Spatiotemporal and multi-scalar assessment of meteorological and agricultural droughts in the Modjo River watershed, Upper Awash Basin, Ethiopia

Abstract

This study presents a comprehensive spatiotemporal and multi-scalar assessment of meteorological and agricultural droughts in the Modjo River watershed, Ethiopia. The analysis integrates 30 years (1993–2022) of hydro-meteorological data from the Ethiopian Meteorological Institute with 24 years (2001–2024) of MODIS-derived vegetation data. Meteorological drought was characterized using the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI) at 3-, 6-, and 12-month timescales, while agricultural drought was assessed using the Vegetation Condition Index (VCI). Prior to analysis, the meteorological records were subjected to quality control procedures, including screening for missing values, outlier detection, and consistency checks to ensure data reliability. Non-parametric Mann-Kendall and Sen’s slope tests revealed a pronounced transition toward temperature-driven aridity. While regional long-term precipitation (SPI-12) remained statistically stable, the SPEI-12 showed a highly significant regional drying trend ( p = 0.0028), suggesting that rising temperatures may contribute to drought intensification by increasing atmospheric evaporative demand and potentially amplifying moisture deficits. This intensification was most acute at Koka station, where the drying magnitude intensified by 35% under the SPEI compared to the SPI, signaling severe threats to reservoir inflows and long-term water security at the Koka Dam. Spatial analysis identified a polarized landscape with a “wetting core” in the south-central region and pronounced aridification at the western and southern peripheries. Agricultural drought analysis via VCI demonstrated high inter-annual volatility (R 2 = 0.11) with decadal troughs in 2002, 2012, and 2022 followed by a dramatic “climatic impact” recovery in 2023–2024. These findings underscore a systemic shift toward chronic moisture deficits driven by atmospheric thirst rather than rainfall failure alone. The study recommends adopting an integrated SPEI-VCI framework for early warning systems, promoting heat-tolerant crop varieties, and implementing temperature-resilient water management to safeguard the watershed’s food security and hydrological stability.