Cumulative departures model for lake-water fluctuations

Lake levels are under the influence of climatologic changes, greenhouse effects, and antropogenic activities. Usual atmospheric and environmental conditions give rise to steady-state time series of hydrometeorologic variables within a lake catchment, in addition to lake-level fluctuations. Under these circumstances, the water balance equation is valid with the substitution of arithmetic averages of input, output, and changes in storage variables. However, unusual lake-level changes, including trends and shifts in means, invalidate the use of averages because the hydrometeorologic time series have unsteady-state conditions. A cumulative departures model of the water balance equation is proposed and applied to precipitation, runoff, evaporation, and lake-level records for the Lake Van catchment location in the eastern part of Turkey. Implementation of the model requires identification of steady-state, long-term periods and their end-times that correspond to the starting points of unsteady dynamic periods, during which there might be unexpected trends and shifts in the mean lake levels.