Spatial distribution and trend change of nitrate in Slovakia during 1968-1993

Abstract

The nitrate concentrations trends were evaluated on the basis of trend analysis in five sub-basins of the Ondava river basin for 25 - years time series 1968/69 - 1992193. A rapid decrease of nitrate concentrations in surface waters was observed in four of them after 1989. The decrease resulted mainly from the lower intensity of agricultural production and fertilization in Slovakia due to economic changes. The application of nitrogen fertilizers decreased from 91 kg per ha of agncultural land (mean for Slovakia) in 1989 to 62 kg per ha in 1991. Therefore, the decrease of nitrate concentrations observed in research basins is expected to occur in other agricultural basins as well. Due to continuing changes in agriculture it is difficult to estimate the trends for next few years but probably the decreasing trend will tum again. In the second part of the paper statistical analysis of measured daily nitrate concentrations was done. The coefficients of theoretical log-normal curves of non-exceedance of the nitrate concentrations were derived in the studied sub-catchments. Both mean annual and characteristic values of nitrate concentrations derived from daily samples of were compared to those estimated from regular monthly samplings of hydrometeorological network. Important differences in the estimatesof the characteristic values were found between both approaches.