| Abstract: | The integration of hydrology and climate is important for understanding the present and
future impact of climate on streamflow, which may cause frequent flooding, droughts, and shortage of
water supply. In view of this, we assessed the impact of climate change on daily streamflow duration
curves as well as extreme peak and low flow values. The objectives were to assess how climate
change impacts watershed-wide streamflow and its extreme values and to provide an overview of
the impacts of different climate change scenarios (Representative Concentration Pathways (RCP)
4.5 and 8.5) on streamflow and hydrological extremes when compared with the baseline values.
We used the Soil and Water Assessment Tool (SWAT) model for daily streamflow and its extreme
value modeling of two watersheds located on the Island of Oahu (Hawaii). Following successful
calibration and validation of SWAT at three USGS flow gauging stations, we simulated the impact
of climate change by the 2050s (2041–2070) and the 2080s (2071–2100). We used climate change
perturbation factors and applied the factors to the historical time series data of 1980–2014. SWAT
adequately reproduced observed daily streamflow with Nash-Sutcliffe Efficiency (NSE) values of
greater than 0.5 and bracketed >80% of observed streamflow data at 95% model prediction uncertainty
at all flow gauging stations, indicating the applicability of the model for future daily streamflow
prediction. We found that while the considered climate change scenarios generally show considerable
negative impacts on daily streamflow and its extreme values, the extreme peak flows are expected to
increase by as much as 22% especially under the RCP 8.5 scenario. However, a consistent decrease in
extreme low flows by as much as 60% compared to the baseline values is projected. Larger negative
changes of low flows are expected in the upstream part of the watersheds where higher groundwater
contributions are expected. Consequently, severe problems, such as frequent hydrological droughts
(groundwater scarcity), reduction in agricultural crop productivity, and increase in drinking water
demand, are significantly expected on Oahu. Furthermore, the extreme values are more sensitive to
rainfall change in comparison to temperature and solar radiation changes. Overall, findings generally
indicated that climate change impacts will be amplified by the end of this century and may cause
earlier occurrence of hydrological droughts when compared to the current hydrological regime,
suggesting water resources managers, ecosystem conservationists, and ecologists to implement
mitigation measures to climate change in Hawaii and similar Islands. |