Average snowpack/precipitation and average inflow

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@JFRCalifornia and other water, um, nerds like me, perhaps this link will be interesting because of all these different water projects which most i've never studied too much about so for late winter reading pleasure if you aren't able to sleep or need a break from the world news:

a lot of websites there to wander through... :) i think i'll have to make an alphabetical checklist or something so i don't miss something.

And every one of those myriad water entities has a horde of attorneys defending their water rights. No wonder it's difficult to change water releases.
Speaking of attorneys, I was at the gym yesterday, asked a lawyer whom I had met awhile back, how much do you charge to answer a couple of questions? He said, $800, what's your second question?
Okay, I'll bite. Maybe not exactly answering the question you ask, but possibly in a sort of roundabout way. First, a few basic statistics to help frame the answer. Inflow to Lake Powell (or before it existed, flow past Lees Ferry) going back in time goes like this. These are 20-year averages, all in maf:

1920-40: 14.9 maf
1940-60: 13.3
1960-80: 10.8
1980-00: 12.5
2000-20: 8.6

So the trend is generally down over the past century. Well understood, not news. And as noted previously, the 30-year average 1991-2020 is 9.55 maf.

Now in a perfect world of math, where the inflow equals the outflow, you would not expect the lake level to change. And yet that math somehow fails. Let's start with this:


Avg Annual Inflow = 9.55 maf
Avg. Annual Outflow = 9.16 maf

So based on that you might expect, assuming there's no other straws in the drink, for the lake to actually rise during that period. But of course it didn't. In 1991, the lake ranged from 3625-3639. And in 2020, the lake ranged from 3582-3610. That is to say, in spite of a net surplus of inflow vs. outflow during that period, the lake lost 30-40 feet.

Let that sink in for a second. Net inflow surplus. Lake level declines.

What accounts for that is a combination of evaporation, seepage, and some limited Lake Powell surface water use. Not sure what those numbers are exactly, but they are clearly enough to say that unless inflow substantially outstrips outflow in a given period, you're going to see the lake slowly decline.

Now going back to the original dataset I started with, let's look at three periods, just to provide context, and maybe a a closer view of what's going on...


Avg Inflow - 11.1 maf
Avg Outflow - 8.9 maf

This was the period when the lake was filling. A net annual surplus of 2.2 maf was just enough to fill the lake in 17 years. So that suggests that much less than that net surplus would not result in much or any rise in the long term. Which brings us to...


Avg Inflow - 12.5 maf
Avg Outflow - 11.6 maf

In this period, the lake started full and ended full--essentially balanced. Average inflow was high, and of course outflow also had to be high in order to make sure the lake didn't spill over. But in the context of Trix's question, this 0.9 maf average annual surplus seems to be the sweet spot of sustainability, where you end up with the same lake level as where you start. Now let's look at the opposite scenario...


Avg Inflow - 8.65 maf
Avg Outflow - 8.72 maf

On the face of it, this suggests the lake level should be in balance, since input = output. But it's obviously not. In 2000, the lake was basically full. But in 2020, it hovered around 3600, a loss of 100 feet. So when input = output, you actually have a net loss. And if you divide 100 feet by 20 years, it comes to about 5 feet per year.

So back to Trix's question, this all suggests that since we need a 0.9 maf surplus to keep the lake from falling, given that outflow is set this year at 7.48 maf, we would need to see an inflow of 7.48 + 0.9 = 8.4 maf...

So there's your answer. This year, we need to see about 8.4 maf inflow to keep the lake from falling... And in years when releases are set at 8.23 maf, we would need 9.13 maf inflow just to stay even... which is more than the average annual average from 2000-20 (8.65 maf). And that is the problem.

For context, here was the inflow of the past 12 years:

2010 - 8.8 maf
2011 - 16.3
2012 - 6.1
2013 - 5.3
2014 - 9.3
2015 - 9.4
2016 - 9.9
2017 - 11.4
2018 - 5.4
2019 - 11.8
2020 - 6.5
2021 - 4.0

I'll let real modelers like drewsxmi take it from here, and really explain the underlying factors in more detail, but that's the layman's version of what we're facing...
Okay, I took a page from drewsxmi and decided to plot the data in order to create a formula that estimates how to correlate net volume change in a given water year with changes to the lake level. If I haven't lost you so far, read on. Geeks only.

The approach was simple enough. I compared the inflow to outflow in a given water year (Oct 1 - Sept 30) all the way back to 1964 and took the difference. As noted in my previous post, some huge inflow years such as 1983 were also huge outflow years, so the net increase wasn't that big in such years. Here were the top five and bottom five net inflow years during life of the lake:

Top Five

1979 +6.34 maf
1997 +5.77 maf
1973 +5.53 maf
1993 +5.09 maf
1983 +3.80 maf

Bottom Five

2021 -4.19 maf
2002 -4.17 maf
2018 -3.60 maf
2012 -3.36 maf
2013 -2.97 maf

So what you know from this is that even in a wildly good year, don't expect a net volume increase of more than about 5 maf. And a really bad year might be a 3-4 maf decrease.

That's the x axis.

The y axis is straightforward. It's the net change in the lake surface level when you compare Oct 1 to a year later. And here are the top and bottom five lake level changes when Oct 1 is the start and finish line:

Top Five

1973 +42.61 feet
1993 +39.50 feet
1965 +38.50 feet
1979 +37.96 feet
1995 +32.60 feet

Bottom Five

2021 -50.54 feet
2002 -38.18 feet
2018 -36.26 feet
2004 -32.92 feet
2012 -31.40 maf

Again, from this you can see the historic range of increases and decreases. Last year was by far the worst. And in the very best years, you might see something approaching 40 feet of net gain. But hard to see the trends from those charts, so I plotted everything back to 1964 on a chart, and here it is:

Lake Rise vs Volume.png

Each blue dot represents one year. The dotted line was generated by Excel to calculate the "best fit", or what we might expect in general. You'll see the data points on the outlying ends of the line tend to be the worst fit--where you get a higher variation in rise or lake fall from what you'd expect from the net volume change. Perhaps this has something to do with other factors such as soil moisture, which during extended droughts tends to decrease quite a bt and suppress lake rise, or exacerbate decreases, such as last year.

In any case, the data makes for a pretty clear trendline. One clear conclusion is that on average, if inflow = outflow, the lake drops by about 5.7 feet. The corollary to that is this: in order to keep the lake at the current level (or higher), the net inflow has to exceed 0.71 maf.

The equation is this:

y = 8.1x - 5.7

y = annual lake level change (feet)
x = annual lake volume change (maf)

[Oct 1 to Sept 30]

So what does this mean for this year?

Well, let's start with this: the lake level on Oct 1, 2021 was 3545.

We also know that BOR plans to release 7.48 maf in WY2022.

Here's the likely range of possible outcomes:

If WY 2022 inflow = 4.0 maf, net inflow = -3.48 maf.
Oct 1, 2022 water level = 3511

If WY 2022 inflow = 5.0 maf, net inflow = -2.48 maf.
Oct 1, 2022 water level = 3519

If WY 2022 inflow = 6.0 maf, net inflow = -1.48 maf.
Oct 1, 2022 water level = 3527

If WY 2022 inflow = 7.0 maf, net inflow = -0.48 maf.
Oct 1, 2022 water level = 3535

If WY 2022 inflow = 8.0 maf, net inflow = +0.52 maf.
Oct 1, 2022 water level = 3543

If WY 2022 inflow = 9.0 maf, net inflow = +1.52 maf.
Oct 1, 2022 water level = 3551

If WY 2022 inflow = 10.0 maf, net inflow = +2.52 maf.
Oct 1, 2022 water level = 3559

You get the idea from there...it's basically add 8 feet to the lake for every net increase of 1 maf, then subtract 5.7 feet...

Now given that total inflow since October 1, 2021 has only been 1.7 maf, we have a lot of ground to make up this spring and summer...

So there you have it... let's hope for more snow... but even if we have something huge happen from here on out, we're unlikely to exceed 3550 on October 1...more likely in the 3525-40 range...
...and to follow up...

I've read all kinds of wild speculation online about how low the lake might get this year, and how the dam won't be able to generate power, and so on. Most of it is way off base. Consider this:

As of March 14, the inflow to the lake during WY2022 (i.e, since Oct 1, 2021) has been 1.7 maf. If somehow someone turned off the tap completely and did not add another single drop to the lake, and the BOR continued releasing water as planned (up to 7.48 maf), the lake would end up with a net deficit of about 5.8 maf for the year. And if that happened, based on the trends shown in the graph on the previous post, the lake would be at 3493 on October 1. Maybe a tad lower because of the martini glass profile of the lake and soil moisture issues. But the bottom line is that it would still generate power this year.

Mind you--that is an impossible scenario. No more water entering the lake. And there'd still be power.

It's also worth remembering that historically, the lake on average adds about half of its inflow from May 1 to July 31. In truly terrible years, it's closer to 25-35% of the annual total, but in any case, it's never been less than about 1 maf inflow in that May-July timeframe, with a historic average closer to 5 maf. So assuming conservatively this is going to be a below average year, there's still going to be significant water added in the late spring.

The reasonable worst case scenario (a repeat of last year) still has the lake at 3511 on October 1. But if the May-July inflow is even about 3 maf or so (plausible given the current snow conditions), the lake would end up the water year somewhere in the neighborhood of 3525-3530. Not great, but not doomsday either.
The storage and pool level data for almost the entire history of Lake Powell can be downloaded in CSV format from the USBoR at https://www.usbr.gov/uc/water/hydrodata/reservoir_data/919/csv/49.csv for pool level and https://www.usbr.gov/uc/water/hydrodata/reservoir_data/919/csv/17.csv for storage. I've plotted them against each other and they make a nice, smooth curve:

If you follow the curve to a storage amount of 0, it hits around 3,370' of elevation, the official minimum storage reserve or "dead pool" level. If you don't want to try following that curve, you can make a guess at storage, then find a matching figure in the storage data, look up the date, then go back to see what the pool level was at that date. Somewhere along the way the storage data was adjusted for sedimentation, so you can add about 211,000 af to the 2022 storage number to get an index into the level data from the 1960's. That's the basis of my predictions in February (see Guess Lowest Lake Level Spring 2022 and subsequent post from Eagle Rock with the sedimentation correction.)

Or you could also just look at the official storage numbers from Lake Powell Water Database, see that the official storage is 5,937,636 af as of 3/13/2022, and try to guess what runoff will be, subtract the absolute worst case that JFR mentioned of 5.8 maf (Average snowpack/precipitation and average inflow), and come up with a really scary number for storage.

My recommendation is do all of your predictions and calculations based on storage, then look up the matching level as your last step.
We visited Mesa Verdi National park last year, will probably go back to view it more it is hard to see in one day. These peoples had a great place for many years then drought. Although no one knows for sure why they left, drought is one of the top thoughts. Climate changes all the time, if not we would still be under ice. I am not a geologist or meteorologist but i do believe the weather has its cycles. We have been in this dry cycle for about ten years now, I don't know how long the Anastasi drought lasted but we I believe we are in the same cycle. Just a thought.
Blue Mesa Lake my not even have a season due to the water they have let go to Powell. Not a lake trout fisherman but i do enjoy kokanee fishing once a year there. Not smart enough to be a geek!!
Blue Mesa Lake my not even have a season due to the water they have let go to Powell. Not a lake trout fisherman but i do enjoy kokanee fishing once a year there. Not smart enough to be a geek!!
Yep, basically if blue mesa goes down AND i cant launch at Powell neither, my boat is for sure going up for sale..... :(
Yep, basically if blue mesa goes down AND i cant launch at Powell neither, my boat is for sure going up for sale..... :(
I live in the Rifle CO area so every good lake is about 4hrs away. Starvation is a good lake for walleye and trout. Not huge but has nice facilities.
Hang on to the boat u will miss it i'm afraid. Until I get to old to go I will try anywhere to fish. None will beat Powell but we all know that!
I live on the front range and had to forego my usual houseboat trip to LP this year (first two weeks of May) since it didn't look like I'd be able to even launch the fishing boat. My regular crew made other plans since we couldn't be sure of even being able to get on the lake at BF. So.....the one crew member (who didn't make other plans) and I are going to Lake McConaughy for part of a week. Not what I'm used to, but better than a shrink. Save the boat! You can find something else if you try.
Phx Channel 12 just ran one of their Verify segments on whether normal snowpack will refill Powell and Mead. Of course their conclusion was "no" and showed just the results for 2020 (85% precip, 32% runoff). Brief comments from some water expert that it would take several years of normal to help fill the lakes. Too bad they didn't get the full story from our experts!
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