Water enters the ground through infiltration, beginning its journey through the subsurface water cycle. This water first encounters the Zone of Aeration (unsaturated zone), where pore spaces contain both air and water, with water clinging to material surfaces. Water is pulled downward primarily by gravity, a process termed percolation, until it reaches a point where the geologic material can hold no more air. This initial stage is entirely vertical, driven by gravity pulling the water deeper into the Earth.
The Transition Point: Entering the Saturated Zone
The downward flow of water eventually reaches a distinct boundary: the Water Table. This boundary represents the uppermost surface of the Zone of Saturation (phreatic zone). Once water crosses the water table, it enters a layer where all voids and pore spaces are completely filled with water.
In this saturated zone, the movement of water shifts from purely vertical percolation to a more complex, generally lateral flow. The water here has become true groundwater. This change occurs because the saturated conditions eliminate the air pockets that allowed simple gravitational drainage in the zone above.
The depth of the water table can fluctuate significantly, rising during heavy rain or snowmelt and falling during dry spells. This fluctuation means the water table is not a fixed line but a dynamic surface separating air-filled pores above from water-filled pores below. The physical laws governing water movement change completely once it is fully submerged in the saturated zone.
Governing Forces of Groundwater Flow
Within the saturated zone, the movement of groundwater is influenced by a combination of gravity and pressure, which together define the concept of hydraulic head. Water moves from areas of higher hydraulic head to areas of lower hydraulic head, similar to how surface water flows downhill. The difference in hydraulic head over a distance creates the hydraulic gradient, which determines the direction and speed of the flow.
The total hydraulic head measures the water’s potential energy at a specific point, combining energy from its elevation and the pressure exerted by the water column above it. In deep, confined environments, high pressure can force water to move upward, temporarily defying gravity, though the overall flow remains toward lower potential energy.
The physical characteristics of the underground material also control flow velocity and direction. Porosity refers to the percentage of open space within the rock or sediment that can hold water. However, the speed at which water moves is governed by permeability—the measure of how easily water can pass through the material due to interconnected pores. For example, clay has high porosity but low permeability, restricting flow, whereas well-sorted sand allows water to move relatively quickly due to both high porosity and high permeability.
Aquifers: Storage and Discharge
Groundwater movement results in the formation of aquifers: subsurface layers that store and transmit usable quantities of water. These formations represent freshwater reservoirs that sustain human activity and ecosystems. Aquifers are categorized based on their relationship with the water table and the layers above them.
An unconfined aquifer, often called a water table aquifer, has the water table as its upper boundary and is directly recharged by downward-percolating water from the surface. These are generally closer to the surface and are more susceptible to fluctuations from rainfall and potential surface contamination. In contrast, a confined aquifer is overlain by an impermeable or low-permeability layer, which restricts water flow and places the water within the aquifer under pressure.
This pressure in confined aquifers can cause water to rise above the aquifer level when a well is drilled, creating an artesian system. The ultimate fate of moving groundwater is discharge, which is how it exits the subsurface system. Discharge occurs naturally through springs, where the water table intersects the land surface, or as baseflow feeding rivers and lakes. Human intervention also discharges water through pumping wells for consumption.