Monthly R-GPCD: what is signal and what is noise?

The California State Water Resources Control Board has proposed using monthly R-GPCD (Residential Gallons per Capita per Day) to administer the Governor's 25% urban water reduction drought mandate.

There's a small problem though: monthly water usage is naturally variable because of different seasons from year to year and water meters aren't always read regularly on a monthly basis!

So how do we make sense of reported monthly water usage at a utility level?  What signals actual conservation and what's just noise?

So with that in mind, here's a few example time series of R-GPCD for reported months in 2013 and 2014/15 with the year over year change reported as a decimal.  

These utilities were selected from ARGO's list of outliers, defined as those with greater than +/- 50 year over year percentage change in any one period.  The full list of utilities from SWRCB is shown in an interactive dashboard at the end of the article.

City of Atwater: what is signal and what is noise?

Bella Vista: what is signal and what is noise?

East Palo Alto: What is signal and what is noise?

Humboldt Bay: What is signal and what is noise?

The full list of water utilities in the SWRCB data collection effort are accessible in the interactive dashboard below.  As an alternative to straight monthly R-GPCD, A.R.G.O. would suggest using a rolling average over at least three months to smooth out natural variation.

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