The results of statistic estimation of measurements done within surface waters monitoring programme were described. The biogens concentrations (the years 1994-1998) and the oxygen indicators (the years 1992-1996) in the measurement point on Odra river in Krajnik Dolny constituted the date base. The time changeability of these indicators was well described by Jog-normal and gamma distributions was shown. The normalised distributions were also given.The autocorrelation and the periodicity were examined. It was stated that the considered time sequences were characterised by the strong autocorrelation and the periodicity. Using the ARIMA process the modelling of measurement series was realised. After the selection of models, the programming of examined indicators changes was accomplished. It was shown that using ARIMA models valuable results can be obtained.

This paper presents the statistical modelling of the monitoring data on oxygen indicators in the Rega River, collected in years of 1992-97. In the first part, the six probability distributions are examined to fit the experimental data. The results indicate that the lognormal and Gamma distributions give an adequate description for all considered data series, as measured by P-P probability plots, and a Kolmogorov-Srnirnov goodness of fit test. In the second part, the detailed statistical analysis was performed based on lognormal distribution. This study showed that the approximation of monitoring data with the appropriate probability distribution could be very useful for evaluating of water quality.