A simple, scientific approach to efficient water use

 
Graphical illustration of the MWELO targets.

Graphical illustration of the MWELO targets.

 

The Model Water Efficient Landscape Ordinance (MWELO) targets provide a scientifically determined allocation of reasonable indoor and outdoor usage. These customized targets provide and effective and equitable framework for conserving our state's precious water resources.

To demonstrate how the state currently performs with regards to the MWELO targets, Data Collaborative staff prepared an estimate of efficiency targets for 340 large water agencies across the state. The chart below compares our aggregate, estimated efficiency target against actual aggregate water production for these utilities.

  • Blue line -- actual residential production
  • Black line -- estimated efficiency target with error bounds

As you can see, in aggregate, California's urban areas met MWELO efficiency targets in 2015 according to our estimates. However, approximately 30% of individual urban retailers did not meet those benchmarks.   

Methodology

The MWELO performance standard is calculated using the formula for existing development:

55 * [Population] * (365/12)  +  0.8 [Monthly ET] [Irrigable Area] 0.62

The variables in brackets are utility specific data aligned with the assumptions outlined above.  55 represents indoor usage of 55 gallons per capita per day and multiplying by 365 and dividing by 12 is used to convert that to monthly usage.  0.8 represents the conservation factor used for current development in MWELO and 0.62 is used to convert into gallons.

1.         This analysis uses the MWELO formula for existing residential and does not factor in the different new development calculation. 

2.         Irrigable area is estimated for each tax parcel using a statistical model incorporating parcel size, population density and neighborhood income. Specifically, a linear regression model was fit to actual measured irrigable area obtained from participating agencies. Variable selection was performed by comparing error scores obtained through five-fold cross validation. This approach yields approximately 70% predictive accuracy after aggregation at the agency level.

3.         Evapotranspiration (ET) for each month is calculated as the inverse distance weighted average of the ten nearest CIMIS stations.  Monthly ET is calculated as the sum of daily ET reported by the CIMIS during that month.

4.         Residential production is simply the total production for each agency multiplied by the reported percent residential usage. These data are both publicly available in the State Water Resources Control Board (SWRCB) conservation reporting data.

5.         All water production data is in gallons and is pulled from the SWRCB statewide conservation reporting excel spreadsheet.  340 agencies out of the 411 agencies in the SWRCB database matched with our statewide parcel shapefiles.