Reconsidering the Utility of the April 1st Snow Water Equivalent Metric for Water Resource Applications

TitleReconsidering the Utility of the April 1st Snow Water Equivalent Metric for Water Resource Applications
Publication TypeConference Proceedings
Year of Conference2019
AuthorsMusselman, Keith N., Addor Nans, Vano Julie, and Molotch Noah P.
Conference Name87th Annual Western Snow Conference
Conference LocationReno, NV
KeywordsApril 1st, earlier snowmelt, maximum SWE, Snow water equivalent, SWE

Snow is the primary source of water in western North America (Li et al., 2017). In mountainous regions, seasonal snowpack accumulates in the winter and subsequently melts in the spring, effectively delaying the timing of downstream water delivery by as much as a half-year. Because of this capacity to store much of the annual precipitation and extend the delivery of meltwater into the arid spring and summer when demand is highest, snowpack is often referred to as a natural water tower. The date on which the snowpack mass, or snow water equivalent (SWE), reaches an annual maximum at a given location is literally and figuratively a watershed moment – dividing winter accumulation from spring melt and providing useful insight into the expected spring runoff. For these reasons, snowpack is vigilantly monitored across the West to inform reservoir operation and seasonal water supply forecasts that critically support agricultural and resource management decisions.

For the past century, April 1st has been used to approximate the average date of maximum SWE. In fact, seasonal water supply forecasts are largely structured on this date. For example, before the 1950s water supply forecasts in the western U.S. were almost exclusively made for April to September – a period that includes both the snowmelt and agricultural irrigation seasons (Pagano et al., 2004). More recently, forecast target periods have been shortened to April to July to exclude the less-predictable summer rainfall (e.g., the monsoon season) in favor of the more-reliable delivery of meltwater. In warmer regions such as Arizona, New Mexico, and Idaho, forecast periods begin earlier to coincide with earlier snowmelt seasons and/or unique user needs such as earlier agricultural planting schedules. The consistent use of the April 1st metric in water resource applications – and compelling examples of warmer regions where operational forecasts deviate from that target date – begs numerous questions. First, how does the date of maximum SWE vary across western North America? Second, how has this date changed over the past 30+ years of observation in response to reported warming and snowpack declines (e.g., Mote et al., 2018)? Third, what is the hydrologic relevance of the date of maximum SWE? And fourth, how might this change this century?

To address these questions, we present a long-term analysis of SWE data from 969 snowpack monitoring stations in western North America. This includes data from the US SNOTEL network, Alberta Environment and Parks, the Government of British Columbia, and the California Cooperative Snow Survey. The average date of maximum SWE varies geographically (Figure 1). The snowpack of the Sierra Nevada and inter-continental regions such as Idaho peak within a few days of April 1st, while the snowpack in the U.S. Pacific Northwest and Southwest peaks nearly a month earlier. In continental regions including Colorado, Wyoming, Montana and the Canadian Rockies, the timing of maximum SWE occurs closer to May 1st. Despite the large regional variability, the median date of maximum SWE computed on all stations and for the full period of record was within a few days of April 1st.