A study published in Nature Water found that climate change is not only altering how much water flows through rivers, but also when that water is available. When the water is flowing from snow fields to rivers is an often-overlooked variable that has significant implications for water users across the United States.

“Water demand varies seasonally, but until now we haven’t fully understood how climate-driven changes in streamflow distribution within the year interact with water rights and their daily administration,” said lead author Steven M. Smith, associate professor and associate department head of economics and business at Colorado School of Mines.

The study examines how streamflow is distributed throughout the entire water year, building on previous research focusing on changes in total streamflow and the timing of peak runoff. That distribution, Mines researchers say, plays a critical role in determining who can access water.

"When the same amount of annual streamflow is concentrated into a shorter period due to higher temperatures, senior water rights holders—those with the earliest claims—may capture less water than when flows are spread more evenly across the year,” Marshall said.

In the Intermountain West, the findings reveal an added complication: years with lower total water availability also tend to have more evenly distributed flows. This combination may intensify drought impacts on junior water users, who receive water only after senior rights are fulfilled.

Water shortages are already impacting the Colorado River Basin as a decades-long drought persists across the West. Lakes Powell and Mead, for example, are less than two-thirds full, leaving scientists, economists, and water law experts fearful that the reservoirs could soon be useless. Dry winter conditions and reduced snowmelt are compounding the problem.

The researchers caution against oversimplifying the equity implications of their study. Senior water rights holders are not always socio-economically advantaged, and those senior rights are often tied to smaller allocations. As a result, changes in water availability do not map neatly onto broader measures of social or economic equity.

The study also highlights regional differences in how streamflow distribution is responding to a warming climate. In the Upper Midwest and New England, streamflow has already become more evenly spread throughout the year due to climate change. In contrast, patterns in the western United States are more complex, the researchers note.

In snow-dominated regions of the U.S. West, warmer years tend to produce a wider distribution of streamflow across the year—conditions that may benefit senior water users while disadvantaging junior users. In non-snowy parts of the region, the opposite pattern emerges: warmer years are associated with more concentrated flows, potentially offering a relative advantage to junior water users.

These findings underscore the importance of considering not just how much water is available in a changing climate, but how it is distributed over time. The issue of how those shifts intersect with long-standing water management systems is also important to consider.

"As water scarcity intensifies across many parts of the country, we hope our findings provide new insight for policymakers, water managers and communities working to adapt to a more variable and uncertain future," Smith said.