Thursday, January 26, 2017

Liveblogging the Deluge: The Post Mortem...Storms That Made a Difference

Don Pedro Reservoir on January 5 before the storms, elevation about 786 feet (76% of capacity).

The atmospheric river storms of January have finally subsided, and we are looking at kind of a new landscape across California. We had a couple of sunny days, so we headed up the highway to have a look at Don Pedro Reservoir on the Tuolumne River coming out of Yosemite. When we visited on January 5, the lake stood at 786 feet, which translates to 76% of capacity. That was 114% of normal for this time of year, which was a relief after five years of intense drought.
Don Pedro Reservoir on January 26 after two atmospheric river storms, elevation 813 feet (90% of capacity).

When we arrived yesterday, the lake stood at 813 feet, or 90% of capacity, which is 133% of normal. It is only 17 feet below the level of the dam. It could have been higher, but the dam operators have been maintaining constant outflows in the range of 7,000-9,000 cubic feet per second, a level that is just short of flood stage. Had they not done this, the lake could have overflowed and flooded downstream urban areas as happened in 1997.
It is a similar picture at the major reservoirs all around California. The monster reservoirs, Shasta and Oroville, are at more than 120% of normal for this time of year. San Luis Reservoir, the critical link in the California Water Project, is 103% of normal, and may fill for the first time in five years. A few are still low, most notably New Melones (68% of normal), and Trinity (84% of normal), but they are way up from where they stood before the storm.
The picture in my corner of the woods is brighter too. The Tuolumne River is running fast and cold, and in many places it covers practically the entire floodplain, and has done so for several weeks now. I believe that most areas downstream are not affected by the high flows (there is a history after all of flooding, and outflow from Don Pedro is regulated so as to prevent it). Instead the river is flushing itself of invasive hyacinth, which has been threatening numerous creatures and plants. Gravel bars are shifting and accumulated silt is being removed, providing better nesting areas for salmon and other fish species. Silt is being redistributed across the floodplain, rejuvenating the soils of the riparian areas. Groundwater aquifers in the vicinity of the river are being recharged.
Tuolumne River in Waterford, about 8,000 cubic feet per second

My rain gauge has been active. My friends in more humid regions may snicker a bit when I talk of how we've had 7.38 inches of precipitation in January. That might not seem like all that much but in 25 years of recording rainfall in Waterford, only two had greater amounts, 7.58" in 1995, and 8.60" in 2011.  We've had a year where that was the total for the entire season (7.26" in 2014). We've already passed the total rainfall normally received in an average year (14.69" so far compared to 13.92" average for the last 25 years).

But the real story? The snowpack.

At the height of the drought, we had a snowpack that was 5% of normal. 5%! It was both unbelievable and appalling.
This year started out with some good healthy storms, but the January events threatened to undo the progress. On the first of January, the statewide average was 70% of normal, which was an improvement over the last five years.The atmospheric rivers began as warm tropical storms that threatened to melt what snow there was. At first they did, but colder arctic are started to move in and the snowpack began to build. After the first set of storms blew through, the statewide snowpack had improved to more than 100% of normal.

And the snow kept falling! The second atmospheric river storm system brought even more snow, and as of today, the snowpack is nearly 200% of normal. If the trend continues (there is NO guarantee of this), we could be on track for a record snowpack. I don't consider it likely, but it would be nice to start recharging the groundwater aquifers that have been severely depleted in recent years.

The record snowpack should not be seen as evidence that global warming isn't happening. Average temperatures are defiinitely up and have been for several decades. The thing is, snow is snow whether the temperature is 15 degrees or 30 degrees. A not-at-all unusual heat wave could undo the progress in the snow levels. The sky spigot could turn off. After the record flooding of 1997, February and March both provided less than a quarter inch of rain over the entire month. And the drought will never be truly over, as demand outstrips supply even in plentiful years. And little is being done to replenish declining groundwater aquifers.

All in all, though, I'm happier to have plentiful rain rather than crippling drought!

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