Outdoor residential water use is increasingly becoming a target for urban water conservation. As western municipalities face more restricted water supplies, water providers are paying residents to take out their lawns permanently. Current programs in Southern Nevada and Los Angeles are yielding water conservation at a cost of $65,000/AF to $87,000/AF.
The math is straightforward. Southern Nevada Water Authority pays $1.50 per square foot of lawn removed. The City of Los Angeles is bumping their payments to $2.00 per square foot of lawn removed. There are 43,560 square feet in an acre. Therefore, Southern Nevada is paying $65,340 per acre and Los Angeles $87,120 per acre.
What is the per acre-foot cost of lawn fallowing? We need an estimate of the amount of water used in lawn irrigation. As a swag (fancy term for wild guess), assume that lawn irrigation uses 1 AF per acre. (Assumption based on residential water use of 2.5 AF/acre, half used outdoors, of which 80% used for lawn irrigation.) Southern Nevada’s cost of water conservation from lawn fallowing is about $65,000/AF. Los Angeles’s cost is about $87,000/AF.
How does the cost of lawn fallowing compare with desalination? Assuming that lawn fallowing is permanent, there is no material default risk from water conservation by lawn fallowing. Therefore, using a long-term interest rate of 4.5% (interest rates), the annualized cost of lawn fallowing is about $3,000/AF for Southern Nevada and about $4,000/AF for Los Angeles. For the Carlsbad plant now under construction, San Diego County Water Authority’s cost is about $2,000/AF (Carlsbad).
How Much More Expensive is Lawn Fallowing than Desalination? Don’t let the above estimates fool you. The annualized cost for lawn fallowing is a fixed nominal amount for permanent conservation. The cost of desalination is not comparable for two reasons: (1) only the capital part of the cost is fixed in nominal terms, the other portions, such as energy costs, will increase over time, and (2) the desalination plant may not be permanent. A comprehensive analysis would need to adjust for these differences. This will increase the comparable cost of desalination. The adjustments are beyond this venue. What do you expect in a blog post? It is Friday and sunset beckons.
Takeaway. Lawn fallowing is not cheap and probably more expensive than desalination.
This post based on from information included in Mr. David Pettijohn’s presentation at 6th Annual OC Water Summit presented by the Orange County Water District and the Municipal Water District of Orange County
May 23rd Addendum: Another advantage of lawn fallowing is that it does not involve extensive regulatory approvals, litigation risks, or infrastructure investment
November 25: There is a discussion on twitter today about this post. Pete Gleick found the “math confusing” and estimate of 1 AF/acre “too low”
Ok with having different ideas about water use for lawns. Don’t understand how math is confusing.
There are four simple steps:
1) How to convert a payment per square foot per acre. Multiple the square foot payment by square feet per acre (43,560 square feet per acre). Conversion factor is a look up. Did I get this wrong?
2) What is the amount of water used for lawns per acre:
a) Residential water use per acre: 2.5 AF/acre. This is the general amount in the west. But, there could be another number. What is it?
b) What fraction of residential water use used outside: 50% is general amount in the west. But, there could be another number. What is it?
c) What fraction of outside use used on lawns (as opposed to plant watering, car washing, hosing down driveways, sidewalks and patios): I assumed 80%. But, could there be another number. What is it?
I said that my estimate was a swag. But 2.5 AF/acre x 50% x 80% = 1.25 AF/acre x 80% = 1 AF/acre.
What is way too low in these assumption that makes the yield from “lawn fallowing” way “too low”
Interested, Peter and, for that matter, anyone’s else’s views.