EOS, Transactions of the American Geophysical Union, v. 84, no. 46, p. F687,
Fall Meeting, December 8-12, 2003
Quantifying the Role of Cloud Water
in the Hydrology of Two Montane Forest Sites on East Maui, Hawaii
Scholl, Martha A.1,
Gingerich, Stephen B.2, Giambelluca, Thomas W.3, Nullet,
Michael A.3, and Loope, Lloyd L.4
Geological Survey, 431 National Center, Reston, VA 20192
Geological Survey, 677 Ala Moana Blvd., Ste. 415, Honolulu, HI
of Hawaii at Manoa, Geography Dept., 2424 Maile Way, Honolulu, HI 96822
Geological Survey, Haleakala Field Station, P.O. Box 369, Makawao, HI
East Maui (Haleakala volcano) rises 3054 m above the ocean, and clouds
intercepting the mountain slopes are an integral part of the climate.
To what extent do the trees and shrubs on the mountainsides extract cloud
water that contributes to soil moisture, groundwater recharge, and stream flow
in the watersheds? Two sites, on
the windward and leeward sides of the mountain, were instrumented to study this
process. Weather stations at each
site measured climate parameters, and stable isotopes of fog, rain, soil water,
stream flow and tree sap were measured monthly to track the proportion of cloud
water in the forest hydrologic system. When
clouds envelop a forest, the precipitation ranges from light mist to rain,
leading to difficulties in measuring cloud water as opposed to rainfall.
Deposition of cloud water depends on wind speed, droplet size, surface
area of the vegetation, and many other factors.
Cloud water deposition was measured on a continual basis using a vertical
screen collector, which did not exclude rainfall.
The fog isotope collectors did exclude vertically falling rain, and
cumulative monthly fog volumes were compared with volumes from isotope rain
collectors. Throughfall collectors
provided information on the efficiency of the vegetation in collecting cloud
water (fog drip). At the windward
site, fog screen/ rain gage volume ratios normalized to collection area ranged
from 0.74-4.9, with a median value of 1.5.
The median fog/ rain ratio from the isotope collectors (normalized to
surface area) was 0.35. The screen
collector cannot be directly extrapolated to forest canopy, but the data suggest
substantial impaction of cloud water on vegetation surfaces exposed to the wind.
Isotopic composition of stream water reflected recent precipitation,
including cloud water, while the soil water had a larger proportion of
precipitation from large rainstorms. Metrosideros
polymorpha tree sap isotopic composition was sometimes identical to fog
drip, almost never similar to soil water, and generally appeared to be a mixture
of fog drip and rainfall. The
leeward site was deforested in the early part of the 20th century,
and has remnants of cloud forest. Most
water input is from infrequent large storm systems, while the clouds that
envelop the site on a more frequent basis have relatively low water content.
Fog screen/ rain gage volume ratios here ranged from 0.21-13.5, with a
median value of 0.70. The median fog /rain ratio from the isotope collectors was
0.21. Soil water isotopic
composition reflected the large rainstorms, with little cloud water input.
Isotopic composition of M. polymorpha sap at this site was similar
to soil water, suggesting that the trees tap a deeper water source than at the
windward site. Results from the study will lead to a better
understanding of the relationship of cloud forests to recharge and streamflow
along mountain slopes.