Water Department completes Central System maximum reliable capacity analysis

Staff from the Maui Department of Water Supply recently completed an analysis of maximum reliable capacity for the Central System.

In accordance with Administrative Rules Section 16-201-4, applications for additional water service will be limited to 1,200 gallons per day unless exempted by the rules.

Chief Engineer James L. Jensen’s summary memo to Director John Stufflebean can be found below:

SUBJECT: Maximum Reliable Capacity and 3-Year Water Usage Forecast Projection CENTRAL DISTRICT

The Department of Water Supply has completed its most recent analysis of maximum reliable capacity for the Central District. This analysis considered the following information:

• Calibrated pump capacity at all wells serving the district
• Pump capacity over run periods of 16 hours and 24 hours for all wells serving the district
• Commission for Water Resource Management (CWRM) permitted limits as of 11/7/2017 and 11/2022 requested modifications for all wells and also for the ʻĪao Water Treatment Plant (WTP)
• 12-month moving average production data for all wells and WTPs serving the district.

Based on this information, current maximum production capacity is 29.29 million gallons per day (MGD). Maximum reliable capacity must account for various uncertainties, including mechanical failure, human error, and weather events; therefore, two failure assumptions are made to assess reliable capacity:

1. The ʻĪao WTP capacity is set equal to its production moving average to account for days lost due to turbidity. This reduces ʻĪao WTP’s production capacity from 3.2 MGD to approximately 2.34 MGD.
2. The Mokuhau Well 3 facility, which produces 2.35 MGD, is assumed to be temporarily out of temporarily out of service.
3. Due to chloride level increases prevalent in over-pumped wells, other wells are not assumed to pump at temporarily higher rates while the Mokuhau 3 well is down.

These three assumptions reduce the system’s maximum reliable capacity from 29.29 MGD to 26.94 MGD.

As of the end of 2024, the combined 12-month moving average production data for all sources is 24.28 MGD. As of now, current demand is 90.2% of maximum reliable capacity.

3-Year Projection
Population growth projections, as described in the 2030 Maui Island Plan, indicate an average annual increase of 2.22% . Over the next three years, anticipated population growth-based demand is:

• 12/31/2025: 24.82 MGD
• 12/31/2026: 25.37 MGD
• 12/31/2027: 25.94 MGD

Current reservations for water service during the next three years, based on gallons per day usage guidelines, are:

• 2025: 0.92 MGD
• 2026: 0.92 MGD
• 2027: 0.89 MGD

Some portion of population growth is attributable to the issuance of meter reservations; therefore, these values will decline as reservations are fulfilled. To account for reservation fulfillment in conjunction with the population growth projection, currently booked reservations are programmed to decline in a linear fashion. As a result, the following adjusted reservations per year are:

• 2025: 0.92 MGD
• 2026: 0.68 MGD
• 2027: 0.46 MGD

Inactive Meters (meters connected but not showing consumption)
Currently, approximately 2300 connected meters are inactive and not measuring consumption. Based on empirical average consumption data for similar sized meters, the projected usage of these inactive meters is 0.977 MGD.

As reservations convert to actual usage, anticipated demand over the next three years is:

• 2025: 25.20 MGD (26.17 MGD including inactive meters)
• 2026: 25.51 MGD (26.48 MGD including inactive meters)
• 2027: 25.83 MGD (26.81 MGD including inactive meters)

Demand at the end of 2027, including potential demand from inactive meters, is projected to be approximately 99.5% of maximum reliable capacity.

Because the 3-year demand projection is between 98.0% and 100.0% of the system’s maximum reliable capacity, a request for water service in excess of 1200 gallons per day will be considered a “large quantity of water” and will be subject to the limitations described in §16 201-04 of the County of Maui Administrative Rules, Title MC-16.