- 3.9.1. Surface water types and reference conditions in the Danube River Basin District

The 64 of surface water typologies and the definition of their (near-)natural reference conditions is a new approach, which requires a detailed scientific analysis of the geographical and physicochemical conditions of surface waters. In addition, it requires a validation with biological data for it to be biologically meaningful as stipulated by the Directive. This is a difficult task and requires iterative steps to arrive at a sound and practically useful system that serves as the basis for the ecological status assessment. The 64 of sound surface water typologies takes several years and the upstream countries having started earlier have provided guidance for the middle and lower Danube countries and shared their experiences. Therefore, similar approaches have been taken in Danube countries, but there has not been a unified or harmonised approach in the 64 of the national typologies. (4.9.1. Surface water types and reference conditions in the Danube River Basin District - p.141)