Propagation from meteorological to hydrological drought in the Horn of Africa using both standardized and threshold-based indices

Abstract. There have been numerous drought propagation studies in data-rich countries, but not much has been done for data-poor regions (e.g., the Horn of Africa [HOA]). This study characterizes meteorological, soil moisture, and hydrological droughts and the propagation from one to the other for 318 catchments in the HOA to improve the understanding of the spatial variability of the drought hazard. We calculate the Standardized Precipitation Index [SPI], Standardized Soil Moisture Index [SSMI], and Standardized Streamflow Index [SSI]. Additionally, we use the variable threshold method to calculate drought duration below a predefined percentile threshold for precipitation, soil moisture, and discharge. The relationship between meteorological, soil moisture, and hydrological drought is examined by finding the SPI accumulation period that shows the highest correlation between SPI and SSMI and SSI timeseries, and by calculating the ratio between the threshold-based precipitation drought duration and soil moisture drought duration, respectively streamflow drought duration. Finally, we investigate the influence of climate and catchment characteristics on these propagation metrics. Results indicate that (1) the propagation from SPI to SSMI and precipitation to soil moisture [P/SM] mean duration ratio are mainly influenced by soil properties and vegetation, with the short accumulation periods (1–4 months) of SPI found in catchments with cropland, high mean annual precipitation, and low sand and silt content, while longer accumulations (5–7 months) are found in catchments with low upstream mean annual precipitation, and shrub vegetation; (2) the propagation from SPI to SSI and precipitation to streamflow duration ratio are highly influenced by the climate and catchment control, i.e., geology, elevation and land cover, with the short accumulation times in catchments with high annual precipitation, volcanic permeable geology, and cropland, and the longer accumulations in catchments with low annual precipitation, sedimentary rocks and shrubland; and (3) the influence of upstream mean annual precipitation is more important for the propagation from SPI to SSI than from SPI to SSMI. Additionally, precipitation accumulation periods of approximately 1–4 months in wet western areas of HOA, and of approximately 5–7 months in the more dryland regions are found, which is useful information for management because of their more direct relation to impacts.