Water Resources Research Act Program

Principal Investigators: Shahram Khosrowpanah, Mark Lander

Abstract: In Micronesia, where tropical cyclones and other manifestations of deep convection are common, very high short-term rain rates are experienced. We have chosen the island of Pohnpei to set up a network of manual and electronic rain gages to study the spatial and temporal distribution of rain. Located at low latitude in the western North Pacific (where deep convection in various organized patterns and stages of development is the dominant producer of rain), the island of Pohnpei is one of the wettest places on earth. Our recently installed rain gage network indicates that the high interior of the island may receive over 300 inches of rain annually. Stream and urban flooding as well as deadly slope failures are recurring problems on Pohnpei. Human activity and other factors are changing the physical and biological character of Pohnpeis unique tropical rain forest. Insufficient spatial distribution of rain gages and lack of appropriate measurements of short-term extreme rain rates have resulted in rainfall statistics for most of the islands of Micronesia that are incomplete, inaccurate, and/or non-existent for many areas. This has created problems in many areas such as: (1) difficulty in estimating the rainfall erosivity factors that are being used for erosion protection and identification of land slide areas; (2) difficulty in developing infrastructure for storage and distribution of surface water; (3) difficulty for disaster managers to better understand the processes that lead to slope failure and local stream flooding; and, (4) difficulty in design of hydro electrical power plants for potential sites. The objective of this project is to acquire, compile, and analyze rainfall data from a transect of manual and electronic rain gages (some already installed and some others to be placed at crucial locations) to produce statistics of rain fall needed to address land and surface-water management problems, and to help identify the character and thresholds of rainfall that lead to dangerous flooding and slope failures. A key parameter we wish to determine are the R-factors that govern erosion on the island. The R factors are related to the peak 15-minute rain rates, and this data is only now beginning to be acquired. With our newly acquired rainfall dataset, we will be able to refine the maps of the distribution of the R factor on the Island. Also, a complete description of the hydrologic cycle on Pohnpei is not complete without some assessment of the magnitude of fog drip in the interior highlands that are often enshrouded in cloud. In order to get better information on the possible contribution of fog drip to the total annual precipitation in Pohnpeis rain forest, we will install a fog-drip collector at our mountain rain gage site.