Modeling the response of Hawaiʻi streams to future rainfall conditions

A full stream pours over rocky steps between lush forest plants as tourists look on from a bridge.
The pictured Wahinepe‘e stream in Maui could be much less picturesque if rainfall amounts decrease over time with climate change. (Photo: Starr CC BY 3.0)

Surrounded by saltwater, Hawaiian communities depend on freshwater streams for consumption, irrigation, traditional Hawaiian practices, and habitat for native fish and other stream life. It is important to be able to predict how Hawaiʻi’s streams will be affected by changing rainfall patterns to enable sustainable management of critical freshwater resources. However, to date, limited data and the uncertain effects of climate change have hindered predictions of future streamflow.

Colored maps of Maui showing patterns of areas that may experience changes in mean annual rainfall (from 12% increases to 76% decreases) and the accompanying changes to stream flows.
Projected rainfall (delivered using CMIP5 and RCP 8.5) and associated low-flow changes by 2100.

Through this project, scientists developed a model that provides a way to estimate future stream low flow (streamflow during a period of prolonged dryness) by categorizing streams based on their physical characteristics. While the model is based on data from gaged streams (i.e., those fitted with measuring devices to monitor streamflow), it allows researchers to also predict streamflow at ungaged (unmonitored) sites and thus develop island- or state-wide predictions of stream response to future rainfall conditions.

Researchers used the model to forecast flow changes in Maui streams under two possible end-of-century future climate scenarios. Results indicate that in areas where rainfall is projected to decrease in the future, stream low flow may decrease by more than 50 percent in some areas, and habitat for native species may decrease by more than 25 percent in some streams. The methods developed for this study can be transferred to other Pacific islands as well as non-island settings.





Maoya Bassiouni 

Hydrologist, USGS Pacific Islands Water Science Center