Friend, colleague and NAE member Leonard F. 'Lenny' Konikow, published this informative paper in the January-February 2015 issue of the NGWA journal Groundwater: 'Long-Term Groundwater Depletion in the United States'.
Download Konikow-2015-Groundwater
Abstract
The volume of groundwater stored in the subsurface in the United States decreased by almost 1000 km3 during 1900–2008. The aquifer systems with the three largest volumes of storage depletion include the High Plains aquifer, the Mississippi Embayment section of the Gulf Coastal Plain aquifer system, and the Central Valley of California. Depletion rates
accelerated during 1945–1960, averaging 13.6 km3/year during the last half of the century, and after 2000 increased again to about 24 km3/year. Depletion intensity is a new parameter, introduced here, to provide a more consistent basis for comparing storage depletion problems among various aquifers by factoring in time and areal extent of the aquifer. During 2001–2008, the Central Valley of California had the largest depletion intensity. Groundwater depletion in the United States can explain 1.4% of observed sea-level rise during the 108-year study period and 2.1% during 2001–2008. Groundwater depletion must be confronted on local and regional scales to help reduce demand (primarily in irrigated agriculture) and/or increase supply.
Lenny does a great job detailing the volumetric depletion rates (cubic kilometers per year) for 40 aquifer systems or subareas during two periods: 1900 - 2000 and 2001 - 2008.
Here is the map for 2001 - 2008:
Lenny correctly noted that using volume per unit time may not give a true picture of an aquifer's depletion. Aquifers of different sizes may have the same volumetric depletion over the same time period, but the aquifer with a smaller area will be experiencing a greater relative depletion. No problem - just normalize volumetric depletion rates by dividing by the aquifer's surface area. This gives a number with dimensions of L/T (length per time), such as meters per year. Lenny calls this (new) parameter depletion intensity.
Here is what the above figure looks like in terms of depletion intensity:
It's easy to see that the High Plains aquifer (HPA) does not look so bad when you consider depletion intensity. Why is that? The HPA underlies a large area so the depletion per unit area is smaller than if you just examine the volumetric depletion. rate. The same is true of the Mississippi Embayment aquifer. The small Coachella Valley aquifer in southern California looks a lot worse when you consider depletion intensity. That's because that although the amount of water pumped out is small, it's taken from an aquifer underlying a relatively small land area.
Lenny is right to call depletion intensity a 'new' parameter, but normalizing volumes by dividing by areas is common in hydrology. For example, hydrologists frequently talk about ET (evapotranspiration) and runoff in terms of length/time as opposed to volume/time. For example, a watershed of 1000 acres may have an mean ET volumetric rate of 500 acre-feet/year or 0.5 feet/year. A watershed of 100 acres may produce 75 acre-feet/year or 0.75 feet per year. The smaller watershed has a greater ET rate. These lengths are really depths distributed uniformly over the watershed areas - equivalent or effective depths.
Keep in mind that the depletion intensity does not account for variations in an aquifer's thickness or pumping from multiple aquifers in the same basin but we're trying to keep things simple here.
Groundwater hydrologists don't use this approach as frequently as surface water hydrologists, although I did so in a post last August when I was discussing an article on groundwater depletion in the Colorado River basin (CRB). I just did not call it a 'depletion intensity'. But the CRB underwent an average groundwater 'depletion intensity' of about 0.03 feet/year.
So is 'depletion intensity' a new parameter and is it useful? Yes and yes!
Excellent paper by Dr. K.
Note: If you read the online version of the paper you can access additional material not in the hard copy version - click here.
"Hey, my kid could have done this!" - Unknown, viewing abstract artwork
"Yeah, but he didn't." - Also Unknown
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