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

Abstract Flooding is a significant threat to life and property in Hawaiʻi. 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 Senʻs slope. Subsequently, we examined the association between PFmax and rainfall. Then, we assessed temporal shifting by combining circular analysis with Senʻs slope. The main identified trends were a decrease in RFmax and PFmax (67% and 61% of all gauges, respectively). The physiography of the Hawaii islands (i.e., windward vs. leeward) has little contribution to both trends. In addition, RFmax trend cannot be fully attributed to PFmax trend, and in many cases, RFmax and PFmax did not occur coincidently. The timing of RFmax and PFmax occurred earlier in the wet season during the El Nino years. Therefore, RFmax and PFmax’s timing have shifted 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.