National water management and the value of nature.

In order to attune its water management to the demands of nature, the Dutch Government uses the ecohydrological DEMNAT for the analysis of scenarios. The input to this model consists of changes in hydrological variables that may be computed with present hydrological models. The output consists of changes in the botanical quality of various ecosystem types, and of resulting values for nature conservation. The applied ecosystem types are defined on the basis of abiotic factors that determine the plant species composition of a vegetation. In the Netherlands these are: salinity, moisture regime, nutrient availability, acidity. Water management measures may cause changes in these four factors and, as a result, change the species composition of a vegetation.Species of the Dutch flora are allotted to the ecosystem types. It is questioned whether the resulting ecological groups are appropriate for describing the plant cover of the Netherlands with the aid of FLORBASE. This national database contains data, per kilometer square, on the presence of indigenous plant species. To answer the question, a comparison is made with a division of species into phytosociological groups by the Dutch standard work of Westhoff & Den Held (1969). On the basis of a correlation analysis, it is concluded that the ecological groups are of better use for the analysis of the national plant cover than the phytosociological groups.The ecological species groups are used in combination with FLORBASE to make nation-wide maps of ecosystem types. On the basis of both the number and the indicative value of diagnostic species, the botanical quality of each kilometer square is assessed for each ecosystem map. The boundaries of the quality classes are obtained by expert judgement. It is possible, however, to compute class boundaries with a mathematical procedure, also for grid cells larger than 1 km <sup>2</SUP>. The maps are corrected for regional differences in the intensity of the plant inventories.It is argued that the conservation value of classification units - such as species and ecosystem types - is especially related to the criterion 'rarity'. The measuring of rarity is discussed and a mathematical formula is presented for the valuation of classification units. Eight methods for the botanical valuation of areas are compared. Those methods which are based on species richness and species conservation values yield results that experts find unsatisfactory. The most favored method, however, is based on the quality classes of the ecosystem maps in combination with the conservation values of the ecosystem types