Controls on drought propagation in drylands

Drought propagation in dryland environments is often conceptualised as a sequential transfer of meteorological deficits into soil moisture, streamflow, and groundwater drought with the propagation rate controlled by catchment and aquifer properties. However, quantifying the spatial and temporal propagation of rainfall deficits remains challenging because few hydrological models explicitly connect climate forcing to all relevant hydrological stores and fluxes, or represent their interactions across spatial scales. Here, we investigate how meteorological drought propagates through overland flow, streamflow, and soil moisture into groundwater storage across dryland catchments in the Horn of Africa. We hypothesise that drought propagation will be related to aridity, catchment scale, and lateral groundwater connectivity. To investigate these ideas, we use the 1 km resolution DRYland water Partition (DRYP) hydrological model to simulate daily water-balance components across the Horn of Africa from 2000–2023. To examine the influence of basin size, catchments are delineated using a range of stream orders (4–8); for example, stream order 5 yields ~1,300 basins with a mean area of ≈900 km² spanning hyper-arid to humid conditions. We focus on regionally widespread meteorological drought events defined using SPEI-12 (derived from CHIRPS rainfall and hPET); catchment-mean SPEI is used to identify basins experiencing prolonged meteorological drought (> 12 months). We then use basin-scale total water storage anomaly (TWSA) and the standardised total water storage index (STWSI) to explore groundwater dynamics in basins experiencing prolonged drought. Our analysis reveals substantial spatial heterogeneity in groundwater response to meteorological drought. Even during prolonged drought events, many basins exhibit neutral or increasing TWSA trends, with coherent spatial clusters of both declining and increasing storage observed within the same drought period. These results demonstrate that meteorological drought does not consistently translate into groundwater drought, even over multi-year timescales. We explore how aridity, basin size, and lateral groundwater contributions can decouple groundwater dynamics from atmospheric water deficits, leading to enhanced drought resilience in some regions (and longer recovery in others). This work highlights the limitations of assuming fixed drought-propagation pathways in drylands and demonstrates the value of high-resolution modelling for improving drought monitoring and water-resource management under increasing climate variability.