VIHMA—A tool for allocation of measures to control erosion and nutrient loading from Finnish agricultural catchments

The empirically based VIHMA model is intended for evaluation of agricultural loading and mitigation measures at the catchment scale. As a starting point for the model construction, results of relevant Finnish long-term field experiments were collected as the database of the model. From the database, specific loading values for different cultivation and tillage methods were calculated using the values of respective tillage methods and conditions (slope, soil texture, hydrological year type) and – in cases of missing empirical data – interpolation and expert judgment. Moreover, based on the data collected in studies carried out at Finnish constructed wetlands (CWs) and at experimental fields with buffer zones (BZs), the model is able to evaluate the environmental benefits achieved by these water protection measures at the catchment scale. The model points out the origin of loading as well as the needs of targeting the protective measures and their load-reducing effects. The use of VIHMA is bound to the distribution of actual information of field cultivation in the target area. Hence, calculations based on changed tillage practices can be assessed only within the limits determined by the lines of production in local farms. On the other hand, the need to decrease the loading is determined by the target state of the recipient body of water. By changing the input data, the potential change in loading, e.g. by increasing the number of BZs or CWs, can be simulated by VIHMA. Influences of different hydrological years with different flow regimes due to different meteorological conditions are taken into account by the choice of the respective empirically-based specific loadings in the model data. The results calculated by VIHMA were found to be well in line with the reported results from long-term catchment monitoring. The model was able to reliably calculate the influences of mitigation measures in large scale in spite of high variation due to different hydrological years.