Drainage class places major emphasis
on the relative wetness of the soil under natural conditions as it pertains
to wetness due to a water table. It refers to the frequency and duration
of wet periods under conditions similar to those under which the soil developed.
Alteration of the water regime by man, either through drainage or irrigation,
is not a consideration unless the alterations have significantly changed
the morphology of the soil. Seven drainage classes have been developed.
Soil drainage characteristics are a useful guide to where ground water
or surface water is most at risk to contamination from nutrients applied
at the land surface. Ground water in areas of well drained soils is vulnerable
to surface application of chemicals, and warrants more complete protection
strategies than in areas of poorly drained soils. Concentrations of mobile
nutrients such as nitrate are generally low in ground water under poorly
Hydrologic Soil Group
This is a set of classes that pertain
to the relative infiltration rate of soil under conditions of maximum yearly
wetness. It is assumed that the ground surface is bare and ice does not
impede infiltration and transmission of water downward (Soil Survey Manual,
1993). Hydrologic soil groups are used in the computation of runoff by
the Curve Number method. Minimum annual steady ponded infiltration rate
for a bare ground surface determines the hydrologic soil groups. They are
based upon the assumption that the minimum Ksat occurs with the top 50
cm of the soil profile. If the minimum occurs between 50 and 100 cm, then
Ksat for the purpose of placement is increased one class.
Permeability class placement pertains
to the amount of water that would move downward through a unit area of
saturated in-place soil in unit time under unit hydraulic gradient. Estimates
are based on models that relate laboratory measurements on soil cores to
the interpretative soil properties and morphology. Soil permeability affects
the entrance of water into the soil profile and redistribution of the contaminants
to ground and surface waters.
This is the range in slope gradient
in percent. It refers to the ground surface configuration for scales that
exceed about 10 m and range upward to the landscape as a whole. Slope has
gradient, complexity, length, and aspect. The scale of reference commonly
exceeds that of the pedon and should be indicated. Slope gradient is the
inclination of the surface of the soil from the horizontal and is measured
in percent. For example, a slope of 45 degrees is a slope of 100% because
the difference in elevation between two points 100 m apart horizontally
is 100 m on a 45-degree slope.
This refers to the loss of water from
an area by flow over the land surface. It differs from subsurface flow
or interflow that results when infiltrated water encounters a zone with
lower perviousness than the soil above. The water accumulates above this
less pervious zone and may move laterally if conditions are favorable for
the occurrence of free water. Runoff classes are developed from soil characteristics
such as soil slope, saturated hydraulic conductivity, climate, and cover.
The concept indicates relative runoff for very specific conditions. The
soil surface is assumed to be bare, and surface water retention due to
irregularities in the ground surface is low. Steady ponded infiltration
rate is the applicable infiltration stage.