Monday, October 18, 2010

Beware of Groundwater Depletion Predictions

Topic: Pet peeve (if not number 1, pretty high up there) - Predictions of when we'll run out of groundwater.

Let's start in my own back yard - the Kansas High Plains - Ogallala country. The following was printed on February 4, 1979 in one of the state's leading newspapers:
State water experts predict that irrigation will be nothing but a memory in many large areas of west central Kansas in eight to 10 years. They give northwest Kansas about 15 years..
Well, northwest Kansas is my area, and I'm glad to report that irrigation is still here. And most of it is still in west central Kansas, too.  In other words, the 1979 predictions were not at all accurate.  The question is why?   The short answer is that most predictions take an average trend - like annual decline rate - and project it forward.  In 1979 the average decline rate in western Kansas was approaching 2 feet per year.  With only 40 to 80 feet of saturated thickness remaining in west central and northwest Kansas respectively, and irrigation needing about 30 feet of water to continue, the math at that time seemed close to correct.

But, in real situations, the declines reduce well yields, which in turn reduce water diversions, which in turn reduce the decline rates.  The assumption of a straight-line trend is faulty.  Click on the water level chart above to enlarge it.  This chart involves 50 obsevation wells in one County in NW Kansas - Sheridan.  Graphed are the 4 wells of these 50 which show:  the most saturated thickness in 1965; the least saturated thickness in 1965; the most decline - 1965-2008; and the least decline - 1965-2008.  The heavy black plotting is the average saturated thickness of all 50 wells in each year.

Several things are obvious. First, where there was good water in 1965, the wells were pumped hard and declines resulted. Where there was not good water, there was limited use and a relatively stable saturated thickness. Second, the average decline rate is slowing and in fact all the graph lines are converging toward that average.  Again, the straight line trend assumption if used in this case in 1975 would have been very wrong.

And furthermore, this is just one section of our district. Looking at the same chart for the 19 observation wells in western Grahan County (the next county east of Sheridan) the decline problem is a non-issue.

Water table declines will always be problematic, but they will rarely be as bad as the press and headline grabbers want them to appear.  So, ask the right questions and get the good data before assuming the end of the groundwater world as we know it today.  Groundwater is very temporal and site specific, so generalizations do no one any good.

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