Recent Reports

Winter Inputs Buffer Streamflow Sensitivity to Snowpack Losses in the Salt River Watershed in the Lo (December 2020)

Recent streamflow declines in the Upper Colorado River Basin raise concerns about the sensitivity of water supply for 40 million people to rising temperatures. Yet, other studies in western US river basins present a paradox: streamflow has not consistently declined with warming and snow loss. A potential explanation for this lack of consistency is warming-induced production of winter runoff when potential evaporative losses are low. This mechanism is more likely in basins at lower elevations or latitudes with relatively warm winter temperatures and intermittent snowpacks. We test whether this accounts for streamflow patterns in the Salt River and its tributaries, which is a sub-basin in the Lower Colorado River Basin (LCRB). Despite significant warming from 1968–2011 and snow loss in many of the Salt basins, annual and seasonal streamflow did not decline. Between 25% and 50% of annual streamflow is generated in winter when runoff ratios are generally higher and potential evapotranspiration losses are one-third of potential losses in spring. Short term streamflow responses to winter inputs were larger and more efficient than spring and summer responses and their frequencies and magnitudes increased in 1968–2011 compared to 1929–1967. In total, 75% of the largest winter events were associated with atmospheric rivers, which can produce large cool-season streamflow peaks. We conclude that temperature-induced snow loss in this LCRB sub-basin was moderated by enhanced winter hydrological inputs and streamflow production.

Landscape restoration minimizes tree growth vulnerability to 21st century drought in a dry forest (October 2020)

With hotter temperatures and less precipitation projected in the future, reducing tree density is a possible strategy to minimize the impacts of drought on forest growth. Many forest restoration programs are focused reducing tree density to minimize wildfire risks, but it is unknown how these efforts will impact drought vulnerability. In this study, we looked at how the Four Forest Restoration Initiative (4FRI) would alter landscape-scale patterns of forest growth and drought vulnerability throughout the 21st century. We found that hotter and drier conditions in the future could reduce tree growth, but the severity of drought and the magnitude of future growth declines was lessened by the thinning treatments. Compared to historical conditions, proportional tree growth by 2050 declines by ~40% if thinning continues at the status quo pace.  By comparison, proportional growth declines by only 20% if the 4FRI thinning treatments are fully implemented, and < 10% if stands are thinned even more intensively. These results indicate that forest restoration projects designed for other objectives can also have substantial benefits for minimizing future drought vulnerability in dry forests and provide additional incentive to accelerate the pace of restoration.

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