Rising groundwater offers hope for drought-stricken East Africa

New research indicates better groundwater supply management could help combat the impact of climate change in East Africa, where countries are currently facing the worst drought and food insecurity in a generation.

The study, led by the University of Bristol, with substantial contributions from the School of Earth and Environmental Sciences at Cardiff University, looked at changes in rainfall within the two rainy seasons in the Horn of Africa - a region hard hit by frequent drought and water and food scarcity.

Findings showed that the total rainfall within the main rainy season (known as the ‘long rains’) is declining across the Horn of Africa Drylands but rising in the other rainy season (known as the ‘short rains’). These trends, which apply to Ethiopia, Somaliland, Somalia, and Kenya, led the researchers to investigate below-ground water storage, a potential lifeline for drinking water provision, during these periods.

Results revealed water storage has been increasing in recent decades, despite the drying effects of the declining ‘long rains’ season which historically deliver more rain cumulatively than the ‘short rains’ season. This suggests groundwater supplies may potentially be harnessed to support food and water-insecure rural populations in the Horn of Africa drylands.

To understand this contradictory finding, the research team examined how the characteristics of rainfall may have changed over time. By analysing rainfall data over the past 30 years, the researchers found that heavy rainfall is becoming more common in the ‘short rains’ season and is generally high in the ‘long rains’ season.

The result emphasises that in drylands, the intensity of rainfall within a rainy season may be actually more important for groundwater replenishment than the total amount of rainfall. Co-author of the study, Professor Michael Singer said, “We were really surprised by these results since they sat in contrast with our expectations under a drying world. We expected that less rain would yield less water storage underground, but we found the opposite, and it turns out that extreme rainfall is likely to be the reason behind it.”

Drylands are regions characterised by low annual rainfall and high evaporative demand, as well as dry rivers and deep groundwater levels. If rain falling on the ground is light it can evaporate quickly back into the atmosphere because it is held in the shallow parts of the soil. However, if it is intense and heavy it produces runoff in the dry riverbeds which can then infiltrate deep into the soil fast and escape evaporation back into the atmosphere.

This mechanism helps explain why, even with the drop in the ‘long rain’ seasonal totals, the combination of high rainfall intensities in both seasons appeared to be contributing to the greater levels of below-ground water storage.

The researchers went on to demonstrate that increasing water storage is not linked to any rise in soil moisture near the surface, confirming it represents banked water deep below ground and likely contributing to a growing regional groundwater aquifer in this region.

The findings provide much-needed hope, as the Horn of Africa drylands are enduring a fifth consecutive season of below-average rainfall, exacerbating the effects of drought including famine, water shortages, livestock deaths, and failed crop harvests. A sixth failed rainy season has also been forecasted for next year, further compounding the scale of devastation.

At this stage, the depth and quality of the available groundwater are unknown. Whether the discovered trends would continue under successive episodes of failed seasonal rains also needs to be ascertained.

The unanswered questions highlight the need for extensive groundwater surveys across the Horn of Africa region to assess whether this apparently increasing water resource may be economically viable enough to help offset the impact of recurrent droughts.

The paper, ‘Sustained water storage in Horn of Africa drylands dominated by seasonal rainfall extremes’ can be accessed online.