This technical report presents the results of the first year of a collaborative effort between WERI and the Conservation Society of Pohnpei to measure the rainfall across the island of Pohnpei. Rain gages were placed in several locations to include the coastal perimeter, the mountain slopes, and the remote highland interior of the island. The discussion includes: general rainfall statistics, a summary of the annual distribution of rainfall, and an examination of the return periods of short-term high-intensity rainfall events (Section 2); the effects of ENSO on the climate and weather of Pohnpei (Section 3); a summary of tropical cyclones affecting the island (Section 4); and, an examination of month-to-month, inter-annual, and inter-decadal variations in mean annual rainfall (Section 5).

The distribution of rainfall on Pohnpei is affected by the topography, and the mean annual rainfall totals among recording stations on Pohnpei differ by as much as 150 inches! The region in the vicinity of Pohnpei's international airport receives the lowest annual total of about 120 inches. The highest measured annual average of approximately 300 inches occurs atop Nahna Laud in the highland rainforest of Pohnpei's interior. Charts of Pohnpei's mean annual rainfall were produced from the first year of data collected from the WERI/CSP rain gage network. Earlier charts of Pohnpei's mean annual rainfall using PRISM were found to be quite accurate. Future refinements are expected.

The Pohnpei rain record is too short to develop accurate return periods of extreme rainfall events (although attempts have been made in this report and by others that may be refined as more data is gathered). More rain records need to be collected in typhoons, and throughout the ENSO cycle to produce reliable tables of return periods for short-term extreme rain events. In any case, intensity-duration-frequency tables have been generated with the short Pohnpei rainfall data sets.