Groundwater provides 99 percent of Hawai‘i’s drinking water and about 50 percent of all freshwater used in the State. Groundwater recharge is water derived from precipitation and other sources, such as irrigation and leakage from surface reservoirs, that moves through or bypasses the plant-soil system and replenishes aquifers. Groundwater availability in Hawai‘i is affected by changes in precipitation and evapotranspiration, saltwater intrusion related to withdrawals, and contamination from anthropogenic sources.
Effects of Climate Variability
Changes in climate can affect groundwater availability in Hawai‘i mainly by influencing groundwater recharge rates. In areas where the future climate becomes drier, reduced groundwater recharge could result in higher-salinity groundwater and reduced base flow in streams.
Observed changes in rainfall and streamflow
Long-term downward trends of rainfall, long-term upward trends in drought duration and magnitude, and long-term upward trends of surface temperature, particularly at higher altitudes generally coincide with observed long-term downward trends of stream base flow. Observed increases in the strength and frequency of occurrence of the trade-wind inversion in Hawaiʻi coincide with observed decreases in rainfall and increases in solar radiation at higher altitudes.
Influence of Ocean-Atmosphere Cycles
Ocean-atmosphere multi-year cycles such as El Niño and La Niña, the Pacific Decadal Oscillation (PDO), and the Pacific North American pattern (PNA) cause some years to be wetter or drier than the long-term average. Hawaiʻi tends to be dry during most El Niño events and wet during most La Niña events. Over the last century, El Niño events have been associated with most droughts in the Hawaiian Islands. The observed historical changes in rainfall, drought duration and magnitude, surface temperature, and trade-wind inversion frequency imply a decreasing trend in groundwater recharge and storage and an overall decline in groundwater availability.
Projected Changes to Rainfall
Several published mid- and end-of-21st-century climate projections for the main Hawaiian Islands estimate decreases in rainfall across large areas on each island. In response to these projected changes in rainfall, groundwater recharge is anticipated to decrease across extensive areas of each island.
Effects of Land-Cover Change
Land-cover change can affect processes that influence groundwater recharge such as cloud-water interception, net precipitation, irrigation, infiltration, direct runoff, and transpiration.
In natural settings, infiltration rates in Hawaiʻi tend to be greatest in areas dominated by forests and shrubland, less in areas dominated by grassland, and lowest in areas with bare soil. The spread of highly-invasive plant species, replacing forests with grassland, and introducing grazing ungulates results in land-cover changes that can have a negative impact on groundwater recharge by decreasing cloud-water interception and infiltration rates, and increasing direct runoff and transpiration rates.
Groundwater provides 99 percent of Hawai‘i’s drinking water and about 50 percent of all freshwater used in the State. Groundwater recharge is water derived from precipitation and other sources, such as irrigation and leakage from surface reservoirs, that moves through or bypasses the plant-soil system and replenishes aquifers. Groundwater availability in Hawai‘i is affected by changes in precipitation and evapotranspiration, saltwater intrusion related to withdrawals, and contamination from anthropogenic sources.
Effects of Climate Variability
Changes in climate can affect groundwater availability in Hawai‘i mainly by influencing groundwater recharge rates. In areas where the future climate becomes drier, reduced groundwater recharge could result in higher-salinity groundwater and reduced base flow in streams.
Observed changes in rainfall and streamflow
Long-term downward trends of rainfall, long-term upward trends in drought duration and magnitude, and long-term upward trends of surface temperature, particularly at higher altitudes generally coincide with observed long-term downward trends of stream base flow. Observed increases in the strength and frequency of occurrence of the trade-wind inversion in Hawaiʻi coincide with observed decreases in rainfall and increases in solar radiation at higher altitudes.
Influence of Ocean-Atmosphere Cycles
Ocean-atmosphere multi-year cycles such as El Niño and La Niña, the Pacific Decadal Oscillation (PDO), and the Pacific North American pattern (PNA) cause some years to be wetter or drier than the long-term average. Hawaiʻi tends to be dry during most El Niño events and wet during most La Niña events. Over the last century, El Niño events have been associated with most droughts in the Hawaiian Islands. The observed historical changes in rainfall, drought duration and magnitude, surface temperature, and trade-wind inversion frequency imply a decreasing trend in groundwater recharge and storage and an overall decline in groundwater availability.
Projected Changes to Rainfall
Several published mid- and end-of-21st-century climate projections for the main Hawaiian Islands estimate decreases in rainfall across large areas on each island. In response to these projected changes in rainfall, groundwater recharge is anticipated to decrease across extensive areas of each island.
Effects of Land-Cover Change
Land-cover change can affect processes that influence groundwater recharge such as cloud-water interception, net precipitation, irrigation, infiltration, direct runoff, and transpiration.
In natural settings, infiltration rates in Hawaiʻi tend to be greatest in areas dominated by forests and shrubland, less in areas dominated by grassland, and lowest in areas with bare soil. The spread of highly-invasive plant species, replacing forests with grassland, and introducing grazing ungulates results in land-cover changes that can have a negative impact on groundwater recharge by decreasing cloud-water interception and infiltration rates, and increasing direct runoff and transpiration rates.