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The new, new water math: Why it (mostly) makes sense

Though I'm often one to criticize our local officials for head-scratcher decisions and bonehead ideas, I'm a bit surprised to see the recent dust-up over the change in how Durham's calculating days remaining of water supply.

Barry kicked this off over at his place yesterday with a piece on the "missing MGD" -- that is, the gap between the 17.75 million gallons/day of water that Durham was using to calculate the days of water remaining, and the fact that the actual water demand was 20.94 MGD.

The plot thickens in today's N&O coverage:

Durham doesn't factor in the full daily demand to calculate how much water it has left in its three main reservoirs. If it did, it would have about 106 days of water in its main supplies, about three weeks less than the 125 it's touting now.

The city's average daily demand for January is 20.95 million gallons. But to calculate the days left, only 17.74 million gallons is divided into the total water available in Lake Michie, Little River and Teer Quarry reservoirs.

There's two issues here. Is the calculation sound? And, was this communicated as well as it could be?

To the first point, I think there's absolutely no question that subtracting the OWASA/Cary inflow from Durham's actual water use to get the 17.75 MGD number is the right way to go.

After all, the crucial immediate question for Durham in the drought is, how long until the Durham-owned reservoirs run dry? The answer is a simple math exercise from any B-school's operations management class. You take the fixed quantity of reserve supply in fixed units, divide by the units per day of usage, and get a number of days remaining.

That calculation would work great for a closed system in which all demand was being met from Durham's own supplies. The wrinkle, though, is a good one: Durham is increasingly drawing water from outside its supplies. Today, as much as 4 MGD from OWASA and Cary; by summer, as much as 11 MGD.

Those flows don't go into Little River and Lake Michie; they go into the Durham water supply to meet daily demand. Since they're flows and not fixed supplies, the only reasonable way to make the math work is to subtract those inflows from Durham's daily demand.

The one asterisk that rides with these numbers is the question of whether Cary's system could continue to supply Durham if the drought worsens. As of today, however, the towns that draw from Jordan Lake are eligible to pull 49 MGD from the huge reservoir; last year's all-time peak was around 28 MGD. And Jordan Lake, as we learned at the recent Duke-Durham drought forum, has barely been touched by the current drought.

Now, I'm no waterologist -- but in the absence of more information that suggests reasons to be concerned, I'm not particularly worried about Jordan Lake yet.

Which brings us to the second issue: Could Durham officials communicated this more clearly?

Sadly, as is often the case, the answer seems to be yes. Not that it was a seeming attempt to hide or deceive; the actual daily water usage numbers on the graphs showed, plain as day, the 21 MGD of actual usage that Barry noted. It wouldn't have been hard for the City to put a quick disclaimer on their web site noting that the inflows from other systems were used in the calculus of water supply remaining.

And I don't begrudge Mayor Bell using the higher number (193 days) in the State of the City speech. After all, the City was called to act, and did act -- bringing Teer Quarry online as a stopgap, and rushing the expansion of water supply interconnections.

The obvious problem here is, as usual, perception. And that's a lesson the City never seems to get right. We're only two months out from the ill-fated lack of communication over the E. Chapel Hill St. parking deck -- the contemporaneity of which with Patrick Baker's move to the City Attorney slot sometimes seems too close for coincidental comfort.

The bigger problem, though, is the risk that a public thinking there's nothing to worry about with the water supply stops caring... stops conserving... stops demanding City leaders make long-term fixes to get us ready for the next drought.

And that's the one area where the new, new water math really doesn't add up.



Kevin - the other question this raises in my mind is how enforceable are the Jordan Lake contracts if we go through another year of drought, and everyone starts pulling their full allotment? Won't that start to put some unanticipated stresses on the system?

Say rainfall remains 25% below normal through October or November. Durham starts running out of water from its reservoirs sometime in May. Demand for Jordan Lake water starts going up, easily reaching (and possibley surpassing) that 49 MGD figure you mention, probably as soon as mid-June or early July.

What happens then?

Will Durham still be able to pull 10 MGD from Jordan, assuming additional infrastructure improvements can be put in place, or does Cary, protecting its own interests, throttle back on the allotment?

It started out as an idle exercise, actually, when i noticed the numbers didn't add up. Combine that with the fact that the city never provided an answer to the question i and a number of other people were asking last year about whether or not the Stage III and Stage IV exemption holders had actually complied with the 30% and 50% mandatory water use reduction, and the issue becomes equally the lack of information being provided, as well as the lack of water.

Michael Bacon

Two things: First, Cary doesn't control the allotment. That's mediated at the state level. Second, Jordan, while not being immune to downswings, drains a much larger area and is a much more stable water supply than the northern Durham lakes.

If nothing else, drawing from Jordan has the "advantage" that it's where our treated wastewater goes. Up until now, Durham's been taking water out of the Neuse basin and dumping it into the Cape Fear basin, without taking any back out of the Cape Fear basin. This way, we're not giving Cary all our treated wastewater.

And no, I can't believe I'm citing that as a good thing either.


not being a lawyer and all that, i can't speak to how any disputes over water are going to be mediated.

but as an amateur reader of things historical, i can pretty much guarantee that if the drought persists for any length of time, the existing water sharing agreements at Jordan Lake are going to come under a lot of pressure.

that may be later rather than sooner, but it will happen.

David McMullen

Yes, it does sound strange to talk about our treated wastewater being a great resource, doesn't it? But hey, we're all downstream from somebody, so we're likely drinking some treated wastewater already. The good folks in New Orleans are downstream from half the country, and they still take their water from the Mississippi. Properly treated wastewater is just fine for drinking.

At various times, I've seen proposals to require all communities to discharge their wastewater at a point upstream from where they take their water in, to encourage proper treatment - poor wastewater treatment then becomes your own problem, not just someone else's. But I recently heard a new wrinkle: Aurora, Colodado is going to do exactly that, pumping all of the city's treated wastewater back into the Platte River above their water intakes, because it actually increases their water supply. They estimate that they can recover 50% of the water they take into their system that way, which doesn't just increase their water supply by 50% - through the magic of convergent series (remember those from calculus?), it effectively doubles their water supply. So it not only makes good ecologic sense to recycle your wastewater, but good economic sense too.

Of course, if ecologic costs were properly factored into economic calculations, that would always be true. But environmental damage is usually treated as an external cost that can be safely neglected on the balance sheet - a mentality we're already starting to pay for in a big way...


If the situation really comes to a head, then Durham will sue Cary in state court. The first thing that Durham will do, is move for an emergency injunction to prevent Cary from restricting water flow, and this will likely be granted. Then, as the issue of Durham's allotment is settled in state law, Durham will have the high ground on the merits.

Michael Bacon

I didn't mean mediated in the sense of arbitration, I meant mediated in the sense of the state ultimately holds all the cards, and if Durham and Cary don't play nice with each other, the state DENR can smack them around some. (Or they can settle in court, as crc32 says...)

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