5.1.5  Overland Runoff

    Overland runoff is computed using the SCS Curve Number technique (SCS 1972, 1982; Kent 1973; USBR 1977).  The factors considered in this method include surface soil characteristics, vegetative cover, land use, antecedent moisture conditions, and geographic locale.  To apply this technique in the module, the following is assumed:

1. The monthly precipitation depth can be equally distributed among the total number of recorded precipitation events.
2. The number of precipitation events per month can be defined as the mean number of days with at least 0.254 mm (0.01 in.) of precipitation (as listed in the LCD).
3. There is no overland runoff when the adjusted average monthly temperature is equal to or below freezing (because precipitation is assumed to be stored on the land surface in the form of snow).
4. The method  is applicable, although snow may be covering the land surface.  This condition rarely occurs, because most snow is usually melted in the first month that the average temperature rises above freezing.

     Using the SCS Curve Number technique, the total monthly runoff from the waste site is estimated as follows:



where

d_roj  is the overland runoff for the j-th month (cm)
n_scs  is the SCS Curve Number used for runoff calculations (unitless)
n_pj  is the number of precipitation events during the j-th month (unitless).

A complete discussion reviewing the development of Equation 5.28 is presented in Whelan et al. (1987).