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We introduce a simple method for estimating the changes of nitrate concentrations in surface waters regarding the land use modification. Stream and drainage water nitrate concentrations in prevailingly agricultural catchments of three different scales located in the Crystalline complex of the Czech massif, Czech Republic, were included in this study. Water quality samples were collected through the years 1992-2006 at monthly and bi-monthly intervals. For the catchment land use analysis, the satellite images LANDSAT 7 (CORINE Land Cover) and digitised cadastre maps of the land register were processed using ESRI ArcMap GIS; both sources were corrected by field survey. We demonstrate on three different basin - scale studies a strong relation between the arable land ratio within a catchment and the coherent stream water nitrate concentration. The results acquired from all the evaluated catchments showed that every 10% decrease of ploughed land proportion in a catchment lowers the nitrate concentration C90 value (90% probability of non-exceedance) in average by 6.38 mg/L.
