Abstract
Mountain runoff ultimately reflects the difference between precipitation (P) and evapotranspiration (ET), as modulated by biogeophysical mechanisms that intensify or alleviate drought impacts. These modulating mechanisms are seldom measured and not fully understood. The impact of the warm 2012–15 California drought on the heavily instrumented Kings River basin provides an extraordinary opportunity to enumerate four mechanisms that controlled the impact of drought on mountain hydrology. Two mechanisms intensified the impact: (i) evaporative processes have first access to local precipitation, which decreased the fractional allocation of P to runoff in 2012–15 and reduced P-ET by 30% relative to previous years, and (ii) 2012–15 was 1 °C warmer than the previous decade, which increased ET relative to previous years and reduced P-ET by 5%. The other two mechanisms alleviated the impact: (iii) spatial heterogeneity and the continuing supply of runoff from higher elevations increased 2012–15 P-ET by 10% relative to that expected for a homogenous basin, and iv) drought-associated dieback and wildfire thinned the forest and decreased ET, which increased 2016 P-ET by 15%. These mechanisms are all important and may offset each other; analyses that neglect one or more will over or underestimate the impact of drought and warming on mountain runoff.
Highlights
The hydrology across a major river basin responds to multi-year dry periods, with the annual water balance given as Q = P − ET − ΔS, where Q is basin discharge, P is precipitation, ET is evapotranspiration, and ΔS is the change in subsurface storage within the basin
A wide suite of spatially distributed measurements of evapotranspiration, precipitation as rain and snow, snowpack and soil-water storage and stream runoff were installed in the Southern Sierra Critical Zone Observatory (SSCZO) well before the 2012–15 drought and continue through present (Fig. 1)[10]
During the 2012–15 drought we observed significant changes to the water balance at three intensive-measurement sites, findings that were consistent with changes observed throughout the 3989 km[2] Kings River basin (275–4250 m elevation)
Summary
The seasonal patterns of evapotranspiration reflect the withdrawal and depletion of subsurface moisture that was recharged earlier in that year. Soil matric-potential measurements at Soaproot point to a failure to recharge at the 2-m depth beginning in 2012, compared with annual recharge for most years at Providence (Fig. 3e,f). This multi-year decline at Soaproot is consistent with the large P-ET deficit for the site. Providence exhibited a multi-year decline in Q and ET, with a storage deficit and decrease in matric potential by 2015; and San Joaquin showed a small but growing storage deficit through the drought These dry summer conditions suggest that net lateral flow into the site was small. The 638 mm yr−1 of evapotranspiration and runoff coming from storage during the dry season at Providence implies that root-accessible water occurs at depths that are well below 2.5 m
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
