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...
