Dependable Erection

Thursday, November 15, 2007

Drought thoughts

I was poking around on the WRAL weather site to check on how much rain we got today (looks like around a quarter of an inch in Durham). they have a little graphic on the right of the page that tells what "average" rainfall through this time of year is, and what our total so far in 2007 is.

The numbers, if you're interested, are 38.43", and 30.92" respectively, or a shortfall of about 7.5". Put another way, our shortage of rain this year over a "typical" year is just under 20%.

I was curious to find out just how uncommon a 20% rainfall deficit is in the Piedmont. I was able to locate the charts below at this site.

This graphic shows annual rainfal totals averaged statewide for the 20th century. I'm pretty sure our numbers in Durham are a touch lower than the statewide average, which, making a rough estimate from the chart, would appear to be about 46" or 47".

This second graph shows seasonal variation in the rainfall received in the Piedmont throughout the 20th century, averaged in 10 year increments. It shows that, on this scale, average precipitation throughout the year has remained constant, but seasonal variations have diminished over the past hundred years. In other words, summer used to be far and away the wettest season, and fall the driest, with the difference between the two seasons of about 9", or more than double. As of 2000, that difference had shrunk to about 3" of rainfall.

I think this is interesting data, and points toward some solutions in and of itself (summer is obviously the time of greatest water use in agricultural and residential areas, while commerce and industry are probably more evenly spread out. If water is not plentiful in the summer, then we probably need to be increasing our storage capacity by a lot more than what we're currently planning.) But it's not the data that i'm really looking for.

What i really want to know is how common or uncommon is a 20% deviation from the "average" rainfall year? If you graph total annual rainfall on the vertical axis, and number of years with that amount of rainfall on the horizontal axis, do you get a normal distribution, or do you get a tall, narrow bell curve, or a wide, flattened one? Anybody who has that data is encouraged to point me in the right direction. Knowing how typical this type of "exceptional" drought is should help guide decision making regarding development issues in particular.

I have to say, just intuitively and without any data backing this up, that a single year of 20% rainfall deficit should not be enough to create "exceptional" drought conditions. What that suggests to someone who has spent time living in Arizona and California, is that we're not mamaging our water wisely, and we're not paying attention to our system's maximum carrying capacity in a lot of our development decisions.



  • The problem this year is not just the lack of rain, but the lack of rain combined with the record number of really hot days.

    By Blogger MK, at 5:02 PM  

  • See, i'm thinking that we don't really have that severe a shortfall. Granted, all of the deficit accumulated during the hottest months of the year, when demand was highest, but i think that the data will show that a 20% deficit at this time of year is not all that rare.

    And what that tells me is that we really need to do a better job of managing our supply.

    That will probably require a significant statewide effort to build some reservoirs up in the mountains where rainfall is greater than it is here, and regional compacts to plan the distribution of that water.

    And some really serious attention to reducing demand at the individual level and the aggregate level, the former with conservation programs and the latter with development controls.

    By Blogger Barry Ragin, at 6:42 PM  

  • The water also evaporates in the reservoirs faster when and where it's hotter.

    By Blogger Lenore, at 8:22 PM  

  • Well, based on your first graph, it looks like a ~20% shortfall occurs roughly 7% of the time. Which isn't at all trivial.

    Of course it also indicates that a 20% surplus occurs with about the same frequency. It's difficult to say without the raw numbers handy, but that looks (to me) like a noise around an average that hasn't moved significantly over a century. So I'm willing to bet that the distribution is normal.

    It seems to me that what matters isn't so much the rainfall in absolute terms, but the depletion of the reservoirs--for whatever reason, be it lack of rainfall, high demand, greater evaporation, or (most likely) all of these. If a 20% shortfall puts us in the danger zone, then we clearly aren't adequately taking advantage of the 20% surpluses when they occur.

    Which is to say, we need more reservoir capacity.

    But yeah, in the short term, not watering our lawns is a damn good start.

    I don't know about you, but in the 7 years I lived in AZ (Tucson) I don't recall ever having any such thing as a water restriction. This despite the fact that we were in a drought pretty much the entire time I lived there. (Which is really saying something in a place where the average is 12"/yr.)

    I'm guessing the fact NO ONE has a lawn in Tucson might have something to do with it.

    By Blogger Brian, at 11:30 PM  

  • i was much younger when i lived in Phoenix (76 - 78) and i can't say i paid much attention to water issues at the time. i do remember going tubing in the Verde River a couple of times. and one time i recall a February rainstorm flooded the river between Tempe and Phoenix. But looking at a current map, it looks like the that dam has been completed since, controlling river flow a little more completely.

    that's also the way it is in California, where pretty much all the water coming out of the dams in the mountains is managed.

    i'm beginning to come around to thinking that the southeast is going to have to do the same with our rainfall. either that or put a pretty strong hold on future development. this year's rainfall deficit seems to me to be neither terribly large nor all that unique; which means that without any steps to increase supply or reduce demand, water shortages are going to be a way of life here for the foreseeable future.

    By Blogger Barry Ragin, at 12:02 AM  

  • Depending on how your data processing fu is feeling today, you can go nuts with NCDC data here:

    I'd say that you'll want to look at monthly averages, not just yearly averages. While we're down for the year in rainfall, that doesn't tell the whole story. We had a relatively normal to vaguely wetter than average spring, and then some time around June or July, the rain just stopped. It's been a long time since I've seen July and August be as dry as they were this year.

    I agree that we need to do a better job of managing our water supply, but all you have to do is look at the number of tree deaths we're seeing (it won't become quite as obvious until the spring, when they just don't leaf out). Most of the trees that I'm seeing in serious condition aren't ones that would have been irrigated in a wetter year, or with better management. I've never seen tree death in NC like I'm seeing it this year.

    That tells me there's more than management issues going on.

    By Blogger Michael Bacon, at 4:05 PM  

  • Cropped URL... try here instead.

    By Blogger Michael Bacon, at 4:06 PM  

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