Projected water table depths in coastal areas around Puget Sound, Washington

To predict water table depths, seamless unconfined groundwater heads for coastal groundwater systems around Puget Sound (Washington State) were modeled with homogeneous, steady-state MODFLOW simulations. The geographic extent examined was defined primarily by watershed boundaries. Steady-state MODFLOW groundwater flow models were used to obtain detailed (50-meter-scale) predictions over large geographic scales (100s of kilometers) of groundwater heads for both current and future sea-level rise (SLR) scenarios (0, 0.25, 0.5, 1, 1.5, 2, 2.5, and 3 m) using 3 spatially varying hydraulic conductivities (K); one based on published K's, one with published K's reduced by a factor of 10 (K*0.1), and one with published K's increased by a factor of 10 (K*10), to assess the sensitivity of model results to K. All models had variable thicknesses with cell tops at the land surface and bottoms at 0m NAVD88 to ensure that steep groundwater gradients in topographically steep and/or high-recharge watersheds did not result in model convergence failure. The models were run with a local mean higher-high water (MHHW) marine boundary condition and with groundwater reaching the land surface removed from the model, simulating loss via natural drainage. Because of the large number of lakes in this region and the influence of lakes on adjacent groundwater levels, large (> 0.5 km2) lake heads mostly were fixed in the model at published present-day levels. In sea-level rise cases where lake elevations would have been below model sea level, lake elevations were set at model sea level. Modeled groundwater heads were then subtracted from high-resolution topographic digital elevation model (DEM) data to obtain water table depths.