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Long-Term Drought: The South’s Old Normal?

April 24, 2012

The mid-twentieth century was the “wettest” period of time in the American South since the year 1665, according to a recent study in the journal Environmental Research Letters.  The co-authors looked back over the last 400 to 1000 years and determined that the 1700s and 1800s were very, very dry compared to the wet 1900s.

How do we know?  Tree-ring data from the Apalachicola-Chattahoochee-Flint River Basin tells us.

So what?

In the late 1800s and early 1900s we started using instruments to measure and record rainfall, stream flow, etc.  By the mid-1900s this instrument data became the basis for water supply management decisions.  At the same time that we collected this data it was raining and snowing more; perhaps the temperature was also cooler.  In general: there was more water circulating in the hydrologic cycle here in the humid South.

In other words, the study suggests we made choices – to build reservoirs like Lanier and Hartwell for example – when it was abnormally wet and the rivers were flowing.  And we continue to use the same data from that wet period to project and plan into the future.  However, if the 1900s was a wet aberration then the future might return to the droughty 1700s and 1800s.   According to the study, this assumption of future water availability “might perversely lead to great vulnerability” during future droughts.  And that data may not provide “the impetus for developing efficient resource use or adaptation” to avoid impending water shortages.

Some folks in the Southeast may want to brush this study off, but they should not.  This long term perspective on drought and river flows will sound familiar to those who know the Colorado River basin’s water allocation history.  In 1922, seven states (Arizona, California, Colorado, Nevada, New Mexico, Utah, and Wyoming) signed the Colorado River Compact and agreed to divvy more than 16.5 million acre feet annually of the Colorado River basin’s water based on flows at a place called Lees Ferry.  This compact has since been tweaked but remains part of the “Law of the River.”

However, in recent decades basin flows suggest the long-term average is closer to 14 million acre feet.  Not unlike the emerging picture of long-term drought in the Southeast, research in the Colorado Basin has since concluded that “unfortunately” the data used in the early 1900s to establish the 16.5 million acre feet metric “as the basis for allocating the river’s flow” represented “an unusually wet period.”

Critics used to prophesize that the Colorado River’s massive reservoirs would fill with silt and one day turn dams into spectacular desert waterfalls.  We are now learning our problem might actually be empty reservoirs regardless of region.  As such, building more reservoirs that will not fill is like opening more bank accounts when you don’t have any cash-flow.  The difference: opening bank accounts will not threaten to bankrupt local communities like an un-justified new reservoir.

-Chris Manganiello

One Comment leave one →
  1. Bruce Henry permalink
    April 29, 2012 9:05 am

    The use of water in the ACF-ACT is based on the last 50 years or so of flow data. All the modeling for these basins is based on this data set. Whenever the suggestion was made that this data might be robust and another, more severe drought scenerio should be considered, the idea was ignored by the COE.

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