Process-oriented Investigation of Snow Accumulation, Snowmelt and Runoff Generation in Forested Sites in Finland

This thesis summarises development and application of a hydrological model for simulating forest canopy processes, snow accumulation and melt, soil and ground water interactions, and streamflow routing. A motivation behind the model development is to outline a methodology for predicting the influence of land use changes on catchment hydrological processes. In addition, the development aims at providing linkages from the hydrological model to atmospheric models through implementation of surface energy balance and to water quality models through quantification of runoff components. The work started with comparison of two existing snow energy balance models using meteorological and snow data from Northern Finland. Based on the comparison the more simple of the tested snow parameterisations was modified to improve its performance in terms of snow heat balance simulation. The modified snow model was then coupled with a canopy scheme to account for the influence of forest on snow processes. The combined model was applied to clear-cut and coniferous forest sites in Southern Finland to identify the differences in snow mass and energy fluxes between open and forest. Finally, runoff generation in a forested catchment (Rudbäck, 0.18 km) was studied by using two different parameterisations. First, the catchment was parameterised as a three-dimensional domain, and secondly, as a vertical two-dimensional hillslope. The models produced similar results in terms of fit against measured daily streamflow, but the computed runoff components were different. Independent calibration of hydrological submodels yielded a more realistic partition of runoff into surface and subsurface components than did calibration merely against streamflow data. It is proposed that the hillslope model can be used to simulate runoff generation in each possibly non-contiguous area that is similar in terms of its land-use. A system where a set of such models is combined together can be used to quantify runoff contributions from pre-classified areas of different land-use, and constitutes a tool for studying hydrological impacts of land use changes.