Groundwater Storage, Porosity, and Specific Yield: Groundwater occupies the cracks and pore spaces between rocks and mineral grains below the land surface. In the saturated zone, essentially all of the pores are filled with water. If a volume of saturated aquifer material is completely dried, the water volume removed reflects the total porosity of the material, or the fraction of pore space within the total volume of solids plus open spaces. This number can be surprisingly large; some minerals and rock formations can have total porosities in excess of 50%. In the unsaturated, or vadose, zone there can be significant amounts of water present, but the voids are not completely filled (see appendix on saturated thickness).
However, some of the pore spaces may be too small or too poorly connected to permit the water they contain to flow out easily. The effective porosity can be thought of as the volume of pore space that will drain in a reasonable period of time under the influence of gravity. Effective porosity is always less than total porosity, sometimes (as in the case of clays) much less. "Good aquifers" tend to have values of effective porosity in the range of 10-30%, although examples of higher and lower values can be found. Figure 1 illustrates the relationship among the types of porosity and the volume of water in storage.
Figure 1: A schematic illustration of an aquifer in which the total porosity in the saturated zone is 30%, half of which is tightly held in small pores or mineral associations, and half of which is in large pores that drain relatively easily. The latter fraction can be pumped out, and is the effective porosity or specific yield. Illustration not to scale.
A characteristic closely related to effective porosity is the specific yield of the aquifer, which is the volume of water per unit volume of aquifer that can be extracted by pumping. Although there are some technical distinctions, effective porosity and specific yield can be thought of as equivalent for most non-technical purposes.
Specific yield (SY) is clearly an important factor in water availability, and is the factor that is used to convert saturated thickness (ST) to the actual volume of groundwater available;
Figure 1 compares the water available for extraction with the total water and aquifer volumes.Volume = Area x ST x SY
At any given location, the porosity of the formation remains essentially constant, but the volume of water in storage, the average local porosity, and the specific yield all vary with changes in saturated thickness (water table elevation). Some of this variation can be explained (and quantitatively predicted) on the basis of straightforward physical principles, but some of it is due to local variations in the aquifer structure. This hydrogeologic variability is difficult to predict or measure with detailed accuracy.
The US Geological Survey has prepared maps and electronic coverages showing estimates of the distribution of Specific Yield for the High Plains aquifer, which have been adapted for use in this project.
Groundwater Flow and Hydraulic Conductivity: Groundwater flow is very slow compared to surface water movement. A rough average number often used for natural flow in the High Plains aquifer is a foot per day. This is thousands of times slower than river flow (typically measured in feet per second), and means that a 'parcel' of groundwater takes over a decade to move a mile, and about a century to cross a township. This natural time scale underscores the importance of long-term planning and management, and helps explain why resource depletion or contamination cannot be quickly or easily rectified.
Groundwater, like surface water, flows 'downhill' in the direction determined by the slope of the water table. Its rate of flow is determined by the steepness of the slope and an aquifer characteristic called hydraulic conductivity. In a porous medium, flow is described by Darcy's Law, an equation that relates the rate of flow to the slope (or gradient) of the water table and the characteristics of the aquifer. This law is illustrated in figure 2, and is written as:
Q = A x K x G,where Q is the volume flow of water (for example, in cubic feet per day -- also called flux), A is the vertical area of the aquifer through which the horizontal flow is occurring, G is the gradient or slope of the water table in the direction of flow (difference in elevation divided by horizontal distance), and K is the hydraulic conductivity -- a constant of proportionality that describes how easily water flows through the medium. The term permeability is closely related to hydraulic conductivity; in strict scientific usage they have slightly different definitions, but for water in unconfined aquifers they are essentially the same.
Figure 2: Illustration of the factors governing flow in groundwater systems -- the head gradient, or slope of the water table, the hydraulic conductivity of the aquifer, and the area through which flow can occur.
Like specific yield, the hydraulic conductivity is related to but not solely determined by the porosity of the aquifer. All of these characteristics may show considerable variation over a variety of spatial scales. Because both specific yield and hydraulic conductivity are typically measured from tests on individual wells, their determination is relatively expensive and applies to the scale of the zone of influence of the well -- which is much larger than local variations in the aquifer, but very small compared to the whole aquifer, basin, etc.
The US Geological Survey maps and electronic coverages showing estimates of the distribution of Hydraulic Conductivity for the High Plains aquifer have been adapted for use in this project.
Qualifications: The USGS maps are a valuable source of aquifer information, and represent the only consistently prepared whole-aquifer assessment of these properties. As with all such descriptions, they suffer from scarcity of data; measurements must be extrapolated much more broadly than would be ideal. In addition, the analyses necessarily treat the aquifer as a single homogeneous layer, while we know that in some areas there is vertical stratification caused by semi-confining layers (see appendix on aquifer types and terminology). Unless the location is close to one of the measurement sites, use of these coverages to describe the aquifer at individual locations could be quite misleading. However, when considering averages at the spatial scale of townships and larger, the data probably represent a significant improvement over "best guess" values.
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