Quantification of precipitation deficits on different time scales in Sierra Leone using standard precipitation index

In this study, precipitation deficits on different time scales have been quantified using the standard precipitation index (SPI) to understand drought patterns in Sierra Leone. Monthly precipitation data spanning from 1960 to 2013 was downloaded from the Climatic Research Unit (CRU) Time-Series (TS) gridded precipitation data (CRU TS4.01) of horizontal resolution (0.5 × 0.5 degrees) grids of month-by-month precipitation developed by CRU at the University of East Anglia was used. This study revealed that annual rainfall has decreased, which indicates a potential drought risk in the country. Variation in drought frequency on different time scales was high, with drought intensity changing from mild to extreme. The frequencies of meteorological drought were higher in districts with high rainfall events and lower in districts with low rainfall events. Severe and mild to moderate agricultural drought frequencies were higher in southern and eastern regions respectively, while the frequencies of mild and moderate hydrological droughts were high in all districts. On the contrary, severe and extreme hydrological droughts were low in some districts. On average, 14% of the country was affected by meteorological, 26.8% by agricultural, and 26% by hydrological droughts. The intensity of drought was observed to increase with timescale. Moderate and extreme agricultural droughts were observed in the northern and southwestern regions, respectively, which confirmed the high impacts of a dry spell on crop production in these regions. Hydrological drought was moderate to extreme in the southern, high intensity in the northern and western, and normal in the eastern parts. The mean return period of drought decreased with increasing SPI time scales. The study revealed that SPI is a powerful drought signal for understanding notable drought patterns and building mitigating mechanisms for future drought occurrences in Sierra Leone.