Ever-increasing industrialization and population growth have been putting considerable pressure on the environment and freshwater resources. Globally, the consistent increase in greenhouse gas emissions, such as carbon dioxide, methane, and nitrous oxide concentrations, has contributed to global warming and thus altered the natural climate system [1
]. Climate change has already altered the hydrologic cycle components and the response of rainfall-runoff processes [3
] and the magnitude, frequency, and timing of high flows (floods) and low flows (droughts), including shift in time of occurrence of peak flow hydrographs [2
]. These impacts are expected to continue in the future and may have adverse consequences on freshwater resources availability and sustainability [3
] and related natural habitats [10
]. Such effects may have more serious implications on the hydrological regimes and riparian ecosystems of Pacific Islands, where there are limited land and freshwater resources.
Like others in the Pacific, the Hawaiian Islands have limited land and freshwater resources available for drinking water supply, food productions, and ecosystem services. The majority of the drinking water supply for the Hawaiian communities originates from groundwater [11
]. However, such resources are generally very sensitive to climate change, particularly to rainfall amounts and patterns [12
], due to the small-size of the feeding watersheds and the corresponding short water residence time. Over the Hawaiian Islands, the most observed impacts of climate change are related to flooding and droughts as well as streamflow and base flow declines. For example, studies by Oki [14
] and Bassiouni and Oki [13
] on long-term trends analysis of observed streamflow and baseflow data of the Hawaiian Islands, generally showed an overall significant decline in trends. More importantly, Oahu Island, the most populated Island in the chain, showed the largest downward shift in base flow compared to the other Islands of the Hawaii state [13
]. This is mainly due to an overall decline in rainfall amount over the Islands as evidenced by several studies [15
] as well as increasing groundwater withdrawal in the most densely populated areas [19
]. These previous studies also indicated that Hawaii is expected to face an overall decline in rainfall amount, but the implications of these changes on future streamflows need to be investigated. It is expected that this may cause significant decline in surface and groundwater resources that can lead to restrictions on water use in some areas during droughts. On the other hand, water demand for agricultural productions and other uses is expected to increase. Other factors, such as population growth (http://data.uhero.hawaii.edu/
) and increased future water demand [19
], can also negatively influence the freshwater availability and sustainability. Furthermore, Diaz et al. [20
] and Giambelluca et al. [21
] reported that the temperature of the Hawaiian Islands is anticipated to increase in the future, which can increase evapotranspiration rate and further reduce availability of future freshwater resources.
In the long run, climate change, when combined with increased water demand and groundwater withdrawals, can also exacerbate the chance of saltwater intrusion to groundwater aquifers, reduce the amount of groundwater discharge to streams and ocean, cause frequent droughts, and further threaten food security [22
]. Thus, the Hawaiian communities are expected to encounter challenges related to freshwater resources availability and sustainability due to hydrological droughts [3
]. In addition, other consequences include an increase in sea level rise [24
] and frequent occurrence of extreme events, such as groundwater inundation and flooding especially in low-lying coastal areas leading to groundwater salinization, loss of infrastructures and inhabitants in that zones [27
]. Such events are anticipated to strongly damage surface and sub-surface drainage and onsite wastewater disposal systems, negatively impact the economy and recreation areas of coastal communities [29
]. For example, Habel et al. [29
] reported that future groundwater inundation and flooding due to sea level rise would threaten approximately $
5 billion of real estate property and municipal infrastructures of Honolulu around Waikiki area. The authors also documented that frequent surficial flooding caused by extreme rainfall events is expected to occur at the coastal communities as a result of groundwater inundation and narrow unsaturated space that limits the amount of water accommodated through infiltration process. Such processes would have significant effect on the economy of Hawaii, as extreme flooding events are projected to cost billions of dollars to repair damaged property and infrastructures of coastal communities [29
Despite the development of numerous layers of protection by adopting the Hawaii Legislature for all freshwater bodies (e.g., see the Hawaii State Water Code 2017) and of enforcing policies and approvals of water allocation for all water users of the Islands (e.g., see the State Commission on Water Resources Management (CWRM)), population growth and climate change are still expected to cause further reduction in freshwater and frequent droughts in the state of Hawaii. For example, approximately 0.3 million cubic meters per day of additional water demand is expected by 2030 [32
The advancements in climate models to predict future climate variables have made increased confidence in their outputs, which are in turn used as inputs to watershed and hydrological models. As a consequence, several studies have shown the impacts of future climate change on freshwater budgets [5
], streamflow [2
], and hydrological extremes [6
] at large-scale continental watersheds. In small-scale Pacific island watersheds, such as in Hawaii, a few studies [3
] have also assessed the impact of climate change on water budgets but focused at monthly and yearly mean values. However, there are no watershed-wide assessment of climate change impact on daily streamflow and its extreme values in Hawaii. Estimating the impact of future climate change on water budget elements is of importance for an overall water resources management approach. However, assessing future extreme values is also of critical need due to their significant impact on the economy, environment, and human life [6
]. Additionally, a detailed overview on the possible impacts of future climate change scenarios on streamflow duration curves and hydrological extremes has not been documented yet. Therefore, predicting climate change impacts on hydrological extremes (floods, droughts) and better understanding these impacts are of major importance for Hawaii. Such studies can ultimately help water management and decision makers as an exploratory tool for evaluating the consequences of climate change on hydrological extremes.
This study assesses the impact of climate change on streamflow and hydrological extremes (peak and low flows) by using the statistically downscaled rainfall data over the Hawaiian Islands for the middle century (2041–2070) and the late century (2071–2100) as reported by Timm et al. [18
] under the Representative Concentration Pathways (RCP) of 4.5 and 8.5 scenarios [1
]. The RCP 4.5 scenario refers to a radiative forcing value of 4.5 W m−2
, while the RCP 8.5 refers to a radiative forcing value of 8.5 W m−2
by the end of 21st century relative to the pre-industrial values [1
]. It is projected that greenhouse gas emissions for the RCP 4.5 scenario will reach its peak values around the mid-century and then decline. However, in the case of RCP 8.5 scenario, the emissions are anticipated to continue rising through the 21st century [1
]. In this study, we also considered temperature and solar radiation changes based on previous studies [20
]. The widely used watershed model, Soil and Water Assessment Tool (SWAT) [45
] was applied to simulate daily streamflows. SWAT was used for two adjacent watersheds, Kalihi and Nuuanu, which are located on the leeward side of Oahu. The specific objectives of this study were to
Simulate daily streamflow and its extreme values of Kalihi and Nuuanu watersheds under current and future climatic conditions;
Evaluate and provide a detailed overview of the effects of two extreme climate change scenarios (RCP 4.5 and 8.5) on simulated streamflow (Flow Duration Curve (FDC)) and hydrological extremes (peak and low flows) of the two watersheds.
These objectives were accomplished by calibrating and validating SWAT against daily observed streamflow data measured at three stations by the US Geological Survey (USGS). The climate change impacts were assessed through changes in three critical variables, i.e., rainfall, temperature, and solar radiation for both RCP 4.5 and 8.5 scenarios. Combining the two RCP scenarios with the possible changes of the aforementioned climatic variables helps to better understand and estimate possible range of changes in FDC and extreme peak and low flow values under future climate conditions.
By using available geo-spatial and hydro-meteorological data, this study set-up, calibrated, and validated the SWAT model for the Kalihi and Nuuanu watersheds of Oahu (Hawaii). The study aimed to use the model for assessing the impact of future climate change on daily streamflow duration curve, extreme peak flows (floods), and extreme low flows (droughts). The model was run with 35 years (1980–2014) of historical data, which were perturbed by using perturbation factors to reflect projected future climate condition. The perturbed future climate data includes rainfall values based on projected rainfall anomalies of the RCP 4.5 and 8.5 scenarios [18
] and temperature and solar radiation values, which were based on expected changes [41
]. In order to assess the relative sensitivity of streamflow and hydrological extremes to climate change, we formulated six climate change scenarios for the middle (2041–2070) and end (2071–2100) of the 21st century.
SWAT adequately reproduced the observed daily streamflow hydrographs and their temporal evolution at three USGS flow gauging stations as the calculated NSE values were greater than 0.5 for both calibration and validation periods. In addition, the model bracketed more than 80% of observed streamflow data at 95% model prediction uncertainty with narrow uncertainty band, indicating the suitability of the model for future daily streamflow prediction.
Predicted climate change scenarios generally indicated an overall decline in daily streamflow values, extreme peak and low flows, and thus downward shift in the respective Flow Duration Curves (FDC). However, some extreme peak flows, which have higher return periods, are predicted to increase by as much as 22% whereas the extreme low flow values are expected to consistently decrease by as much as 60%, when compared to the baseline values. More importantly, larger negative changes in extreme low flows are projected in the upstream part of the watersheds where higher groundwater recharge is expected. The latter expected change is probably due to more warming as a result of increase in temperature and solar radiation values, including consistent decrease in rainfall, during the dry season in the mountainous regions.
When compared to the RCP 4.5 scenario, the RCP 8.5 scenario is projected to cause increasingly severe changes on extreme peak and low flow values, especially during the later period of the 21st century. Surprisingly, both extreme negative and positive changes are predicted under the RCP 8.5 scenario. These effects may have serious implications on freshwater availability and groundwater sustainability of the Hawaiian and similar Islands. In general, our findings are consistent with previous studies on Oahu that focused on future climate change impacts on monthly and annual water budgets [3
Overall, the study’s findings show that while the intensity of extreme peak flow events with high return periods is anticipated to increase, floods are less likely to become more frequent in the future, in contrast to hydrological droughts. The severity of drought is likely amplified toward the end of this century. Further, the timing of hydrological droughts occurrence is expected to occur earlier in comparison to the present hydrological conditions, suggesting a need for groundwater conservation efforts. Due to such severe consequences of climate changes, water resources managers, planners, and ecological conservationists may need to prepare appropriate mitigation measures, such as enhancing groundwater availability through artificially recharging groundwater aquifers from water captured by water harvesting structures, as proposed by Leta et al. [4
]. This study can also be used as a supportive tool to help water resources managers and decision makers for evaluating the implications of climate change on streamflow and its extreme values.