The Data Collaborative has applied statistical techniques to estimate the actual water savings achieved through turf removal rebates. The results of that approach (using bayesian structural time series) are shown below and are described in full in the CaDC study published by KDD, which is available here.*

Predicted monthly savings for each household in the data set. The dark green line corresponds to median savings. Seasonal variation leads to swings in average savings from -1.5 to -2.7 gallons per square foot.

Predicted monthly savings for each household in the data set. The dark green line corresponds to median savings. Seasonal variation leads to swings in average savings from -1.5 to -2.7 gallons per square foot.

For the CaDC study of the water savings associated with turf removal, mean predicted savings for single-family residential accounts are estimated at 24.6 gallons per square foot per year for the households used in the study. The study utilized a data set of 545 unique single-family residential turf rebates across 3 California water utilities, totaling 635,713 square feet of converted turf grass.

The blue line shows the observed usage for one household. The red line shows the expected usage based on the consumption of households with historically similar usage patterns. The vertical dotted line represents the time the turf removal was performed. A 29 percent reduction from expected water use is visible after the removal.

The blue line shows the observed usage for one household. The red line shows the expected usage based on the consumption of households with historically similar usage patterns. The vertical dotted line represents the time the turf removal was performed. A 29 percent reduction from expected water use is visible after the removal.

When this technique is combined with automated monthly data ingestion, it provides the ability to generate ongoing estimates of program savings to adaptively manage the historic investment in turf rebates. This sort of approach also opens the door to more advanced analyses such as outlier detection of accounts with suboptimal water savings.  

More broadly, the approach can be applied to estimate the causal impact of any targeted conservation program by comparing the usage patterns of affected and unaffected customers and how these patterns change after a program takes effect.  Several other studies are ongoing and we are working with the CaDC technical working group to develop an expanded version of this analysis.

Of course, the ultimate effectiveness of this historic investment in turf rebates will come down to two key things: 1) how long the lawns will stay converted and 2) how many neighbors were influenced to change their landscape.  The chart below shows the cost effectiveness of the turf rebate program broken out by those two axes.  The calculations used to generate this chart are available here.

The ultimate effectiveness of the turf rebate program will come down to those two factors: lifespan and peer effect.   If landscapes do not stay converted and lawns stay the default in ornamental landscaping, the turf rebate program will go down in history as not cost effective.  

Yet if this historic investment is the tipping point in making California friendly plants mainstream, it will go down in history as a visionary and extremely cost effective program.   So below is a strategy on continuing the turf market transformation momentum to deliver on exactly that.  

Turf rebates have succeeded in their goal: converting early adopters away from water thirsty lawns to California friendly landscapes.

Yet even with the historic investment in turf rebates, we are only at approximately 1.6%  of turf market transformation in terms of the total square footage of turf.  Smart conservation pricing and public education are cost effective ways to continue the momentum in making California friendly plants mainstream. 

The goal of course is not to eliminate lawns.  Yet ornamental landscaping like street medians does not need to be a water thirsty lawn.  In fact, per the latest Governor’s executive order lawns in street medians cannot be irrigated with potable water.

*CORRECTION: An early version of the translation of the water savings associated with turf removal into a dollar per acre foot value had a unit conversion error and yielded an incorrectly low dollars per acre foot figure.  This has been corrected in the graph shown and final paper linked above.  Assuming a 30 year lifespan, no peer effect and a 5% hyperbolic discount rate, the correct translation of 24.6 gallons per square foot into $1422 dollars per acre foot of water saved from a $2 per square foot turf rebate.