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 temperature and solar radiation. Ritchie (1972) outlines a procedure to compute soil and plant
evaporation separately. The Penman-Monteith equation requires air temperature, solar radiation, wind
speed, and relative humidity. Additional methods for PET estimation are available from Dunne and
Leopold (1978) and Schwab et al.(1993).
1.1.1.3 Infiltration and Runoff
The SCS Curve Number Equation (CNE) (Ogrosky and Mockus, 1964; Mockus, 1972) may
be used in determining runoff volume for source areas. The CNE may also be used in combination with
the loading function for determining dissolved-phase nutrient loads in rural and urban runoff (Section
1.1.1.9 and 1.1.2.2-1.1.2.3).
The CNE is a research-based method for determining watershed runoff volume. The essential
features of the equation are illustrated in Figure 1.1.1. SCS1 (1986) provides guidance on use of the
CNE.
R
Q
Rainfall
F
Ia
Infiltration Curve
Time
Figure 1.1.1. Relationship Between Precipitation, Runoff and Retention in the
SCS Curve Number Equation (after McCuen, 1982)
CNE variables as defined by McCuen (1982)
R = rainfall (in)
F = actual retention, the difference between precipitation volume and runoff volume
Q = runoff (in) from source area k
S = potential maximum retention
1
Urban Hydrology for Small Watersheds (Technical Release 55, PB # 87-10158j0/AS A08) is available through:
National Technical Information Center
5285 Port Royal Road
Springfield, VA 22161
(703) 487-4650
1.1-2
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