Cloud Water Interception by a Montane Tropical Forest in Leeward Kohala, Hawai‘i

Cloud water interception (CWI) is a significant source of water for many tropical montane cloud forests (TMCFs) in Hawai‘i and elsewhere. As vast regions of the Hawaiian Islands have undergone extensive deforestation over the past several centuries, many have suggested that the loss of CWI and other ecohydrological characteristics of tropical forests may have resulted in lower rates of groundwater recharge and subsequent streamflow. Leeward Kohala is one such region where many hectares of closed-canopy TMCF and wet-mesic woodland were lost to the cattle industry, which planted exotic pasture grasses in their stead. Because leeward Kohala’s perched groundwater system is geologically-isolated from the intact forests of windward Kohala, the resulting loss of CWI could have significantly lowered recharge rates and water table levels throughout the leeward slope. But despite reports of reduced flow and increased flashiness in leeward streams, CWI in Kohala’s remaining leeward forests has not yet been quantified.Rainfall (RF) and throughfall (TF) were measured along an elevation gradient spanning the natural transition from mesic koai‘a woodland to closed-canopy ‘ōhi‘a forest atop leeward Kohala, Hawai‘i. By applying a simplified canopy water balance model, combined rates of CWI minus canopy interception losses (Ei) were determined at seven sites for twenty distinct measurement periods. These rates ultimately reflect the additional amount of (or loss of) precipitation contacting the land surface due to the presence of forest vegetation rather than flat pastureland. Near Kohala’s leeward crest, RF totaled 1,104.5±13.7 mm over 133 days while forest cover provided an additional 568.0±15.4 mm of water to the land surface. CWI contributions were progressively lower farther down Kohala’s leeward slope, with Ei appearing to outweigh CWI by 41.3±14.3 mm at the current man-made transition from ‘ōhi‘a forest to pasture. Two lower-elevation sites in remnant koai‘a woodlands exhibited a CWI-elevation trend distinct from that of the ‘ōhi‘a sites, with site D recording 50.9±16.8 mm of additional water input at the forest site compared to 457.2±14.0 mm of nearby pasture RF over the study period. At site E, the driest site, Ei exceeded CWI in almost all measurement periods (73.9±16.6 mm net water loss in the site E forest over the study period). Mean TF/RF across sites was 1.07, indicating CWI generally outweighed canopy Ei and added significant amounts of water to leeward Kohala.Spatial variations in CWI were driven by forest structure in addition to elevation: despite receiving the same amount of RF over the study period, different rates of TF and CWI were recorded between the three forested plots at site B. In general, denser forest tended to exhibit lower TF at both the site scale and gauge scale, although this relationship weakens in wetter periods due to higher canopy saturation. Wet periods with strong trade winds featured the highest overall CWI rates across the elevation transect, and CWI rates at all sites generally decreased when conditions become drier and less windy. While CWI, TF and RF are positively related to elevation, periods with weaker trade winds present flatter versions of these trends due to the waning influence of the orographic moisture responsible for both RF and CWI. Broadly speaking, wetness and elevation appear more explanatory than trade wind strength or forest structure for CWI rates in leeward Kohala.

Keywords: Earth Systems Program
Kohala
Hawaii
Tropical Montane Cloud Forest
Cloud Water Interception
Throughfall
Canopy Water Balance
Forest hydrology
Ecohydrology
Text
Thesis
Advisor: Vitousek Peter M.
Konings Alexandra G.