The weather has a great effect on the water balance and, indirectly, affects water quality of river systems. At the same tirne, man-made changes in the Iandscape and other human activities have a great impact. To be able to distinguish the human irnpacts from the effects of natura! weather fluctuations we need observations and measurements but also analysis tools.

In this thesis the PULSE and HBV hydrological models have been used as the analysis tools. Examples are given from forest management, in particular clearcuning, drainage and biomass increase, and from mining and agricultural activities. The models include conceptual descriptions of the most significant hydrological processes and are capable of coping with. weather-dependent fluctuations. Observed air temperature, precipitation and an estimate of the potential evapotranspiration are input data to the models.

Simple hydrochemical and nitrogen leaching subroutines have been Iinked to the PULSE water balance model.These subroutines have been used to quantify weather-dependent and human effects on pH downstream from a mine tailings deposit and on nitrogen Ieaching from different non-point sources, especially from arable land.

The applications illustrate the advantage of this type of model for analysis of man-made irnpacts and short-term climatological fluctuations. As the models are restricted to stationary conditions they cannot be used for forecasting of long-term changes due to changes in atmospheric deposition, land use or climate, unless the local effects of these changes are known. Other methods of analysing effects of man-made changes have also been tested, such as conventional comparative investigations, regression analysis and trend analysis. The use of these methods is exemplified by an analysis of human effects on erosion and sediment transport. It was found to be much more difficult to quantify effects with these sirnpler methods