“Impact of Artificial Reservoir Water Impoundment on Global Sea Level” by Chao, et. al.: Useful but flawed

May 9, 2010

B. F. Chao, et. al., made a useful but flawed contribution to the question of sea level rise with their Science article “Impact of Artificial Reservoir Water Impoundment on Global Sea Level.”  The gist of their paper was that all the water captured by man-made reservoirs is water that would otherwise be contributing to higher sea levels.  This is an important point, they say, when considering global warming induced sea level rise.

They correctly point out that the sea level would be higher if this impounded water had been allowed to naturally flow to the sea.  Therefore, they conclude, global warming induced sea level rise is even worse than thought.  Read on, and I think you will agree with me that, amazingly, they missed one obvious point.


Chao, et. al., did an inventory of reservoirs, their creation dates and their capacities for the 20th century.  Their online supplementary material includes an exhaustive 600 page list of over 29,000 reservoirs.  They add up the total impounded water in these reservoirs to be 10,800 cubic kilometers.  They point out that this would have contributed another 30 mm to the sea level during the 20th century.  (See simple conversion factor here.)

They find that the vast majority of the water impoundment happened in the second half of the 20th century.  So, the amount of uncounted sea-level rise in the second half of the century was greater than the fist half of the century.  This confirms the alarmist point of view about dangerously accelerating sea level rise rates.

Here is Chao’s plot of the Church and White observed sea level with and without the Chao reservoir correction…

…but fatally flawed

What about the opposite effect: depleted natural reservoirs, such as lakes and aquifers?  Water that is pumped from an aquifer or lake, but not recharged, ultimately adds to the sea level.  This water would mistakenly be considered to be part the sea level rise due to global warming.

Ground water depletion has a host of serious ramifications apart from sea level rise.  For example the Indian states of Rajasthan, Punjab and Haryana (114, million people, or less than 2% of the world’s population) are depleting their aquifers by about 20 cubic kilometers per year, which may have devastating effects to these important agricultural regions.  In the United States (5% of the world’s population) as much as 800 cubic kilometers of water have been overdrawn from the aquifers in the 20th century.

In just 40 years the Aral Sea in central Asia has shrunk by nearly 1000 cubic kilometers (as this very nice interactive web page illustrates).

The University of Wisconsin’s Professor J. Van Klump estimates that the yearly global ground water overdraft is 200 cubic kilometers per year!!  This would account for over half a millimeter of sea level rise per year (“200 cubic kilometers” x “2.78 microns sea level rise per cubic kilometer“), and entirely counteract the effect of artificial reservoirs.

So, Chao made a useful contribution to the understanding of sea level, but his sea level correction was fatally flawed because he neglected the counter-effect of ground water depletion.

My question

Why wasn’t this point obvious to Chao, et. al.?  My guess is that I am not the only one who thought about ground water depletion when reading about Chao’s artificial reservoir water impoundment and its effect on sea level.  It is hard to believe that it did not occur to Chao, et. al.  It seems quite simple and obvious.  Their paper would have been more scientifically accurate and useful if it had not neglected ground water depletion.

The only conclusion I can draw is that the prevailing alarmist view of the day makes it easier to get a paper that inflates sea level rise rates published than a more complete paper that does not.

Update 10/24/10
A new Geophysical Research Letters paper (Wada, Y., L. P.H. van Beek, C. M. van Kempen, J. W.T.M. Reckman, S. Vasak, and M.F.P. Bierkens (2010), Global depletion of groundwater resources, Geophysical Research Letters, in press) confirms my estimate (and more).  In fact, Wada’s data shows the effect of ground water depletion at the present time to be GREATER than the effect of artificial reservoir storage.


  1. I believe your analysis is correct and Chao, et. al., missed the obvious due to their preconceived bias.

  2. Just a minute: If I put a dam across a river, a lake forms, interrupting the river’s flow for, say, a year. Then the water overflows the dam at precisely the rate of river flow, restoring the downstream bed to more or less original conditions.

    I don’t understand the problem.

    • Craig,

      thank you for your comment.

      Each gallon of water that is trapped behind the dam is water that does not flow to the ocean. As more and more dams are built, more and more gallons of water are trapped behind the dams.

      A reservoir with a one cubic kilometer capacity may have many cubic kilometers of water passing through it over time, as you suggest. But it still retains one cubic kilometer of water.

      Chao, et. al., added up over 10,000 cubic kilometers of water retained behind the dams over the last century. If these 10,000 cubic kilometers of water were released today, the oceans would rise about 30 mm

      Best regards,

  3. I am curious to know if the 10,000 number is for all damns filled to overflowing. Many dams I have seen are considerably drawn down, especially in my travels around California in the summer time.

    Also, many years ago I recall seeing a picture of a street in Mexico that was once level but the picture showed a wavy or bumpy street with the claim that the ground was slowly sinking due to less water in the aquifer, thus causing the un-level ground.

    I recall many years ago reading about Australian aquifers at risk of being depleted in the somewhat near future.

    If I as a layman could figure this stuff out even back then, including noticing the shrinking of the Black Sea, well I think the global warming bias by the authors is obvious.

  4. […] the “correction” to the sea level from Chao that made the sea level rise rate for the second half of the 20th century appear to be higher  […]

  5. One nit… the Church & White “observed sea level” wasn’t actually observed. Church & White admitted adding an adjustment (I’d call it a fudge factor) which greatly increased their reported rate of global mean sea level rise.

    Here’s the admission quoted from their paper:
    “An additional spatially uniform field is included in the reconstruction to represent changes in GMSL. Omitting this field results in a much smaller rate of GMSL rise…”

    Geographically weighted averaging of the best actual (GLOSS-LTT) tide gauge data yields a global average coastal sea level rise rate of about 1.1 mm/year, if you don’t add GIA and fudge factor adjustments like Church & White’s:

    Note that the alarmists’ adjustments “correct” primarily in one direction: up. They correct for Glacial Isostatic Rebound (which, for most locations, increases the reported rate of sea level rise), but they do not correct for land subsidence due to water, oil & gas wells. The IPCC’s AR4 admits this (though without mentioning how it biases the result) in the final paragraph of AR4 section
    “Trends in tide gauge records are corrected for GIA using models, but not for other land motions.”

    The measured rate of coastal sea level rise hasn’t accelerated AT ALL in response to the last 80 years of GHG emissions, so it is irrational to expect it to do so in the next 80 years.

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