Statistical tests for drought tendency investigation and their sensitivity

Drought is a recurrent feature of the climate, its impacts can be especially serious in economy. There is no general definition of drought, as it is a water deficiency relative to the climatological expected value. Therefore, several methods have been developed for quantifying droughts. One commonly applied way is the calculation of drought indices. Widely accepted meteorological measures of drought include Palmer's Drought Severity Index and the recently developed Standardized Precipitation Index. Long time series of these indices have been calculated for several stations in Hungary, which reveal changing patterns of drought occurrence. Two statistical tests have been applied for these time series, both requires a threshold value assigned to the occurrence of the event in question. One test is a new tool for detecting statistically significant changes in the lengths of spells. The other test is able to detect changes in the frequency of occurence.

These tests both captured the changes resulting from the subsequent droughts during the 1980's and 1990's in Hungary. On the other hand, the last four years have been wet anomalous. The effect of these years on the overall characteristics of drought index time series gives a good possibility for studying the sensitivity of the two statistical tests.