West Maui system’s Maximum Reliable Capacity and 3-Year Water Usage Forecast

Maximum Reliable Capacity and 3-Year Water Usage Forecast Projection WEST MAUI SYSTEM

The Department of Water Supply has completed its most recent analysis of maximum reliable capacity for the West Maui System, which serves Napili in the north and Lahaina in the south. This analysis considered the following information:

• Calibrated pump capacity at all wells serving the system

• Pump capacity over run periods of 16 hours and 24 hours for all wells serving the system

• Pumpage limits prescribed by groundwater management area designation by the Commission for Water Resource Management (CWRM)

• Maximum capacity for surface water treatment plants (WTP)

• 12-month moving average (MAV) production data for all wells and WTPs serving the district.

Based on this information, current maximum production capacity for the system is 7.17 million gallons per day (MGD). Maximum reliable capacity (MRC) must account for various uncertainties, including mechanical failure, human error, and weather events. Probability and duration of various system events were considered, including downed well pumps and periods of low to zero intake flow at treatment plants. Based on the probability and duration analysis, the following operational and failure assumptions are made to assess reliable capacity:

1. Mahinahina Water Treatment Plant is down.

2. One Waipuka well is down.

These assumptions reduce the system's capacity from 7.17 MGD to 4.26 MGD in the MRC scenario.

Prior to the August 8, 2023 wildfire, the combined 12-month moving average production data for all West Maui operated sources is 5.50 MGD. As of now, current demand is 129.1% of MRC.

3-Year Projection

Pre-wildfire population growth projections, as described in the 2030 Maui Island Plan, indicate an average annual increase of 2.13% . Assuming a eventual repopulation of the burn zone and a continuation of prior projections, over the next three years, anticipated population growth-based demand is:

• 12/31/2025: 5.50 MGD

• 12/31/2026: 5.61 MGD

• 12/31/2027: 5.73 MGD

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

• 2025: 0.00 MGD

• 2026: 0.00 MGD

• 2027: 0.00 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, currentlybooked reservations are programmed to decline in a linear fashion. As a result, the following adjusted reservations per year are:

• 2025: 0.00 MGD

• 2026: 0.00 MGD

• 2027: 0.00 MGD

Inactive Meters (meters connected but not showing consumption)

Currently, approximately 117 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 0.10 MGD.

Department of Hawaiian Homelands (DHHL) Allocation

In accordance with the Memorandum of Agreement for Licenses entered into between DWS and DHHL on November 15, 2019, DHHL is registered an allocation of 200,000 gallons per day for its Honokowai and Village of Leiali'I Phase I-B projects.

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

• 2025: 5.80 MGD including inactive meters and DHHL allocation

• 2026: 5.92 MGD including inactive meters and DHHL allocation

• 2027: 6.04 MGD including inactive meters and DHHL allocation

Demand at the end of 2027, including potential demand from inactive meters, is projected to be approximately 141.8% of MRC.

Because the 3-year demand projection is in excess of 100% of the system's MRC, additional source is not currently available to the West Maui water system for processing new or additional service.

Source Development

The Department is currently constructing a new groundwater source, the Kahana Well. This well is projected to produce approximately 0.96 MGD and is anticipated to be available for placement into service in early 2026. This well is being developed for the purpose of offsetting reduced inflow to the Mahinahina WTP due to Interim Instream Flow Standards.

Because Mahinahina is assumed to be down in the MRC scenario, the Kahana Well source will be additive to reliable capacity. When this source is connected and producing, the MRC of the West Maui system will increase from 4.26 MGD to 5.22 MGD, which will reduce the 3-year projection from 141.8% of MRC to 115.7% of MRC.