RMAXINF

I have been trying to find any documentation that describes what the
maximum infiltration value (RMAXINF) when using the Integrated Horton
Equation With Maximum Infiltration (INFILM=2) will perform.

I assume it is used for continuous simulations to set a maximum amount
of infiltration, thus the regeneration would start at the end of the
rain event. Is this correct, or do you know where I can find
documentation on this?

Thanks
Jeff Rogers
RogersJW@CDM.COM

Jeff,

In cases such as this (poor documentation) it is best to go to the
source - the code itself. The programming for RMAXINF is in subroutine
Wshed. Look in particular at the code for totinf and rmaxinf for the
explanation of the regeneration.

Regards,
Bob Dickinson
DickinsonRE@CDM.COM

Jeff,

RMAXINF was coded into the two infiltration methods as a simple means of
modeling areas where the water table is close to the ground surface.
When the total volume infiltrated equals RMXAINF (i.e., when enough
infiltration has occurred to cause the water table to rise to the
surface), the infiltration rate is set to zero. The RMAXINF option can
be used for single event simulation with Horton or Green-Ampt. However,
I believe that RMAXINF can only be used for continuous simulation when
using Horton, since this is the only method for which regeneration of
the parameter has been coded. RMAXINF begins when surface storage on the
pervious plane is depleted and uses the same exponential recovery that
is used to regenerate the maximum Horton infiltration rate.

Greater control of modeling a rising water table can be obtained using
the groundwater routine in Runoff, but the input requirements are much
more extensive.

Brett Cunningham
BCunningham@JEA.NET

To follow up on Bob's suggestion, maximum infiltration capacity is
regenerated when there is no rain or standing water on the surface. It
is regenerated at the same first- order rate as used to regenerate
Horton infiltration capacity, namely, REGEN*DECAY. The program keeps
track of parameter TOTINF, which is the cumulative infiltration that has
a max of RMAXINF. TOTINF "decays" during dry time steps at the rate
REGEN*DECAY until it reaches 0.01*RMAXINF at which point it is set to
zero.

Thus, regeneration occurs at the same rate at which Horton regeneration
is occuring. This is less than satisfactory, because regeneration is not
a function of ET, but it does regenerate.

The value of RMAXINF itself could be set from the SCS storage variable S
= 1000/CN -10, if you are a curve number believer. Better is to have
some knowledge of soil moisture, theta, and depth to the shallow water
table, d. Then RMAXINF = d*(porosity - theta). (Porosity-theta) is given
in generalized values for soils as "available water capacity, in./in."
on SCS (or NRCS) soil survey interpretation sheets or in tables of
"estimated soil properties" in county soil surveys.

Wayne
Wayne.Huber@ORST.EDU

Procedures for estimating RMAXINF from soil survey data are described in
http://www.stormwater-resources.com/Library/087MSoil.doc

Regards,
Mike Gregory
GregoryMA@CDM.COM

Mike, you are quite correct. Thanks for the clarification. This should
not be used for RMAXINF.

Wayne

> Just to clarify your statement below, I believe the "available water
capacity" quoted in county soil surveys is the plant-available water
(i.e., capillary water held in tension between the wilting point and the
field capacity). What we want is the drainable porosity which is held
between field capacity and saturation.
> Regards,
> mike g.