Impacts of potential evapotranspiration on drought phenomena in different regions and climate zones

The present study is aimed at examining whether potential evapotranspiration (PET), which is important for drought assessment, influences a drought index (standardized precipitation evapotranspiration index; SPEI) for different regions and climate zones. The study regions were East Asia, Europe, the United States (US), and West Africa, and the climate zones considered were the arid, semiarid, subhumid, and humid zones. We examined the pattern of water deficits, spatial trend of the SPEI, area ratio of spatial extent, and temporal trend to provide an understanding of drought characteristics. Two datasets, Climate Research Unit (CRU) and National Centers for Environmental Prediction (NCEP), were used for assessing the drought phenomena. Two types of evapotranspiration obtained using Thornthwaite and Penman-Monteith equations were used to estimate the PET. Negative water deficit values were clearly observed in the arid and semiarid zones of the majority of regions, whereas positive water deficit values were observed in the subhumid and humid zones of the regions. The SPEI spatial trend largely presented a decreasing trend in East Asia and West Africa, a neutral or decreasing trend in Europe, and a neutral or increasing trend in the US. The area ratio of the spatial extent showed large values of a neutral or decreasing trend in East Asia and Europe, a neutral or increasing trend in the US, and a decreasing trend in West Africa. The temporal trend of the spatial extent primarily exhibited no trend or an increasing trend in the aforementioned regions, except in the case of the majority of West Africa. Although the results obtained from the two datasets appear to be slightly different, they show that the PET is predominant in regions, especially in the US. The PET trends are identified through comparisons and used to understand the drought phenomena while considering various geographic regions and climatic zones.