Final Comprehensive Conservation Plan - U.S. Fish and Wildlife ...

Hakalau Forest National Wildlife Refuge
Comprehensive Conservation Plan
Similar to other oceanic islands, rainfall is the greatest source of freshwater on the Island of Hawai‘i.
Rainfall contributes roughly 8,000 million gallons per day (mgd) to the water budget of Hawai‘i
Island (Lau and Mink 2006, TMA 2007). This rainwater recharges two vital water resources:
groundwater and surface water.
Groundwater, which occurs beneath the surface, is the primary water resource in Hawai‘i.
Groundwater can occur as thin basal lens, as well as high-level aquifers that do not float on seawater
(Juvik and Juvik 1998). Unlike older islands in the archipelago, Hawai‘i Island does not have
sedimentary coastal plain or caprock. Lack of a caprock allows fresh water to outflow and the
seawater to intrude the freshwater lens. As a result, basal water levels throughout the island are low
(Bauer 2003).
Surface water is water flowing in stream channels. This water originates from surface runoff derived
from rainfall, groundwater seepage, and channel water that seeps into the banks during high stream
stages (Lau and Mink 2006). Streams are classified as intermittent or perennial based on flow
conditions. Perennial streams are streams that normally have surface flow throughout the year, at
least in some part of the course (Hawai‘i Cooperative National Park Studies Unit 1990). Perennial
streams, which are generally sustained by groundwater in high level aquifers, are usually restricted to
the windward sides of islands that receive more rain (Nishimoto and Kuamo‘o 1997, Juvik and Juvik
1998). The largest perennial stream on Hawai‘i Island (and in the State) is Wailuku River. This river
is 22.7 mi long and discharges 180 mgd (HDBEDT 2007).
Water on the island is primarily derived from groundwater rather than surface water (TMA 2007). In
2000, the County of Hawai‘i used 44.55 mgd of groundwater and 8.86 mgd of surface water
(HDBEDT 2007). Major water systems are located in the Kona, Ka‘ū, and Puna areas. Water is
supplied to these systems from wells, springs, and roof catchments (TMA 2007). Hawai‘i Island
consumes a relatively small percentage of the State’s water. In 2006, the County of Hawai‘i
consumed 11.9 percent of the total freshwater consumption of the State of Hawai‘i (HDBEDT 2007).
Wetlands are critical components of an area’s hydrology and provide a variety of ecological
functions. The Service defines wetlands as “lands transitional between terrestrial and aquatic system
where the water table is usually at or near the surface or the land is covered by shallow water”
(Erickson and Puttock 2006). According to this definition, unvegetated areas including beaches,
mudflats, and ponds are considered wetlands. Hydrology, vegetation, and soil type are used as
indicators to determine the presence of a wetland (Erickson and Puttock 2006). Although small,
isolated wetlands occur on the Refuge, they would not be under the jurisdiction of the U.S. Army
Corps of Engineers as waters of the U.S. because they are not connected or adjacent to navigable
waters (USFWS 2008a).
Flooding is common in certain areas of the island due to ponding, surface runoff, high seas, storm
surge, and tsunami inundation. Hawai‘i Island is particularly vulnerable to flooding because it is
relatively young and water courses are generally not well-defined (County of Hawai‘i 2006). The
Federal Emergency Management Agency’s National Flood Insurance Program has prepared Flood
Insurance Rate Maps (FIRM) that depict flood hazard areas through the State. The maps classify land
into four zones depending on the expectation of flood inundation. The entire Hakalau Forest NWR is
within Zone X, defined as areas outside of the 100 and 500 year floodplains. In addition, the County
General Plan (2005) lists flood prone areas. Statewide flood control is managed by the Department of
Land and Natural Resources’ Engineering Branch, Land Division.
Chapter 3. Physical Environment 3-9