Average snowpack/precipitation and average inflow

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Trix

Keeper of San Juan Secrets
I know our resident staticians/water wizards have provided their analyses of inflow (runoff) but I couldn't find any recent threads to satisfy my current curiosity.
The USBR has their three inflow scenarios based on inflow as percent of average. The Feb study has most probable as 7.26 maf at 76% of average, so average would be 9.55 maf (7.26 ÷ 0.76). I couldn't find the USBR period for their average (last 30 years, 1990-2020, 1980+, dunno).
The LPWDB and USBR "snowmap" have percentages of average and one would assume they are based on same average.

Soooo, if the snowpack/precipitation numbers can be sustained through the spring months at their current levels (🙏) around 100%, then can we expect about 9.5 maf rather than 7.26 maf and water levels above most probable? Ignoring: dry soil, windy spring, changing releases, yada, Yada.

Next week's 24 month study should be interesting.
 
Not to be a downer and hijack your post, but....I sound like a broken record here....even if we have an average snowpack, runoff will be lower than 7.26 maf. Because of the warming temperatures, evaporation has increased exponentially--thus the dry soils and paltry runoff we have experienced recently, even when snowpack #s are near or above average. Here in the Upper Green River basin in Wyoming, we have been near or above average snowpack in most of past 10 years. During this same time we have experienced record low late season stream flow and soil moisture levels. The water is simply not making it to and down the rivers like it used to. The snowpack data seems to be less predictive of stream flow and runoff than it used to be.

This is abundantly clear for Lake Powell when you look at water year 2020. Snowpack was slightly above average, but the runoff to the lake was pretty disappointing, the lake didn't even rise more than a foot during runoff season....
 
Dorado, yep, know recent runoff has not matched snowpack percentages for a menu of probable reasons. Not a hijack, just more info.
So we know (recently at least) average snowpack doesn't produce average inflow. But both averages are over the same historical period (I presume last 30 years ending in a recent year). And the disconnect results from several hydrological and weather factors.
Thus, from recent evidence it appears that we would need about a 130% of average snowpack to produce average inflow. Would love to sit in on a briefing with USBR statisticians to hear their assumptions and see their formulas.
Or maybe one of our WWW (Wayneswords wizards) can enlighten me (us). Maybe a WWW has already calculated or posted a table of snowpack versus water level.
 
A further complication: runoff is not just a function of snowpack and soil moisture, as already discussed above. It's also a function of consumptive use upstream of Powell. For example, between 2016 and 2018, annual Upper Basin water usage increased 467,000 acre-feet (that's excluding the increase for Arizona, which comes out of Powell, and the change in Powell's evaporation, which obviously also comes out of Powell). See https://www.usbr.gov/uc/envdocs/rep...adoRiverBasin2016-2020-CULReport-508-UCRO.pdf, pp. 9 (summary table) and 23 (evaporation by reservoir) of the pdf. So that's almost half a million acre-feet of water that got to Powell in 2016 but not in 2018. I have no idea how much Upper basin consumption has changed since then.
 
I know our resident staticians/water wizards have provided their analyses of inflow (runoff) but I couldn't find any recent threads to satisfy my current curiosity.
The USBR has their three inflow scenarios based on inflow as percent of average. The Feb study has most probable as 7.26 maf at 76% of average, so average would be 9.55 maf (7.26 ÷ 0.76). I couldn't find the USBR period for their average (last 30 years, 1990-2020, 1980+, dunno).
The LPWDB and USBR "snowmap" have percentages of average and one would assume they are based on same average.

Soooo, if the snowpack/precipitation numbers can be sustained through the spring months at their current levels (🙏) around 100%, then can we expect about 9.5 maf rather than 7.26 maf and water levels above most probable? Ignoring: dry soil, windy spring, changing releases, yada, Yada.

Next week's 24 month study should be interesting.
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:

1991-2020

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...

1964-80

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...

1980-2000

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...

2000-2020

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...
 
Like throwing a pumpkin Yamamoto split tail into the stick ups, I pulled a large mouth JFR out of the weeds!
Thanks for the deep dive.
Is there a database of basin snowpack by date whereby you could retrieve the spring (say March 15 or Apr 1) snowpack percentages to correlate to annual or decadal inflow?
Thanks,
Keith
 
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...
JFR, you hit that one out of the ballpark, not much more that I can say beyond that. (There might be a systemic error in measuring inflow to Lake Powell, because 1 maf per year seems like a whole lot of evaporation and seepage. Or was the Navajo Power Plant using that much for cooling?)
 
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:

1991-2020

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...

1964-80

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...

1980-2000

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...

2000-2020

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...
I find it really interesting that in 2020 on April 6 (the date on average when the peak snowpack occurs), snowpack above LP was above average. It was still above average through May.
If you can have an above average snowpack, but only get 6.5 maf at LP that is pretty scary for the future....
 
Something else to consider is that prior to 1983 there was much more limited tracking of snowpack, which meant that the runoff that year was a surprise to the USBoR. (At least that's the excuse we hear for all of the water down the spillways and associated damage.) Since that time snowpack is tracked much more carefully.
 
I find it really interesting that in 2020 on April 6 (the date on average when the peak snowpack occurs), snowpack above LP was above average. It was still above average through May.
If you can have an above average snowpack, but only get 6.5 maf at LP that is pretty scary for the future....
Where are you finding historical snowpack by date?
 
when looking at the various prediction graphs it is amazing to see how rapidly things can change in the system. in a few months time. i'm not really up on chaos theory, but to me that looks like about what we've got and since the water levels are weather related and weather is known to be chaotic it does make sense to me that this should be a difficult problem.

which is why we have GCD and LP to begin with...

the downward trend is hoped to be temporary. a few '83-84's please Momma Nature. how do i put in my order? :)

glad to see more snow in the forecast.
 
JFR, you hit that one out of the ballpark, not much more that I can say beyond that. (There might be a systemic error in measuring inflow to Lake Powell, because 1 maf per year seems like a whole lot of evaporation and seepage. Or was the Navajo Power Plant using that much for cooling?)
Well, I’ve seen studies that estimate evaporation at anywhere from 0.4-0.8 maf per year (probably closer to the lower number) and seepage around 0.4 maf. If true, that collectively explains 0.8 maf right there, and maybe a bit more. Water use from Page and until recently the Navajo power plant is negligible in the context of these numbers…

But I would say that if my thumbnail analysis that inflow needs to be 0.9 maf more than outflow to achieve balance throws a monkey wrench into some of the wild guess “studies” that GCI commissioned to show that evaporation exceeds 1 maf/yr… the flow data right in front of us suggests otherwise…

I plan to dig a bit deeper into this before I draw too many more conclusions that come from used napkins….
 
Where are you finding historical snowpack by date?
If you click on the water database site link here at the bottom of the WW page, then snowpack data, it has link to "snow data", then click "snowpack above lake powell", then click "snowpack above lake powell" on the top link. To my point, 3 of past 6 years have been above average snowpack....
 
@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.
 
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