Effects of climate change on snow accumulation and melting in the Broye catchment (Switzerland)

The study used a daily step conceptual hydrological model to examine the effects of climate change on snowfall accumulation and on snow cover melting in the Broye catchment (moderate relief- altitude from 400 to 1500 m a.s.l.). Five elevation bands representing a range of climatic conditions were used together with three realistic climate change scenarios based loosely on GCM's predictions and which reflect feasible changes by extending time periods. For a very moderate climate change (rise in air temperature of ca 1 °C), possibly in a near future, the reduction of snow cover duration, mean water equivalent and monthly maximum water equivalent is the most sensitive in the lower part of the catchment and during the first and last months of the snow season. In the higher part of the basin and during the colder months January and February, similar reduction rates can be expected in case of larger climate changes. The floods due to the melting of snow cover are lower. Sometimes rainfall, considered as snow in the present day conditions, generates additional floods during the winter season. For winter sports resorts below 1500 m a.s.l., even the very moderate climatic change scenario (temperature rise around 1 °C) leads to economically very difficult conditions. Finally, a climatic change detection index based on snow cover duration is proposed.     

Impact of Climate Change on Hydrological Regimes and Water Resources Management in the Rhine Basin

The International Commission for the Hydrology of the Rhine basin (CHR) hascarried out a researchproject to assess the impact of climate change on the river flow conditionsin the Rhine basin. Along abottom-up line, different detailed hydrological models with hourly and dailytime steps have beendeveloped for representative sub-catchments of the Rhine basin. Along atop-down line, a water balancemodel for the entire Rhine basin has been developed, which calculates monthlydischarges and which wastested on the scale of the major tributaries of the Rhine. Using this set ofmodels, the effects of climatechange on the discharge regime in different parts of the Rhine basin werecalculated using the results ofUKHI and XCCC GCM-experiments. All models indicate the same trends in thechanges: higher winterdischarge as a result of intensified snow-melt and increased winterprecipitation, and lower summerdischarge due to the reduced winter snow storage and an increase ofevapotranspiration. When the resultsare considered in more detail, however, several differences show up. These canfirstly be attributed todifferent physical characteristics of the studied areas, but different spatialand temporal scales used in themodelling and different representations of several hydrological processes(e.g., evapotranspiration,snow melt) are responsible for the differences found as well. Climate changecan affect various socio-economicsectors. Higher temperatures may threaten winter tourism in the lower wintersport areas. The hydrologicalchanges will increase flood risk during winter, whilst low flows during summerwill adversely affectinland navigation, and reduce water availability for agriculture and industry.Balancing the required actionsagainst economic cost and the existing uncertainties in the climate changescenarios, a policy of `no-regretand flexibility' in water management planning and design is recommended, whereanticipatory adaptivemeasures in response to climate change impacts are undertaken in combinationwith ongoing activities.     

Estimated annual regime of energy-balance components,evapotranspiration and soil moisture for a drainage basin in the case of a CO2 doubling

Assuming a doubling of the atmospheric CO₂ concentration, parameters of an empirical formula for calculating the daily net terrestrial radiation under the climatic conditions of Belgium are determined. The developed method takes into account information yielded by climate models about the CO₂ impacts. Annual regimes of the energy-balance components are calculated for a drainage basin in Belgium. A daily step conceptual hydrological model (developed at the Royal Meteorological Institute of Belgium) was run to estimate the effective evapotranspiration and the soil moisture in the 2 × CO₂ case; results of this simulation are compared with the present-day conditions.This research was supported by a Commission of the European Communities Grant [CLI-104B(RS)]     

Streamflow response of Belgian catchments to IPCC climate change scenarios

The IRMB (Integrated Runoff Model—F. Bultot) daily step conceptual model has been applied to eight Belgian catchments with areas ranging from 100 to 1200 km². These catchments are characterized by various infiltration rates and ground water storage capacities. The outputs of six GCMs (General Circulation Model) distributed by the IPCC (Intergovernmental Panel on Climate Change) and an earlier scenario have been used to perturb time series of hydrometeorological input data relevant to simulate the water cycle. This paper focuses on the impacts on streamflow and its surface and underground components, as well as on the occurrence of flood days and low flow days. Impacts are shown to be catchment and scenario dependent. Due to the scenario diversity, streamflow impacts are found to be either positive or negative. The trends are common to scenarios with the same patterns or to catchments with similar characteristics. For all but two scenarios, all the catchments present an increase of flood frequency. Nevertheless, for all the scenarios, catchments with prevailing surface flow are undergoing an increase in flood frequency during winter months.     

Impact of a CO2-induced climatic change on river flow variability in three rivers in Belgium

A hydrological conceptual model developed by the Royal Meteorological Institute of Belgium has been run to assess the potential hydrological impacts of an hypothetical doubling of the atmospheric CO₂ concentration. The simulated scenarios were derived from the predictions of climatic change currently provided by General Circulation Models (GCMs). Three typical drainage basins in Belgium have been selected for assessing their common responses and bringing out possible specific behaviours attributable to catchment characteristics. The study dealt essentially with modifications of the streamflow and with alterations of the flood and low-flow regimes. In catchments with prevailing surface flow the considered change in climate could induce:

• An increase in flood frequencies during the winter season together with a strengthening of the extreme river stages leading to greater flooding risks;
• A decrease in streamflow during the summer season and, as a corollary, increased risks of water pollution;
• Possible restraints, in summer and autumn, on water availability from local groundwater storages.

In catchments with high infiltration rate and with strong aquifer the impact could be:

• An increase in groundwater storage, bringing about an increase in the base flow throughout the year, which in turn involves increased flood risks;
• A reduction of the number of low-stage occurrences in summer, resulting in reduced river pollution;
• A possible increase in water availability from the aquifers.

     

Trend and Correlation Analysis of k-Day Extreme Precipitationover Belgium

Summary k-day extreme precipitation depths (k=1,2,3, … 30) for the climatological network of Belgium (165 stations) are analysed to detect a possible evolution in the occurrence of extreme rainfall events during the 1951–1995 reference period. The calendar year and the hydrological summer and winter are considered separately. Spearman’s rank correlation coefficient shows a strong spatial correlation between extreme k-day precipitation events, depending on the time of the year (lower during summer than during winter) and increasing with k. In some cases the distances of de-correlation exceed 200 km which is comparable to the size of the country. Due to this correlation, tests for trends have been carried out on the leading principal components (PC) derived from the covariance matrix. Various PC selection rules have been applied to identify the number of components to analyse. The number of components needed to reproduce a given proportion of the total variance varies, with larger values for summer than for winter and a decrease with growing k. The Fisher test is used as a global test. It combines the individual Mann-Kendall trend tests carried out on the selected PC scores. Significant trends have been found in extreme winter k-day precipitation for all the values of k and none in extreme summer precipitation. The results for the annual k-day precipitation depths are between those for the two seasons: no trend for small k because summer events dominate and a significant trend for k larger than 7 due to the winter events. Analysis of a few stations with long-term series shows no significant trend for the period 1910–1995, these series also reproduce almost the same trends as those found for the shorter 1951–1995 period. Received April 23, 1999 Revised December 6, 1999