IFS in East Maui – Part III: Base or Storm Flow?

Scott Werden @ Wiki Wai

This post is a continuation of our series on East Maui Stream IFS (Instream Flow Standard). Part I covered the legal rulings on two groups of East Maui streams, Part II dove into how IFS is determined, and Part III (this post) will look at an issue that has caught the attention of people who work with water: base flow vs. storm flow.

Review: Base Flow

The IFS system for East Maui designed by the Water Commission pays very close attention to base flow and total flow. To recap from Part II – base flow is reliable and steady, which is good for commercial agriculture so there is a built-in assumption that EMI gets the base flow. But, there is a question of which portion of the flow (base flow or excess flow, i.e. storm flow) will deliver the most water?

Average flow is not used much in discussions about IFS and East Maui water policy but it is extremely useful in analyzing long term benefits. Average flow (also called mean flow) is functionally equivalent to the total volume of water delivered since the total is just the mean times 365 (days).

This example is based on a East Maui stream, the Kopiliula. We chose the Kopiliula because there is lots of flow data for it (from the USGS) and we can calculate the statistics that the Water Commission uses. We also have info on the diversion design, and can use that to model what happens to the stream as it goes through it. The diverter for this stream is type “A” as that term is defined in Appendix D of the 2021 EIS. The Water Commission has set IFS for the stream at 3.2 MGD.

Two views of the Kopiliula stream diversion. The intake to the ditch is the grate which the stream flows over. Excess flow goes over the edge of the dam shown in the left picture.

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The table below contains TFQ values calculated from the stream’s flow data and using the description of the diverter and the IFS. The row marked “Upstream” is above the diversion. The row marked “Downstream” is the stream flow after diversion. The row marked “Ditch” is the flow in the ditch from the diversion. Numbers in each row are Million Gallons per Day (MGD).

The important thing to notice is (1) the ditch flow is very steady (a high Q₉₀) while the downstream flow is erratic, and (2) the downstream mean (orange text in the table) is significantly higher than the ditch mean! In other words, the total water that goes into the ditch is much less than the total water that goes into the streambed.

Total flow is never discussed by the Water Commission staff because it has little bearing on the in-stream uses. But it has huge bearing on off-stream uses. Off-stream users could store up that huge amount of excess flow in reservoirs. It can be argued that the diversion model commonly used in East Maui is backwards – instead of diverting base flow and leaving the excess flow in the stream, the opposite should happen.  Doing so will make the stream biota happier and should make Mahi Pono happier, although they will have to build more reservoirs.

Conclusions

Base flow is steady but excess (storm) flow ultimately delivers more total water. It is clear that diverting excess flow should be a consideration for off-stream uses. Capturing and using excess flow requires reservoirs. Instream uses cannot build reservoirs but off-stream users can and this should be considered when defining who gets the base flow – the ditch or the stream? This has caught the attention of the Maui DWS who recently did an analysis (See section 3.2.1) on options for new water sources for Maui, and capturing excess flow was one of their options.

Further Reading and Resources

Document Name	Description
Low-Flow-Streams-CWRM-PR202201	Research paper on base flow characteristics for Huelo streams.
PR200910-Kopiliula	Instream Flow Assessment Report for the Kopiliula Stream