Plants Are Secretly Stealing Colorado River Water, Study Finds OR Colorado River Shrinking as Plants Tap Groundwater During Droughts

The Colorado River basin is facing a deepening crisis, and a new study reveals a surprising factor contributing to dwindling water supplies: plants are increasingly tapping into groundwater reserves during periods of drought. This behavior, dubbed a “drought paradox,” means vegetation continues to thrive even when surface water is scarce, but at the cost of reducing the amount of water that eventually flows into the already strained river system. The findings, from research conducted in the East River watershed of Colorado, have significant implications for water management strategies across the American West and beyond, where over 1.4 billion people globally rely on snowmelt-driven rivers for their water supply.

More than 10% of the U.S. Population depends on the Colorado River alone, making the situation particularly acute for states like Arizona, California, and Nevada. These states, along with Colorado, New Mexico, Utah, and Wyoming, are grappling with how to equitably share a shrinking resource, a challenge further complicated by the fact that the river’s flow has already decreased by 35% over the past seven years. Negotiations over post-2026 water management rules are ongoing, but a consensus remains elusive, as evidenced by missed deadlines and calls for greater concessions from all states involved. Arizona, California, and Nevada have already proposed reductions in their allocations – 27%, 10%, and 17% respectively – but further agreement is needed.

Unraveling the Drought Paradox

Traditionally, scientists believed that during hot, dry periods, plants would reduce water loss through a process called evapotranspiration – the combined loss of water from the soil through evaporation and from plants through transpiration (releasing water vapor from leaves). The assumption was that reduced evapotranspiration would aid conserve water in the system and maintain river flows. However, recent studies have challenged this notion, revealing that plants often increase or maintain transpiration even when water is scarce.

To investigate this phenomenon, researchers Reed Maxwell and Harry Stone from Princeton University installed an array of sensors across a 200-acre area within the East River watershed. Over two years – 2023, a year with high snowpack but a hot, dry summer, and 2024, with moderate snowpack and a cooler, wetter summer – they meticulously measured water movement throughout the snowmelt-to-streamflow pathway. Their findings were striking: even during the hottest, driest conditions, when soil moisture reached record lows, evapotranspiration remained high. This indicated that plants were accessing water from deeper sources – groundwater reserves – effectively diverting water that would otherwise contribute to river flow. The Bureau of Reclamation is currently working on new management rules for the river, and these findings will likely inform those discussions.

“Dry summer, wet summer. they’re getting their water,” Maxwell explained. “But they’re finding it from other sources. They’re taking it from shallow groundwater.” This suggests that the impact of climate change on water availability is more complex than previously understood. Even as reduced snowpack is a major concern, the increased water demands of plants, even during drought, are also playing a significant role in diminishing river flows.

Implications for Water Management

The study’s findings have important implications for how water resources are managed in the Colorado River Basin and other snowmelt-driven regions. Current water models often assume that evapotranspiration decreases during drought, leading to an overestimation of available water. By demonstrating that plants are actively tapping into groundwater reserves, the research highlights the need to recalibrate these models to account for this hidden source of water loss.

Brad Udall, a senior water and climate research scientist at Colorado State University, who was not involved in the study, emphasized the strength of the research, noting that the direct measurement of evapotranspiration on an hourly basis provided valuable data. He also pointed out that the findings support the growing understanding that increased evapotranspiration, driven by rising temperatures, is contributing to reduced river flows. Udall’s research has consistently shown the impact of warming temperatures on water availability in the region.

The Colorado River Basin has already warmed by 2.5 degrees Fahrenheit (1.4 degrees Celsius) over the past century, and this warming trend is expected to continue. As temperatures rise, plants will likely rely even more heavily on groundwater, further exacerbating the water shortage. Udall estimates that the region could spot a 40% decline in water availability by mid-century if current trends continue.

Understanding Evapotranspiration and its Role

Evapotranspiration is a crucial component of the water cycle, representing the transfer of water from the land to the atmosphere. It’s a natural process, but its rate is heavily influenced by factors like temperature, humidity, wind speed, and vegetation type. During drought conditions, plants typically close their stomata (tiny pores on their leaves) to reduce water loss. However, the “drought paradox” suggests that some plants are able to maintain or even increase transpiration by accessing deeper water sources, effectively bypassing the limitations imposed by surface drought. This adaptation allows them to survive, but it comes at a cost to the overall water supply.

What Comes Next: Refining Water Budgets and Collaborative Solutions

The Princeton study, currently available as a preprint article awaiting peer review, underscores the need for a more comprehensive understanding of the complex interactions within the Colorado River Basin’s water cycle. Maxwell emphasizes that a more accurate water budget, one that accounts for increased summer transpiration and groundwater depletion, is essential for making informed decisions about water allocation.

Moving forward, continued monitoring of evapotranspiration rates, coupled with improved groundwater modeling, will be crucial. Fostering collaboration among the seven basin states – Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming – is paramount. As Governor Gavin Newsom of California, Governor Katie Hobbs of Arizona, and Governor Joe Lombardo of Nevada stated in a joint statement, “Our stance remains firm and fair: all seven basin states must share in the responsibility of conservation.” The future of the Colorado River, and the millions who depend on it, hinges on a shared commitment to sustainable water management.