Estimation of Drought Severity on Independent and Dependent Hydrologic Series

The drought severity is the most important parameter for the design of water storage systems in order to alleviate the water shortages during drought periods. The largest drought severity, i.e. severity of a worst drought at a desired truncation level and for the desired return period can be estimated using the truncated normal distribution of the deficits in individual drought years, Poisson distribution of the number of drought spells over a period of T years and geometric distribution of drought duration. The analysis can be done for random or Markovian structure of drought variable (annual rainfall or runoff sequences) distributed normally or lognormally. The study indicated that the severity at a high truncation level is larger than that at a low truncation level over any desired return period T. Severity is also larger in the case of autocorrelated drought variable. A frequency formula for the largest drought severity can be formulated analogous to the flood frequency formula commonly found in the hydrologic texts. Copyright Kluwer Academic Publishers 1997