Thursday, April 25, 2013

Rain, Irrigation and Declines in GMD 4

One would hope that irrigation, precipitation and groundwater level changes are all relational - the more it rains, the less we irrigate and the slower the water level decline goes.  The following are data for the 10 Counties in NW Kansas for the years 2000 through 2012.  

Col 1 = Year; Col 2 = Annual average precip (10 stations); Col 3 = Reported water use (GMD 4 total, all uses except domestic); Col 4 = Acres reported irrigated; Col 5 = Inches of water reported applied per reported acre irrigated; and Col 6 = January 1 (following year) water level change.  Unfortunately, 2012 data for water use and acres irrigated are not yet available.

The water level change data come from 275 observation wells across the entire GMD area that are measured each January. 

Year  Precip     Wtr Use      Ac Irr      In/Ac      WL Chg
2000   16.72      497,737      386,055        1.29      -1.16
2001   19.79      424,223      380,152        1.12      -0.41
2002   11.30      527,661      386,350        1.37      -1.51
2003   14.06      484,311      386,979        1.25      -1.14
2004   20.13      479,461      385,161        1.24      -0.6
2005   21.15      397,666      381,202        1.04      -0.57
2006   19.37      435,017      379,479        1.15      -0.29
2007   17.07      417,848      377,010        1.11      -0.89
2008   21.65      406,801      377,691        1.08      -0.42
2009   25.59      301,350      376,254        0.80        0.1
2010   17.45      368,030      376,969        0.98      -0.5
2011   22.48      435,900      380,667        1.15      -0.59
2012   11.76


I see a pretty strong correlation (inverse) between rainfall and annual declines - Col 2 and Col 6.  When the rainfall increases, the water level decline rate decreases.  Just eyeballing the numbers, it appears to me that with average annual rainfall (18.35 inches) we'd expect about a .6 foot decline.  With 23-24 inches of rainfall, a .25 foot decline, and with 12-15 inches somewhere around a 1.5 foot decline.

I see a little less correlation between the reported water use and the water level change, but there is still some relationship.  It is quite true in the extremes, anyway - the wettest year (2009) saw the least water used and the smallest water level change while the driest year (2002) saw the greatest water use and the largest water level change.

Most can probably see the inherent sense of all this, but may wonder why there is not a perfect relationship between rainfall, pumpage and declines.  Well there are a number of reasons why we'll never see such an absolute relationship:

1)  The rainfall data provided is annual precipitation.  Six months out of the year the rainfall is far less relevant to crops and irrigation.  There is probably a more relevant relationship between in-season rainfall and what we're trying to show, but this is hard to flesh out when our region gets just 60-65% of its total rainfall in season.

2)  The rainfall data is highly variable - meaning that while the entire NW Kansas average may have been 20 inches, we easily could have had irrigation areas that only got 14 while other areas got 23.   

3)  The quality of rainfall is never known in the data.  You might get 20 inches of annual rain, but if it came in 5 hard rains of 4 inches each over a three hour period, most of it ran off and did not contribute to soil moisture conditions that would allow for reduced irrigation.

4)  Any water level change is a function of natural recharge and pumping withdrawals. While more rain generally means increased recharge (and reduced pumpage) it is not an exact relationship (see reason 3) above).

5)  Cropping is in constant flux and different crops affect withdrawals differently - both the timing and quantity.

6)  A late freeze in the Spring, hail, excessive insect or weed pressures all affect an irrigated crop and the amount of water applied.  It may rain 15 inches, but if a late Spring freeze takes out your corn crop, irrigation is greatly reduced that year.

Anyway, I think you get the picture.  But I have to say, all-in-all, there is a pretty good relationship in GMD 4 between rainfall, irrigation use and groundwater declines.  Now, if it'd only rain - at the right time, in the right amount and with the perfect intensity...


  1. such a nice informative post. thanks for sharing it..


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