Now, with the availability of sensors, loggers, giving a fairly accurate temperature data for different aquifers, there is the possibility of a more strict approach to the selection of subannual and intraseasonal periods in reservoirs. Concepts such as "the moment of establishing (or, conversely, destruction) stable stratification", "the beginning of the spring warm-up" or "beginning of autumn cooling" specify with numbers and formulas. Are there any specific criteria (from physics, thermodynamics or any science disciplines), of "sustainability", "violations", "constancy", "impermanence", etc.?