Image from Levia, D. F., and S. Germer (2015)
A primary goal of the Honolulu Board of Water Supply (BWS) and its Watershed Program is to ensure an adequate supply of fresh water for current and future generations.
In this regard, the capacity of O`ahu’s watersheds to capture and store precipitation is critical: it is the sole natural source of fresh water supply for the island.
Our BWS Canopy Interception Study attempts to quantify differences in rainfall capture between native and invasive forests found in the Wai`anae Mountains.
This aspect of the water budget of O`ahu’s watersheds (and of watersheds in general) has not been well-studied. Until recently, the only similar research in Hawai`i was located in high-elevation “cloud forest” terrain on the island of Hawai`i, where fog drip is a primary component of precipitation. Because that research was not representative of the majority of forested areas on O`ahu, this study was established as a first step to address O`ahu's forests.
RF = TF + SF + CI → CI = RF – TF - SF
RF = gross rainfall TF = throughfall
SF = stemflow CI = canopy interception
By measuring gross rainfall, throughfall and stemflow in the field at native and invaded forest sites, the canopy interceptions of the native and invaded sites can be calculated and compared.
Image based on Rainfall Atlas of Hawaii (University of Hawaii, 2011)
For this study, the native and invaded forest test plots are located in the Mākaha watershed of O`ahu, part of the Wai`anae Mountains. This is considered a high-priority watershed by BWS in terms of watershed protection and restoration efforts, ranking high with respect to groundwater recharge and groundwater production.
The Mākaha watershed is largely undeveloped; land cover is generally shrubs and evergreen forest. Elevation across the watershed ranges from sea level to 4,025 feet at Mount Ka`ala, the highest point on O`ahu.
Historical rainfall patterns across the Mākaha watershed are shown above. As expected, average annual rainfall is greater along the mountain slopes compared to lower elevations, reflecting the capacity of the higher mountain slopes to capture the predominant northeast tradewind-borne precipitation. The rainy season on O`ahu extends roughly from October through April.
Example test plot layout, for native plot
Gross Rainfall Equipment
Because the native and invaded test plots are located within close proximity to each other, a single tipping bucket rain gauge (standard 8-inch diameter, 0.01-inch / tip) installed near the plots collects gross rainfall data to represent both the plots.
For each test plot, a total of three troughs lead to a fabricated tipping bucket (150 mL / tip) rain gauge. The rain gauge is nested in a stainless steel can, levelled and anchored to the ground for stability.
Troughs were installed under trees typical of the test plots (species, diameter, height) in configurations that represented the average canopy and gap proportions and minimized overlap of throughfall collection between troughs.
Stemflow collectors connected to a fabricated tipping bucket (150 mL / tip) rain gauge were used for each test plot.
Trees selected for stemflow data collection represented the range of typical species, diameter, and height found commonly on the test plots.
Stemflow collectors were supported through lengths of 7/8-inch diameter poly tubing protected in 1-inch diameter high density polyethylene (HDPE) pipe, leading toward the rain gauge, nested in a stainless steel can, levelled and anchored.