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Size of RBDs

The size of the RBDs varies considerably from very small ones below 1,000 km² to the largest one, the Danube with over 800 000 km² (Table 2.2). Obviously, the international RBDs are generally larger. The average size of current reported (national) RBDs is about 30 000 km². There are 42 and 29 RBDs with an area greater than 15 000 and 50 000 km², respective. These two size categories cover 25 % and 65 % of the reported area.  More than half of the population are found in the RBDs larger than 50 000 km².