Modeling tools are needed to assess (i) the amounts of loading from agricultural sources to water bodies as well as (ii) the alternative management options in varying climatic conditions. These days, the implementation of Water Framework Directive (WFD) has put totally new requirements also for modeling approaches. The physically based models are commonly not operational and thus the usability of these models is restricted for a few selected catchments. But the rewarding feature of these process-based models is an option to study the effect of protection measures on a catchment scale and, up to a certain point, a possibility to upscale the results. In this study, the parameterization of the SWAT model was developed in terms of discharge dynamics and nutrient loads, and a sensitivity analysis regarding discharge and sediment concentration was made. The SWAT modeling exercise was carried out for a 2nd order catchment (Yläneenjoki, 233 km2) of the Eurajoki river basin in southwestern Finland. The Yläneenjoki catchment has been intensively monitored during the last 14 years. Hence, there was enough background information available for both parameter setup and calibration. In addition to load estimates, SWAT also offers possibility to assess the effects of various agricultural management actions like fertilization, tillage practices, choice of cultivated plants, buffer strips, sedimentation ponds and constructed wetlands (CWs) on loading. Moreover, information on local agricultural practices and the implemented and planned protective measures were readily available thanks to aware farmers and active authorities. Here, we studied how CWs can reduce the nutrient load at the outlet of the Yläneenjoki river basin. The results suggested that sensitivity analysis and autocalibration tools incorporated in the model are useful by pointing out the most influential parameters, and that flow dynamics and annual loading values can be modeled with reasonable accuracy with SWAT. Sensitivity analysis thus showed the parameters which should be known better in order to result in more realistic parameter values. Moreover, the scenario runs for CWs made with SWAT revealed the high demand of land area for this protective measure to be substantially effective.;
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