You have entered the website of the Water and Environmental Research Institute of the Western Pacific (WERI) at the University of Guam. We are currently six faculty and five staff and our mission is to seek solutions through research, teaching and outreach programs, to issues and problems associated with the location, production, distribution and management of freshwater resources in Guam, the CNMI and the FSM.
The rainfall on the island of Guam, at 13.5 º N, 144.8 º E, has a pronounced temporal asymmetry: roughly one-third of the annual rainfall accumulates during the dry season months of January through June, while two-thirds of the annual rainfall accumulates during the wet season months of July through December. The mean annual total rainfall on Guam varies from about 85 inches (2170 mm) at drier spots to a little over 100 inches (2540 mm) in wetter areas.
Despite the relatively high annual rainfall amounts. Guam suffers deleterious effects of drought almost every dry season: desiccation of grasslands, desiccation and defoliation of some species of trees, significant reduction of streamflow, and significant reduction of the water level in many of Guam's wells. Wildfires burn thousands of acres during many dry seasons. Every three or four years, the dry season is especially dry and prolonged. Wildfires and stress to local crops are thereby aggravated and prolonged.
The seasonal asymmetry of the rainfall on Guam is governed primarily by the seasonal shift of the monsoon trough and its associated zone of monsoonal cloudiness. During January through June, northeasterly tradewinds blow on Guam. In a tradewind regime, the air is subsiding, clouds lack vertical development, and rainfall comes in the form of sporadic tradewind showers. During the rainy season, the monsoon trough becomes active in the western North Pacific, allowing the deep convective clouds, meso-scale convective cloud systems, and tropical cyclones (with their torrential rains) to affect Guam.
The interannual variation in the rainfall on Guam is strongly affected by episodes of El Niño Southern Oscillation (ENSO). Exceptional dryness during the dry season and a prolongation of dryness into the early part of the rainy season are an effect that ENSO episodes have upon Guam and all of Micronesia.
As with many meteorological phenomena in the tropics, persistence also plays a role in the variation of rainfall on Guam. Oddly, the persistence works only one way. If the dry season of a particular year is very dry or very wet. The wet season of that same year tends to be likewise; however, if a wet season of a particular year is very dry or very wet, the condition of following dry season is just as likely to be the same as the opposite.
Through use of the relationship of Guam's rainfall to ENSO and the one-way persistence of dry-season anomalies into the wet season, some regression equations were developed which have some skill (when applied to the dependent data sets) at predicting Guam's annual and seasonal rainfall totals. A 77% success rate was thereby achieved in a simple forecast of whether the dry-season rainfall would be above or below normal. Given the evolution of, and accurate forecasts of, the ENSO indices and, also, given the ongoing rainfall anomalies, skillful forecasts (6 to 12 months in advance) of the annual and seasonal rainfall totals on Guam may be possible.
