Groundwater is water located beneath the ground surface that completely fills or saturates the pore spaces and cracks in soil and rock like a sponge. The upper boundary of the underground zone where soil or rock are completely saturated with water is known as the water table. The water table can rise as water moves into the groundwater (e.g., from rain that infiltrates or flows horizontally from elsewhere through the permeable soil/rock), or fall as water moves out (e.g., through evaporation or discharge to a stream/spring/the ocean. The CoSMoS-Groundwater model is focused on the shallow water table that is influenced by the rise and fall of the ocean.

The CoSMoS-Groundwater model shows the predicted present-day and future long-term average water table depth with sea level rise from 0-3 meters (0-9.8 ft) relative to the present-day Mean Higher High Water (MHHW). To make accurate predictions, it accounts for present-day topography, allowing for rising groundwater to discharge to streams or seeps rather than assuming that the water table will rise uniformly. The amount groundwater rises with sea level rise is also controlled by the subsurface geology (see detailed information for Groundwater Geology). Projections of depth to the water table are intended as a screening tool to identify locations that may experience increasing groundwater hazards as seas rise.

Hazard Exposure Reporting and Analytics (HERA) Web Tool

Shaded areas illustrate modeled depths to the coastal water table under a given sea level rise scenario:

• The dark blue areas are below Mean Higher High Water (MHHW), indicating marine inundation.
• The light blue areas indicate places where the water table is predicted to rise to at least the land surface (emergent groundwater), flooding the affected area.
• The yellow areas indicate places where the water table will be very shallow, within 1 meter (3.3 feet) of the ground surface, resulting in potentially soggy ground.
• The orange (1-2 meter depth, 3.3-6.6 feet) and brown (2-5 meter depth, 6.6-16.4 feet) areas will have less risk of surface flooding, but may still pose hazards to buried infrastructure, for example.

Subsurface geology determines how easily groundwater can move through connected subsurface openings, set by a property known as hydraulic conductivity. In more permeable material like gravel, groundwater flows easily and doesn’t mound or “pile up”, resulting in groundwater tables that are farther below the ground surface.

Where materials are less permeable, such as silt/clay, it is harder for groundwater to flow through the subsurface, and water moving down into the ground (known as recharge) piles up, creating a shallower water table.