Impact of water sampling frequency on estimating water quality status in the Ondava River

The influence of the number of the water samples on classifying surface water into water quality classes with regard to nitrate was determined. The characteristic values c90 (percentile 90%) of the nitrate nitrogen concentrations were computed according to following three ways:–from the monthly time series of nitrate nitrogen concentrations monitored in Ondava River by Slovak Hydrometeorogical Institute (SHMI) at basic network sampling site under Stropkov (period 1987–1991);–from the daily time series of nitrate nitrogen concentrations monitored by Institute of Hydrology of Slovak Academy of Science (IH SAS) at Ondava: Stropkov sampling site;–from the theoretical log-normal distribution estimated from the daily time series.