Snow surveys conducted around every April 1st coincide with the average date of peak snowpack. This year, the surveys were completed at the end of March and revealed a large increase in snowpack over the previous month—from 50% of average to 76% of average!
Posts Tagged ‘Hydrology’
Last week the Los Angeles Department of Water & Power (DWP) released its preliminary runoff forecast based on record-breaking March 1st snow surveys: 195% of average runoff for the April 1, 2017 through March 31, 2018 runoff year. This volume of runoff is very similar to 1983, the wettest runoff year on record. Due to the warm storms in January and February, the lower-elevation snowpack below about 9,000 feet above sea level is much lower than in 1983, so we are assuming that record runoff is a high-end scenario. That forecast also assumes median precipitation over the next year.
Based on 1983 as a high end, 1995 as a probable scenario, and 2006 as a lowest possible scenario, we modeled the likely rise in Mono Lake based on those past year inflows and probable reservoir operations this year. The result? A 3.8-foot rise in Mono Lake is likely over the next year. Expected Grant Lake Reservoir operations add about half a foot to our forecast.
All three scenarios have little or no rise before May and a similar rise in May and June, since snow can only melt so fast, (more…)
After the wettest February since 1986 at some survey sites, Mono Basin snowpack is more than double the March 1st average!
Snow water equivalent (SWE) ranges between 205% and 244% of average at the five snow survey sites in the Mono Basin (called snow courses). Gem Pass, Ellery Lake, and Saddlebag Lake have the highest March SWE on record. At the lowest-elevation snow survey site—Gem Lake at 9,150 feet above sea level—SWE was about 10 inches shy of the 1969 record, but it had reached the 1983 amount. The Tioga Pass snow course was 5 inches shy of the 1983 record. In the map below showing the snow courses, portions of the Lee Vining Creek (top) and Rush Creek (bottom) watersheds are outlined.
February 1 snow surveys conducted over the past week revealed an approximately 200% of average snow water content in the Mono Basin. This puts the April 1 average at just over 120%.
It is still early in the season, but even with a dry February and March, it will be an above-average year, and with an average end to the season it will be a wet year. With a wet February and March, new April 1 records could be set!
At the end of the wettest January in the 29-year record for Lee Vining, how do Mono Basin season totals for precipitation and snowfall compare to other years? The season precipitation total (since October) of approximately 15.84 inches of water is among the wettest years recorded since 1989. It is the second-wettest year to date (1997 was the wettest) that we’ve recorded in 29 years as of the end of January.
At Cain Ranch, five miles south of Lee Vining and more representative of the precipitation that falls on Mono Lake itself, the 2017 seasonal total of 9.44″ to date is sixth-wettest in 86 years. For both Cain Ranch and Lee Vining, these totals fall into the wettest 7% of years. (more…)
When Mono Lake is between 6377 and 6380 feet above sea level, and the final May lake level forecast (and any subsequent projections) shows that it will stay above 6377 feet, the Los Angeles Department of Water & Power (DWP) is permitted to export 4,500 acre-feet of water that year. Any time Mono Lake falls below, or is projected to fall below 6377 feet, exports must stop.
Operations plan guidelines state that the water should be exported late in the summer, and this year, DWP exported this water September through early November, allowing more water to remain in Grant Lake Reservoir during the summer—a good thing that kept the reservoir higher during recreation season and likely kept water temperatures cooler for fish in Rush Creek. (more…)
At Cain Ranch, a precipitation station five miles south of Lee Vining that is representative of precipitation patterns on Mono Lake, October 2016 precipitation was 2.1 inches. This was the fifth-wettest October since 1931 and about 325% of average. Two of those other wet Octobers occurred in 2009 and 2010.
October 2016 was also the sixth-wettest month at Cain Ranch in the last decade, and curiously three of those months were the Octobers mentioned above. The other three months that were wetter than October 2016 were December 2010 and 2012 and May 2015. Aside from the wet Decembers, this pattern is unusual, since December and January and February are usually the wettest months of the year.
The rain in the last week raised all the creek flows and also raised Mono Lake from 6377.12 to 6377.19 feet above sea level, putting it back on track with the lake level forecast DWP made in the spring.
The strong 2015–2016 El Niño winter is likely to turn out below-average in the Mono Basin. The 90% of average February 1st snowpack, and an 80% of average preliminary runoff forecast based on that snowpack, were followed by a dry February. In the next week, March 1st snow surveys will reveal how bad things are. A wet March might be able to catch things back up to average, but it would take a very wet spring to shift the snowpack and snowmelt runoff above average at this point. From 1980–2010, the lowest March snowpack that resulted in above-average runoff was 110%. (more…)
October 1st marks the start of a new water year. The 2016 water year runs October 1, 2015–September 30, 2016. It measures the fall-winter precipitation and the rain that falls in the following growing season all in one 12-month period.
Here is how last year stacks up for Lee Vining precipitation: 11.84 inches, or 84% of average. The remarkable thing is that the fall-winter was the lowest on record, at only 34% of average, meaning the snowpack was the lowest on record. Summer was highest on record, at 285% of average.
At the Cain Ranch weather station, five miles south of Lee Vining, as of July 13, 1.57 inches of rain had fallen in July. That makes this month already the second-wettest July on record after the 1.98 inches of rain in July 1965. Records at this location began in 1931. Rain fell on all but two days between July 1 and 10, and while it has been dry there since July 10, it still has been raining in other parts of the Mono Basin almost every day.
Lee Vining Creek above the Los Angeles Department of Water & Power (DWP) diversion dam experienced its peak flow of about 124 cubic feet per second on July 11, from rain and melting fresh snow. Below the dam, the minimum flow is being released, and the floodwaters are being diverted to Grant Lake Reservoir, which has been slowly rising since May.
Aside from brief floods due to thunderstorms, the Mono Basin’s creeks are dropping to the very low levels that were otherwise expected this summer. If the thunderstorms stop, we will start seeing new low flow records later this month, especially in watersheds without glaciers, such as Walker Creek. The April–September snowmelt runoff forecast issued by DWP in May predicted 19% of average runoff, with a lower bound of 7% and an upper bound of 32%. Nineteen percent is less than half of the runoff measured in 1977, the driest year on record; 32% is still much drier than the driest year on record. Thanks to the recent wet weather, Mono Basin runoff is on track to reach 32%.