A quiet transformation is taking place across the highlands of Ethiopia—one that stretches beneath the surface, deep into the earth's hidden water reserves.
Imagine a tree so thirsty it can drink a groundwater reservoir dry. This isn't a scene from a fantasy novel, but a reality unfolding in the Ethiopian highlands, where the rapid expansion of eucalyptus plantations is dramatically altering the water landscape. In the Lake Tana Basin, a region crucial for the water security of millions, scientists are tracking how these fast-growing trees are tapping into groundwater reserves with surprising speed 1 .
For smallholder farmers, eucalyptus has become a financial lifeline—its wood valuable for construction, fencing, and charcoal production. But this economic benefit comes with a hidden cost: each tree acts like a natural straw, drawing up enormous quantities of groundwater and leaving less for streams, wells, and future generations 3 . Recent research reveals the startling extent of this phenomenon, showing that eucalyptus trees can consume 2.5 times more water than agricultural crops 1 .
Eucalyptus trees possess unique biological features that make them exceptionally efficient at drawing water from the ground. Unlike shallow-rooted crops, eucalyptus develops an extensive root system that can reach deep into the soil to tap groundwater reserves far below the surface 3 .
These remarkable trees function like natural pumps, with several key adaptations driving their high water consumption:
More surface area for water transpiration
More pores open for longer periods
Access groundwater beyond crop reach
Continuous water uptake even in dry seasons 1
Unlike seasonal crops that complete their life cycle in months, eucalyptus trees remain active year-round, maintaining their water consumption even through the dry season when water resources become most precious 1 .
To understand exactly how eucalyptus plantations affect groundwater resources, researchers conducted an elegant natural experiment in Ethiopia's Fogera plain. Their approach was simple yet powerful: track the daily rhythm of groundwater levels in areas dominated by eucalyptus trees 1 5 .
Scientists installed automated water level recorders in monitoring wells across the study area. These sophisticated instruments measured groundwater elevations every hour, creating a detailed picture of how water levels change not just seasonally, but throughout each day and night 1 5 .
The research focused particularly on the dry monsoon phases when rainfall isn't replenishing the groundwater, and trees rely almost exclusively on water stored underground. For two consecutive years, the team collected data, comparing areas with eucalyptus plantations to control areas with traditional agriculture or natural vegetation 1 .
Identified areas with eucalyptus plantations and control sites with traditional agriculture
Deployed automated water level recorders in monitoring wells
Hourly measurements taken continuously for two years
Compared daily and seasonal patterns between different vegetation types
The resulting data told a compelling story. Each day, groundwater levels followed a distinct pattern: reaching their highest point in the early morning, then gradually declining throughout daylight hours to a low point in the evening 1 .
This daily fluctuation isn't random—it's the literal pulse of the trees' water consumption. During daylight, eucalyptus trees open their stomata to absorb carbon dioxide for photosynthesis, simultaneously losing water through transpiration. This water is drawn up from the roots, creating a suction effect that pulls water from the surrounding soil and eventually lowers the groundwater table 1 .
The findings from the groundwater monitoring revealed a dramatic story of water consumption that far exceeds what researchers had anticipated.
| Measurement | Value | Context |
|---|---|---|
| Average daily groundwater decline | 3.1 cm/day | Linear decline throughout dry season |
| Daily evapotranspiration rate | 9.6 mm/day | Nearly double reference evapotranspiration |
| Peak transpiration rate | 1.65 mm/hour | Occurs at midday following solar irradiance |
| Seasonal evapotranspiration (Oct-May) | ~2300 mm | Compared to ~900 mm without eucalyptus |
The data reveals a stunning disparity in water consumption. During the seven-month dry period from October to May, eucalyptus plantations consumed approximately 2300 mm of water through evapotranspiration—the combination of direct evaporation and plant transpiration 1 5 . In areas without these trees, this measurement was only about 900 mm 1 . This means eucalyptus plantations are removing an extra 1400 mm of water—equivalent to a 1.4-meter deep layer from the entire land surface—that would otherwise remain in the groundwater system each year.
Perhaps most strikingly, the daily evapotranspiration rate of 9.6 mm under eucalyptus is nearly double the reference evapotranspiration for the area and 2.5 times higher than the rate measured under fallow agricultural fields 1 . This means each eucalyptus tree consumes as much water as 2.5 equivalent areas of farmland during the dry season.
| Time of Day | Transpiration Rate | Relationship to Environmental Conditions |
|---|---|---|
| Nighttime | Minimal | Stomata largely closed |
| Early morning | Rising | Follows increasing solar radiation |
| Midday | Peak (1.65 mm/hour) | Parallels daily solar irradiance maximum |
| Evening | Declining | Follows decreasing solar radiation |
This pattern closely follows the daily solar irradiance curve, with water consumption rising and falling with the sun's intensity 1 . The clear connection to sunlight demonstrates that the trees' photosynthetic activity—and resulting water uptake—is driving the groundwater fluctuations.
The impact of eucalyptus expansion extends far beyond the immediate groundwater table. At the watershed level, the consequences are equally significant.
In the 39 km² Amen watershed, also in the Lake Tana basin, researchers tracked how streamflow patterns changed as eucalyptus plantations expanded. Between 2001 and 2024, eucalyptus coverage in this watershed increased from 238 hectares to 799 hectares—a dramatic transformation of the landscape 3 .
The hydrological effects were profound. During the critical dry monsoon phase from January to March, the actual evapotranspiration from eucalyptus was up to 30% greater than from other land uses 3 . This increased water consumption resulted in decreased water storage throughout the watershed over the 23-year study period.
One of the most significant impacts appeared in how the watershed responds to rainfall. The region primarily experiences saturation excess runoff—meaning runoff occurs when the ground becomes completely saturated with water 3 . As eucalyptus trees remove more water from the system, the ground takes longer to reach this saturation point.
The data reveals this clearly: in the 2002-2009 period, it took an average of 160 mm of cumulative effective rain for significant runoff to begin. By the 2015-2018 period, after substantial eucalyptus expansion, this requirement had increased to 274 mm 3 . This means that now, more rain is needed before water begins flowing into streams—a crucial change that affects both surface water availability and the recharge of aquifers.
Additionally, base flow—the sustained flow between rainfall events that feeds streams during dry periods—decreased significantly 3 . This decline in dry-season streamflow has direct implications for both human water use and aquatic ecosystems.
| Parameter | Change | Implication |
|---|---|---|
| Eucalyptus coverage | 238 to 799 ha (2001-2024) | Rapid land use transformation |
| Dry season ET | Up to 30% higher than other vegetation | Reduced water storage |
| Rainfall needed for runoff | Increased from 160 mm to 274 mm | Delayed stream response to rain |
| Base flow | Significant decrease | Reduced dry season streamflow |
The research presents Ethiopia with a complex challenge: how to balance the economic benefits of eucalyptus cultivation with the protection of precious water resources. For smallholder farmers, eucalyptus represents a reliable source of income in a region where economic opportunities can be scarce 4 . Yet the cumulative impact of individual planting decisions could have far-reaching consequences for regional water security.
The implications extend beyond Ethiopia's borders. The Ethiopian highlands generate 85% of the transboundary water in Egypt and Sudan 3 . Changes in water availability in this region therefore have diplomatic and geopolitical dimensions, particularly in relation to major projects like the Grand Ethiopian Renaissance Dam 3 .
Avoiding eucalyptus planting in critical recharge areas and riparian zones to protect groundwater resources.
Recognizing the trade-offs between economic benefits and water resource impacts in land use decisions.
Developing water management strategies that account for groundwater consumption by different vegetation types.
Tracking both groundwater and streamflow changes as plantations continue to expand across the region.
Understanding how researchers study groundwater impacts reveals the sophistication of modern hydrology. The key tools and methods include:
These instruments measure groundwater elevations at regular intervals, providing the high-frequency data needed to detect diurnal fluctuations 1
Satellite imagery from Landsat and other platforms helps track land use changes over time, showing how eucalyptus coverage has expanded across watersheds 3 4
Using portals like WaPOR, researchers can calculate how much water different vegetation types are returning to the atmosphere 3
This technique helps distinguish between surface runoff and groundwater contributions to streamflow, revealing how watershed behavior changes with vegetation 3
This approach classifies land cover types from satellite imagery, even with limited ground training data 3
The story of eucalyptus and groundwater in Ethiopia's Lake Tana Basin illustrates a broader global challenge: how to manage natural resources when economic and environmental priorities appear to conflict. As climate change intensifies water scarcity in many regions, understanding these plant-water interactions becomes increasingly crucial 3 .
Future research might explore whether modified management practices could reduce the water footprint of eucalyptus plantations, or how these trees might be strategically placed to actually benefit water management in some contexts—such as using their remarkable water consumption for bio-drainage in waterlogged areas 3 .
What remains clear is that in the delicate balance between economic needs and environmental sustainability, every drop counts—and sometimes, the most significant water management decisions aren't about pipes and pumps, but about which trees we plant, and where.
The silent decline of groundwater beneath eucalyptus plantations serves as a powerful reminder that our most precious resources often lie hidden, and that protecting them requires understanding the invisible connections between the forests we grow and the water we need.