UHERO Insights
Native Forests as Critical Infrastructure: The Economic Case for Watershed Management in the Ala Wai
INSIGHTS ARE PRELIMINARY MATERIALS CIRCULATED TO STIMULATE DISCUSSION AND CRITICAL COMMENT. THE VIEWS EXPRESSED ARE THOSE OF THE INDIVIDUAL AUTHORS. WHILE INSIGHTS BENEFIT FROM ACTIVE UHERO DISCUSSION, THEY HAVE NOT UNDERGONE FORMAL ACADEMIC PEER REVIEW.
By Conrad Newfield, Yu-Fen Huang, Yinphan Tsang, Emma Yuen, Leah Bremer, Kimberly Burnett, Nathan DeMaagd, Jean Fujikawa, Nate Dube, Serene Smalley, and Erin Bishop
Take Home Messages
- Native forests provide natural flood protection. Upland forests slow stormwater, stabilize soils, and reduce downstream flooding. Without conservation actions, invasive species like Albizia and Miconia can increase flood risk by clogging streams, increasing erosion, and decreasing the capacity of forests to act as a sponge.
- Removal of invasive species by watershed conservation organizations measurably reduces flood risk and improves water quality. Empirical data show that targeted removal of Albizia between 2021–2022 was correlated with lower runoff and reduced turbidity for equivalent rainfall events.
- The cost of inaction could nearly double flood damages. Modeling projects that 10 years of uncontrolled spread of Miconia and Albizia would increase expected annual flood damages from $68M to $134M, while more than doubling Ala Wai Canal annual dredging costs from $1.4M to approximately $3M.
- Sustained funding for invasive species committees and watershed partnerships is critical. These benefits depend on continued management by groups like Koʻolau Mountains Watershed Partnership and the Oʻahu Invasive Species Committee. A shift from temporary grants to durable, long-term funding is an urgent priority.
Recent flooding events from multiple back-to-back Kona Lows devastated areas across Hawaiʻi. While we cannot stop high rainfall events, land care and management, including watershed protection and stream and ditch management, play an important role in flood outcomes. Healthy native forests can reduce the impacts of flooding through slowing water flow and reducing erosion. In contrast, where left unchecked, invasive species like Albizia (Falcataria falcata) and Miconia (Miconia calvescens) can reduce these important hydrologic services. Albizia, in particular, is also known to clog streams, exacerbating flooding. Watershed conservation efforts that protect native forests and control the spread of these and other invasive species can reduce runoff and peak flows during flood events, leading to better outcomes for ecosystems, people, property, and livelihoods.
On March 23rd, 2026, while many of us were working on campus or from nearby homes, a flash flood took us all by surprise and overtopped Woodlawn bridge, lifted parked cars, flooded many homes (including our lead hydrologist’s!) and left a path of mud through Noelani Elementary School, which closed for the following week (Fig. 1). Biking around Mānoa after the flood, Conrad, a researcher on this team, documented Albizia logs jammed in and alongside Mānoa stream. This recent event was very reminiscent of the 2004 Halloween flood, an estimated 27-year extreme (daily) rainfall, which caused $80 million in damage to UH when water, blocked by a large Albizia log, jumped that same bridge and ran down through the Mānoa campus.[1]
Figure 1. Parking lots in Mānoa valley following the March 23, 2026 storm.
The question on all of our minds, in addition to the recovery efforts, is: what role exactly does watershed management play in flood outcomes downstream? Over the past few years, our team of resource managers, hydrologists, geographers, and economists has been studying the impacts of invasive species control and watershed conservation efforts in the Mānoa watershed, and right before the flood hit, we produced our preliminary results. The simple answer is, it matters a lot. Specifically, we found that without the efforts of the Koʻolau Watershed Partnership, the Oʻahu Invasive Species Committee (OISC), the Department of Land and Natural Resources Division of Forestry and Wildlife (DOFAW), and others’ efforts to stop the spread of Albizia and Miconia, expected annual damages from flooding could double from $68 million to $134 million. And, as we all know from the recent flooding, flood damage is not just about the immediate economic damages, but the long-term hardship for families, communities, farmers, and our environment.
The nitty gritty
Healthy forests in Makiki, Mānoa, and Pālolo watersheds act as a natural sponge, intercepting intense rainfall and stabilizing the soil on steep valley walls. This natural hydrology subdues peak streamflows and limits erosion despite the topography. However, these ecosystem services are under threat from many invasive species, including Albizia and Miconia. Albizia is actively targeted by the Koʻolau Mountains Watershed Partnership (KMWP) as part of a grant from the National Fish and Wildlife Foundation (NFWF). Albizia is a brittle, fast-growing species that forms enormous canopies. During storms, its branches break easily, physically blocking stream channels, creating dangerous backwater flooding in residential areas, and littering coastlines post-flood (Fig. 2). Miconia is one of the 100 worst invasive species globally (GISD 2026[2]) and the Oʻahu Invasive Species Committee (OISC) was founded 25 years ago to work toward Miconia eradication. Miconia, with its large leaves and shallow root system, disrupts watershed function by creating a bare understory. This effect, in combination with heavy rain, exacerbates erosion and flooding that strip away topsoil, causing massive sediment surges that threaten downstream water quality and coral reefs.[3]
Fig. 2. Broken logs in and near streams following the March 23, 2026 floods in Mānoa.
KWMP has been actively working to remove Albizia and OISC has been actively removing Miconia. We wanted to understand how this work might influence flooding and erosion downstream. To do this, we combined hydrologic monitoring with a linked land cover-hydrological-economic model.
First, we collected empirical hydrological data, including rainfall, streamflow, and turbidity from Makiki, Mānoa, and Palolo. We separated the data based on time ranges, before the project started (before 2021), and after the major invasive plants removal (2023-2025). Then we compared the streamflow and turbidity changes before and after the invasive plant removal. The results show decreased streamflow for a given rainfall amount in Makiki and Mānoa (Fig. 3), where Albizia and Miconia were detected and removed most often. Additionally, we find turbidity has decreased in Makiki stream, for a given streamflow. Notably, these changes happened with just a few years of management, which suggests that the runoff response is relatively fast.
Fig. 3. Makiki and Woodlawn sub-watershed before-and-after, log streamflow vs. log rainfall results
Second, we created maps of where Miconia and Albizia are today and asked the question of what the landscape would look like in 10 years without continued removal and conservation actions. To project the future risk of inaction, we developed spatially explicit spread models for both target species into a 10-year unmanaged spread scenario (henceforth “spread scenario”) and compared it to a baseline of current, partly invaded forests. Our modeling suggests that, without removal, both species would spread rapidly across the area (Figs. 4 and 5).
Fig. 4 . Miconia baseline (green dot) and its 10-year spread scenario (yellow dot)
Fig. 5. Present-day Albizia (Panel A) vs. Simulated 50-year spread of Albizia (Panel B)
We then used the calibrated and validated hydrological model (SWAT+) to project out what the difference in flooding (peak flows) and erosion (sediment yield) would be under an unmanaged future by integrating the hydrological impact from these two invasive species. We find that major floods would happen 50-150% more often under the spread scenario, depending on the size of the flood. More specifically, the amount of rain that would trigger a “26-yr flood” at Woodlawn drops from a 26-year rainfall event (266 mm) to a 12-year rainfall event (210 mm), meaning catastrophic flooding becomes roughly twice as likely in any given year.
We then used data from the U.S. Army Corps of Engineers to assess the economic implications of this. Collectively, we found that if Albizia and Miconia are allowed to spread unchecked for just 10 years, expected annual flood damages in the watershed would nearly double, increasing by approximately $68 million, to $134 million annually. Furthermore, the resulting sediment increase would double both the occurrence frequency and annualized cost of dredging the Ala Wai Canal. Dredging would be required roughly every 8 years instead of every 17, pushing annualized costs from $1.4 million to around $3 million.
Our findings demonstrate that upland forest protection and invasive species control are critical for reducing flood risk and supporting resilient ecosystems and communities. To ensure that this work continues, we must look for ways to move from a reliance on temporary grant funding to more durable funding for the Oʻahu Invasive Species Committee (OISC), Koʻolau Mountains Watershed Partnership (KMWP), and other groups who care for our watersheds (Fig. 6). The health of our watersheds is intricately linked to community well-being, and we need to develop reliable and dedicated public and private funding streams to support this work over the long-term.
Fig. 6. OISC and KMWP crews managing invasive species
Acknowledgements: Mahalo to the National Fish and Wildlife Foundation for funding this project and to Flint Hughes of the USDA Forest Service Institute of Pacific Islands Forestry for providing invaluable science and framing of the Albizia problem.
[1] Chu, P., X. Zhao, Y. Ruan, and M. Grubbs. 2009. Extreme Rainfall Events in the Hawaiian Islands. J. Appl. Meteor. Climatol., 48, 502–516, https://doi.org/10.1175/2008JAMC1829.1.
[2] https://www.iucngisd.org/gisd/
[3] Nanko, K., Giambelluca, T.W., Sutherland, R.A., Mudd, R.G., Nullet, M.A. and Ziegler, A.D., 2015. Erosion potential under Miconia calvescens stands on the Island of Hawai‘i. Land Degradation & Development, 26(3), pp.218-226.