Aerial view of U.S. Naval Base Guam, 5 March 2016

By Marc Kodack

The U.S. Geological Survey recently released a report that assessed “the influence of future climate change and sea level rise on freshwater resources (surface-water and groundwater) of Guam.” These changes have serious implications for the local population, as well as for Department of Defense installations including Naval Base Guam and Andersen Air Force Base. The work was funded by the DoD’s Strategic Environmental Research and Development Program.

Andersen AFB is in the northern part of the Guam. It depends of water from the Northern Guam Lens Aquiver (NGLA) which is recharged predominately by rainfall (97%), supplemented by other sources including irrigation. Withdrawals are mostly through wells, with Andersen accounting for approximately 9.5% of all the withdrawn groundwater. Naval Base Guam is in the southern part of the island. The base and surrounding communities depend on water drawn from the Fena Valley Reservoir (FVR) which captures surface water flow from the surrounding region.

As part of the research, USGS wanted to determine the effects of climate change across Guam. They evaluated a number of global climate scenarios eventually selecting a business-as-usual model projected to 2080-2099. The selected scenario was supplemented with simulations of local, higher spatial resolution climate models for an area in and around Guam and the surrounding ocean. All this climate information was then incorporated into modeling efforts applied to the northern and southern portions of Guam to understand surface and groundwater affecting Guam’s hydrology.

For the NGLA, USGS waned to examine future water recharge rates. To do this, it used a groundwater flow and transport model using different withdrawal scenarios including current (2010) rates, higher rates based on projected population growth, and drought. “In general, the freshwater lens becomes smaller as withdrawal increases, or recharge is reduced. During drought wells throughout the NGLA become more saline and the acceptable yield is reduced” by almost 75%.

For the southern part of Guam, USGS modelled “future monthly FVR [Fena Valley Reservoir] water levels in response to various combinations of projected water-withdrawal rates.” To calculate these estimates, USGS used a precipitation runoff watershed model, stream flow, air temperature estimates [which affect evaporation], bathymetric information, existing stream gauge data, and sedimentation rates including changes that can occur from a typhoon striking the island which can reduce reservoir capacity by 2-3% in only a few days.

Based on all these modelling efforts “Guam’s water sources in a future climate condition (2080-2099) are projected to diminish…Future average temperature increases, and average rainfall decreases will lead to reduced stream flow in southern Guam and reduced groundwater recharge to the…NGLA…Average temperatures in southern Guam will increase about 3.2 [degrees] C (5.8 [degrees] F), overall rainfall will decrease about 7%, and streamflow will consequently decrease 18 percent in parts of southern Guam…Reduced future streamflow will decrease water availability from the FVR…Higher sea level and reduced future recharge will decrease water availability from the NGLA” and raise salinity to unacceptable levels. If future droughts also occur, salinity levels will further increase.

To address these projected climate related changes to the NGLA, USGS simulated reducing production well depth [to reduce pumping of higher saline waters], reducing withdrawal rates, and, balancing areas-of-greatest-supply with areas-of-greatest-demand. These combined actions would allow sufficient volumes of water to continue to be withdrawn to support local populations on the northern end of the island including Andersen AFB.

To address these projected climate related changes for the FVR, the water supply intakes could be lowered, the spillway height could be raised, or a combination of both to keep the reservoir water level above the intakes. With these changes, local populations on the southern part of Guam, including those on Naval Base Guam, would continue to have access to a sufficient water supply.

Without sufficient potable water supplies either today or in the future neither Andersen AFB or Naval Base Guam would be able to function as required to execute their various missions. The USGS’s modelling is based on multiple assumptions, including those for climate, which may change based on, on-going and future scientific research and data collection. However, despite any modelling uncertainty, modelling should be used in long-range planning to understand climate-informed possibilities that might be faced by an installation. Given the extended lead time needed for structural/materiel solutions that might be proposed to actually be constructed, e.g., military construction funding, modelling, such as that performed by USGS, can help shape what these solutions will be able to address with respect to climate change possibilities, if these solutions were ever funded.