Annual low flows generated by mixed processes

In areas where the annual low flow series is generated by more than one distinct hydrological process, a compounded Gumbel distribution can provide a satisfactory fit to the empirical data. The annual low flows of rivers in Northern Ontario are the result of storage depletion following either long periods of below freezing temperatures at the end of winter or early spring (spring low flows), or high evaporation in late summer and early fall (fall low flows). Use of the compounded Gumbel distribution adequately represents the return periods of low flows for the rivers studied. Parameters estimated for the distribution show noticeable spatial trends both in terms of the seasonal mean and the standard deviation of the low flows.