IFS in East Maui – Part II: Determining IFS
Scott Werden @ Wiki Wai
This post is a continuation of our series on East Maui Stream IFS (Instream Flow Standard). The first part covered the legal rulings on two groups of East Maui streams, this part will go into the details of how each stream’s IFS is derived. The last part examines the difference between diverting base flow and storm flow.
Instream Flow Standard
Accord to State law (HRS § 174C-3), “‘Instream flow standard’ means a quantity or flow of water or depth of water which is required to be present at a specific location in a stream system at certain specified times of the year to protect fishery, wildlife, recreational, aesthetic, scenic, and other beneficial instream uses.”.
Stream Flow Metrics
The Water Commission uses statistical measures, like “90% of the time the water flow must be at least X”. They have a shorthand for that, that looks like this: B90 or T90. The letter indicates the type of flow, the number indicates the percentage of days the flow is at or above. So, for instance, B90 = 5.0 cfs means 90% of the time (or 330 days out of the year) the base flow must be at least 5.0 cfs. A high percentage value, such as B90, is the most commonly found flows so this number is going to be used for beneficial uses that require reliable flow.
What is Base Flow?
The Water Commission divides raw stream flow into two regimes:
- Base Flow. This is flow that is due to springs, cloud drip, and leakage of soil. It excludes runoff from rainfall. It should be present most of the time.
- Total Flow. This is base flow + the runoff from rainfall (we call the latter, “excess flow” here). It is sporadic but accounts for a significant amount of water in East Maui.
Why do this? Base flow values are the most dependable values for stream flow since it does not depend on rainfall runoff. Everybody wants to have access to base flow because it is most reliable. The Water Commission recognizes that and uses base flow quality for setting IFS values. But here is the fly in the ointment – base flow statistics are used to specify instream flow yet the base flow is preferentially diverted for off-stream uses.
So how does all this relate to the real world of East Maui Streams? The following table should help illustrate that. This table is derived from the 2018 Decision & Order from the Water Commission for several (but not all) East Maui Streams.
| Category of Instream Use | Formula for IFS | Example Streams |
| Connectivity | 20% of BFQ90 | Hanawi, Nua’ailua |
| Habitat or H90 | 64% of BFQ90 | Kopiliula, Waikamoi |
| Traditional Use | 100% of all Total Flow | Honopou, Waiokamilo |
Notice that Connectivity and H90 are both calculated from the 90% percentile of base flow. That is the flow that is present in an undiverted stream for at least 330 days out of the year. A percentage of that must be present in the diverted stream, and that is the IFS.
Confirming that IFS is Being Met
The law, specifically HRS § 174C-3, requires the IFS to include the time frame over which the flow value must be met or exceeded. This is not exactly adhered to in the 2018 D&O issued by the Water Commission, which states the following in regards to confirmation:
Monitoring of the IIFS will be through 12-month moving averages. This method recognizes that requiring a specific amount of flow at all times at a specific location is incompatible with the objectives of providing sufficient flow to meet irrigation and domestic requirements and/or providing sufficient habitat for growth, reproduction, and recruitment of native stream animals.
This is significant – the Water Commission is admitting it is not complying with the law when it only requires a one-year average rather than a minimum flow over a certain number of days, and it admits it is prioritizing an off-stream use of water (irrigation and domestic use) over instream use, even though off-stream use has a questionable legal status.
Diverting Streams
The story of IFS is not complete without talking about how streams are diverted. There are several different designs but many of the streams use a combination of a dam with a spillway, and a sluice gate. Appendix D of the 2021 EIS (Volume 2), calls this diversion type A, and it is the most common diversion design in East Maui. The dam-with-spillway is designed to separate base flow from the high-volume rainfall runoff, as described in the following excerpt from Volume 1 of the EMI 2021 EIS:
The EMI Aqueduct System was designed and is intended to be operated to capture and convey a major portion of the base flow from streams in the License Area to supply the former sugarcane operations in Central Maui. The EMI Aqueduct System is not designed to capture and convey short periods of high streamflow known as freshets that occur when it rains heavily in the upslope areas of the watershed. Such larger flows quickly overtop or bypass the diversions and remain in the streams.
Ho’olawanui diversion. Input grate to the ditch. The stream bed is behind the man and with low to moderate flow it all goes in the grate and is diverted. High flow makes it past the grate and stays in the stream bed.
A sluice gate allows a portion of the diverted base flow to re-enter the stream bed but the gates are mechanical and have to be set by hand. In principle, each sluice gate can be set such that the instream flow complies with the IFS for that stream. But very few streams have active monitoring of downstream flow so the public has no visibility into whether the diversion is complying with the stream’s IFS, and since the Water Commission allows one-year average to be used, there is no real incentive to tuning the sluice gate to maintain some semblance of steady flow in the stream.
The IFS system for East Maui designed by the Water Commission is to a great extent differential to the needs of EMI – base flow is reliable and steady, which is good for commercial agriculture so by default, EMI gets the base flow. This prioritization has questionable legal basis to it, but that aside, there is a question of which portion of the flow (base flow or excess flow) will deliver the most water? We will defer answering that question to the next part in this series, which you can access here.
Further Reading and Resources
| Document Name (and link to it) | Description |
| sb20260519C1 | Progress report on modifying diversions in order to comply with the 2022 PAIFS ruling. |
| CWRM – Summary of Diversions in Huelo sb20220719B2 | Summary of stream diversions in Huelo |
| Stream Portal | Wikiwai Map and list of IFS and other data for all streams that have been diverted by EMI. |