A Review of the Hydrologic Response Mechanisms During Mountain Rain-on-Snow

Abstract

Mountain rain-on-snow (ROS) generates large flooding events worldwide. Climate warming will enhance the frequency, magnitude, and widespread nature of these events. Past studies indicate rainfall, not snowmelt, typically drives much of the runoff response during ROS. However, there is substantial event-to-event variability—resulting from shifting atmospheric drivers and nuanced physical mechanisms governing water flow through a snowpack. Historically, turbulent fluxes were assumed to dominate the energy balance for snowmelt during ROS. Recent research nonetheless suggests that other components of the energy balance might be larger drivers depending on: 1) the time of year; 2) the elevation; and 3) the aspect of the slope. This mini review summarizes the literature on the physical processes governing ROS and proposes that moving forward we utilize the terms “active” and “passive” to describe a snowpack’s contribution (via snowmelt) to terrestrial water input (TWI) during ROS. Active snowpacks readily contribute meltwater to TWI via the energy balance, bolstering rainfall-runoff totals. Passive snowpacks do not melt, but simply convey rainwater through the snow matrix. In both snowpack cases, preferential flow paths enhance transmissivity. This proposed classification scheme will help researchers and water managers better communicate and interpret past findings, and aid in forecasting discussions of future events.

Decline in Seasonal Snow During a Projected 20-Year Dry Spell

“…Increasingly ephemeral snowpacks and persistent drought pose challenges to water management paradigms built on the assumption of abundant seasonal snowpacks [8,12,19,54]. With less snowpack cold content [27], ephemeral snowpacks are more prone to melt and have the potential to increase flood hazards [55]. Elevated midwinter runoff, whether via radiation-driven melt or rain falling instead of snow, often occurs at the expense of later season flows as this water is not stored in the mountain snowpack and is more readily available for ET, lowering runoff efficiency.…”

Watershed memory amplified the Oroville rain-on-snow flood of February 2017

“…First, the degree to which snowmelt amplifies runoff during ROS is crucial yet highly variable ( 12 ). Snowmelt contributions are often quantified by comparing the snowmelt volume to the sum of rainfall and snowmelt, or terrestrial water input (TWI), which can range from 0 ( 13 , 14 ) to 60% ( 15 ).…”

“…Snowmelt contributions are often quantified by comparing the snowmelt volume to the sum of rainfall and snowmelt, or terrestrial water input (TWI), which can range from 0 ( 13 , 14 ) to 60% ( 15 ). While rainfall primarily drives TWI ( 12 ), even small snowmelt contributions (e.g., ∼10%) at unfavorable times or locations can dictate whether or not a water engineering emergency occurs. Snowmelt is the product of the energy balance, and can only begin once energy inputs exceed the snowpack’s heat capacity (i.e., its cold content) ( 16 ).…”

“…Case studies indicate that turbulent and longwave radiative fluxes during extreme ROS can dominate snowmelt. Several examples of these “active” ( 12 ) contributions to TWI range between 21 to 56% in the United States Pacific Northwest ( 17 ), 13 to 26% in the Swiss Alps ( 18 ), 25% in the California Sierra Nevada ( 19 ), and 2 to 60% in Black Forest, Germany ( 15 , 20 ). These large ranges indicate the diversities in both storm meteorological drivers and snowpack states, which vary considerably across elevation, aspect, and vegetation ( 4 , 13 , 15 , 17 , 18 , 21–24 ).…”

“…Snowpacks in such ROS events are considered “active,” meaning that melt contributions to TWI exceed 10%. Snowmelt from “passive” snowpacks contribute less than 10% to TWI—a nominal threshold representing the small yet inevitable heat advection delivered to snow during rainfall ( 12 , 22 ). In the Sierra Nevada, rainfall contributions dominate TWI (∼77 to 95%) ( 4 , 30 ).…”