Last revision: 11/09/2000 - js
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Saturated Thickness -- Concepts and Measurement

R. W. Buddemeier

Boldface items are linked to other atlas sections; italic items are linked to glossary definitions

A Close-up Definition --

The detailed figure below shows the rock and pores spaces in a volume of the aquifer. It gives an exaggerated illustration of saturated and unsaturated zones in an aquifer, and how those relate to the upper and lower boundaries and to saturated thickness. See the appendix section on groundwater storage and flow for how this relates to porosity, specific yield, and water volumeIn the unsaturated, or vadose, zone, addition of water by recharge from the surface causes the water content to rise until it reaches the field capacity of the soil -- the concentration of water at which gravity-driven drainage flow occurs.

 Figure 1: Saturated and unsaturated zones in an aquifer composed of coarse, unconsolidated material. In the saturated zone, all of the pore spaces are filled with water. In the unsaturated (also known as the vadose zone), some water is usually present, but the amount is variable and it is always less than the field capacity.

Under natural conditions, the elevation of the boundary between the unsaturated and saturated zones -- the water table -- fluctuates as a function of recharge, discharge, and evapotranspiration. These variation can be surprisingly large, in part because of the effect of porosity in amplifying the elevation change due to recharge. If the effective porosity of the aquifer is 17%, for example (a common value in the High Plains), recharge of 2 inches of free-standing water will cause the water table to rise by approximately one foot.

The figure below illustrates the idea of saturated thickness -- that volume of the aquifer in which the pore spaces are completely filled (saturated) with water. At a particular point, the local saturated thickness is the difference in elevation (or depth) between the water table and the bedrock surface (the base of the aquifer). However, bedrock and water table elevations are often measured at different points, and because both vary over space, the saturated thickness at any given point must usually be estimated from what we know about the two surfaces rather than on observed values. This is often a more useful measure of the actual amount of underground water than observations at a few specific points, because it provides good estimates for a whole region instead of precise measurements at very limited locations.

Natural Condition -- No Groundwater Pumping

Figure 2: Saturated thickness in cross section -- the light blue area represents the average volume of permanent saturation, and the dark blue area is the range of variation of its upper limit, which is affected by topography, recharge, discharge, and the effects of vegetation.

The elevation of the water table varies over time even under natural conditions.  Seasonal and year-to-year variations in recharge result in fluctuations, and where the water table is shallow the evapotranspiration of plants can have very significant effects during the growing season. Estimates of saturated thickness therefore depend on the times, places and numbers of measurements taken. Uncertainties in the absolute value will be at least several feet and perhaps tens of feet, but adoption of a standard set of assumptions and measurement protocols can provide an estimate that is useful as a baseline for comparison regardless of its accuracy.

Human Modification --

Human modification of the land surface, and especially groundwater pumping and/or stream diversion, have even greater short-term and long-term effects on the water table and saturated thickness. For a discussion of the effects of pumping on short-term water table elevation changes, see the appendix on drawdown and pumping. Long-term effects are addressed in the atlas section on Change in Saturated Thickness, and in the appendix on decline rates.

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Funded (in part) by the Kansas Water Plan Fund