Simulation of the hydrological impacts of climate change on Hawaiian streams

Abstract:

Seasonal Hawaiian climate projections (statistically downscaled CMIP5 models for wet and dry seasons) for two scenarios (RCP 4.5 and 8.5 for mid and late 21^st century) are used to simulate streamflow using Soil Water Assessment Tool (SWAT) models for three watersheds (Punalu‘u, Kaluanui, Kamananui) on the island of O‘ahu, Hawai‘i. These watersheds were chosen for their varying hydrology, geology, and climate, and their conservation importance for endemic migratory fishes. More than 10 years of high-resolution gridded daily rainfall, temperature, and stream discharge data were used to parameterize and calibrate/validate the models, respectively. The impacts of climate change on ecologically important hydrologic indices (e.g., flow intermittence, flood magnitude) are investigated. The RCP 8.5 late century scenario projects the greatest declines in streamflow (on average -30% change) in the study watersheds, resulting in increased flow intermittence in streams with low baseflow index. Drier summers predominate, and high and low flows are reduced in most cases. Climate-induced reductions in flow pulse, increases in flow intermittence, and reduced linkages with the ocean are likely to have ecological responses including impacts on dispersal capability and juvenile recruitment to streams of native aquatic fauna that depend on mountain to sea connectivity.