Quantifying the importance of fog drip to ecosystem hydrology and water resources in tropical montane cloud forests on East Maui, Hawaii
Martha Scholl, U.S. Geological Survey, Water Resources Division, Reston, VA, USA
Stephen Gingerich, U.S. Geological Survey, Water Resources Division, Honolulu, HI, USA
Lloyd Loope, U.S. Geological Survey, Biological Resources Division, Haleakala National Park, HI, USA
Thomas Giambelluca, University of Hawaii at Manoa, Geography Department, Honolulu, HI, USA
On the windward side of East Maui, the forests are a crucial watershed, providing water for irrigation, residential and commercial use. Despite the extremely high rainfall in the area, evidence from previous studies (Juvik and Ekern, 1978; Scholl et al., 2002) indicates that fog may be a substantial part of the water budget of these forests, especially at altitudes between 600 and 2000 meters. There have been no studies done to quantify fog input to the water cycle in this area.
On the leeward side of Maui, the original dryland cloud forests were destroyed during the 1800s, due to logging, grazing and fire. Archaeological evidence shows that the area was once inhabited by native Hawaiians, so presumably there were water sources at the higher altitudes during that time. It is likely that fog drip from forested areas was the source of the water. Fog is the major source of moisture during much of the year, as rainfall occurs mostly during large winter storms.
The objectives of the research are to answer the following questions: How much does fog contribute to cloud forest water budgets? Do plants utilize fog water, rain water, or both? Does fog recharge the deep ground-water system? What factors contribute to fog-interception efficiency of the forest canopy? There is growing interest in reforestation on the leeward side of the island. We hope that gaining an understanding of the role of fog in forest hydrology on the windward side will help in learning what factors are most important in restoration of forests in leeward areas.
We measured the amounts of water input from fog at two sites, one each on the windward and leeward sides of Haleakala volcano. Water samples were analyzed for stable isotope composition. Previous work (Ingraham and Matthews, 1995; Scholl et al., 2002) has shown that rain and fog have unique isotopic signatures, so that stable isotopes of water can be used to track the fog water through the hydrologic cycle.
Sites and Instrumentation
The leeward site is in the Auwahi land unit (ahupua'a) on the south-facing slope of Haleakala volcano. The area is mostly pasture land, but has one of the few remaining areas of native forest, at 1220 m elevation in the fog belt (click on small photo for larger version).
The windward site is in the upper Piinaau Stream drainage basin at 1950 m altitude. The vegetation is mixed height, consisting of ohia and koa stands and shorter ferns and brush (click on small photo for larger version).
The location map above shows the two fog research sites. East Maui (Haleakala volcano) rises to an altitude of 3055 m at the summit. The north and east slopes comprise the windward side, as trade winds blow from the north-northeast direction. Orographic rainfall occurs almost daily. The south and west slopes are the leeward side, with a much drier climate; rainfall occurs from storms or fronts, generally in winter months.
Samples of fog, rain, soil water, tree sap, (and stream water at the windward site) were collected for stable isotopes, and amounts of fog and rain precipitation and throughfall were measured. Other meteorological measurements were made by a weather station at each site. Below is a list of instruments/measurements at each site:
Progress and Results
Weather station data and isotope samples were collected from the Waikamoi site beginning in August 2001; the site was fully instrumented by November 2001, and data collection ended August 2003. The Auwahi site weather station and fog collectors were deployed as of November 2001, and the visibility sensor was installed in December 2001. Data collection ended in November 2003.
Graph showing oxygen-18 composition of fog compared to stream water and volume-weighted average rainfall, windward East Maui, from data collected prior to this study (click on small picture to see large version):
Table showing relative amounts of cloud water deposition on the standard fog collector at the windward (Waikamoi) and leeward (Auwahi) sites. [HP = horizontal precipitation; fog/no rain = fog collector catching precipitation while standard rain gage was not] Note: these data are preliminary and are subject to revision. (click on small picture to see large version):
Links to relevant sites:
Ulupalakua Ranch Company, The Nature Conservancy of Hawaii, and Haleakala Ranch Company have gone out of their way to contribute to this research, providing access to the sites and help with equipment transport and fencing. We thank them for their generous support.
Falconer, R.E. and P.D. Falconer, 1980, Determination of cloud water acidity at a mountain observatory in the Adirondack Mountains of New York State, J. Geophysical Research, 85 (C12), 7465-7470.
Ingraham, N.L., and Matthews, R.A., 1995, The importance of fog drip water to vegetation: Point Reyes Peninsula, California, J. Hydrology, 164, 269-285.
Juvik, J.O. and P.C. Ekern, 1978, A climatology of mountain fog on Mauna Loa, Hawaii Island, Technical Report no. 118, Water Resources Research Center, University of Hawaii, Honolulu, HI, 63 p.
Schemenauer, R.S. and Pilar Cereceda, 1994, A proposed standard fog collector for use in high-elevation regions, J. Applied Meteorology, 33 (11), 1313-1322.
Scholl, M.A., Gingerich, S.B., and Tribble, G.W., 2002, The influence of microclimates and fog on stable isotope signatures used in interpretation of regional hydrology: East Maui, Hawaii, J. Hydrology, 264, 170-184.
Scholl, M.A., Ingebritsen, S.E., Janik, C.J., and Kauahikaua, J.P., 1996, Use of precipitation and groundwater isotopes to interpret regional hydrology on a tropical volcanic island: Kilauea volcano area, Hawaii, Water Resources Research, 32 (12), 3525-3537.
U.S. Department of the Interior, U.S. Geological Survey, Reston, VA, USA