UCLA

Midlatitude mountain snowpacks are a critical freshwater resource. However, climate warming is endangering its natural water tower function by decreasing the fraction of winter precipitation falling as snow in an intensifying precipitation regime. In a warmer future, snowpacks may therefore shift from seasonal reservoirs to posing more flood hazards, stressing current ecosystems and water resource management practices. Successful adaptation and hazard preparation to snowpack shifts is predicated on a first-principles understanding of its driving processes. However, most long-term observations and model-projections are incapable of resolving process-level snowpack behaviors, leaving an incomplete toolkit for assessing and interpreting environmental threat levels in snow-fed watersheds. This dissertation addresses this problem by investigating process-level snowpack behaviors during warm storms in California’s Sierra Nevada through sub-daily snow, soil and hydrometeorological measurements, and with supporting observations from atmospheric reanalyses and satellite remote sensing in three studies. The first study delineates an hourly relationship between air temperature, atmospheric moisture, and changes to snow water equivalent (SWE) spanning the Sierra during water years 2010-2019. Conditions balancing precipitable water (humidity) and snowfall requirements (temperature) favor SWE accumulation. SWE increases are observed above 1.0˚C when moisture supplies are modest or snowpacks sufficiently deep, and are otherwise immediately followed by SWE loss. The second study focuses on this SWE “oscillation” in the Northern Sierra during rain-on-snow (ROS) events in January and February 2017. Snowmelt was a weaker driver in ROS flooding than previously documented. Snowpacks may rather “passively” route rainwater through snow, where the saturation and liquid drainage of snow causes observed SWE to rise and immediately fall. The final study elaborates on this case study to other ROS events in water years 2017-2019. Results describe both the limitations common to standard daily observations, and the importance of storm sequencing on augmenting ROS impact. Sierra Nevada watersheds remember large prior storm inputs to soils and streams that imminent storms stand on top of. These observations present a nuanced and integrated perspective to understanding how snowpacks respond to warm storms, which may be beneficial to future water resource and flood risk forecasting interpretations and discussions.