Hahn_Arroyo_Under_Repair

Originally uploaded by heinemanfleck.

I did another water ride this morning, up the Hahn Arroyo bike trail to see the bits that washed out in last Saturday’s downpour.

Water in a sense defines landscape (or at least contributes strongly to its definition), and the concrete of the Hahn is a great illustration of one of the fundamentals of life here in the desert southwest.

Albuquerque spreads across a series of alluvial fans flowing out of the Sandia Mountains. With no permanent running water in the arroyos, it’s clear that the alluvium all got here in big debris flows – flood events. When you lay a city down on top of that, you’ve got to do something to accommodate the water from those flood events, which is why we’ve got the concrete-lined Hahn Arroyo and a bunch like it threading down from the mountains.

There’s an excellent paper in the latest Bulletin of the American Meteorological Society about the public communication problem associated with “normal temperature.” The difficulty is that the numerical average – the thing you always get on the TV news and in the newspaper – only captures part of the statistics that matter in measuring daily temperature. The range of typical variability is also important. So it was three degrees F above “normal” yesterday in Albuquerque. How abnormal is that? In truth, “normal” represents a range, not an absolute number.

Here in the desert southwest, precipitation makes for a very interesting case study. In weather page we run ever day in the morning paper, we include a precipitation table that includes a comparison between how much rain fell the day before, and the “normal” for the day – for example 0.04 inches (about a millimeter) for July 15. If by “normal” we mean the arithmetic mean, I suppose that’s technically correct, but in terms of public communication, it’s fundamentally wrong.

This time of year in Albuquerque, we typically get measurable rainfall one out of every three days. That means zero rainfall two out of every three days, which is by a reasonable plain English reading of the word, “normal.” At this time of year, spatial variability is equally important, with spot thunderstorms creating huge differences in rainfall amounts over relatively short distances – say, one side of town to the other.

That variability, both spatial and temporal, is what whacked the Hahn Arroyo last Saturday night. It was an epic downpour. It was raining too hard at my house to actually go out back and check the rain gauge, so I watched from the back window with binoculars. The gauge’s central chamber, which holds an inch (25 mm) filled up in less than 15 minutes.

I don’t have the statistics to tell you where that fits in the envelope of variability, but geomorphology suggests it’s the sort of event that made the landscape underlying Albuquerque. There’s one particularly epic boulder field up in the foothills half the size of a football pitch covered with rocks as big as small cars that seem to have all come down in a single flood event. There’s probably not been a storm like that since the Pleistocene, but my life of anecdotes suggests storms like last Saturday’s happen at least once every few years here during our summer rainy season.

When Saturday’s flood waters hit the Hahn Arroyo, they tore out massive hunks of concrete, rendering unto the downstream landscape that which was thought to be the product of impervious human engineering. So all week long, crews have been feverishly working to repair the damage. By Friday, they had the new concrete poured, and the rain has held off long enough for the results to cure. Human engineering seems again to have the upper hand over climatic variability.