What is the water table, and why can its depth vary?
Quick answer
The water table is the approximate upper surface of the saturated zone underground. Below it, the saturated zone contains groundwater; above it is the unsaturated zone. Its depth can differ across places and fluctuate over time as recharge changes. A general explanation cannot tell you the water-table depth at a particular site or in a particular well. [S1]
The question, the details, the next step.
What exactly is the water table? Is it the same thing as groundwater?
They are related, but they are not the same thing. The water table is the approximate upper surface of the saturated zone underground. Groundwater is the water within that saturated zone. Think of the water table as a boundary and groundwater as the water below that boundary. [S1]
Above the water table is the unsaturated zone; below it is the saturated zone, where groundwater is present. These are useful labels for a simple cross-section. [S1]
Could you show me a labeled cross-section? I want to see how it relates to the land surface.
Here is a simplified sketch. The ground line and sloping water table are illustrative, not measurements of a real place:
Land surface
_________/^^^^^^^^^^^^\________
Unsaturated zone
. . . . . . . . . . . .
~~~~~~~ Water table ~~~~~~~~~~~
Saturated zone
groundwater below boundaryRead the sketch from top to bottom: land surface, unsaturated zone, water table, then saturated zone. The distance from the land surface down to the water table is its depth below ground at that point. A regional water table can reflect the broad shape of the land, but this sketch cannot show the actual depth at a specific location. [S1]
Why would its depth vary from one place or time to another?
There are two useful comparisons: place to place and time to time. Across a region, the water table’s shape can reflect land-surface topography, so it need not be a flat, uniform surface. At a given place, its depth can fluctuate seasonally and from year to year as recharge changes. Precipitation and surface water are among the recharge influences described by the U.S. Geological Survey. [S1]
For example, imagine two points on uneven land. Even if you draw one connected, gently sloping water table beneath them, the amount of ground between each point’s surface and that boundary may differ. At one point, the boundary might also be higher or lower at another time. This example explains the concept; it does not predict the direction or size of change at any particular site.
How can I use this idea without mistaking the diagram for a local estimate?
Keep the sketch conceptual and label what is known versus assumed. For example, you can copy this caption:
“Conceptual cross-section, not to scale. The water table marks the approximate upper surface of the saturated zone. Groundwater is shown below it; the unsaturated zone is above it. The drawing does not give a measured depth for a particular place or well.” [S1]
Then label the four features: land surface, unsaturated zone, water table, and saturated zone. General information explains what the water table is and describes broad reasons it may vary; it does not establish a local level, a well’s current measurement, or how much the water table changes at a particular location. [S1]
A practical checklist
Label the top boundary of the sketch as the land surface.
Mark the unsaturated zone above the water table.
Draw and label the water table as the approximate upper surface of the saturated zone.
Label the groundwater-containing region below it as the saturated zone.
Add “conceptual, not to scale” and do not present the drawing as a local measurement.
A few more useful answers
Does the water table always stay at the same depth?
No. It can fluctuate seasonally and from year to year as recharge changes. General information does not establish the amount or timing of change at a specific place. [S1]
Does a diagram show the depth of groundwater at my location?
Not unless it is based on a relevant measurement for that location. The cross-section here is a conceptual illustration, not a well reading or local estimate.
Is the line drawn for the water table always flat?
Not necessarily. Its regional shape can reflect land-surface topography, so a simple diagram may show it sloping rather than flat. [S1]
Sources
References used for this guide.
- [S1] General Facts and Concepts about Ground Water — U.S. Geological Survey Circular 1186 pubs.usgs.gov · Checked