A GIS -based analysis of catchment properties within a drumlin field

When considering water protection, the spatial distribution of catchment properties is relevant, since the areas adjoining the water body contribute more to water quality than the remote areas near the water divide. This study presents an approach to describe catchments spatially; two-dimensional descriptions allow calculations of horizontally distributed properties of catchments as a function of distance to the water body by using the concept of the typical water flow path. The study area, situated in a drumlin field in central Finland, included 14 third-order catchments that were further divided into 782 head-water catchments using a 25 m x 25 m digital elevation model. For each head-water catchment, a typical water flow path with slope, elevation, length, relative width, and soil type was calculated. The flow path geometry was diverging towards the water body and there was a 25 m section with a gentle slope adjacent to the water body followed by a steeper slope. Mineral soils dominated throughout the typical water flow path. On the basis of the results, we proposed characteristics for an average head-water catchment with the following properties: area 109.1 ha and length of the water flow path 954 m with maximum elevation 11.5 m. The peatlands were concentrated within a distance of 50 m to the water body; the rest consisted of mineral soil. The results can be utilized, for example, in calculating the impacts of forest management practices on the quality of runoff water. The results offer a baseline for evaluating how typical previously studied catchments are in relation to larger catchment populations. They also enable a regional estimate of the distribution of the relative proportion of areas situated along the water flow path.