Changes in exceptional hydrological and meteorological weekly event frequencies in Greece

The aim of the present study is, primarily, to compare the frequencies of exceptional hydrological and meteorological weekly events, employing the self-calibrated versions of Palmer’s drought indices (PDSI and PHDI, respectively), from 17 stations across Greece in the decade 1997–2006 with these of 1961–1990; on second level, to identify the trends and define the time lags between these two indices for the study period 1961–2006. The changes in the spatial distribution of exceptional weekly event frequencies between the most recent decade and the baseline period were similar for both drought indices. When 1997–2006 was compared with 1961–1990, the number of stations with a frequency of exceptionally dry weekly events >93rd percentile, increased by seven stations (41%) for PDSI and nine (53%) for PHDI, at the expense of exceptionally moist weekly spells. PDSI was found to lead PHDI by three to 20?weeks. If exceptional weekly events continue to be more frequent in the future, major implications for natural water resources are expected.