At yesterday’s City Council meeting, Greg Meszaros presented a wide variety of information including a graph titled “Projected Peak Day Demand” as part of the Water Treatment Plan No.4 presentation. This graph appeared to get a significant amount of the attention from City Council members as the discussion unfolded. Unfortunately, there were three substantial problems with this graph.
The Starting Point is Incorrect
According to the graph, the 2009 peak day demand is a bit over 240 MGD. However, slide No.5 from the Water Conservation Update presentation the Council received earlier in the day indicates otherwise. Slide No. 5 titled “Daily Water Pumping 2000-6/2009” indicates that at the end of June daily water pumping had not exceeded 200 MGD. I’ve pasted a replica of the chart from that slide below. If indeed the current peak day demand stays closer to 200 MGD at current population levels (40 MGD below the demand graph’s starting point), it significantly undercuts the need for the additional 50 MGD added by WTP4. That’s why it is important to get the starting point right.
The Growth Correlation is Not Supported by Recent Data
A second implication of slide No.5 is that the assumption of peak day demand and population growth is not a firm correlation. As the graph above points out, peak day demand growth was not a firm linear pattern even before the significant implementation of conservation efforts. Population was indicated as the key driver – that peak day demand did not grow consistently during a period of constant population growth undercuts this notion.
The Assumed Growth Rate is Not Explained
The titles of the different trend lines indicate that they should have consistent relationships. Since the actual assumptions used to create the data were not disclosed at the council meeting, I went ahead and tried to recreate the chart using the data points for the 16 MGD conservation line (the green line.) The annual growth assumption between 2014 and 2025 is 1.35% for the 16 MGD line. It is not clear at all from the presentation where this assumption comes from, especially given that the low and high population growth scenarios are 1.8 and 2.2%. Please see recreation of chart below.
Because the chart provided by Mr. Meszaros does not have gridlines, it is not clear that the relationships remain consistent across time. Any changes in the relationship would reveal important assumptions that should be understood by the Council and public.
One option that Council might want to consider is to make the data and assumptions public and encourage the Austin community to submit their own models. It would be highly-innovative if the Council created something similar to the Netflix Prize (www.netflixprize.com) and crowdsourced the development of alternative demand estimates and awarded public recognition to the best models. Given the significant capital on the line in this investment, it might make sense to engage the many civic-minded, quantitatively-gifted people in the city to create a more powerfully explanatory model of peak day pumpage.