The variables Ia and F represent evapotranspiration and ground water portions of the mass balance.
Empirically Ia was found to be equal to 0.2 times the potential maximum retention. For a complete
derivation of the CNE see McCuen (1982).
The CNE is given below to estimate runoff volume (Q) for each source area. A snowmelt term (M)
(Haith and Tubbs, 1981) is added to the original CNE to yield:
(R + M - 0.2S)2
(R + M + 0.8S)
S = 1000 - 10
SCS Curve Number Determination
The magnitude of the CN is a function of the hydrologic condition of each source area. Characteristics
that determine the CN are cover complex classification, soil group classification and antecedent
moisture condition. Values of the CN for rural lands are summarized in Table 1.1.1. A complete listing
of CNs along with guidance on their selection is given in (SCS, 1986).
Cover Complex. The cover complex determination is composed of a sequential assessment of
cover type, treatment, and hydrologic condition.
Cover type includes such land use categories as fallow, row crops, small grains,
legumes, rotation meadow, pasture, brush, woods or farmsteads. For the first four
cropping cover types, combinations of treatments describe the land use.
Treatment aspect of the cover complex considers the percentage area covered with
crop residue and the type of tillage system or combination.
The hydrologic condition (Good, Fair or Poor) is based on the following factors
affecting infiltration and runoff (SCS, 1986).
-density and canopy of vegetative areas
-amount of year-round cover
-amount of grass or close-seeded legumes in rotations
-percentage of residue cover on land surface (good>20%)