Implications of processing spatial data from a forested catchment for a hillslope hydrological model

Finland has committed to both increasing timber production and decreasing the nutrient loading caused by forestry, which calls for development of methods to assess environmental impacts of forest management. A simulation model based on the concept of a typical hillslope is applied to describe water and nitrogen processes in a forested catchment. Application of the model requires that spatially distributed catchment data are processed to create parameterisation for a vertical two-dimensional profile. In such a two dimensional catchment description, behaviour of the system at different distances to a stream can be considered. This study explores 1) how changing the location of a clear-cut area is reflected in model results, and 2) how the inevitable simplifications when representing a catchment as a single hillslope may affect the model outcome. The results suggest that description of the catchment with a single two-dimensional profile is a reasonable approximation as long as areas having a high fraction of subsurface runoff (andgt; 60-70%) are not combined with areas where the surface runoff component is dominant. At low hydraulic conductivities the nitrate load was strongly controlled by the distance from the cut area to the stream, and the load increased almost linearly with the inverse of the distance. But when the conductivity value became sufficiently large, the effect of the cutting location became smaller, and the relationship to the inverse of the distance was obscured by snowmelt timing differences in open and forested environments.