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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 2. 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.

The present study suggests that many soils in the Netherlands, in natural as well as in agricultural areas, may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. Preferential flow has wide-ranging significance for rapid transport of solutes, such as agrichemicals, towards the groundwater and surface water, making it essential to understand this phenomenon.The persistence and degree of water repellency was examined in topsoils of nature reserves and cultivated soils, using the water drop penetration time (WDPT) and alcohol percentage tests. The severity of water repellency measured on dried soil samples, the so-called "potential" water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. In some cases, however, the severity of potential water repellency was found to be sensitive to the initial moisture content of the soil and the temperature during drying. Measurement of the "actual" water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The "critical soil water content", below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies.Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm 3steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This thesis provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

This is a report of the hydrogeological excursion to Slovakia, held in the period from September 8 up to September 15, 1996. This report is a compilation of the work of the participating students, parts of the excursion guide and also information, provided by the Slovak excursion guides.

Verdrogingsproject gebied Staatsbosbeheer

Afvoer van het Renkums beekdal