Friday, November 11, 2011

Let's Take A Step Back And Look Long Term

This hydrograph is from the long term monitoring well located on the Kansas State University Experiment Station just west of Colby, Kansas.  It has been measured since the late 1940's with much of the record having a continuous water level chart available.  When you look closely, you see a pretty steady decline rate since 1959 except for a slight, but noticeable bump starting in 1990 and lasting until 2000.

You should be aware that from 1977 through about 2000 this region went through a remarkable conversion of the irrigation systems used to irrigate local crops - from the traditional flood systems to center pivot systems.  These new systems reported pumping reductions - sometimes up to 30%.

If pumping dropped so significantly, why didn't the water level decline rate slow accordingly?  It only slowed during the 1990's because every year of this decade but for one was above average rainfall.  Pumpage in the 90's was down even more and recharge was up a bit.  As soon as rainfall returned to normal, the decline rate did so as well.

The answer is that the water table decline rate is related solely to consumptive water use, not pumpage.  The high pumpage rates under the older, less-efficient flood irrigation systems also meant more deep percolation - recharge.  When the more efficient irrigation systems came in, less water was pumped, but less water was also recharged - the higher system efficiencies meant that a higher percentage of the pumped water went to crop production and healthier crop canopies.  With slightly higher ET use on slightly reduced irrigated acres, total ET stayed about level.  All this means that consumptive crop water use during this transition was staying about the same - thus the decline rates stayed the same.

The only way in our neck of the woods to slow the decline rate is to reduce consumptive water use - meaning crop ET.  The only way to do this is to grow lower ET crops on the same acres, reduce acres, start deficit irrigating the same acres or some combination of all these things.  As the declines continue and eventually well yields drop off, irrigators will start making these decisions by default and the economic engine of the region will begin to slide down - over time.  This is especially true since all wells will not drop off at the same rate or over the entire region at the same time.  It doesn't make it any better, but the notion that one morning the entire Ogallala will be dewatered and stop producing agricultural irrigation water is far fetched.

This is what the HPA process is all about - allowing local water users to decide to reduce CU earlier in this scenario to extend the economic life of the aquifer as they see fit.  I don't see strongly "right" or "wrong" answers here, just local preferences - IF the locals can publicly make these decisions. Let's hope they can discuss these issues in earnest and chart their best course.

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