Shark Water Redux

Last week I was able to attend the annual gathering of the Colorado River Water Users Association (CRWUA) in Las Vegas.  It appears that my recent post about the multi-decadal depletion of Lake Powell (“A Shark in Fresh Water“) is generating a good bit of discussion, and some questioning of my numbers (my thanks to those that asked!).

Therefore some elaboration is in order.

“Did Lake Powell really lose ~600,000 acre feet per year?”

Not in each and every year.  But over the past 19 years, the reservoir did in fact lose 600,000 acre feet per year on average. The calculation is a simple one that you can repeat for yourself.  Go to this Bureau of Reclamation site, obtain the Lake Powell volumes for January 1, 2000 and December 31, 2018, subtract, and divide by 19 years.  Your answer will be just under 600,000 AF/year.

This is an accurate and true estimate of what actually happened in Lake Powell over that time period. However, this is an averaged loss; most of the loss took place between 2000-2005, with a particularly sharp drop in 2002.  It is important to note, though, that an equally severe drop occurred during 2018.

“Yeah but that’s because the Bureau was releasing too much water from Lake Powell”

This was a common critique of my blog, and it’s a valid point. Again, the 600k/year loss is accurate based on what really happened, but things would have looked quite different if the Bureau had been releasing an average of 8.23 million acre feet (MAF) per year over the 2000-2018 period, per the Colorado River Compact mandates.

My model suggests that Lake Powell would have reached full capacity in June of 2017, dropped below the brim in 2018, and then topped up again in June 2019 (the graph below shows only storage levels as of January 1st each year).

(in next week’s blog I’ll show you what those lowered Powell releases would have done to Lake Mead)

The Bureau’s numbers don’t quite add up

Upon further scrutiny, it appears that the Bureau of Reclamation’s estimates for inflows and outflows (reservoir releases) at Lake Powell are slightly inaccurate.  For example, if you compare the Bureau’s estimates of total reservoir releases with the river flows measured by the US Geological Survey at Lee Ferry located just downstream of Lake Powell, you’ll find that the Bureau’s daily release estimates are 1.6% lower than Lee Ferry flows.  If you accept the USGS measurements as being the more accurate, you’ll then find that you have to further adjust (lower) the Bureau’s daily estimates of inflows into Lake Powell by -2.5% to get your water balance model to match what actually happened at Lake Powell (see above).  Taken together, using the Bureau’s estimates results in under-estimating Lake Powell declines by ~4% per day.

Those daily errors or adjustments might not seem like much, but over the course of 19 years (2000-2018) they really add up! Adjusting the reservoir releases to match USGS measurements causes an additional loss of 140k AF/year.  By adjusting the inflows to match actual Lake Powell declines causes an additional loss of 224k AF/year.

By my estimates, inaccuracies in the Bureau’s estimates leads to an under-estimation of Lake Powell depletion by 364k AF/year, on average, during 2000-2018.

None of this changes the pain of the climate shark in our future

None of the above makes the future bottom line look any different. In fact, the modest adjustments in the Bureau’s inflow and outflow estimates discussed above only make the future look even more challenging than I suggested in my blog.

My slightly-adjusted projections: with 20% less inflow due to climate change and assuming Lake Powell releases of 8.23 MAF/year we will need to reduce consumptive use in the Upper Basin by 1.65 MAF/year. With 35% less inflow, that number soars to 2.9 MAF/year.

I’ll say it again: We need to start aggressively reducing consumptive use IMMEDIATELY if we want to stave off a massive water crisis and ensure a secure water future for everyone depending upon the Colorado River.