Retardance refers to how vegetation and its standing residue slow runoff.
	Two data points are used to describe the curve.  One data is the retardance at a high yield.
	The other data point from one of three conditions is used.
	Condition 1: The vegetation has no effect at a low yield, which is the situation for the lowest curve in figure.  An example is that the retardance for corn is assumed to be zero at about 100 bu/ac.
	Condition 2: The vegetation has no retardance at a zero yield, which is the middle curve.  An example is grass.
	Condition 3: The vegetation has retardance at a zero yield, which is the upper curve.  An example is small grain like wheat.
	Retardance is described in two directions, on the contour and up and downhill.  The retardance relationship in the above figure is used to describe retardance on the contour.