RAPID: How Critical Zone Processes Mediate the Impacts of Severe and Sustained Drought

The extreme heat wave and drought that is currently gripping the western United States is unprecedented, having led to record-high temperatures and/or record-low precipitation volumes across large geographic areas. The ongoing “megadrought” has been described as the West’s driest period at least 1200-years, having enormous societal consequences.

This project rapidly mobilizes to characterize the impacts of the unprecedented and severe drought conditions, where there is a unique opportunity to observe hydrological responses during this event. It explores how three watersheds in northern California respond to prolonged drought. Water storages will be measured during the drought in soils, underlying weathered rocks, groundwater, and stream water, and compared with long-term records of observation under other climatic conditions.

The study will reveal how moisture storage in the critical zone (which extends from the vegetation top, through the soil and the underlying weathered bedrock to fresh bedrock) mediates the effects of droughts, and the consequences for stream flow and vegetation survival. Results will be shared with the scientific community and with local stakeholders.

The research takes place at the Eel River Critical Zone Observatory in northern California, which is equipped with instrumentation to observe coupled hydrological, ecological, and geological processes. The study will identify how watersheds respond to multi-year drought and wet season shortening, patterns that are predicted for California’s future.

Seasonal depletion of rock moisture (moisture stored in the weathered bedrock) by plants leads to a delay in the onset of winter runoff as the first rains restore moisture levels before passing subsequent rains to recharge and runoff. It is hypothesized that forest resilience to drought in summer dry environments depends on the annual rainfall relative to the subsurface moisture storage capacity.

The current drought presents a new opportunity to test storage capacity hypotheses because the winter rains ended months earlier, initiating subsurface draw-down sooner, effectively extending the summer dry period. Field sampling campaigns will be conducted to measure water storages at selected locations representing variable critical zone properties, spanning a range of conditions and potential drought sensitivity.

The exceptionally dry conditions facilitate sampling of the oldest, least mobile waters released to streams and trees. Using these new data, hypotheses will be tested about the sensitivity of water storage to the style of climate variability predicted in California, informing how watersheds respond to drought.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.