Defining Snow Drought and Why It Matters
However, among references to snow drought, we observe conditions that reflect a lack of winter precipitation or a lack of snow accumulation during near-normal winter precipitation We propose a new classification to differentiate “dry snow drought” from “warm snow drought.” These two uses of snow drought have different scientific underpinnings and different implications for water supply forecasting and management.
Despite large differences in the amounts and timing of winter precipitation, the snow water equivalent (SWE) in both regions was less than average.
Snow drought in the Pacific Northwest reflected a lack of snow accumulation due to warm temperatures that increased rainfall and melted snowpacks, despite near-normal precipitation.
With less precipitation and warmer temperatures, streamflow in the Sierra Nevada lacked both large winter flows and its usual spring snowmelt pulse and fell to extremely low levels early in the summer.
However, the same term, snow drought, was used to describe snow conditions in both regions.
Defining and Quantifying Snow Drought We propose more precise terms to distinguish between the two different snow droughts observed in the Pacific coast states in winter 2015: dry snow drought for precipitation-driven snow drought and warm snow drought for temperature-driven snow lack.
If the SWE and precipitation are nearly equal (SWE:P is close to 1) and SWE is below normal, winter precipitation must also be below normal, and the lack of SWE is likely a reflection of low precipitation—a dry snow drought.
During a dry snow drought, streamflows are low, and inflows to reservoirs are reduced all year long.
In the case of the Oroville Dam, water managers are currently battling with a reservoir that is now filled to (and beyond) capacity but that contained only 46% as much water on the same date in 2015 in the midst of the snow drought described above (and only 30% as much water by water year’s end).
The (Unknown) Hydrological Effects of Snow Drought The definitions of warm and dry snow drought help to frame science that is fundamental to water management challenges facing snow-dominated regions: Which form of snow drought (dry or warm) is likely to dominate in different areas under future climate scenarios?