The Colorado River has recently made national news due to the collective overdraft of the river system by neighboring states. Overuse of the Colorado River is caused by both over-allocation of the water (i.e., states have more water rights than the river can actually yield) and the impacts of anthropogenic climate change. Reductions in Colorado River allocations that are required will impact communities across multiple Southwestern US states.

The Science

When the Colorado River Compact was written in 1922, annual water yields (the amount of water that flows to the Colorado River and is available for consumption every year) were estimated to be 17.5 million acre-feet per year. These observed yields were from a relatively wet period in the Colorado River Basin’s history, despite earlier data showing evidence of previous long-term droughts and drier years. This meant that the estimated water yield for the basin was too high (it is likely closer to 15 million acre-feet per year). Note that climate variability, where the weather over the course of months to years may be wetter or drier than average, is relatively normal, and it is super-imposed on anthropogenic climate change. While the Colorado River Compact’s over-estimated water yield did not cause any major problems in its early years, the growth of Southwestern states in both population and agricultural industry has made current withdrawal levels from the river unsustainable. This overdraft of water from the Colorado River system of dams and reservoirs is called the structural deficit: water withdrawal for agricultural, industrial, and residential use exceeds the natural flow entering the reservoirs.

Meanwhile, the effects of anthropogenic climate change have exacerbated the existing structural deficit. In a warming climate, increased temperatures allow the atmosphere to hold more moisture, and the amount of moisture that the atmosphere can hold increases exponentially with increased temperature. In a semi-arid environment like the Southwest, this warming increases evaporation. If precipitation stays the same, then increased evaporation will decrease the amount of water flowing into streams and rivers, thus taxing the Colorado River System even more. Furthermore, a warming climate is also subject to greater extremes, meaning that droughts, which can affect the Colorado River on timescales of months to decades, will become more extreme as well. Thus, climate change presents two additional threats to the Colorado River, where water availability is reduced on average and stress on the system caused by droughts will be worse, further increasing the need for long-term storage of water.

The Policy

Water is a precious resource for any community’s public health, safety, and economic vitality. In 1922, seven Western states signed the Colorado River Compact: Wyoming, Colorado, Utah, and New Mexico (Upper Basin), as well as Nevada, Arizona, and California (Lower Basin). The compact provided for an allocation of water to the Upper and Lower Basin states. This occurred at a time of unusually high flow for the river, so in short order, the river was “oversubscribed,” which has caused stress among the states, especially since the construction of the Central Arizona Project (CAP) that brought water and growth to the Phoenix Valley area, Tucson, and Yuma, Arizona farmers.

Today, after more than 20 years of drought conditions, Lake Mead, the primary reservoir for the Lower Basin, is rapidly reaching low levels that could trigger serious impacts for the Southwest. The states have been asked by the U.S. government to co-create a viable plan, to no avail so far. If an agreement can’t be reached, the federal government may step in to mediate or impose a solution. Currently, California is the holdout to an agreement due to its priority rights over other Lower Basin states.

Local Efforts

As climate changes, drought and discord continue to threaten water supplies to communities reliant on the Colorado River. But many communities are working to amplify conservation efforts and transition to alternative water supplies. Water utilities are deploying conservation incentives, such as broad outreach and education, rebates for low-efficiency fixtures and irrigation controllers, conversion of turf to more drought-mindful landscapes, and water restrictions, as well as more data-driven efforts to identify the largest water users and customer categories and provide recommendations to make simple, effective reductions. In addition, water utilities will responsibly convert to greater groundwater use, alternative water sources such as recycled water, and system infrastructure that brings water from new sources.

Water sources and policy challenges in the western United States are different from water issues along the East Coast. Next quarter we will explore the impacts of climate change on water in the Mid-Atlantic region.