Last Week Pasadena Star News Public Editor Larry Wilson started a running discussion going at the JFleck at Instain blog on the question of how much groundwater recharge occurs from watering lawns?
http://www.inkstain.net/fleck/?p=3737#comments
The answers ranged from zero for those mostly amateurs trying to expound from Albuquerque on the Raymond Basin in Pasadena, to about 25%, or 7,000 acre feet of water (enough to supply 14,000 households or about 35,000 people) according to David Powell, a retired water engineer residing in Pasadena based on prior studies.
To put this in perspective, the City of Pasadena is mandating a 10% cutback in water use which is about 3,500 acre feet per year (enough to supply 7,000 households or about 17,500 people). So the calculation of groundwater recharge is a big deal.
Below is the thread of discussions from the JFleck Blog on this question:
Easy answer: For a southern US climate, unless the soil is made of gravel or sand, expect almost all infiltrated water to be lost as evapotranpspiration, not g/w recharge.
I agree completely with Daniel, but let me elaborate a bit.
If the water being used to irrigate was pumped from local groundwater sources the argument is absolutely absurd because you are suggesting that water was pumped from the ground solely for the purpose of putting it back in the ground - a complete waste of energy. If you are irrigating with an imported water source, you might be able to make the argument with a straight face, but in order to induce significant recharge would require significant overwatering of turf areas, in which case the excess water is more likely to run off into nearby drainages and/or be lost to evapotranspiration. If there is extra water available from an imported source it makes far greater sense to intentionally recharge by using designed recharge facilities ensuring maximum benefit from that recharge. Of course if the goal is to maximize water utility revenues, maybe it does make more sense to just encourage wasteful use.
@Daniel
Do you have references?
Thanks.
Eric, in hydro circles, there’s really no question - as Chris’s comment suggests. But if you want some more meat, see the relevant refs in Collins and Bras 2007 (WRR) - sorry, that’s the fastest way of answering your question. (It even applies to Yucca Mtn
Hey Folks,
I agree with the general tone here, but I’ll point out that people *frequently* claim some sort of public benefit from actions that are actually privately beneficial. It’s called the “baptists and bootleggers” paradigm.
(I’m gonna steal this example for my blog
Great answer from Chris.
Don’t forget the water quality issue. Should any inflitrating lawn water make it to the aquifer, it could be loaded with pesticides and fertilizer. Homeowners tend to overapply that stuff to their lawns.
Gentlemen:
Perhaps I need provide some additional information to all those making rash comments above about Mr. David Powell’s preliminary professional opinion that it is plausible that conservation activities could deplete the Raymond Basin in Pasadena, California beyond its safe yield level.
First, Mr. Powell’s qualifications: David Powell, P.E. is a Cal-Tech engineering grad, former head of the San Diego office of the California Department of Water Resources, former chief engineer for Bookman-Edmonston Engineering, qualified as an expert by a Special Master on Arizona vs. California Colorado River case, among many other eminent qualifications. He is presently retired and lives in Pasadena and is well-apprised on groundwater issues dealing with the Raymond Basin.
As I understand it, Powell is well versed with the mathematical and hydrological models previously conducted in the Raymond Basin by several water engineering firms. Those studies indicate the City of Pasadena has been overdrafting the basin beyond its safe yield level for quite some time. Moreover, the EIR for a proposed conjunctive use water storage program by the Metropolitan Water District of Southern California indicated that a significant percentage of landscape irrigation beyond evaporation losses was necessary to recharge the Raymond Basin to maintain its safe yield level.
Next a piece of anecdotal information: The neighboring City of La Canada is over a portion of the Raymond Basin. La Canada suffered a precipitous drop in its water table when it shifted from septic systems and leech fields to sewers. This is a possible historical indicator of what could happen to the Raymond Basin.
The Raymond Basin is not suffering due to what is somewhat euphemistically being called “California’s Wettest Drought.” Rather, the Pasadena Dept. of Water and Power Water Fund is suffering a revenue shortfall due to water conservation and a large number of unoccupied new downtown new housing units which are not generating water sales. Thus, water rates have to be increased due to conservation.
Doing something “green” is not necessarily good for the Raymond Basin and Pasadena’s Arroyo Seco watershed and ecology wherein the famed Rose Bowl is located. The Arroyo Seco is a large area of sand, gravel and silt which includes the Devil’s Gate Dam and Basin upstream from the Rose Bowl.
Actual water use in Pasadena over the last 10 years is flat in absolute measures - read here - “Downtown New Development Soaks Up Water from Others is an Urban Myth” - link:
http://pasadenanow.com/main/2009/06/10/opinion-downtown-new-development-soaks-up-water-from-others-is-an-urban-myth
It might actually be about the same cost for Pasadena to buy imported water than to enact new ordinances which criminalize over-watering - read here: “Would It Be Better to Buy Imported Water or Hike Water Rates?” link: http://pasadenasubrosa.typepad.com/pasadena_sub_rosa/2009/06/would-it-be-better-to-buy-imported-water-or-hike-water-rates.html
Now that you know a little more about the situation in Pasadena I hope that you can realize that Mr. Powell’s preliminary professional opinion is well founded. The situation with Pasadena’s Raymond Basin is probably paradigmatic to the other seven large water basins in Southern California and the rest of California. That the environmental professions have entirely missed the unintended consequences of water conservation on local ecologies and aquifers is yet another story.
I invite you to join Mr. Powell and I in educating the public about this paradoxical issue where doing something green (conservation) could likely harm the natural environment.
I’m sorry, maybe it’s just that I’m a dumb journalist, and not a hydrologist, but this makes no sense to me. Maybe Wayne or someone can correct me where my thinking has gone off of the rails.
Let’s say I pump up 100 gallons of water from the Raymond Basin and put it on my lawn. And let’s say 75 gallons of it ends up percolating back down into the aquifer. So the aquifer has been depleted by 25 gallons - the evapotranspiration of my lawn.
Would the aquifer not have been better if I’d left the whole 100 gallons down there? We can tweak the assumptions all we want, but the amount getting back down to the aquifer is always going to be less than the 100 gallons I pumped up.
What am I missing?
jfleck, what you need is a more efficient lawn irrigation system so that you only have to pump 60 gallons to water your lawn. the lawn still uses the 25 gallons to grow, but you’re now only recharging 15 gallons.
BUT, you can tell everyone that you’ve reduced your pumpage!! This is a good thing, yes? The really great news is that you can now add some more lawn, pump 70 gallons, consumptively use 30 gallons (25 for the original lawn and 5 for the new lawn), recharge nothing, and still claim to be a water conservationist because you’ve reduced your pumpage from 100 gallons to 70.
Water use efficiency improvements in a closed basin rarely effect the consumptive water use that drives groundwater depletions, but they sure look good!
[...] Fleck asks if we should agree with those who claim that their outdoor irrigation is okay because it “recharges” local groundwater. [...]
[...] Fleck asks if we should agree with those who claim that their outdoor irrigation is okay because it “recharges” local groundwater. [...]
If the choice is between pouring a gallon of water on your lawn, and pouring a gallon of water down the toilet, then sure, you’ll get more recharge out of pouring it on your lawn (AFAIK Pasadena doesn’t do groundwater recharge with treated wastewater, as some cities do; everything from the sewers goes straight to the ocean - though it’s been 5 years since I’ve worked on the Raymond Basin groundwater model, so I could be misremembering). I just can’t imagine a scenario in which this is the actual tradeoff - seems much more likely that people would simply reduce their water use.
On a certain level the way it is characterized by Powell is true - if an imported water source is used to irrigate landscaping, some of that water will recharge the aquifer. And if that irrigation is stopped, that recharge will no longer occur. The La Canada example is instructive - most indoor water use is not consumptive, so when you are using a septic system typically 90-95% of your water use infiltrates to recharge the aquifer. But outdoor use is usually considered to be mostly consumptive, i.e. 70-80% is lost to evapotranspiration with inefficient irrigation, more if you are efficient. This makes recharge via lawn irrigation very inefficient hydrologically. Economically, I suppose it depends on the marginal cost of the extra water applied to induce recharge vs. the cost of recharging some other way (assuming an imported source, if the source is local groundwater it’s a loser from the get-go).
So if the choice is between importing water and having some of it recharge by way of irrigation vs. not importing the water in the first place - Powell’s argument has some merit. But you have to consider what other uses that water could be put to, such as recharging by some more efficient means or a different use entirely. His argument presents a false choice, in my opinion, where the correct answer must be to continue the status quo - but if all possible choices are considered the answer is different.
Chris - Thanks for the helpful elaboration.
So, to pursue my metaphor, if we take the 100 gallons of Colorado River water and water our lawn, we get 20 to 30 gallons of increase in the Raymond Basin. If we stop watering our lawn entirely, and instead use the extra Colorado River water to offset 100 gallons of Raymond Basin pumping, we get a 100 gallon increase in the Raymond Basin.
Am I doing this right?
Yes, John, it’s not rocket surgery.
And I think DZ has the right approach, though I might switch “politics” for “money”.
I seem to have become the subject of some controversy and discussion. Unfortunately, the dicussion seems to have gotten off-track from what I was attempting to point out, and to have attributed to me views which I have not expressed.
Let me first say that: I am not opposed to water conservation; I am not proposing that users deliberately overirrigate their landscaping; I do not consider that overwatering of landscaping creates new water supplies; I do recognize that withdrawal of ground water from return flow uses energy; and I do recognize that some portion of the applied landscape irrigation water represents irrecoverable losses in the form of evaporation losses and runoff to paved surfaces which do not result in ground water recharge..
In order to understand the point I was trying to make, we need to understand the setting. Water users overlying the Raymond Basin obtain their water from two sources: extractions from the ground water basin; and imported water supplies purchased from The Metropolitan Water District of Southern California. Those supplies are insufficient to meet demands at historical levels. So to get through the current period of water supply deficiency, it is necessary to reduce the net draft on the local and imported water sources.
I have no objection to irrigating more efficiently. It is probably a good idea. But let us not fall into the trap of thinking that we are thereby substantially reducing the net draft on available resources. The reduction in water applied to landscaping is in large part offset by a reduction in that portion of the groundwater basin recharge derived from return flow. What we are doing is transferring a portion of the water normally obtained from Importation to increased overdraft on the Raymond ground water basin .
The only real question is what portion of the over-irrigation appears as return flow to groundwater and what portion represents irrecoverable losses. It is my opinion that the vast majority returns to ground water and that irrecoverable losses are minor. There seem to be those who hold the opposite view. I should be most interested in learning the basis for their view.
One final note. I have found it beneficial to have some indication as to the qualifications of those who offer their opinion on technical subjects. I have done a lot of ground water studies. I was in charge of a ground water management program when I was the head of the engineering department of a Water District in the San Francisco Bay Area. Later, while in private practice, I acted as the Engineer for a water agency which was the manager of one of the largest ground water management programs in Southern California.
@Daniel
Thanks for the reference in Water Resources Research.
Is there ‘rocket surgery’? I have heard of rocket science but not surgery.
Also the reference Collins and Bras. Collins does not turn up much in Google. Bras does.
My qualifications in hydrology-few.
My qualifications in chemistry, math, biochemistry, engineering, and modeling-lots.
I am just trying to learn a bit here from the rest of the folks and construct a model in which all the pieces fit together, hence my questions about following each water molecule to see where they go.
Yeah, yeah Eric. I’m trying to catch up with Bras. Just give me a few decades.
Dave, since there are number of themes now going along here, which ones do you disagree with?
I know you don’t think much of my first comment, but in response to the question being asked at the top, it is widely accepted hydrological knowledge. Where evaporative demand is high (SoCal), hydraulic conductivity is low and porosity is high (pretty common; non-gravel/sand; lawns), and soil is permeated by plant roots (lawns; down 10-30 cms) you should expect very little drainage below the root zone unless you’re really crazy with the irrigation (but in that case you’ll have a whole lot more runoff).
That said, I don’t actually see how this conflicts with your specific assertion. It’s possible the PSN journalist thinks there’s a conflict. Do you?
@Daniel
Thanks for very clear explanations.
Daniel:
Let me try to answer what I think are your questions.
Regarding the Pasadena Star-News editorial, I would disagree with the characterization that I am anti-conservation and the suggestion that I say that conserving could be counter-productive. I think my view could be more properly expressed by saying that the illusion of conserving is not the equivalent of actually conserving. I thought this was brought out rather thoroughly in my earlier posting.
I would strongly disagree with the view that return flow from applied irrigation water is not a significant part of the replenishment of groundwater. I would suggest that you do a little research into the views held by technical people in California. Contact the California Department of Water Resources, or the U. S. Bureau of Reclamation. See what academics at UC Davis and UC Berkeley have to say on the subject. In that regard, I might specifically direct you to the paper “UC cooperative extension groundwater hydrology program”–it is available on line. Also, you might talk to those who actually operate major groundwater management programs, such as the Water Replenishment District of Southern California or the Orange County Water District. After you have done that, let us know if you are still willing to contend that your view is widely accepted in the hydrological community.
Let’s now direct our attention to the specifics of the area being eiscussed– the Raymond Basin. You indicate that my contention would not be valid unless the area is underlain by gravel or sand. Just what, pray tell, do you think makes up the upstream portion of alluvial fans? Also, I would expect that before expounding on the flaws in the opinions of others, one would try to ascertain what hydrologic studies have already been made. I would refer you to the EIR for the Pasadena Groundwater Storage Program. The hydrology appendix for that EIR was based on groundwater modeling done by Geoscience Support Services. I assume this is the Raymond Basin groundwater model mentioned in an earlier posting by Maria. The EIR can be found at:
http://www.ci.pasadena.ca.us/planning/environmental/PasadenaGroundWater/PasadenaGroundWaterStorage_Home.asp
If you study the hydrology appendix and the response to my comments on the EIR. you will find that the water balance includes a contribution to ground water of some 7,000 acre-feet per year of return flow from applied water. That is nearly 1/4 of the adjudicated safe yield of the Raymond Basin.
I look forward to hearing your response.
Reply to J.Fleck on “wouldn’t it be better to leave the whole 100 gallons in the basin in the first place?”
There are two portions of a water basin: the portion that is “unsafe” to pump without depleting it and the “safe yield” which is the portion that will naturally replenish itself each year. So the answer to your question is NO, it is not better to leave 100% of adjudicated safe yield in the basin. Mr. Powell could educate you, as he has me, that allowing the basin to replenish without pumping to its safe yield could lead to a condition called “mounding” which could put pressure on the outer edges of the basin and the basin would start losing water.
According to Mr. Powell’s post below, if about 25% of the safe yield or 7000 acre fee of water is recirculated back into the Raymond basin each year that would reflect about 20% of the City of Pasadena’s total water use per year. Pasadena is being asked to cut back at least 10% in water use this year with likely another 10% cutback at least looming the future.
The cost or producing groundwater in Pasadena is about $125/AF. The new Tier 4 & 5 water penalty rates by the Metro Water District, the regional water importer, are about $800 to $1000/AF. Do the math. Pasadena could save $675 to $875/AF by relying on groundwater, or $2.3 to $3 million per year. Pasadena’s reliance on groundwater in 1999 was 60%; today dropping to 30%.
You might want to read my article “The Real Green Paradox: Staying in City May Reduce C02 But Importing City Water is Unsustainable” here http://greeneconomics.blogspot.com/2009/02/interesting-e-mail-on-water-and.html
David -
How many acre feet of water need to be applied to Pasadena landscaping in order to create that 7,000 acre feet of return flow? My point is that it is, necessarily, greater than 7,000 acre feet.
John:
First, one small correction. The Raymond Basin underlies not only the City of Pasadena–it also underlies portions of the Cities of La Canada-Flintridge, Arcadia and Sierra Madre together with the community of Altadena and other unincorporated territory. The 7,00 acre foot figure is for the entire basin.
I can’t answer your question directly. The 7,000 acre foot figure is that used by Geoscience, and their hydrology as contained in the EIR does not permit a determination of the corresponding irrigation water used on landscape area. I have not tried to independently estimate it, but my educated guess would be somewhere around 12,000 to 15,000 acre feet.