Critical period of reservoir systems for planning purposes

Being able to predict the duration of the critical period (CP) and hence the precise mode of behaviour of a reservoir system prior to analysis is advantageous because then the input data interval can be selected to match the requirement. In this sense, purely over-year systems (CP»12 months) can be adequately analyzed using annual time-series data, whereas for purely within-year systems (CP«12 months), only the `critical' 12 months in the data record need be considered. These are the two extremes; in reality most reservoirs exhibit a mixture of both behaviours and to capture these, time series data at monthly (or shorter) time scale are required. In this paper, we present results of Monte Carlo simulation experiments investigating the CP of within-year and over-year reservoir systems and its relationship with the currently used test for discriminating between the two patterns of reservoir behaviour. Based on data from two different climatic regions, the results show that the current test is incomplete without incorporating other characteristics of the reservoir storage-yield-reliability problem. A procedure for incorporating these other characteristics is demonstrated and suggestions are offered as to how the study could be extended.