 scenarios is used to drive a distributed hydrological model and assess the projected changes in flood hazard in Europe through the current century. Changes in magnitude and frequency of extreme streamflow events are investigated by statistical distribution fitting and peak over threshold analysis. A consistent method is proposed to evaluate the agreement of ensemble projections. Results indicate that the change in frequency of discharge extremes is likely to have a larger impact on the overall flood hazard as compared to the change in their magnitude. On average in Europe, flood peaks with return period above 100 years are projected to double in frequency within a time range of three decades.JRC.H.7-Climate Risk Managemen

Climate Change Impact on Flood Hazard in Europe: An Assessment Based on Regional Climate Scenarios

Simulations with global and regional climate models predict that future climate change will lead to an increase in frequency and intensity of extreme precipitation events in Europe, especially in the north and in the winter also in central Europe. Some models project an increase in heavy rainfall amounts even in areas that in general are expected to become much drier. This trend is likely to lead to more frequent and more intense river flooding in many parts of Europe. To analyse changes in flood hazard at the European scale we employed the hydrological model LISFLOOD that has been developed for operational flood forecasting using a grid scale of 5 km. This model was driven by data from several regional climate models, including an experiment of the RCM HIRHAM that was performed with a very high horizontal resolution of 12 km. It was found that, under the SRES A2 emissions scenario of the IPCC, in many European rivers the extreme discharge levels may have increased in magnitude and frequency by the end of this century. In several rivers, most notably in the west and parts of eastern Europe, the probability of what is currently a 100-year flood may double or increase even more, meaning that the return period decreases to 50 years or less. A notable exception to this was found in the northeast, where warmer winters and a shorter snow season reduce the magnitude of the spring snowmelt peak. Also in other rivers in central and southern Europe a considerable decrease in extreme river flows was found. The results from the 12-km HIRHAM simulation were compared with those obtained with two experiments of the same model at a lower resolution of about 50 km for the SRES A2 and B2 scenarios. Disagreements between these model experiments indicate that the effect of the horizontal resolution of the regional climate model is comparable in magnitude to the choice for a particular greenhouse gas scenario.JRC.H.7-Land management and natural hazard

Assessment of Conceptual Model Uncertainty for the Regional Aquifer Pampa del Tamarugal - North Chile

In this work we assess the uncertainty in modelling the groundwater flow for the Pampa del Tamarugal Aquifer (PTA) ¿ North Chile using a novel and fully integrated multimodel approach aimed at explicitly accounting for uncertainties arising from the definition of alternative conceptual models. The approach integrates the Generalized Likelihood Uncertainty Estimation (GLUE) and Bayesian Model Averaging (BMA) methods. For each member of an ensemble M of potential conceptualizations, model weights used in BMA for multi-model aggregation are obtained from GLUE-based likelihood values. These model weights are based on model performance, thus, reflecting how well a conceptualization reproduces an observed dataset D. GLUE-based cumulative predictive distributions for each member of M are then aggregated obtaining predictive distributions accounting for conceptual model uncertainties. For the PTA we propose an ensemble of eight alternative conceptualizations covering all major features of groundwater flow models independently developed in past studies and including two recharge mechanisms which have been source of debate for several years. Results showed that accounting for heterogeneities in the hydraulic conductivity field (a) reduced the uncertainty in the estimations of parameters and state variables, and (b) increased the corresponding model weights used for multi-model aggregation. This was more noticeable when the hydraulic conductivity field was conditioned on available hydraulic conductivity measurements. Contribution of conceptual model uncertainty to the predictive uncertainty varied between 6% and 64% for ground water head estimations and between 16% and 79% for ground water flow estimations. These results clearly illustrate the relevance of conceptual model uncertainty.JRC.H.7-Land management and natural hazard

Global warming and windstorm impacts in the EU

Windstorms are amongst the most damaging natural hazards in Europe, with approximately 5 €billion of estimated annual losses in the EU. The number of reported windstorms significantly increased over the last decades, yet there is no consensus about a climate-induced trend in windstorms over Europe. Climate model projections of extreme wind are highly uncertain, but they suggest that windstorms will not become more intense or happen more frequent with global warming over most of the European land. As a consequence, it is expected that risks from windstorms in the EU will not rise due to climate change. Future impacts of wind extremes could be reduced by a range of measures, such as the development and implementation of enhanced windstorm-resilient standards and building codes.JRC.E.1-Disaster Risk Managemen