Hydrology and Earth System Sciences | Um et al. 
This study aims to understand how different reference periods (i.e., calibration periods) of climate data for estimating the drought index influence regional drought assessments. Specifically, we investigate the influence of different reference periods on historical drought characteristics such as trends, frequency, intensity and spatial extents using the standard precipitation evapotranspiration index with a 12-month lag (SPEI-12) estimated from the datasets of the climate research unit (CRU) and the University of Delaware (UDEL). For the 1901–1957 (P1) and 1958–2014 (P2) estimation periods, three different types of reference periods are used: P1 and P2 together, P1 and P2 separately and P1 only. Focusing on East Asia, Europe, North America and West Africa, we find the influence of the reference periods to be significant in East Asia and West Africa, with dominant drying trends from P1 to P2. The reference periods influence the assessment of drought characteristics, particularly for severity and spatial extent, whereas their influence on the frequency is relatively small. Finally, self-calibration, which is the most common practice with an index such as SPEI, tends to underestimate the drought severity and spatial extent relative to the other approaches used in this study. Although the conclusions drawn in this study are limited to two global datasets, they nevertheless highlight the need for the reference period to be clarified in drought assessments to better understand regional drought characteristics and their temporal changes, particularly under climate change scenarios.
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