This thesis developed a decision support system for implementation of agricultural measures for groundwater protection against pollution by nitrates. The system consists of 3 phases: analysis of the existing situation, evaluation of the effectiveness of measures, and optimization and placement of measures. Influence of different sources of input (measured or calculated soil hydraulic properties data) and calibration data (river discharge, amount of water in the soil, a combination of both) on the functioning of 12 versions of the SWAT model was studied. We found that not all model calibration methods are suitable for all situations. Only in some cases differences in the model results were confirmed. We conclude that the differences are not so large as to clearly indicate the best option, so it may be more prudent to use data that are easier to access or otherwise more appropriate in a given situation. The proposed decision support system was tested in vulnerable areas of the Drava and Krško basins by evaluation of the environmental and economic impact of different agricultural practices; the first by simulating nitrate leaching and the second by calculating the contribution margins. By simulating the expected agricultural practices, we determined which parts are more vulnerable and where it would make more sense to introduce stricter measures. By comparing the effects of a wider range of alternative agricultural practices, we determined which ones are most suitable for individual soil types in the area. It turned out that universally effective practices do not exist: some are more effective in some, others in other cases. Effective agricultural practices were allocated in both areas according to three optimization criteria: environmental, economic and combined. We found that all three, even the economic one, which maximizes the economic impact of agriculture, improved the nitrate leaching situation in the whole area, so we conclude that using the proposed system and placing measures based on both their environmental and economic effectiveness in the area can reduce nitrate leaching from agricultural land and increase the economy of agricultural production.
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