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Water Year 2020-21 is Over: Key Takeaways

JFRCalifornia

Escalante-Class Member
Water Year 2020-21 (I'll use shorthand and just call it "2021") came to a merciful finish on September 30, and we can finally put that one behind us. If you don't like to be depressed, go ahead and skip this thread, but if you like numbers and are interested in seeing some trends in the big picture, read on.

When all was said and done, this was the worst year in terms of inflow to Lake Powell (or before the lake, river flow past Lees Ferry) since records were kept. It narrowly nudges out 2002 for the title.

Here are the five worst years on record for inflow:

2021 - 4.0 maf
2002 - 4.1 maf
1990 - 5.2 maf
2013 - 5.3 maf
2018 - 5.3 maf

To provide some context, the Colorado River Compact assumed 15.0 maf on average, and later we assumed 16.5 maf to account for Mexico's share. But water delivery requirements are actually couched in terms of a 10-year rolling annual average, so let's look at the numbers that way. Let's start with the idea that historically, the river has rarely exceeded a 10-year rolling average more than 15 maf. In fact, the last year the happened was 1933 (!). After that, the 10-year average went into a slow but steady decline. Here was the rolling 10-year annual average for every ten years since 1920:

1920 - 18.2 maf
1930 - 17.4 maf
1940 - 11.7 maf
1950 - 14.9 maf
1960 - 12.1 maf
1970 - 10.5 maf
1980 - 11.1 maf
1990 - 13.2 maf
2000 - 11.3 maf
2010 - 8.2 maf
2020 - 9.1 maf

If that isn't a clear long-term trend, I don't know what is. It's worth noting that because of the back to back terrible years of 2020 and '21, that 10-year rolling average right now (2021) sits at 7.9 maf, its lowest point ever. Sobering thought. In terms of long-term delivery requirements, there's no way this is going to work. And for the record, the only year since 2000 that exceeded 15 maf was in 2011 - inflow was 16.3 maf. That was pretty much an average year in the early 20th century.

So is there a light at the end of the tunnel? That is, is there typically a "bounce back" after a terrible runoff year? I looked at that. First off, good and bad years tend to appear in clumps. From 1906-52, there were only 3 years where inflow was less than 11 maf!! And only one year less than 7 maf. The script has flipped since 2000. Since then, there's only been 5 years with more than 11 maf, and 9 years with less than 7 maf. So it's not really reasonable to extrapolate short-term or mid-range trends from century old data--river dynamics have changed quite a bit since then. I won't speculate as to cause, but facts are facts.

But what about "bounce back" potential? Well, I looked at all the years where inflow was less than 7 maf, and wanted to know what the average inflow was the next year. (By the way, the average inflow of the 14 years that have been under 7.0 maf is 5.6 maf.) Here's what we've got:

Since 1906, the year following a "bad" year under 7 maf averages 8.0 maf. Since 2000, that number is reduced to 7.2 maf. So on average, its fair to expect next year to be better than this one, but not great. Less than the 10-year rolling average. In other words, one of the best indicators for next year is what happened this year. For those looking for a ray of hope, the biggest bounce back since 2000 after a bad year was just two years ago in 2019. In 2018, inflow was only 5.3 maf. In 2019, it was 11.7 maf. You had something comparable in 2004-05 and 2010-11. Here's those years in comparison:

2004 - 5.6
2005 - 11.3 (5.7 maf change)

2010 - 8.8
2011 - 16.3 (7.5 maf change)

2018 - 5.3
2019 - 11.7 (6.4 maf change)

This all also begs the question--what's the biggest increase in inflow from one year to the next, regardless of how bad the first year was? Here's the top 4:

1983 - 10.6 maf (jumped from 11.2 in 1982 to 21.8 in 1983)
1909 - 10.5 maf (climbed from 12.5 to 23.0)
1941 - 10.0 maf (went from 8.0 to 18.0)
1957 - 9.5 maf (jumped from 10.0 to 19.5)

In modern times (since say, 1990), the biggest jump was from 2010 to 2011... 7.5 maf. That ranks 9th on the list.

What that tells me that in our wildest but reasonable best hopes, we might see an increase in inflow next year of 6-7 maf compared to this year. That would mean an inflow of 10-11 maf for the next water year, since this past year was 4.0 maf. Not too bad, but nothing great in historical terms. Comparable with 2016 or 2017, not quite as good as either 2005 or 2019. A definite rise in the lake. But data from the past 20 years tells me the more likely outcome will be closer to an inflow of 6-8 maf next year. Not great, but an improvement. Best recent comparable years would be 2000, 2006 or 2007. Possibly enough to hold the lake level more or less in place compared to today (comparing Oct 1, 2021 to Oct 1, 2022), especially considering releases will be less in the coming year (7.48 maf instead of 8.23 maf). But if the inflow ends up being less than 7.5 mf, we're going to see a slightly lower lake level next October than we are seeing now.

But let's hope for the best!

There's your short summary of the likely outlook for the coming year...
 
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drewsxmi

Well-Known Member
Interesting. 2018 was worse in terms of maximum snowpack, but the 2021 snowmelt looks like the earliest in the past 7 years. I seem to recall there being an extended period of hot, dry, windy weather last spring, but water managers were warning about very low allocations for irrigation beginning early in the spring. Looking at the last 7 years, 2021 does not look that much worse than the seven year average.
 

210 FSH

Active Member
Water Year 2020-21 (I'll use shorthand and just call it "2021") came to a merciful finish on September 30, and we can finally put that one behind us. If you don't like to be depressed, go ahead and skip this thread, but if you like numbers and are interested in seeing some trends in the big picture, read on.

When all was said and done, this was the worst year in terms of inflow to Lake Powell (or before the lake, river flow past Lees Ferry) since records were kept. It narrowly nudges out 2002 for the title.

Here are the five worst years on record for inflow:

2021 - 4.0 maf
2002 - 4.1 maf
1990 - 5.2 maf
2013 - 5.3 maf
2018 - 5.3 maf

To provide some context, the Colorado River Compact assumed 15.0 maf on average, and later we assumed 16.5 maf to account for Mexico's share. But water delivery requirements are actually couched in terms of a 10-year rolling annual average, so let's look at the numbers that way. Let's start with the idea that historically, the river has rarely exceeded a 10-year rolling average more than 15 maf. In fact, the last year the happened was 1933 (!). After that, the 10-year average went into a slow but steady decline. Here was the rolling 10-year annual average for every ten years since 1920:

1920 - 18.2 maf
1930 - 17.4 maf
1940 - 11.7 maf
1950 - 14.9 maf
1960 - 12.1 maf
1970 - 10.5 maf
1980 - 11.1 maf
1990 - 13.2 maf
2000 - 11.3 maf
2010 - 8.2 maf
2020 - 9.1 maf

If that isn't a clear long-term trend, I don't know what is. It's worth noting that because of the back to back terrible years of 2020 and '21, that 10-year rolling average right now (2021) sits at 7.9 maf, its lowest point ever. Sobering thought. In terms of long-term delivery requirements, there's no way this is going to work. And for the record, the only year since 2000 that exceeded 15 maf was in 2011 - inflow was 16.3 maf. That was pretty much an average year in the early 20th century.

So is there a light at the end of the tunnel? That is, is there typically a "bounce back" after a terrible runoff year? I looked at that. First off, good and bad years tend to appear in clumps. From 1906-52, there were only 3 years where inflow was less than 11 maf!! And only one year less than 7 maf. The script has flipped since 2000. Since then, there's only been 5 years with more than 11 maf, and 9 years with less than 7 maf. So it's not really reasonable to extrapolate short-term or mid-range trends from century old data--river dynamics have changed quite a bit since then. I won't speculate as to cause, but facts are facts.

But what about "bounce back" potential? Well, I looked at all the years where inflow was less than 7 maf, and wanted to know what the average inflow was the next year. Here's what we've got:

Since 1906, the year following a "bad" year under 7 maf averages 8.0 maf. Since 2000, that number is reduced to 7.2 maf. So on average, its fair to expect next year to be better than this one, but not great. Less than the 10-year rolling average. In other words, one of the best indicators for next year is what happened this year. For those looking for a ray of hope, the biggest bounce back since 2000 after a bad year was just two years ago in 2019. In 2018, inflow was only 5.3 maf. In 2019, it was 11.7 maf. You had something comparable in 2004-05 and 2010-11. Here's those years in comparison:

2004 - 5.6
2005 - 11.3 (5.7 maf change)

2010 - 8.8
2011 - 16.3 (7.5 maf change)

2018 - 5.3
2019 - 11.7 (6.4 maf change)

This all also begs the question--what's the biggest increase in inflow from one year to the next, regardless of how bad the first year was? Here's the top 4:

1983 - 10.6 maf (jumped from 11.2 in 1982 to 21.8 in 1983)
1909 - 10.5 maf (climbed from 12.5 to 23.0)
1941 - 10.0 maf (went from 8.0 to 18.0)
1957 - 9.5 maf (jumped from 10.0 to 19.5)

In modern times (since say, 1990), the biggest jump was from 2010 to 2011... 7.5 maf. That ranks 9th on the list.

What that tells me that in our wildest but reasonable best hopes, we might see an increase in inflow next year of 6-7 maf compared to this year. That would mean an inflow of 10-11 maf for the next water year, since this past year was 4.0 maf. Not too bad, but nothing great in historical terms. Comparable with 2016 or 2017, not quite as good as either 2005 or 2019. A definite rise in the lake. But data from the past 20 years tells me the more likely outcome will be closer to an inflow of 7-8 maf next year. Not great, but an improvement. Best recent comparable years would be 2000, 2006 or 2007. Possibly enough to hold the lake level more or less in place compared to today (comparing Oct 1, 2021 to Oct 1, 2022), especially considering releases will be less in the coming year (7.48 maf instead of 8.23 maf).

But let's hope for the best!

There's your short summary of the likely outlook for the coming year...

Awesome Write up! Thanks!
 

JFRCalifornia

Escalante-Class Member
So JFR give us a ballpark estimate of the increase in lake levels (in feet) under each of the scenarios you describe
Okay, this is all back of the napkin stuff, but here's my take on that question....

If my "mostly likely" scenario of 6-8 maf inflow for WY 2021-22 holds, that compares to the prescribed outflow of 7.48 maf. That's basically a wash (assuming we come close to 7.5 maf inflow), so the lake level on Oct 1, 2022 would be about the same as now, or 3545. And that would suggest a summer peak similar to this year, or about 3560 or so. A little less if inflow turns out to be on the low end of that estimate. And so assuming we bottom out next spring at about 3515-20, that's a 35-45 foot spring rise. Normally I'd say that's a good rise, but from such a low start you don't end up any better off than where we are right now.

Under my "super optimistic best case" scenario of 10-11 maf inflow, when you subtract the 7.48 maf outflow, the net increase on Oct 1 next year would be somewhere around 3 maf. That means a lake level of about 3585 on Oct 1, 2022. And that implies a summer peak of about 3600. If that happened, that would be a spring rise of 80-85 feet or so, and that's never happened (best ever was 58 feet). But I'd say it's possible. Very unlikely but possible...

How does this compare to USBR's latest estimates? Well, their "most likely" scenario shows a summer peak of about 3550 and an Oct 1, 2022 level of closer to 3535. A little less optimistic than my "most likely scenario". On the other hand, their "best case" is way more optimistic than mine. They show a possible summer peak of 3615 and staying there until Oct 1, 2022... I seriously doubt that's possible, but hope I'm wrong...
 
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Dougie

Well-Known Member
Thanks. USBR forecasts are so widely disparate between worst and best that it’s almost impossible to use them. 7-8 MAF is my guess as well, and this easily avoids the doomsday 3490 scenarios, at least into 2023. But we are truly guessing...like trying to forecast who will be in the Super Bowl next February (bad analogy but it’s the one that came to mind). May not help Bob London much to illustrate the point.
 

drewsxmi

Well-Known Member
Thanks. USBR forecasts are so widely disparate between worst and best that it’s almost impossible to use them. 7-8 MAF is my guess as well, and this easily avoids the doomsday 3490 scenarios, at least into 2023. But we are truly guessing...like trying to forecast who will be in the Super Bowl next February (bad analogy but it’s the one that came to mind). May not help Bob London much to illustrate the point.
Looking at the USBoR forecasts, they are doing strict statistical analysis on the historical records of inflows, and assigning probability levels based on what has happened in the past. Their "worst case" and "best case" projections each have a 10% probability of occurring, based on historical data. I don't think that they include any wishful thinking in their forecasts. The big question for me is how they weight the data. Are they giving data from 50 years ago as much weight in the statistical analysis as the past 10 years, or giving more emphasis to recent years? In any case, there can be a huge variance in runoff. Think of the snow drought years in the Sierra Nevada versus the year that Oroville Dam overflowed, all in the last 10 years.

I guess my suggestion for planning is to be prepared for each of the extremes, but expect the "most likely" scenario. That means a much bigger recovery than spring 2021, followed by levels about the same as 2021 from June onwards. Don't make a huge investment in Bullfrog unless the expected returns are massive and you like taking risks.
 

JFRCalifornia

Escalante-Class Member
... one more thing to consider...

If you're looking for the bright side in the big picture, there's this: this coming year, the USBR will be releasing 7.48 maf, not 8.23 maf as it has for the last few years. That extra 0.75 maf in Lake Powell will make a difference of about 9-10 feet in the lake over the course of the coming year. In other words, had the release only been 7.48 during this past water year, the lake would now be closer to 3555 than the 3545 as it currently is.

And one more thing...

In my initial post I posed the question--after a bad year, what's the likelihood the next year will be better? And as described above, the answer is "fairly likely", because extreme outcomes are rare, and in this case, even an average year would be an improvement. But what about the opposite question: how often is a bad year followed up by one that's even worse?

Thankfully, not that often. In the 14 times that annual inflow was less than 7 maf, it's improved 9 times in the next year, and gotten worse 5 times. The two times it's dropped significantly were these:

2001 to 2002 (6.9 to 4.1 maf)
2020 to 2021 (6.4 to 4.0 maf)

Of course, one of those two times was this year. So you might ask, what happened following 2002? And the answer is, there was an improvement in 2003. Not a huge one, but 2003 saw an inflow of 6.3 maf--which was still way below average, but better than horrific. And that's a reasonably likely scenario for 2022 too. Sort of the low end of the "most likely" range described in the first post in this thread.

But whatever happens in 2022, the good news is that there will be an extra 0.75 maf that will remain in Lake Powell, and not released downstream...
 
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