Based on field measurements made at dry (Auwahi) and wet (Waikamoi) cloud forest sites on the island of Maui, a preliminary analysis of fog gage measurements and wet-canopy water balance estimates was made. Accounting for effects of wind-blown rainfall and varying wind direction, estimates of cloud water flux were derived based on fog gage observations. Throughfall (TF) measurements, incident rainfall estimates, and calculated amounts of wet-canopy evaporation were used to estimate event totals of cloud water interception (CWI) by the vegetation at each site. Measured TF was about 37% of incident rainfall at Auwahi, and 119% at Waikamoi. At both sites TF was dominated by rainfall, but was significantly influenced by fog at Waikamoi only. Fog contributed at an average frequency of once every two days at Auwahi and about twice in three days at Waikamoi. Derived CWI totals were equivalent to 151 mm year–1 at Auwahi and 1073 mm year–1 at Waikamoi. At Auwahi, however, the majority of intercepted water was re-evaporated from the wet vegetation, and never reached the ground. Total CWI was related to fog screen catch and cloud water flux at Waikamoi, but not at Auwahi.

Research on fog climatology and cloud water interception (CWI) in the montane cloud forests of Hawai'i spans nearly 50 years, from the pioneering work of Wendell Mordy and Paul Ekern on Lâna'i in the 1950s, through the continuing efforts of James Juvik since 1972. This work has helped to improve understanding of the spatial patterns of fog occurrence and to quantify CWI in forests and other vegetation. Reported CWI or fog incidence estimates are as high as 4982 mm year−1 at particularly exposed locations, although most windward sites within the cloud zone are in the range between 280 and 1130 mm year−1, with leeward sites receiving between 100 and 500 mm year−1, and less than 250 mm year−1 in high-elevation areas above the trade-wind inversion. Most of the early work was based on mechanical fog gage measurements whose well-known limitations make accurate estimation of actual CWI by a forest canopy difficult. Advancing the current level of understanding will have to come from studies incorporating other methods, such as the wet-canopy water budget and stable isotope mass balance approaches, in addition to the continued use of recording fog and throughfall gages.