Monthly Rainfall Maps for the Islands of Kauai, Oahu, Molokai, Maui, and Hawaii, 2008-2010


Dr. Thomas Giambelluca, Professor of Geography


National Institute for Water Resources, Water Resources Research Institute Program




The Hawaiian Islands have one of the most diverse rainfall patterns on Earth. The mountainous terrain, the persistent trade winds, the heating and cooling of the land, and the regular presence of a stable atmospheric layer at an elevation of around 7,000 ft interact to produce areas of uplift in distinct spatial patterns anchored to the topography. The resulting clouds and rainfall produced by this uplift lead to dramatic differences in mean rainfall over short distances. Knowledge of the rainfall patterns is critically important for a variety of resource management issues, including groundwater and surface-water development and protection, controlling and eradicating invasive species, protecting and restoring native ecosystems, and planning for the effects of global warming.

The Rainfall Atlas of Hawaii is a set of maps containing spatial patterns of rainfall for the major Hawaiian Islands (Giambelluca et al. 2013). The maps represent the best estimates of the rainfall for a 30-yr period (1978 to 2007). Using the data set developed for the Rainfall Atlas, a set of monthly maps was generated from 1920 to 2007 to allow analyses of year-to-year variations (Frazier 2012). In recent years, it has become apparent that monthly maps from 2008 to 2010 are needed for numerous water-resource investigations being undertaken by federal, state, and county agencies in Hawaii. The updated maps are needed to extend the available rainfall record through 2010 to enable analyses of current hydrological conditions in Hawaii. Monthly rainfall maps are foundational products used to develop water budgets and estimates of groundwater recharge. Understanding changes in the spatial distribution of rainfall is critical for the protection and management of water resources in Hawaii. Due to Hawaii’s small land masses and isolation, water-resource options are limited, making assessment of water availability even more critical to federal, state, and local decision-makers. The objective of this project was to develop monthly rainfall maps for 2008 through 2010 for the islands of Kauai, Oahu, Molokai, Lanai, Maui, and Hawaii using an approach that was previously used for the Rainfall Atlas of Hawaii.

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