Temporal Shifts in the Magnitude of Peak Streamflow and its Associated Rainfall across the Hawaiian Islands

Climate-driven shifts in the frequency of extreme rainfall events have altered the magnitude of floods, with consequences for both the evolution of landscapes and the protection of life and property. Stormflows, especially those that occur after an extended period of drought, transport upslope sediment and pollutants to the coasts, where they adversely impact the nearshore ecosystem. In Hawaíi, previous studies showed that extreme rainfall events are becoming more common on the Island of Hawai'i, but less frequent on O'ahu and Maui. However, little research has examined temporal and spatial trends of peak streamflow (peak flow) events associated with heavy rainfall events.

In this study, we analyzed annual peak flow and annual maximum rainfall trends of 112 stream crest gages from the U.S. Geological Survey, along with 82 rain gages from the National Centers for Environmental Information across the Hawaiian Islands from water years 1970 to 2005. We applied the non-parametric Mann-Kendall test for significance and Sen's Slope for changes of magnitude. Annual peak flow and rainfall trends were addressed respectively first. Overall, we found a declining trend , especially in leeward areas, while there was an increase in windward areas on Kauái, Maui, and Molokái. We found that the date of annual peak flow did not coincide with the date of annual maximum rainfall. To better link trends in annual peak flow and rainfall, we paired daily, 2-day, and 5-day accumulated rainfall to the peak flow, and examined trends in paired-rainfall with annual peak flow. We discuss how annual peak flows have changed over time, patterns in their spatial distribution, and how they are associated with rainfall trends to add to the current knowledge of flood risk and management for Hawaíi.