The types of subsurface drainage that is represented are those that are deep drained by tiles or by deep lateral ditches.
	Main effect is how drainage system affects runoff, which is represented by inputting values for two hydrologic soil groups, one when no drainage system is present and the other is when the a highly efficient system is used.
	The fraction of the area relates to the fraction of the area where the soil loss is being computed that is treated with the drainage system. For example, a low value for the fraction drained would be entered where only wet spots are drained.