Watershed Sublimation
Much of the snow that was expected to melt and fill the Catskill and Delaware reservoirs vanished into thin air in the past few weeks…
This wasn’t a mysterious mountain magic trick but rather a physical process known as “sublimation.” That is when a solid transitions to a gas without the intermediate step of becoming a liquid. Think of dry ice and the ghostly vapors it gives off—that’s sublimation. Snow, however, doesn’t give off such overt signs when it turns into water vapor. The process occurs quietly. There’s no specific way to measure it other than by just paying attention to the weather and what’s on the ground.
Our snow surveys showed that snowpack grew consistently during January and reached well-above average during February. The February 23 survey found a total of 123.7 billion gallons (BG) of water trapped in the snowpack across the entire system, compared to the historical average of 42.5 BG. Despite such high quantities of stored water, very little melted and February runoff in the watershed streams finished 65 percent below normal. The loss of snowpack became evident in the March 2 survey, which showed 86.3 BG remaining in the snowpack. One-third of the snow had melted, but watershed streams still did not show an increase in runoff.
Warmer temperatures began to melt some snow pack during the second week of March, causing runoff to increase. Still, the loss due to sublimation was pretty substantial. The March 15 snow survey showed a significantly different story with only 29.3 BG compared to the historical average of 52.3 BG for that time of year.
So how did the snowpack dwindle without causing a significant increase in streamflow? Temperatures stayed below freezing much of the time (no melting) and many days were sunny (energy that turned snow into vapor). Wind and low humidity also played a role in increasing the loss of water from the snowpack to the atmosphere. Appreciable rainfall, high humidity (to lessen evaporation) and temperatures above freezing are perfect conditions for high amounts of runoff.
Luckily, the loss of the frozen assets with minimal return isn’t cause for any significant concerns about the water supply. Atmospheric conditions in the near future should be favorable for spring rain, and computer modeling shows good probability of the reservoirs being refilled by late spring.
