Warming is Driving Decreases in Snow Fractions While Runoff Efficiency Remains Mostly Unchanged in Snow-Covered Areas of the Western United States

Abstract

Winter snowfall and accumulation is an important component of the surface water supply in the western United States. In these areas, increasing winter temperatures T associated with global warming can influence the amount of winter precipitation P that falls as snow S. In this study we examine long-term trends in the fraction of winter P that falls as S (Sfrac) for 175 hydrologic units (HUs) in snow-covered areas of the western United States for the period 1951–2014. Because S is a substantial contributor to runoff R across most of the western United States, we also examine long-term trends in water-year runoff efficiency [computed as water-year R/water-year P (Reff)] for the same 175 HUs. In that most S records are short in length, we use model-simulated S and R from a monthly water balance model. Results for Sfrac indicate long-term negative trends for most of the 175 HUs, with negative trends for 139 (~79%) of the HUs being statistically significant at a 95% confidence level (p = 0.05). Additionally, results indicate that the long-term negative trends in Sfrac have been largely driven by increases in T. In contrast, time series of Reff for the 175 HUs indicate a mix of positive and negative long-term trends, with few trends being statistically significant (at p = 0.05). Although there has been a notable shift in the timing of R to earlier in the year for most HUs, there have not been substantial decreases in water-year R for the 175 HUs.

Western Canadian freshwater availability: current and future vulnerabilities

“…Since most of the downstream flow originates in mountainous regions, these "alpine water towers" are crucial to society and ecosystems, especially as demands for water resources grow (e.g., Immerzeel et al 2020). Recent and projected increases to temperature and continued and increasing variability in precipitation have sparked concern regarding a changing alpine water tower source with respect to shifts in the high-elevation snow regime and resultant impacts on the timing, duration, and amount of runoff (e.g., Stewart 2009;Dong and Menzel 2018;McCabe et al 2018). This, along with projected increased water use, has major implications for future freshwater availability (Barnett et al 2005;Immerzeel et al 2020).…”

Winter Inputs Buffer Streamflow Sensitivity to Snowpack Losses in the Salt River Watershed in the Lower Colorado River Basin

“…In the Upper Colorado River Basin (UCBR), which currently supplies water to 40 million people, concerns were recently heightened when warmer temperatures since the 1980s were associated with annual streamflow declines [8][9][10]. However, other studies in the western US reveal an apparent paradox: significant increases in temperatures and declines of snow storage or snowfall have not resulted in consistent streamflow losses [11][12][13][14]. Thus, temperature effects on snow and streamflow are not fully resolved and require further examination.…”

Brief Communication: Early season snowpack loss and implications for over-snow vehicle recreation travel planning

“…The observed decreasing trend towards reduced early season snow fraction (S f ; 0.6% year -1 , p<0.0001; Figure 2c), implies that both increasing numbers of dry days and a shift towards increased rainfall are likely contributing to later onset of SWE min . The reduction in precipitation falling as snow is primarily driven by warming temperatures (McCabe et al, 2018), which may be controlled by regional atmospheric and oceanic circulations that favour higher snow level storms (Hatchett et al, 2017). The higher snow levels (and hence lower S f ; Figures 2a-b) reduce snowpack accumulation during precipitation events and can allow for snowpack loss due to turbulent heat fluxes and heat input by rain.…”

“…Ongoing and projected climate change is accelerating the decline of the cryosphere throughout Earth's mountain regions (Huss et al, 2017). Reductions in winter season snow, ice, and permafrost cover and volume primarily result from rising air temperatures (Brown and Mote, 2009) and shifts in precipitation from snow to rain (McCabe et al, 2018). These changes have cascading effects from mountains to lowlands with wide-ranging socioeconomic and ecologic impacts (Huss et al, 2017).…”