Hi! I have some questions about the Green-Ampt equation.
What does exactly the head (or suction head) mean in this equation? I get confused here and can't find a good explanation. I found one equation of head online, but not clear why it is expressed like this.
h = h0 + L + S
where h0 = depth of surface ponding (usually neglected)
L = depth of water already infiltrated
S = suction head at the wetting front
What is the ponding in this method? If normally the depth of surface ponding is negligible, why is Green-Ampt equation assumed that there is a ponding?
Replies
Here are the definitions
// Input: infil = ptr. to Green-Ampt infiltration object
// tstep = runoff time step (sec),
// irate = net "rainfall" rate to upper zone (ft/sec);
// = rainfall + snowmelt + runon, //(5.0.022 - LR)
// does not include ponded water (added on below)
// depth = depth of ponded water (ft).
// Output: returns infiltration rate (ft/sec)
// Purpose: computes Green-Ampt infiltration for a subcatchment.
//
// Definition of variables:
// IMD = initial soil moisture deficit at start of current rain event
// (void volume / total volume)
// IMDmax = max. IMD available (ft/ft)
// Ks = saturated hyd. conductivity (ft/sec)
// S = capillary suction head (ft)
// F = cumulative event infiltration at start of time interval (ft)
// F2 = cumulative infiltration at end of time interval (ft)
// Fs = infiltration volume needed to saturate surface (ft)
// Fmax = max. allowable infiltration (ft)
// T = cumulative event duration (sec)
// Tmax = max. discrete event duration (sec)
// L = depth of upper soil zone (ft)
// FU = current moisture content of upper zone (ft)
// FUmax = saturated moisture content of upper zone (ft)
// DF = upper zone moisture depeletion factor (1/sec)
// DV = moisture depletion in upper zone (ft)
//
// f = infiltration rate (ft/sec)
// ivol = total volume of water on surface (ft)
// ts = remainder of time step after surface becomes saturated (sec)
// iv2 = available surface water volume over time step (ft) //(5.0.021 - LR)
// c1, c2 = terms of the implicit Green-Ampt equation
{
double F = infil->F;
double F2;
double DF;
double DV;
double Fs;
double f;
double ivol, iv2;
double ts, c1, c2;
// --- add ponded water onto potential infiltration
irate += depth / tstep;
if ( irate < ZERO ) irate = 0.0; //(5.0.021 - LR)
ivol = irate * tstep;
// --- add ponded water head to suction head //(5.0.019 - LR)
c1 = (infil->S + depth) * infil->IMD; //(5.0.019 - LR)
Hi Bob, thank you very much for your answers, but I still have no idea what the soil suction head is. Could you please tell me the definition of it?
Sorry, I must not be understanding your questions about GA
Here are a few other sources
http://www.hydrology.bee.cornell.edu/BEE3710Handouts/GreenAmpt.pdf
http://www.alanasmith.com/theory-Calculating-Effective-Rainfall-The...
http://www.water-research.net/Waterlibrary/Stormwater/greenamp.pdf
Infiltration.ppt
Subject: How Does the Green Ampt Initial Moisture Defiict Work in InfoSWMM and SWMM 5?
This graph shows the values of the internal SWMM 5 parameters for Green Ampt Infiltration for the pervious area of a Subcatchment during a simulation. The parameters are:
· Soil Moisture = IMD Max – (FUMax – FU)/Upper Soil Zone Depth
· FU or current moisture content of the upper zone of the of the soil
· FUMax which is the saturated moisture content of the upper zone in feet and stays constant during the simulation
· IMD Max is the user defined Initial soil moisture deficit and is a fraction
Figure 1. How Soil Moisture changes over time.
Figure 2. Soil Moisture and IMD are related – the Soil Moisture has a maximum of IMDMax.
Subject: How Does Green Ampt Cumulative Event Infiltration work in SWMM 5?
This graph shows the values of the internal SWMM 5 parameters for Green Ampt Infiltration for the pervious area of a Subcatchment during a simulation. The parameters are:
· F or FTOT which is the cumulative event infiltration at the start of a time interval in the internal units of feet in SWMM 5,
· FU or current moisture content of the upper zone of the of the soil
· FUMAX which is the saturated moisture content of the upper zone in feet and stays constant during the simulation
Figure 1. How FTOT, FU and F change over time
Figure 2. A closer look at how FTOT or F and FU Change over time in a Green Ampt Pervious Area Simulation.
Subject: How is Capillary Suction Head Used in SWMM 5 Green-Ampt?
How sensitive is the infiltration loss and rate to the capillary suction head parameter in the SWMM 5 Green-Ampt infiltration method. Figure 1 shows how the total infiltration loss and total loss rate vary as you change the suction head from 12 to 6 to 3 inches. Internally the suction head is used in infil.c of SWMM 5 by adding the suction head to the ponded water on the pervious area in the parameter c1 of the implicit Green-Ampt SWMM5 solution.
C1 = (Suction Head + Depth of Ponded Water) * IMD or Initial Moisture Deficit
Figure 1. The sensitivity of the total infiltration loss to the capillary suction head in a continuous simulation