Flawed Data = Flawed Conclusions

June 23, 2022

Tonight I tried to have Craig Sutherland, a Kerr Wood Leidel engineer, and one of the authors of the Lake Maxwell Hydrology Reports, accept the fact that KWL’s modelling of Lake Maxwell is flawed, due to bad data. He didn’t seem to understand what I was trying to say.

So, let me try and state it clearer. KWL’s 2021 Report on Maxwell states on Page 4-5:

“Over the 105-year record, at a demand of 48% of the total licenced demand volume, which is the approximate current NSSWD consumption, it was found that Maxwell Lake does not refill 21 out of the 105 years. This can be seen in Figure 4-1, which shows the change in month end storage / lake level over the long-term record. Although the lake does not refill every year of the record, there is sufficient storage to meet the current demand, which is consistent with the 2015 Maxwell Lake report findings. Increasing the demand to 60% of the total licenced demand volume results in Maxwell Lake being unable to refill 41 out of the 105 years and reaching zero live storage 6 out of the 105 years(Figure 4-2).”

Both computer projections are erroneous for the following reasons.

The current NSSWD, average annual consumption (2015 to 2020) is approximately 48 million gallons per year = 34.5% of the 146,000,000 gallons per year license, and NOT 48% as stated by KWL.

Subtracting from 48% the actual, average percentage of license withdrawal of 34.5% equals 13.5%.

13.5% divided by the 34.5% equals 39% which means KWL’s simulations are OVER ESTIMATING the number of years which the current average withdrawal would mean that Lake Maxwell would not refill by a whopping 39%.

KWL needs to rerun the simulation at 34.5% and make an amendment to their report.

Now is begged the question “how” did KWL arrive at the 48% number in the first place.

To answer that question we have to first go back in time to December 2014.

On December 17, 2014, four months before the issuance of KWL’s first study dated April 2015, NSSWD Staff reported to the Board, and I quote, “The control valve that was installed at the Ganges Hill tanks to eliminate tank overflow has reduced daily flows by 30 to 40,000 imp gallons/day.” That “leak” was the equivalent to 11 million to 14.5 million gallons per year.

Staff’s estimate was later substantiated by NSSWD’s 2015 Water Audit (https://northsaltspringwaterworks.ca/wp-content/uploads/2018/03/NSSWD-Water-Audit-2015.pdf) which indicated that there was a drop in bulk withdrawal of over 14 million gallons per year between 2013 and 2015.

Since that leak was repaired in 2014, bulk withdrawal from 2015 to 2020 has remained consistently within the 46-47,000,000 gallon per year range, and metered consumption (the amount that users actually use) has remained relatively constant in the 43,000,000 gallon per year range.

Tonight I asked KWL the question as to when they were informed of the 2014 leak repair, and was told they were not informed about it at the time of the preparation of their 2015 report, and then not until 2018.

However, in spite of evidently been informed of the 2014 leak repair in 2018, KWL’s updated 2021 report states (on page 6-1) “NSWWD currently withdraws about 258,870 m3/year from Maxwell Lake on average (2010-2020).”

And here is where we run into the data problem. From 2010 to 2014 the average annual bulk withdrawal was 69,720,154 gallons per year. From 2015 to 2020 the average annual bulk withdrawal was only 46,672,472 GPY.

So, when KWL included the 2010 to 2014 data in the 2010-2020 data set, to calculate what they refer to as “the approximate current NSSWD consumption” they are skewing the “current” past 6 years consumption by including the pre-leak repair withdrawals…by a whopping 39%.

The importance of this is underlined by KWL’s above statment, “there is sufficient storage to meet the current demand.” PLEASE NOTE. The “current demand they are talking about in that statement is 48% of licensed withdrawals, not 34.5%. So, the conclusion that can be drawn by that statement is there is at least 13.5% of the licensed volume available = 146,000,000 x .135 = 19,710,000 gallons available which could service over 450 dwellings.

With all due respect to KWL, when flawed/skewed data is used (which you know is flawed/skewed data by 39%), to model a future scenario, you cannot call this good or reliable science to base a decision on, and, I trust that when I send KWL and NSSWD Board a link to this article in a moment, we can perhaps get KWL to go back to the drawing board and use accurate data to run their models again.

As a ratepayer, I paid good money for these studies, and, I expect my elected representatives on the Board to make sure I get accurate reports on which they can make an informed decision.