AbstractThe data for solar radiation (S), air temperature (Ta), surface water temperature (Tw) and pH recorded at 0.5 hr intervals at Slapy Reservoir were subject to spectral analysis. The complete series covered the month of July 1978, where three characteristic sections were distinguished, of which one was characterized by low variations, more irregular values and a slowly decreasing trend and another with high and regular variations as well as a steeply increasing trend. These sections were analysed separately in addition to the complete series. The period of 24 hrs was most important and represented forS,Taand pH 60, 38 and 18 % of the total variances, respectively. The deterministic periodic component represented 65–95 % of the total variance. The cross‐correlation and cross‐covariance between selected pairs of variables suggested lags of up to 4.5 hrs, the longest lag being observed forTwandS. An autoregressive model of the interrelation between the signals was formu