Groundwater and Sea Level Rise: Avoiding The Water Budget Myth Trap

November 8, 2010 | Posted by Michael "Aquadoc" Campana
2 Comments

A few months ago I posted about two papers dealing with global groundwater depletion that had been published in the journal Geophysical Research Letters:

Paolo D’Odorico, Francesco Laio, and Luca Ridolfi (2010). Does globalization of water reduce societal resilience to drought? Geophysical Research Letters, 37: DOI 10.1029/2010GL043167

Marc.F.P. Bierkens et al (2010). A worldwide view of groundwater depletion Geophysical Research Letters: DOI 10.1029/2010GL044571

Now, friend and hydrogeologist extraordinaire Clay Cooper sent me information about another one, this time focusing on the contributions of groundwater pumping to global sea level rise:

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, Geophys. Res. Lett., 37, L20402, doi:10.1029/2010GL044571.

Here is a summary of the paper from a New York Times blog.

The abstract (the emboldened portions are mine):

In regions with frequent water stress and large aquifer systems groundwater is often used as an additional water source. If groundwater abstraction exceeds the natural groundwater recharge for extensive areas and long times, overexploitation or persistent groundwater depletion occurs. Here we provide a global overview of groundwater depletion (here defined as abstraction in excess of recharge) by assessing groundwater recharge with a global hydrological model and subtracting estimates of groundwater abstraction. Restricting our analysis to sub-humid to arid areas we estimate the total global groundwater depletion to have increased from 126 (±32) km3 a?1 in 1960 to 283 (±40) km3 a?1 in 2000. The latter equals 39 (±10)% of the global yearly groundwater abstraction, 2 (±0.6)% of the global yearly groundwater recharge, 0.8 (±0.1)% of the global yearly continental runoff and 0.4 (±0.06)% of the global yearly evaporation, contributing a considerable amount of 0.8 (±0.1) mm a?1 to current sea-level rise.

You can purchase this paper for $9 USD at the GRL WWW site.

The reason Clay sent me the abstract is the second sentence, which would seem to indicate that these authors, likeothers before them,  have fallen prey to the ’groundwater and the water budget myth’ syndrome. Here is what I wrote on 1 Deecmeber 2008:

What most people don’t realize is that once you start the development (i.e., pumping), the water budget, usually calculated under steady-state conditions, becomes invalid. Why? Because you have imposed a new stress on the system – pumping – and that means that the water budget has changed. But most water managers don’t realize this, and blithely assume a steady-state budget when transient conditions actually apply.

Theis’ paper, “The source of water derived from wells: essential factors controlling the response of an aquifer to development”, stated that the source of water discharging from a well comes from:

  1. increase in recharge;
  2. decrease in discharge (springflow, ET, baseflow to a stream); and/or
  3. change in water storage in the aquifer.

As long as some water comes from storage, well water levels will continue to drop. Only when pumpage is balanced by (1), (2), or some combination of (1) + (2) will water levels cease to decline and a new equlibrium is reached. It may take many years for this to happen.

In that same post  I also wrote the following:

I am partial to the Bredehoeft et al. paper. When I discovered it 26 years ago while at the Desert Research Institute, I promptly photocopied it and sent it to the Nevada State Engineer, with the notation, “Have your ground water staff read and understand this paper.” What got me in trouble was the “and understand” part. I almost got fired for that little episode. I doubt they read it down in Carson City.

So do the authors of this most recent paper succomb to the water budget myth trap? The first emboldened passage in their abstract suggests that they have. But the second emboldened passage implies that they are aware of it: in arid and subhumid regions the likelihood of capturing discharge and increasing recharge are less likely than in humid regions when you start pumping groundwater.

I am more comfortable the assumption that increasing recharge is less likely than is capturing discharge. In dry regions with minimal streamflow, discharge such as ET or spring discharge can be captured.     

In reading the paper I noted that the authors do reference John Bredehoeft’s work so I assume that they are cognizant of the pitfalls of simply looking at groundwater abstraction and natural recharge to assess depletion. So the paper has my imprimatur.

Still puzzled by all the fuss about groundwater and the water budget myth? Read my original post and the sources therein. 

“These concepts must be kept in mind to manage groundwater resources adequately. Unfortunately, many of our legal institutions do not adequately account for them.” — Bredehoeft et al., 1982, p 57


Comments

2 Comments so far

  1. Tim McGivern on November 12, 2010 2:38 pm

    Thank you. I’ll pass this along.

  2. Re-thinking Groundwater: The Water Budget Myth « Its All One Big Adventure on November 12, 2010 3:02 pm

    [...] wanted to bring up an important topic that got some blog time on the AWRA blog a couple days ago. The post has to do with how we manage groundwater withdrawals. It deserves [...]

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