Summary Report No. 125
Historisch vereinbarte minimale mittlere Monatsabflüsse der Elbe im tschechisch-deutschen Grenzprofil bei Hřensko/Schöna – Eine Analyse der Niedrigwasseraufhöhung im Grenzprofil infolge des Talsperrenbaus im tschechischen Einzugsgebiet der Elbe
M. Simon, J. Böhme (März 2012)
Historical agreed minimum discharges of river Elbe at the czech-german border profile Hřensko/Schöna
Discharges of the river Elbe at the border profile between the Czech Republic and Germany at Hřensko/ Schöna play an essential role for the suitability of the German river section downstream for shipping. A cascade of reservoirs upstream the border profile was mainly built between 1950 and 1964. Since then a significant supplemental low water management at the boarder profile is possible. Although the low wa¬ter control at the German-Czech boarder profile is currently managed by a network of German and Czech institutions the legal basis of this management regime is currently not fully understood. In particular, the question raised weather current management targets for low water supplement can be seen based on still valid German-Czech agreements between the former states German Democratic Republic and Czech Slovakian Socialistic Republic.
This report summarizes the chronological follow up of the construction of the Czech reservoir system and the related agreements between the German and Czech site.The authors have assembled the development of the effectively agreed minimum discharges at the border profile ČSSR/GDR at Hřensko/Schöna. The available joint coordination statements between ČSSR and GDR during the period April 1983 and May 1988 were reviewed and compared to the recorded discharges.
The capacity for low water supplements at Hřensko/Schöna increased since 1900 from 143,58 Mil. m³ to 2 566,24 Mil. m³ in 2010. The greatest parts of the total storage capacity are located in the Vltava and in the Ohře river basin with storage capacities of 1 894,03 Mil. m³ and 404,35 Mil. m³, respectively.
The Vltava cascade is currently managed in a way that a minimum discharge is guaranteed of about 40,0 m³/s at barrage Vrané. This compares to a natural discharge situation with minimum flows between 12,0 to 15,0 m³/s. The barrage system in the Ohře river system can currently ensure minimum discharges of about 8,00 m³/s compared with natural discharges between 1,50 and 2,00 m³/s. The boarder profile at Hřensko/ Schöna is directly affected by the reservoir control in Vltava and Ohře.
The GDR made several efforts to agree with the ČSSR on minimum discharge levels for the river Elbe at the border profile at Hřensko/Schöna. After years of negotiations minimum monthly medial discharges (Min MQ month) were defined for a hydrological year starting from November until October. However, these monthly medial values still allowed for shortfalls at the daily scale.
Based on the monthly medial values two coordination arrangements were concluded between the ČSSR and the GDR about the supply of minimum discharge at their joint border profile at Hřensko/Schöna. The first started in April, 1983 and was valid till 1990. The agreement was updated and extended in May, 1988 till 2000. The lastly agreed monthly minimum values range between 86,8 m³/s in August and 185,2 m³/s in April.
Although an agreement about these values exists, it never had a legally binding character. However, these values are still used as orientation for the control of Czech reservoirs in practise. In the past, the agreed minimum discharges felled only once below the limit. In July 1964, the observed monthly discharge was 89,1 m³/s which is 7.1 m³/s short of the agreed value of 96,2 m³/s.
Climate change might lead to a more frequent short fall of the once agreed minimum values. Considering this fact and the informal nature of the current target values for control a new formal framework for the minimum flow regime at the German-Czech boarder might be in the interest of the German water users downstream.