Evaluating The Meteorological Drought Characteristics Over Pakistan Using In Situ Observations And Reanalysis Products

Drought is one of the most frequent natural disasters occurring in Pakistan and has a great influence on livelihood, agriculture, and economy. The availability of long-term high-quality reanalysis products over Pakistan has been of great concern in recent decades. Here, we conduct drought assessment in Pakistan based on the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI) at 3, 6, and 12 months timescales during 1983-2018. We use long-term in situ observations to evaluate the accuracy of reanalysis products, including Climatic Research Unit (CRU TS), National Centers for Environmental Prediction version II (NCEP-2), European Centre for Medium-Range Weather Forecasts Version-5 (ERA-5), and Modern-Era Retrospective analysis for Research and Applications version II (MERRA-2). The main results are summarized as follows: (a) drought indices and drought areas assessed from reanalysis products are relatively more representative of historical droughts that had occurred in southern Pakistan and overestimation is evident for drought severity in western than eastern Pakistan; (b) statistically significant increasing trends (1984-1998 and 2000-2010) in monthly drought areas and occurrence are evident by CRU TS and MERRA-2 in dominant arid and semiarid regions; (c) climate variables and drought features of southern Pakistan are best represented by CRU TS and MERRA-2, while that of southwestern and western parts are best represented by ERA-5; (d) the Nash-Sutcliffe efficiency (NSE) results range from -2 to 1, where the NSE of SPEI values (-1.0) show relatively weaker than SPI values (0.5) in most parts of the regions, specifically in the southern Pakistan; (e) a strong positive linear relationship on a monthly scale is evident in CRU TS, MERRA-2, and ERA-5 exhibiting relatively high correlation coefficient (0.84), except for NCEP-2. Furthermore, the SPEI results are found to be better than SPI; thus, this study suggests SPEI may be more suitable than SPI in monitoring droughts under climate change.