Tutuila Groundwater Recharge Under SWB2 Model

Groundwater is the primary water source on the island Tutuila in American Samoa, and accurate
quantification of groundwater availability is essential for well-informed management of this limited
resource. A water budget approach using SWB2, a soil water-balance model was applied to Tutuila
with the primary objective of calculating spatially and temporally distributed net-infiltration, which
directly controls groundwater recharge rate. Other water budget components such as
evapotranspiration, canopy interception, runoff, and mountain front recharge were also quantified
with the SWB2 model for average present-day climate conditions. Additionally, the potential effects
of future climate change on water resources availability were simulated by integrating dynamically
downscaled climate predictions for 2080 to 2099 derived from externally supplied global climate
model results. Notable improvements in this model over previously developed water budget
models for Tutuila include flow-routing based on land topography, inclusion of the mountain front
recharge process, and consideration of direct net infiltration from anthropogenic sources such as
on-site wastewater units and leaking water delivery lines. Model results indicated approximately
54% of Tutuila’s rainfall infiltrates as groundwater recharge, 8% is lost to canopy evaporation,
another 15% is lost to evapotranspiration from soils, and 21% is removed through surface-water
features as stormflow-runoff. The model was able to simulate these processes with a high-spatial
and temporal resolution with a 20 by 20 m grid-cell size, and a daily-resolution output time step.
Climate scenarios suggested an increase in net-infiltration of 17 to 27% may be expected by the end
of the century depending on the emissions scenario used.