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By Christopher Wada, Kimberly Burnett, and Sittidaj Ponkijvorasin
In Hawai‘i and other parts of the world, native forest and valuable linked natural resources are being threatened by forces including land use change, invasive species, and climate change. Management strategies for protecting these resources are commonly divided into two categories: (1) active restoration where interventions such as planting seeds and seedlings or removing invasive species are implemented to accelerate recovery, and (2) passive restoration where stressors such as ungulates are removed that cause degradation, usually followed by fencing to protect areas no longer under stress. The North Kona coast of Hawai‘i Island is an ideal site for considering a variety of such management strategies to combat freshwater scarcity, given projected increases in freshwater consumption due to planned development, negative impacts of feral ungulates in upland groundwater recharge capture areas, uptake of groundwater in lowland areas dominated by invasive kiawe trees, and the abundance of submarine groundwater discharge (SGD) in the region, which supports a number of groundwater dependent ecosystems (GDEs). Although most people tend to focus on the direct consumptive value of groundwater, it became clear through discussions with the Commission on Water Resource Management (CWRM) that maintenance of proper ecological balance is also an important factor in groundwater management decisions.
To that end, UHERO researchers, in collaboration with Sittidaj Ponkijvorasin from Chulalongkorn University (Econ PhD from UH Manoa), developed an integrated groundwater and watershed management model using data from Kīholo aquifer on the west coast of Hawai‘i Island as part of the NSF-funded ‘Ike Wai project. The model included a minimum SGD threshold to ensure maintenance of limu manauea in the nearshore, which is valued culturally, ecologically, and as a food source. This means that in the absence of other management interventions, pumping would likely have to be lower in the future to maintain the required amount of SGD, which would give rise to some economic costs (making up the shortfall using a groundwater alternative like desalination would be costly). However, results from numerical simulations of the model suggest that an integrated mountain-to-sea approach to groundwater management has the potential to offset a large share of those costs. The study finds that investment in upland ungulate-proof fencing is preferred to lowland invasive species (kiawe) removal if one is limited (e.g. by budgetary considerations) to using a single conservation tool, but using both management instruments simultaneously is nearly always optimal. Under baseline assumptions, the cost (or tradeoff) of protecting the valuable GDE (limu) is $192 million in present value terms over 50 years, and optimal coordination of fencing and invasive species removal reduces that cost to $98 million, i.e. by more than 50 percent.
In general, different combinations of management instruments may be optimal. Net present value is not always necessarily maximized by using every available management instrument simultaneously, and it is important to understand both the value of the water a particular instrument is providing, as well as the cost of that approach. When coordinated optimally, mountain-to-sea management lowers scarcity of the groundwater resource in a cost-effective manner and helps to reduce costs imposed by safe minimum standards designed to protect GDEs like limu. In practice, such coordination is often challenging, however, given that resource management within different areas of the mountain-to-sea system are typically undertaken by a number of different government or private entities.
Pongkijvorasin, S., C.A., Wada, and K.M. Burnett. 2020. Optimal multi-instrument management of interrelated resources and a groundwater dependent ecosystem. Journal of Environmental Management, 269, https://doi.org/10.1016/j.jenvman.2020.110723.
Support for this work was provided by the National Science Foundation’s Research Infrastructure Improvement (RII) Track-1: ‘Ike Wai: Securing Hawaii’s Water Future Award # OIA-1557349, and a travel grant from Chulalongkorn University Office of International Affairs Scholarship for Short-term Research.