Application of SWAT model to small agricultural catchment in Poland
Poland is obliged, like the other EU countries, to implement the Water Framework Directive - WFD (2000/60/WE) by the end of 2015. The main objective of WFD is to provide normative quality of all water resources. To reach this goal reduction of water polluter emission to the environment is needed. Our project focuses on pollution from agricultural sources which share in global pollution is high and growing still. As a pilot area, where the WFD is going to be implemented, small agricultural Zgłowiączka catchment was chosen.
The state monitoring of surface water quality for the catchment is conducted in three points along the Zgłowiaczka River. In each of these three points, nitrates concentration periodically significantly exceeds the allowable value of 50 mg NO3·dm-3. The highest average monthly values of nitrates concentration in years 1990-2007 occur in February, March and April, which indicates on agriculture as a source of pollution. The Zgłowiaczka catchment is an area where reduction of nitrogen run-off from agricultural lands to water resources is especially needed. The main topic of the research carried out in the Polish-Norwegian project is to propose different means for reduction of migration of nitrate to surface water based on modeling approach. In the paper a conception of creating buffer zones using SWAT model is presented. We considered fitting the buffer zone width, depending on the flow rate of water flowing from the fields to the stream. Using SWAT model interface a map of potential flow under the conditions of the intensive precipitation was generated. The next step was distribution over the whole Zgłowiączka catchment, places with high density of the temporal streams network. It was done using GRASS program. The map of stream "density" was done by assigning the raster number which is the sum of raster in the neighbourhood (radius of neighbourhood smaller or equal 25 raster). The choice of the most endangered subbasins was done on base of visual evaluation of the surface flow density map.
It is visible in the results that filter strips on endangered areas are far more effective and therefore more required. If the width of the vegetated buffer strips is not sufficient, it will not attain the desired effectiveness. Conversely, if the width is too great, it will cause agricultural land waste, preventing farmers' interest in cooperating with environmental preservation efforts. For the above reasons, it is important to set a reasonable width range. According to the results we are suggesting wider buffer zones in endangered subbasins and narrow in other subbasins.
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