Southern Sierra Critical Zone Observatory and Kings River Experimental Watersheds: A Synthesis of Measurements, New Insights, and Future Directions

Abstract

Core Ideas Water quality and stream flow have temporal and spatial trends in response to variable climate. Our work reveals how Sierra Nevada forests responded to and recovered from multiyear drought. Regolith thickness trends reveal water storage capacity differences with elevation. Monitoring shows deep‐water changes via plant utilization or capillary flow during drought. Sensor networks within the Southern Sierra Critical Zone Observatory (SSCZO) and Kings River Experimental Watersheds (KREW) document changes in the water cycle spanning the west slope of the southern Sierra Nevada in California. The networks were established to document water dynamics throughout the critical zone spanning profile, hillslope, catchment, and watershed scales at key locations that reflect systematic differences in bioclimatic conditions imposed by a strong elevation gradient. The critical zone observatory attempts to constrain the hydrologic budget via representative measurements of streamflow, eddy flux covariance, snow depth, meteorological conditions, and water content and water potential in soil and deep regolith. These measurements reveal the complexity of interactions among all aspects of the water balance (runoff, storage, evapotranspiration [ET], and precipitation) through daily, seasonal, and annual timescales. Multiyear drought, catastrophic wildfires, insect outbreaks, and disease have caused widespread tree mortality in the Sierra Nevada. These disturbances offer a window into the future for this region, which is expected to undergo significant change in response to global warming. This hydrological observatory provides valuable hydrometric attributes and fluxes across the stream–groundwater–vadose zone–soil–vegetation–atmosphere continuum.

Information content of snow hydrological signatures based on streamflow, precipitation and air temperature

“…In addition, differences in S MCT and S PQ between the Providence and Bull catchments were found too small compared to what was expected given the differences in SWE maxima references. These results are the consequences of the underlying assumptions of both methods – subsurface storage changes and evapotranspiration should be small compared to snow storage – which are not true in our study catchments (Bales et al, 2018; O'Geen et al, 2018). Larger differences between the two sets of catchments were obtained with S MCT estimates compared to S PQ estimates, suggesting that the MCT approach is less sensitive to these assumptions violations.…”

“…These catchments are part of the Kings River Experimental Watersheds operated by the U.S. Forest Service (Hunsaker & Safeeq, 2017, 2018). A detailed description of the catchments and a synthesis of the measurements can be found in O'Geen et al (2018).…”

“…Precipitation mostly occurs ‘during large frontal storms that move off the Pacific Ocean from West to East’ (Goulden et al, 2012). A large part of the precipitation is lost to the atmosphere through evapotranspiration (Bales et al, 2018; O'Geen et al, 2018) as illustrated by the runoff coefficient ranging from 17.7% (P303) to 42.9% (B203) calculated by Safeeq and Hunsaker (2016).…”

“…It assumes that subsurface water storage changes and evapotranspiration (and sublimation) are small compared to snow storage. These assumptions are not true in the case of the Southern Sierra catchments as there is large evapotranspiration and possible significant subsurface storage changes (see Section 2.1 and Bales et al, 2018; O'Geen et al, 2018). However, in spite of these limitations (discussed further in Section 4), we argue (and test if) it can still be used to derive relevant information on snow storage.…”

Lithologic, Climatic and Depth Controls on Critical Zone Transformations

“…Slope of the regression line between heat energy and weathering proxies was fivefold steeper at the rain‐dominated site, suggesting temperature plays a major role in transformations near the surface (Table 4). Given the modest difference in MAT between our two sites (4.6 ° C), it is possible that microclimate contributed to the observed variability in regolith weathering at sites; however, air temperature monitored continuously at north and south aspects at these sites since 2008 were similar (O'Geen et al, 2018). Alternatively, subtle differences in mafic mineral content within parent material such as hornblende could be responsible for the observed trends.…”