Estimating the necessary duration of observation period in studying winter river runoff

A procedure for determining the minimal representative observation period at hydrological gages is proposed as required for studying the regularities in river runoff formation in rivers in the drainage basin of the Votkinsk Reservoir. Data of hydrological gages with long observational series were used as an example to show, by the division of these series into intervals and analysis of the obtained deviations from long-term mean values, that such minimal period can be taken to be 40 years long.