Sanders, Marlies 
built around reed fens and the reed fens were irrigated with wind pumps. Base-rich and nutrient-rich surface water is let in 
to maintain the water level necessary for water retention. This water causes fens to become eutrophied, which leads to an 
increased biomass production. When parts of fens become isolated from surface water (hydrological isolation), it acidifies 
due to the dominance of acidic and oligotrophic rainwater. 
The usual method to quantify the influence of land use and hydrology on biodiversity is fieldwork, but this is time 
consuming and thus very expensive, especially in large, inaccessible areas like wetlands. The objective of this study was to 
identify the possibilities of remote sensing and GIS to model hydrological isolation. The intend was to provide land use 
management with spatial quantitative information to support sustainable use of fens in The Netherlands. 
Using GIS and remote sensing the study attempted to accomplish the following tasks: 
e modelling hydrological isolation in GIS 
e using remote sensing to obtain input for the hydrological model 
e applying the model to assess plant species habitat. 
2 METHOD 
2.4 Hydrological isolation 
A gradient of surface water influence determines plant species distribution. The surface water network is the source of 
base-rich water. A lateral influx of surface water compensates water loss to the surrounding polders. Water supply (and 
hence, base supply) decreases as the hydraulic resistance between any site and the supply source increases or the driving 
force decreases. The driving force is the hydrostatic pressure maintained by water loss and thus proportional to the 
hinterland area. The hydraulic resistance is inversely proportional to the permeability. The volume of water that actually 
flows to a site, depends on the hydraulic resistance and the driving force, as described by Darcy's law. In this model it is 
assumed that in an average year the water table does not change and there is no upwards seepage (Van Wirdum, 1991). 
Another assumption is that the water loss of a site is caused mostly by water that flows to other sites (hinterland) which 
makes precipitation, evapotranspiration and infiltration at that site negligible, because of its small area compared to the 
hinterland area. This implies that the amount of water that could potentially flow to a site (Darcy's law), is equal to the 
amount of water loss of the hinterland (equation 1). 
H*(E+D-P) = k*A*dh/dL (1) 
H = hinterland area (m?) A = cross section (m?) 
E = evapotranspiration (m/day) dh = water level rise (m) 
D = infiltration or discharge (m/day) dL = length (m) 
P = precipitation (m/day) k = permeability (m/day) 
dh/dL = driving force 
All site factors of equation 1 can be derived from maps or reports, enabling dh to be calculated. Rearranging equation 1 
gives: 
dh=H*(E+D-P)*dL/(A*k) Q) 
According to Vegt (1978), the degree of the hydrological isolation depends on the magnitude of water level fluctuations. 
The magnitude of the water level fluctuations was also considered the best indicator for hydrological isolation (Hi) in the 
framework of this study. When the water level is within reach of plant roots it is assumed to supply bases. The water level 
of a site can be calculated by accumulation of the water level rise (dh). For every site in the entire reserve, the Hi value can 
be calculated in a GIS by accumulation of dh values of sites forming the shortest route to the water source. In formula, the 
Hi value of site y is: 
  
1310 International Archives of Photogrammetry and Remote Sensing. Vol. XXXIII, Part B7. Amsterdam 2000. 
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