Drainage Class 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 drained soils.
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 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.
Slope 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.
Surface Runoff 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.