Scott Werden @ Wiki Wai
Demand and Capacity
When talking about water security there are two side to the coin: Capacity and Demand. Capacity is how much we have now and how will that change in the future. Demand is what are all the uses for water and how will that change in the future. We are only going to focus on the capacity side in this post. Demand is an important concern and will be discussed in a future post.
Factors Affecting Capacity
There are four sources of water on Maui and each of these is divided into whether the source is purely public or involves private infrastructure to distribute the water. This is illustrated in the table below. The figures represent the contribution of that category to the entire water budget for Maui Island. Two sources (private recycling and public desalinization) are negligible so we ignore them leaving only six sources.
| Public | Private | |
| Ground | 11% | 21% |
| Surface | 4% | 62% |
| Recycled | 2% | – |
| Desalinization | – | <1% |
Explanation of sources:
Groundwater.
These are wells. Public groundwater is almost entirely from wells owned by MDWS or the State. Private groundwater are residential wells and irrigation wells. The latter do not enter the public system but are important because they reduce demand on surface water.
Surface water.
This is streams and springs. Public surface water means water that comes from a stream but is delivered by MDWS. Private surface water is almost entirely stream water that is diverted for agriculture. The latter is the single biggest water source/use on Maui.
Recycled water.
Some waste water treatment facilities (WWTF) can produce various grades of recycled water. Most is done within three public WWTF (Kahului, Kihei, and Lahaina). The interesting fact here is that the capacity of these plants is about 25 mgd (million gallons per day) but only 2.65 mgd is actually used. It is far under-utilized. See the table in the Appendix.
Desalinization.
Removing salt (and other minerals) from sea water. Most of this is private. Desalinization is not significant as of 2026 for Maui.
Risks
Now that we understand the relative importance of each source, we can look at the risks each one has. We breakdown risk into the six categories in the table below. Each source is assigned a value (low, medium, high) for that risk factor. We total (low=1, medium=2, high=3) each column to get an overall risk for each source type. Low total is low risk.
| Surface | Ground | Recycled | De-Sal | |
| Climate Change | High | Medium | Low | Low |
| Economic Cost | Low | Medium | Medium | Very High |
| Environmental | Med | High | Low | High |
| Private Ownership | High | Low | Low | Low |
| Growth Risk | Med | Low | Low | Low |
| Cultural Risk | High | Low | Low | Low |
| Overall Risk | 14 | 10 | 7 | 11 |
Discussion on Risks
Surface Water
Climate change is a huge risk to surface water, both by diminished rainfall and erratic rainfall. Existing surface water systems were built for base flow with occasional storm flow. It is uncertain how these will change as climate change progresses.
Most surface water delivery systems are in private hands (as of 2026) which means they will bear the brunt of adjusting to changing climate. It is not known if they will respond adequately to these changes, so private systems have an added risk. Surface water is the most utilized water source on Maui so when it is mismanaged, particularly in times of draught, it puts pressure on groundwater sources to make up the loss. Since groundwater is mostly used for municipal water systems, that means mismanaging surface water ends up affecting domestic and municipal availability. This is best seen in the Central aquifer sector which is drastically over-pumped (see the table in the Appendix), partly due to agricultural pumping.
Cultural risk used to be very high for surface water but some of the problems have been addressed and it has come down but we still put it at high, largely due to West Maui.
Groundwater
Groundwater is largely more stable than surface water but it is more expensive (due to pumping costs and decontamination), and this is the primary risk. Secondary risk is environmental – it is unknown what the effect will be of pumping large volumes of groundwater. Climate is a lesser risk for groundwater than surface water because erratic rainfall is not an issue but total rainfall will be an impact. Knowing the hydrogeology of the aquifer will better quantify the degree of climate risk.
Recycled Water
Recycling has very little risk and very good opportunity for growth. The greatest risk is economic since a lot of infrastructure is needed to make use of it.
Desalinization
Very high economic risk due to the cost of power. High environmental cost due to the disposal problem of waste salt.
Summary
Which option gives Maui the most water security? Clearly recycled water has the most upside with the least downside, but it is limited on how much can be harvested so it is in the category of low-hanging fruit. Groundwater has only a single “High” risk, for environmental concerns (mostly due to insufficient knowledge of the aquifer). But this risk is solved with more USGS work. Surface water is almost tapped out so there is very little upside to be gained and it still has a lot of environmental and cultural risk. But it is clearly at risk in just maintaining the status quo due to climate change.
What are the policies that would best secure water for Maui’s future?
Put all private surface water systems under public control. As the climate changes, we cannot leave the necessary adjustments up to the whims of private entities; this puts municipal water at risk.
Invest in recycling water. Because the capacity far exceeds the actual use of it, there is insufficient delivery infrastructure and financial incentives to use recycled water are needed. The MDWS can (and should) address both of these.
Tightly manage aquifers that are being over-pumped. There is a risk of poisoning the aquifer by pulling seawater into it. The CWRM and MDWS need to monitor this closely.
Get better recharge models of the Central, Ko’olau, and Lahaina ASEAs to better gauge the sustainable yield of groundwater. Groundwater is the next best opportunity for growth after recycled water but it has its limits and we need to know what those are before we start pumping.
Appendix
Much of the data for this post is from the 2019 Maui Water Use and Development Plan (WDUP).
How Water Is Used
| Agricultural | Irrigation | Municipal | Private | Total | |
| Groundwater | 57.3 | 4.3 | 25.1 | 4.1 | 91.2 |
| Surface | 172 | 11 | 183 | ||
| Recycled | 2.6 | 2.6 | |||
| Total | 229 | 6.9 | 36 | 4.1 | 277 |
| 82% | 2.5% | 13% | 1.5% | 100% |
See page 169 of WUDP
Surface Water Diverters
| Company | Amount | Destination(s) |
| WWC | 42.7 | Agriculture Municipal |
| EMI | 117 6.6 | Agricultural Municipal |
| MLP | 10.0 3.3 | Agricultural Municipal |
| Private | 3.0 | Irrigation |
| DWS | 10 | KAP |
See page 168 of the WDUP
Groundwater Sources
| Aquifer Sector | Sustainable Yield | Pumpage | Percent |
| Wailuku | 36 | 20.7 | 58% |
| Lahaina | 34 | 6.2 | 18% |
| Central | 36 | 63 | 241% |
| Ko’olau | 175 | 1.0 | 1% |
| Hana | 122 | 0.6 | 0 |
| Kahikinui | 34 | 0 | 0 |
| Total | 427 | 91.2 | 21% |
See page 166 of the WDUP
WWTF That Recycle Water
| Plant | Designed Capacity | Amount Used | Percent of Capacity |
| Kahului | 7.9 | 0.25 | 3.2% |
| Kihei | 8.0 | 1.5 | 18.7% |
| Lahaina | 9.0 | 0.9 | 10% |
| Total | 24.9 | 2.65 | 10.6% |
See page 171 of the WDUP