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TRANSBOUNDARY WATER MANAGEMENTwell-founded prognoses can only be made if the processes in the riversystem are understood. This, in turn, requires a thorough knowledge ofthe historical development of hydrological parameters. Therefore, CHRhas undertaken a detailed analysis of changes in the run-off regimes ofthe Rhine and its tributaries in the twentieth century and their potentialcauses: natural climate fluctuations, anthropogenic climate change anddirect human interventions such as river regulations and embankments,barrage weirs, reservoirs, water transfers and changes in land use.Run-off regimeThe flow of a watercourse, whether it is a small brook or large river,strongly depends on the amount of precipitation in the catchment.The water volume that is not lost to evaporation or plant transpirationeventually runs off; fluctuations in run-off are controlled by thetemporal distribution of precipitation and evaporation. If precipitationfalls as snow, it is released with a time delay when melting orstored as ice in glaciers for even longer periods of time. In a nutshell,‘run-off regime’ means the intra-annual run-off of a stream that canbe regularly expected.The major tributaries in Germany (Neckar, Main and Moselle)consistently show a pluvial regime. Due to the distribution of rainfalland the seasonal differences in evaporation intensity, meanrun-off reaches its maximum in the winter months and its minimumin August and September. As the river proceeds, the run-off regimeof the Rhine reflects the natural and man-made impacts resultingfrom its gradual catchment expansion. In the process, none of thejoining tributaries succeeds in imposing its own regime characteron it – but the plethora of feeding rivers downstream increases thecomplexity of the run-off regime in the Rhine.Owing to the dominant alpine influence, the Basel Rhine gaugeshows a typical nival regime that is superimposed by tributaries with apluvial character further downstream. As these tributaries bear significantlyless water than the Rhine, the basic nival character of the Rhineregime continues to exist up to the confluence with the Main river.Not until reaching the Middle Rhine do the rivers Main and Moselleeventually cause major changes. From the Andernach gauging stationRestoration of gravel banks of the Thur River, a tributary of the Rhine in SwitzerlandImage: CHRonwards, pluvial elements prevail. Now the highestrun-off occurs in February, the lowest in September.Past achievementsThe Rhine Alarm model was developed with ICPR. Itwas initiated by the Rhine Ministers conference afterthe Sandoz accident in 1986 and accepted duringthe 8th Conference of Rhine Ministers. Participatinginstitutes are the Federal Office for the Environmentin Bern (Switzerland), the Service de la Navigation inStrasbourg (France), the Federal Institute of Hydrology(BfG)in Koblenz, the Albert-Ludwigs-University ofFreiburg in Germany and Dutch organizations such asRijkswaterstaat in Lelystad and the Technical Universityand Deltares in Delft.The Rhine Alarm model delivers effective forecastsat various alarm stations in cases of strong water pollutionin the catchment. Forecasts of the travel time anddistribution of harmful substances are highly importantfor all water users such as water boards and watersupply companies, in order to implement the necessarymeasures in time.The model covers the Rhine River from LakeConstance to the North Sea, including the main tributariessuch as the Aar, Neckar, Main and Moselle. Inputto the model calculations includes the location andconditions of the initial pollution, decomposition anddrift capacity of the harmful substances, discharges and/or water levels, geometry and dispersion. Calibration ofthe model was done by tracer tests. The model calculates,the concentration as a function of time as well asthe point of time and scope of the maximum concentrationfor every alarm station along the Rhine. Theforecasts of progress time and concentrations are accurateto about 89 per cent and 95 per cent respectively.The knowledge of the Rhine Alarm model was incorporatedinto the set-up of the model for the DanubeRiver. For the Danube model a cross-flow of the pollutionmodule has been implemented which is alsoincluded in the latest version of the Rhine Alarm model.So, what has started with a catastrophe in the past hasresulted in a well-implemented alarm model whichhelps ensure better, timely reaction in urgent situations.Knowledge exchange with other river basins (SouthAmerica to the Rio Bermejo) in Argentina took placethrough a twinning project, with a site-mission and asymposium held in 2007. 3 CHR’s counterpart was theBinational Commission for the Development of theBermejo River Basin (COBINABE) in Argentina. Duringthe field visit the sediment problems in the upper part andmiddle reach of the Bermejo river basin were addressed.The basin of the Rio Bermejo has various types ofclimates, with precipitation ranging from 200 mm/yearto over 2,000 mm/year within a few dozen kilometres.Population density is quite low in terms of centralEuropean countries. Main traffic routes are constructedalong the river stretches and are prone to natural hazardactivities. Soil loss and adequate land use is a majorproblem for local populations in the river basin. Traffic[ 58 ]