From 2000 to 2021, climate change will cause the loss of more than 40 trillion liters (10 trillion gallons) of water in the Colorado River Basin — roughly equivalent to the entire storage capacity of Lake Mead — according to a new study modeling the impact of humans on the region’s hydrology.
Without climate change, the drought in the basin would likely not reduce reservoir levels in 2021 to the point of requiring supply cuts under the first-ever federally declared water shortage, according to the study, published in the journal Water Resources Researchwhich publishes original research on the movement and management of Earth’s water.
“While we know that warming has an effect on the availability of water in the Colorado Basin, we were surprised to learn how sensitive the basin is to warming compared to other large basins in the western US, and how high the sensitivity is in a relatively small area in the important snowpack regions of the basin,” said Benjamin Bass, a hydrological modeler at the University of California-Los Angeles and lead author of the study.
“The fact that warming removed so much water from a basin the size of Lake Mead during the recent megadrought is a wake-up call to the effects of the climate change we live in today.”
The Colorado River Basin, which is the area drained by the Colorado River and its tributaries, covers about 647,500 square kilometers (250,000 square miles) in seven states across the US West and supplies water to about 40 million people, as well as supporting agriculture and natural ecosystems. The regional drought that began around 2000 was the driest in 1,200 years and has reduced river flows and depleted reservoirs, raising concerns about water scarcity as the climate continues to change.
Previous analyzes of the change in water resources in this region have focused on the effects of climate processes alone, without considering the impact of the complex responses of plants to the increase of carbon dioxide in the atmosphere. The new study improves on previous hydrologic modeling of this region by including changes in runoff as a result of carbon dioxide-driven vegetation shifts.
It is the first to calculate how much runoff will change per degree of warming based on historical warming in snowpack regions of the Colorado River Basin compared to non-snowpack regions, quantifying how anthropogenic climate change has dried out traditional snowpack regions and rapidly reduced runoff that feeds the Colorado River.
To see how the basin’s hydrology will change between 1880 and 2021, the researchers used a land surface model capable of analyzing water, changes in vegetation and the response of plants to the increase in carbon dioxide in the atmosphere. They use standard data sets to represent atmospheric conditions, streamflow gauges and other records to capture regional hydrology, and ground and satellite data to document actual vegetation changes.
By including all of these types of data, “we have all the major players in terms of runoff sensitivity to climate change,” according to Bass.
Model analyzes show that from 1880 to 2021, temperatures in the Colorado River Basin warmed about 1.5 degrees Celsius (2.7 degrees Fahrenheit) as a result of anthropogenic climate change. This warming led to a 10.3% reduction in runoff under current conditions. Without including the effects of plants, water loss is now around 13%, which points to the importance of including plant processes in water modeling, the authors said.
That 10.3% reduction in runoff ended during the historic drought of 2000 to 2021, when the cumulative amount of water lost in runoff nearly equaled the full capacity of Lake Mead, the largest reservoir on the Colorado River.
The researchers also discovered that parts of the basin that are usually snow-covered in winter are now losing water about twice as fast as regions that are usually snow-free. The transition is of immediate concern for water managers because snowpack makes an outsized contribution to the basin’s water supply: Only about one-third of the basin is covered by snow each year, but snow-covered areas are the source of about two-thirds of the basin’s total runoff.
Rapid water loss in snowpack regions is a sign that the Rocky Mountain West is transitioning to a drier climate rather than just going through a daily drought, according to Bass.
Bass points out that the 1922 Colorado River Compact, which governs the use of river water in the seven western US states through which the Colorado flows, was written with the assumption that the region’s climate was stable.
When representatives of these states signed the contract, “they expected 15 million acre-feet of water (about 18.5 trillion liters or 4.6 million gallons) to be provided from the basin, on average, every year,” Bass said.
“If they outline the compact now, they have to adjust that amount because of the warming effect. Going into the future, we can get some natural changes, wet or dry changes, but this study emphasizes that there is a decreasing trend in runoff.
More information:
Benjamin Bass et al, Drying of Snowpack Regions in Colorado River Basin Causes Water Losses Despite Ameliorating Vegetation Effects, Water Resources Research (2023). DOI: 10.1029/2022WR033454
Provided by the American Geophysical Union
Citation: Study: Colorado River Basin loses water equivalent to Lake Mead due to climate change (2023, July 24) retrieved on July 24, 2023 from https://phys.org/news/2023-07-colorado-river-basin-lost-equal.html
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