Water Resources Research Act Program

Details for Project ID 2002GU3B

A rainfall climatology for Saipan: distribution, return periods, and inter-annual variations.

Institute: Guam
Year Established: 2002 Start Date: 2002-03-01 End Date: 2003-02-28
Total Federal Funds: $17,181 Total Non-Federal Funds: Not available

Principal Investigators: Mark Lander, Mark Lander

Project Summary: Improved information on rainfall depth-intensity-duration-frequency and their areal distribution is needed for use in designing storm water control systems. Previous estimations (based on rainfall records of limited duration) of Saipans mean monthly and mean annual rainfall, and the distribution of rainfall throughout the island will be re-examined. Prior calculations of return periods of heavy rainfall events on Saipan can be shown to be erroneous and are in need of a complete overhaul. This proposal seeks to develop a climatology of Saipans rainfall to include: (1) an estimation of the distribution of rainfall at mean monthly to mean annual time periods, (2) calculations of return periods of heavy rainfall events, and (3) an examination of interannual variations in mean annual rainfall. Previous estimations of Saipans mean monthly and mean annual rainfall, and the distribution of rainfall throughout the island will be re-examined. Prior calculations of return periods of heavy rainfall events on Saipan can be shown to be erroneous and are in need of a complete overhaul. Interannual variations of Saipans rainfall are closely linked to the El Niouthern Oscillation (ENSO) phenomenon. The CNMI and Guam are in an ENSO core region that features very dry conditions in the year following El Nind an increase in the level of threat from typhoons during an El Niear. The long-term variations of rainfall on Saipan are very similar to those on Guam. As on Guam, the mean annual rainfall on Saipan varies substantially (15%) at different locations on the island. As a first approximation, the heaviest rain tends to be at the higher elevations -- at Guam it occurs on the eastern slopes of the southern mountains and, at Saipan, it appears as if the mean annual rainfall is heaviest at stations in the central high ground (e.g., Capitol Hill and Mount Topachau). The recurrence interval of heavy rain events, however, can not be considered to have a similar distribution as the annual mean since the causes of extreme daily rainfall events are typhoons, monsoon squall lines, and other so-called meso-scale weather systems that produce rain amounts that are independent of the island topography. The proposed project directly supports three of the CNMIs stated needs in Water Quality and Water Quantity, and Education and Professional Training: ● Improved information on rainfall depth-intensity-duration-frequency and their areal distribution for use in designing storm water control systems. ● Develop regional (Guam/CNMI) manual of best management practices for control of storm-water runoff. ● Task force organization to address special water resource issues.