Every Last Drop A Newsletter From the Keep Long Valley Green Coalition

Decoding Winter

by Allison Weber

Water year, runoff year, percent of normal, average or median, snow water equivalent...living in the Eastern Sierra certainly comes with a lot of water related jargon. What do water managers mean when they use these words and how can we, as citizens, interpret this information?

In honor of being in the midst of one water year and towards the end of a runoff year, let's break these terms down to better understand annual reports, newspaper articles, and your friends' social media posts about the lack of snow.

Runoff: This is a term that has gotten a bad reptutation for its connotation with agricultural pollution, but in the water management world it usually refers to the result of precipitation, whether by rain or snow, that is not lost to evaporation, transpiration from vegetation, or infiltration into the soil. The water that remains is the amount of water that "runs off" and makes its way to feed streams and rivers. It is this water that feeds the Owens River and is spread for irrigation in Long Valley.

Water Year: A "water year" begins when the next precipitation season begins, when summer heat subsides and rain and snow begin to come knocking once again. This is considered in California to be on October 1st, with the last day of the "water year" being September 30th.

Runoff Year: Occasionally used interchangeably with water year, generating confusion, this term refers to the period in which snow begins to melt and therefore becomes runoff, feeding local streams, creeks, rivers, and lakes. Our local water departments consider a "runoff year" to begin in April and end in March of the next year.

Percent of Average: Percent of average often refers to the average for that specific date or point in the year, but can also refer to the seasonal average, as in the average amount of snow in a year. The comparison is a percentage, with 100% meaning that the value on that day, or in that year if referring to a seasonal average, is exactly equal to the average, calculated by adding up all of the individual values and dividing this total by the number of observations.

Percent of Normal: Also referred to as the percent of the median, this term refers to the comparison of current conditions to the median of a historic 30-year time period on that same date in the year. The median is the middle value in a set of numbers, with half of the set above and the other half below the median. Using the median instead of the average can help account for extreme conditions, such as especially dry or especially wet years.

Snow Water Equivalent: The snow water equivalent measures the amount of water contained in the snowpack. This can be very different depending on weather conditions.

Snow Drought: A snow drought occurs when there is a below normal snowpack, no matter if precipitation in general is below, at, or above normal. This happens when more precipitation falls as rain rather than snow in the winter.

Watershed: A watershed is a land area that channels rainfall and/or snowmelt to creeks, streams, and rivers. In the Eastern Sierra, you can see this in action on a warm spring day as you watch snow melt down a hill side to join a stream. Watersheds can be big or small, and many small watersheds make up larger watersheds.

Atmospheric River: This weather term made its' debut as a common household name last winter due to the many times it was said over and over in the news; California was slammed by 31 atmospheric rivers from late 2022 through March 2023, resulting in record breaking amounts of rain and snow. These "atmospheric rivers" are defined as flowing columns of condensed water vapor in the atmosphere that, as one makes landfall, can produce significant rain and snow.

Infographic on atmospheric rivers from the National Oceanic and Atmospheric Administration (NOAA).
Infographic on prevailing wind patterns across the globe from NASA/JPL-Caltech.

To understand the hot topic terms that are El Niño and La Niña, or more officially, the El Niño-Southern Oscillation (ENSO) cycle, we have to first understand what is "normal" in the Pacific Ocean, where these weather patterns are generated. Normally, trade winds, winds flowing from the east to the west and circling the Earth near the equator, take warm water from South America towards Asia. Upwelling then occurs, in which cold water rises from the depths of the ocean to replace the warm water.

El Niño Weather Pattern: Literally "little boy" in Spanish, this is a pattern which breaks up the status quo. In an El Niño year, trade winds weaken, causing warm waters to be pushed back east, toward the west coast of the Americas, instead of flowing west towards Asia as normal. The warmer waters cause the Pacific jet stream to move southward, causing the north to be warmer and drier and the south to be wetter.

La Niña Weather Pattern: Literally "little girl" in Spanish, this pattern breaks up the norm in an opposing way. During La Niña, trade winds become stronger than usual, pushing even more warm water towards Asia and creating a stronger upwelling effect. These cold waters in the Pacific Ocean push the jet stream northward, bringing cooler temeperatures and storms with it, and leaving the south more dry and warm.

While these weather patterns are often discussed in the news, the truth is they can be unpredictable for the Eastern Sierra's central location. You can see in the two above infographics that our beloved home is neither in the obviously dry or obviously wet regions for each pattern.

Miracle March: Miracle March refers to high precipitation in the month of March, referred to as a miracle both for the water it brings for the health of the landscape as well as the snow it brings for winter sport loving enthusiasts.

At the risk of jinxing it, it is being suggested that another atmospheric river is heading our way for next week, bringing more snow and with it more snow talk. If the savvy Every Last Drop reader was confused on any of these terms, they now will be ready for conversations around town or reading water management reports. What else in the water world should we decode? Write to us at info@keeplongvalleygreen.org

Volume 4 - Issue 1 | January 2024


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