title: A 155-year tree-ring based record of warm and dry snow droughts for the Cascade Range, USA
Creators: name: Dye, Laura
Publisher: name: University of Nevada, Las Vegas
Year: 2021

description: Contributing 50-80% to the surface water for the region, mountain snowpack is a pillar of the hydrologic cycle in the western US, and is projected to decrease to 44% by 2100 in the Cascade Mountains alone. The western US has recently been identified as a global snow drought hot spot, exhibiting more frequent, intensified, and lengthened snow drought events in recent decades. Only recently has the need to understand the types of snow droughts and their driving mechanisms become critical to understand and quantify. Recent research suggests ‘warm’ snow droughts caused by winter precipitation falling as rain rather than snow play a primary role, and may be unusual relative to past ‘dry’ snow droughts caused by a lack of winter precipitation. In light of the recent widespread 2014-16 snow drought, water resource managers have identified the need for long-term probabilistic estimates of warm snow droughts in particular. The steep and fast-draining watersheds characteristic of the Pacific northwest, combined with aging dam and reservoir infrastructure, leaves this region especially vulnerable to shifts in the hydrologic cycle. Overall, snow droughts pose significant societal, economic, agricultural, and ecosystem impacts, and are a mounting challenge for water resource management throughout the region.

Tree-ring chronologies provide precise, annually-resolved, multi-century paleoclimate information which can enhance our knowledge of the natural long-term variability in the climate system beyond the short instrumental record. To date, no dendrohydrologic snowpack reconstructions have distinguished between warm and dry snow droughts, nor have they examined the durations and magnitudes of warm snow droughts in the pre-instrumental record. Here, I present a novel dendrohydrologic approach that pairs energy- and moisture-limited tree ring chronologies (total, early, and latewood) to develop complementary, independent snowpack reconstructions of warm and dry snow droughts for the Cascade Range, USA. By examining the two reconstructions in tandem, I quantify and compare the magnitudes and durations of warm versus dry snow droughts over the past 155 years, relative to the observed period of record (1952-2018). My results suggest that while more warmsnow droughts have occurred in the pre-instrumental record, the magnitude of these events has intensified in recent decades, indicating that the observed period of record may not capture the full range of Cascade Range snow drought variability. Furthermore, reconstructions demonstrate that only single- and two-year snow drought durations have occurred in the past 155 years. This provides an important benchmark for multi-year snow droughts in the region, and thus, suggests that (¸ 3 year) snow droughts such as the recent 2014-16 event may be unprecedented in the Cascade Range. These individual warm and dry snow drought reconstructions give context to what the natural hydroclimate system is capable of, which may serve water resource managers and climate adaptation specialists in planning for future warm snow droughts risk in the following decades.