Canopy Water Balance of an Elfin Cloud Forest at Alakahi, Hawaiʻi

Abstract

The contribution of cloudwater or fog, to the water balance of tropical mountain forests has been the subject of increasing attention in recent decades. This study estimates the water falling to the forest floor as a result of cloud water interception (CWI) by the canopy in a short stature tropical montane cloud forest (TMCF) with abundant epiphytic vegetation. The study site, located in the Alakahi region of Kohala on the island of Hawaiʻi, receives abundant orographic rainfall and is frequently immersed in clouds. Cloudwater interception was estimated from meteorological data using the Gash interception model to calculate canopy water balance at the event scale for precipitation events with, and without fog precipitation recorded by a mechanical collector in the forest canopy. Cloudwater interception was derived as a residual by comparison of measured and predicted throughfall during events with fog precipitation. Storage capacity of the major canopy bryophytes and ferns, as well as that of the mossy mat carpeting the forest floor, was also estimated from observed distribution and storage capacity. The calculated contribution of CWI was equivalent to 11% of the total throughfall. The annualized 219 mm is substantially lower than for CWI previously estimated at a nearby exposed, forest-edge location where estimated CWI and throughfall exceeded rainfall. Canopy epiphyte water storage capacity accounted for 3.53 mm and the mossy mat on the forest floor was estimated to store up to 22 mm. The epiphyte storage during precipitation events was far less than the estimated storage capacity; however, epiphytes represented a significant potential moisture source for interception loss between precipitation events.