Major Water Cycle Components of a Tropical Watershed.

Population growth, climate change, and human induced land-use changes triggered high demands on one our water quality and quantities. The impacts of these changes have more pronounced effects on small steep islands (Hawaiian Islands) with a strong spatio-temporal variability of the major components of the water cycle.  In 2004, an experimental watershed experiment began on upper part of the Makaha valley, O’ahu with an area of 5.5 km² to quantify the spatio-temporal variability of the major components of its water cycle; which includes gross and canopy intercepted rainfall, stemflow, evapotranspiration, groundwater resources and recharge, streamflow, and soil water content storage. Some of these components have been monitored since 2005 at six locations on the study area that best represent the spatial variability of topography, land use and rainfall. Rainfall gradient show strong spatial variability that varies between 200 and 440 mm km^-1; however, the ET gradient is near constant with -205 mm km^-1. Rainfall intensity and frequency increased with elevation. Throughfall, stemflow, and canopy interception varied spatially due to land cover and rainfall. Annual average canopy interception ranged from 22 to 32% across the watershed.  The upper part of the watershed is the major contributor to streamflow and groundwater recharge.