Hydrological characteristics of a tropical montane cloud forest in a summit bog in Hawaii

Tropical montane cloud forests are ecosystems found on tropical mountains at the elevation of cloud condensation. The frequent fog immersion affects many aspects of the ecohydrological processes of these unique forest ecosystems. Fog presence may reduce the incoming solar radiation and increase humidity, therefore causing low evapotranspiration rates. Fog may also serve as a water source when the liquid droplets are captured by the forest canopy through a process called cloud water interception, which in turn induces prolonged canopy wetness and enhances net precipitation. Because of the reduced water loss and the additional input of fog water, tropical montane cloud forests are believed to increase the water budget and are beneficial to water resources. This study examines the relationship of hydrometeorological variables with soil moisture and water table at a tropical montane cloud forest in the summit bog at over 1200 m on Mt. Kaala in the Hawaiian Archipelago. The site is equipped with a long-term climate and fog monitoring station. Cloud water interception was also measured using an explicit canopy water balance model which accounts for canopy storage and interception evaporation. Water table and soil moisture data were compared to climatic, hydrological, and fog-related variables such as relative humidity, rainfall, potential evapotranspiration, cloud water interception, fog frequency, cloud water flux, and cloud liquid water content. Preliminary analyses show that, at the daily scale, the water table was most closely related to total precipitation, i.e., the sum of rainfall and cloud water interception. Soil moisture is typically near saturation at the site and correlated poorly with other variables. The water table also exhibited different patterns depending on soil moisture thresholds. These results show that although the infiltration rate changes with soil moisture, it is generally high enough for the water table to respond to short-term precipitation fluctuations. On the monthly scale, the water table was most strongly related to rain frequency. Because higher rain frequency may be related to higher rainfall amounts and lower evaporation, this suggests the water table may be sensitive to changes in both water input and losses.