Water management activities, such as irrigation withdrawals and dam operations, strongly influence the flow of water through a watershed. Researchers from the U.S. Department of Energy’s Pacific Northwest National Laboratory conducted the first regional modeling study to explore how these activities might modulate the effects of long-term changes in temperature and precipitation on regulated river flows. They found that close to half of the watersheds in the western United States could experience noticeable flow changes earlier in the century than they would in the absence of water management. While results vary substantially across different watershed regions and seasons, the overall magnitude of change tends to be smaller in highly managed watersheds.
This work is the first large-scale investigation of the “emergence of change” in highly regulated versus unregulated rivers—that is, it accounts for the influence of water management as well as for changing environmental conditions. Water management activities are generally thought to reduce flow variations, which was confirmed. However, the work revealed that water management can accelerate the emergence of these smaller changes. The findings highlight significant regional and seasonal differences in how long-term Earth system changes will affect water availability. These results can be used to help understand and prepare for possible shifts in the timing and magnitude of future flows in the western United States. The research also highlights the need to account for human influences when constructing models of energy systems and other sectors that are strongly affected by changes in water availability.
This study focused on seasonal and regional variations in the emergence (the “when”) of flow volume changes. Using a large-scale hydrologic model driven by three different Earth system models, researchers used statistical techniques to identify when noticeable shifts in flow volume occurred in each water basin across the western United States. They examined two different scenarios of projected temperature and precipitation changes over the 21st century. They then compared hydrological simulations that included water management with those that did not consider the influence of dams, irrigation, and other human influences on water flow. The research team also analyzed historical conditions for context and comparison. The results showed that in 40 percent of river basins, water management activities accelerate the emergence of noticeable changes in annual flow volume (that is, the changes appear earlier in the century than in simulations that did not consider water management activities). In 10 percent of basins, however, water management delays the onset of noticeable changes. Even though water management generally accelerates the emergence of changes in flow, the magnitude of those changes over the 21st century is smaller in heavily managed basins. The picture gets more complex when looking at individual seasons, but in general, compared to unregulated conditions, the overall sensitivity of flow changes to water management tends to decrease in the spring and increase in the winter and summer.