Mapping Rainfall Trends in Hawai';i

Spatial patterns of rainfall in Hawai';i are among the most diverse in the world; ranges on a single island rival those of continents. As the climate warms, it is essential to understand how rainfall has changed so that we can better understand possible future climate changes. This is especially important in a small island context where resources are limited, and understanding the potential impacts of climate change on freshwater supplies is crucial. Utilizing Hawai';i's extensive network of rain gauges (over 2,000 stations have operated since the mid-1800s), data screening, homogeneity testing, and gap filling were performed to produce a serially complete dataset for as many stations as possible. This dataset was used to develop a set of month-year rainfall maps for Hawai';i from 1920-2007. Maps of rainfall values and anomalies (departures from the most recent 30-year mean) were derived for all major Hawaiian Islands at a 250 m resolution. Using this time series of maps, linear trends for the entire period (1920-2007) and the most recent 30-year period available (1978-2007) and the corresponding significance levels (p-values) were calculated at every pixel across the state, accounting for the effects of autocorrelation, for each month and each 3- and 6-month season. These trends and p-values were then mapped to produce spatially continuous trend maps of Hawai';i. The results show drying trends on all islands, with Hawai';i island experiencing the largest significant long-term declines annually (island mean percent change of -1.39% per decade since 1920) and Maui island experiencing the largest short-term declines annually (-8.10% per decade change since 1978). The seasonal analysis reveals that the largest changes are seen in winter and summer months (clear drying trends on all islands), while spring and fall seasons experience more neutral or positive changes (fewer significant trends). Most of the significant declines in long-term winter rainfall are seen in the windward mountainous high-rainfall areas, while most of the significant summer declines are seen in dry leeward areas. These results are in agreement with previous studies of trends at individual stations or for indices representing the entire state. However, this study is the first to show the spatial details of rainfall change over time, which is critically important information for water resource managers in Hawai';i.