Dependable Erection

Thursday, January 24, 2008

Drought numbers

This is going to seem a little picky to some of you, and maybe it is.

I've been monitoring the city's "Current days of supply" figure as posted on the website pretty regularly for the past couple of months. Right now it says:
Using the 30-day running average demand as of January 20, 2008 of 17.74 MGD:

* Days of supply of easily accessible, premium water remaining (Lake Michie, Little River Reservoir and Teer Quarry combined): 125 days
* Days of less accessible water below the intake structures remaining: 68 days
* Total days of supply = 193

Initially, i was concerned about the 17.74 MGD figure. The 30 day moving average includes the Christmas/New Year week when demand was lower than normal. It would be much better to use a shorter period of time in this case. Especially since the 125 day figure of "easily accessible" water gets us into late May, when demand figures to be higher, as well as such things as evaporative loss.
The city also helpfully publishes a bar graph of recent daily demand.

I've gone ahead and added tick marks showing 16, 17, 18, 19, 21, & 22 MGD levels, to make the graph more useful and easier to read.

(Click on each graph for a full size view)

I can't for the life of me figure out how a 17.75 MGD figure can be extrapolated from a period of time in which demand has not dropped below 19 MGD for a single day. Michael suggested in conversation that perhaps the graph is measuring outflows from the city's water plants, while the daily demand figure is calculated on meter readings. I hope that's not the case. Whether the city bills for all of the water it ships or not is irrelevant. It's how much water that leaves the plant that matters.

The city does give a "month-to-date" figure for January '08 of a shade under 21 MGD, which looks more accurate to me based on the chart. Using that figure, our "easily accessible" water supply drops to about 105 days, almost 3 weeks less than the number on the web site. I think that's a big deal.

It would also be useful to project forward what demand might be. For example, we can tell that demand year over year in January has dropped about 13% from '07. It's not unreasonable to expect that figure to remain consistent, within a couple of points at least, over the next few months.

Good luck trying to find those numbers, though. Someone's got them, but they're not posted anywhere online that i can find.

I'm going to hazard an educated guess that our actual "easily accessible" water supply, based on anticipated demand, is somewhat less than 90 days, which only gets us to mid-late April, not late May. And that our total supply, which includes that "less accessible" water, is not 193 days, but is closer to 135 - 140 days.

Historically, the median inflow at Lake Michie and Little River reservoirs for this time of year has been a combined 170 CFS, more than enough to replenish supply for the year. Today, they're a combined 11 CFS, which is less than half of what we need just to meet daily demand.

Seems to me the city needs to be a ittle more realistic about how bad the situation really is. If the remainder of the winter and early spring stays dry, those supplies are going to drop even more precipitously than they did last August.

UPDATE: I've gotten an email suggesting that the difference between the 17.75 MGD figure and the 20.94 MGD figure represents the water Durham is purchasing from Cary. With the new connections, that's about 4 MGD, so that explanation makes a certain amount of sense.

Would be nice if the city put that on the website to make it clear, though. It would also be nice to see month-by-month numbers for the past couple of years. That way we could make some projections and set soem realistic targets for reducing demand.

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  • I wrote to Vicki Westbrook with the city to pose your questions. I'll post her answers.

    I think our demand will be greater than 13% lower as we heat up, since most landscape watering will be prohibited.

    The 13% reduction of demand is pitiful. It is apparent that most of Durham has its head up its butt.

    By Anonymous steve bocckino, at 12:08 PM  

  • Reply to my query from James Lim, Conservation Coordinator, Water Management, City of Durham

    "Our “days of supply” calculation is based only on the demand that the City places on our two supply lakes and the Teer Quarry. A portion of our demand is being met through the Jordan Lake allocation we are accessing via the Town of Cary and purchases from OWASA. Because this portion of our demand is not actually placed on our own lakes, it is subtracted from the calculation. This is why you see a lower number used in the calculation. Should conditions change regarding our purchases from OWASA and Cary, these calculations will be amended to reflect how the demand is being met. It’s important to note that “days of supply” is a moving number with some assumptions tied to it, including an assumption of no additional stream flow, no additional rain, and no change in demand. It’s likely that this number will fluctuate as conditions change.

    Please let me know if you have addiitonal questions."

    By Anonymous steve bocckino, at 2:21 PM  

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