Characterizing rainfall regime changes and estimating the timing of high streamflow events across the five main Hawaiian Islands

Abstract

Changes in extreme precipitation and associated floods attract a lot of attention because of their devasting consequences on societies and ecosystems. Despite rainfall is recognized as the dominant driver of streamflow, we still lack the understanding on how extreme rainfall has changed spatially and temporarily, and we still lack the understanding on the relationships between rainfall and streamflow responses in Hawaiʻi. These unresolved questions and less-understood relationships between rainfall and streamflow can undermine our infrastructure planning and emergency responses. This study utilized hourly rainfall and Generalized Extreme Value (GEV) distribution to examine the changes in rainfall regime across the five main Hawaiian Islands. Temporal patterns of annual maximum hourly rainfall were also examined using circular statistics. Lastly, occurrence dates of high streamflow events induced by rainfall were estimated by establishing relationships between rainfall events, soil moisture conditions, and streamflow data using multilinear regressions. Dominant decreasing trends in extreme rainfall were highlighted across the main Hawaiian Islands, at the exception of Hawai‘i Island and Kaua‘i. Circular analysis indicated that annual maximum hourly rainfall occurred principally during the wet season (April-November), and more specifically from mid-December to early January. On another hand, occurrence dates of high streamflow events induced by rainfall events were correctly identified in the two study sites, when incorporating antecedent soil moisture conditions to rainfall observations. For locations where in-situ measurements of soil moisture contents are not available, this study also provided an empirical solution to estimate with a great accuracy soil moisture contents of the surface layer. This added knowledge on extreme rainfall trends and their consequential streamflow will improve our understanding of the patterns, timing, and locations of these extreme events, and assist to design and implement policies and infrastructure to minimize their threats in Hawaiʻi.