FLOOD11629 ‎(2)‎

• County: Statewide
• Basin: Statewide
• Title: Hydrologic Basin Response - Phase II B - Parameter Estimation Guidelines
• Date: 3/1/2007
• Floodplain - Doc Type: Floodplain Report/Masterplan

SECTION 3: RAINFALL LOSSES
Green & Amnt Infiltration Eauation
introduction: This model, first developed in 1911 by W.H. Green and G.A. Ampt, has
sincE: the early 1970's received increased interest for estimating rainfall infiltration losses. A
sound and concise explanation of the Green and Ampt equation is provided by Bedient and
HubE:r (1988). Use of the Green and Ampt equation as coded in HEC-1 involves the
simulation of rainfall loss as a two phase process, as illustrated in Figure 3. The first phase
is th~~ simulation of the surface retention loss. This loss is called the initial loss (IA) in HEC-
1. C~uring this first phase, all rainfall is lost (zero rainfall excess generated) during the
peric-d from the start of rainfall up to the time that the accumulated rainfall equals the value
of Ip,. It is assumed, for modeling purposes that no infiltration of rainfall occurs during the
first phase. The second phase of the rainfall loss process is the infiltration of rainfall into
the :;oil matrix. For modeling purposes, the infiltration begins immediately after the surface
retention loss (IA) is completely satisfied, as illustrated in Figure 3.
A~plicabili : The Green and Ampt equation should be applied for the one percent storm
(100-year) and more frequent storms. This method is also suitable for use with less
frequent storms but the parameters IA and volumetric storage of infiltrated rainfall may not
be significant for large rainfall amounts associated with storms less frequent than the one
percent storm.
Metlhod Description: The first phase of the rainfall loss process is simulated with the IA
parai~neter. Values of IA for various land surfaces in Colorado are shown in Table 8 for
natu~~al areas and Table 9 for developed areas. The second phase of the rainfall loss
estimation process is simulated with the Green and Ampt equation and an estimate of
watershed impervious area (RTIMP). The three Green and Ampt equation infiltration
paraireters as coded in HEC-1 are:
hydraulic conductivity at natural saturation (XKSA~;
wetting front capillary suction (PSIF); and
3. volumetric soil moisture deficit at the start of rainfall (DTHETA).
19 March 2007 2$