Hydrology of Hawai`i

To understand the hydrology of an area, one must first understand the area's climatology and geomorphology. Hawaii has a wide range of climatological areas which depend on elevation and geographic position on each individual island. The windward or North through Northeast facing sections of the islands generally have a consistent year-round supply of trade winds that bring brief showers. The wetter period of the year in windward areas depends on the individual island and the elevation, but generally occurs in the spring months. The higher slopes of the windward areas and higher elevation area on many of the islands receive enough rainfall to support rainforest vegetation. The highest peaks, such as Mauna Loa on the Big Island of Hawaii, receive several inches of snowfall and the landscape can appear more like tundra than tropics. The leeward sides of the island, which are the Southwest through Southeast sides of the islands, are more arid. Parts of the islands are extremely dry, for instance some southwest locations receive less than 12 inches of rainfall per year and support desert-like vegetation. In general, the wetter season is winter and the dryer season is summer. There are exceptions, such as those areas of the Island of Hawaii, that have a summer rainfall maximum induced by land and sea breeze convection.

Hawaii is a volcanic island chain in the Pacific Ocean. Each of these islands has ground water available in some locations depending on the age and the geologic structure of the island. The interests of the National Weather Service are primarily with the surface water hydrology and as it relates to the protection of lives and property from flood related weather events. The major key to surface water hydrologic forecasting and analysis in Hawaii is to understand the size and land use characteristics of the individual drainage basins. All drainage basins in Hawaii, given the right meteorological conditions, can produce dangerous flash floods. Many basins in the state, such as those on Oahu, are very urban in nature with streets, roads, and buildings producing large areas of impervious surfaces that generate near 100 percent runoff. In those areas stream flow is, for the most part, a measure of the rainfall amounts. In other areas of the state, it is more difficult to characterize the nature of the surface water hydrology. Areas, such as portions of Kauai and the Big Island of Hawaii that are basically uninhabited, soil type and vegetation are extremely variable and will dictate the amount of surface water runoff that is expected. A major consideration that must be made when observing the hydrology of any volcanic island, including Hawaii's islands, is the specific size of the drainage basin. When compared to the drainage areas on large continents, the stream flow generated in Hawaii, even in very heavy rainfall periods, is small. However, no matter how small the scale, one must also remember the scale of the area in question. An island that is small only requires a small amount of flooding to produce dangerous conditions.

Roger V. Pierce
Previous Senior Service Hydrologist
NWS Forecast Office, Honolulu, HI