Personal Water Supply Update

Here is a great photo of Vernal Fall — I wish I had taken it! It is a file photo from BayTurtle photo galleries, but I hope to show you some from my trip to Yosemite this past weekend.
The flow in the Merced River — which is the stream in this waterfall — is quite a bit higher than what is seen in this photo. However, I was a little discouraged because I have witnessed higher flows at Vernal later in the season. This is a possible indication that the snow pack is not as extensive as in past seasons.
However, it is hardly proof positive of a low snow pack. When the California Department of Water Resources does their monthly snow surveys, they measure the snow level and the water content at a few dozen locations in the Sierra Nevada range. The reason for the multiple sites is that precipitation is often quite variable from one location to another — sometimes even when the sites are close to one another. So one possible explanation for a lower-than-expected flow at Vernal Fall is that the immediate watershed that feeds this stream has a low snow pack, even though adjacent watersheds may have significantly more snow.
Another possible explanation is timing. The flow over the fall is from liquid water, but the snow is frozen. Melting is necessary to produce stream flow. Heat impacts the rate of melting, so a cool Spring will not produce as rapid a melting as a warm one. If this is the case — and I believe it is a factor this year — that is good news. The slower melting rate means that stream flows will persist later into the Summer and Autumn, and continue to add water to our reservoirs. So far this year, that is the impression you get when examining the DWR Lake Oroville storage chart.
Another aspect of timing is night versus day. The flow actually changes during the course of the day, since day time temperatures produce a lot of melting, while night temperatures — even at this time of year — may actually cause some freezing to occur. So we would expect to see higher flows during the day, especially late in the afternoon, following the warmest hours of early afternoon.
But there is still another aspect of timing: how far is the source away from the stream? Vernal Fall normally flows year-round, because the Merced River has a very large watershed. Snow that melts on a sunny slope near the boundary of Yosemite National Park has to travel several miles along the Merced and its tributaries before it passes over Vernal Fall. Early in the Spring, there is snow right next to the Fall, so this transit time is minimal; later in the season, the transit time could be several hours. (My “quick-and-dirty” estimate of this transit time is around 8 hours.)
In short, I can’t tell you precisely the best time to visit Yosemite to witness the highest possible flows in the waterfalls. Really, the best time to go is … whenever you can get there!
I’ve heard from a couple of folks about their State Exam experiences last Saturday. I would love to hear from each of you who took the test. As I mentioned last week, the State does not want you to divulge exact questions, so that’s not what I am asking of you. Instead, please let me know what areas of the test most surprised you, and where you had the most difficulty. That type of feedback is invaluable to me, as I prepare for future training classes. Your help is greatly appreciated.

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