Shifting magnitude and timing of streamflow extremes and the relationship with rainfall across the Hawaiian Islands

Flooding is a significant threat to life and property in Hawaii. As climate warming continues to alter precipitation patterns and hydrological processes in the tropics, characterizing the shifting patterns in magnitude, seasonality, and location of floods would improve our understanding of the consequences and better prepare us for future flood events. In this study, 84 rain gauges and 111 crest gauges across five major Hawaiian Islands were analyzed from 1970 to 2005. We estimated trends in the annual maximum daily rainfall (RFmax) and the annual peak flow (PFmax) using the Mann-Kendall test and Sen's slope. Subsequently, we examined the association between PF(max )and rainfall. Then, we assessed temporal shifting by combining circular analysis with SeMs slope. The main identified trends showed decreases in RF max and PF max (67% and 61% of all gauges, respectively). The physiography of the Hawaiian islands (i.e., windward vs. leeward) has little contribution to both trends. In addition, PF max trend cannot be fully attributed to RFmax trend, and in many cases, RFmax and PF max did not occur coincidently. We also found the timing of RFmax and PFmax occurred in early wet season during the El Nino years and shifted even earlier from 1970 to 2005, likely due to the change of El Nino. These findings have implications for assessing flood risk. Our finding will aid watershed management and flood mitigation, and can increase resilience of downstream communities and near-shore environments.

Automated summary

The study analyzed data from rain gauges and crest gauges across five major Hawaiian Islands from 1970 to 2005, finding decreasing trends in RFmax and PFmax. The physiography of the Hawaiian islands had little contribution to these trends, and RFmax and PFmax trends were not fully correlated and often did not occur concurrently. The timing of RFmax and PFmax occurred earlier in the wet season during El Nino years and shifted even earlier from 1970 to 2005.