Abstract
Abstract. This study aims to understand how different reference periods (i.e., calibration periods) of climate data used to estimate drought indices influence regional drought assessments. Specifically, we investigate the influences of different reference periods on historical drought characteristics, such as the trend, frequency, intensity and spatial extent, using the standardized precipitation evapotranspiration index (SPEI) with a 12-month lag (SPEI-12), which was 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 to compute the SPEI: P1 and P2 together, P1 and P2 separately and P1 only. Focusing on East Asia, Europe, the United States and West Africa, we find that the influence of the reference period is significant in East Asia and West Africa, with dominant drying trends from P1 to P2. The reference period influenced the assessment of drought characteristics, particularly the severity and spatial extent, whereas the influence on the frequency was relatively small. Finally, self-calibration, which is the most common practice for indices such as the 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 by the use of two global datasets, they highlight the need for clarification of the reference period in drought assessments to better understand regional drought characteristics and the associated temporal changes, particularly under climate change scenarios.
Highlights
Drought is a complex, slow onset and natural phenomenon that affects more people than any other hazard and seriously influences water resources, agriculture, society and ecosystems (Hagman, 1984; Wilhite, 2002; Ionita et al, 2015)
We investigate the influences of different reference periods on historical drought characteristics, such as the trend, frequency, intensity and spatial extent, with the Standardized Precipitation Evapotranspiration Index (SPEI) estimated using two historical global climate datasets from the Climate Research Unit (CRU) and the University of Delaware (UDEL)
We focused on the differences in meteorological conditions between periods of 1901–1957 (P1) and P2 to enhance our understanding of similar or different drought index values according to the different reference periods
Summary
Slow onset and natural phenomenon that affects more people than any other hazard and seriously influences water resources, agriculture, society and ecosystems (Hagman, 1984; Wilhite, 2002; Ionita et al, 2015). Because drought impacts are largely nonstructural and spread over relatively large regions, the onset and end of a drought, as well as its severity, are often difficult to determine (Wilhite, 2002). Based on recent changes in the 21st century and projected climate warming, such drought phenomena will likely worsen (Sheffield and Wood, 2008; Dai, 2011a). Sheffield et al (2012) stated that severe and prolonged drought events have been observed since the 1970s, and these changes are related to higher temperature and lower precipitation. Drought can be defined and explained using absolute or relative terminology, which allows terms and measures to be compared (Dai, 2011b; Trenberth et al, 2014). The relative measures include the Palmer drought severity index (PDSI), the standardized precipitation index (SPI), the Standardized Precipitation Evapotranspiration Index (SPEI) and others. Various drought indices have been widely used in many drought studies
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