modules_GreenAmpt (03/31/10)

This module is defined in ClassGreenAmpt. It handles unfrozen soil infiltration.

Observations

• none

Variables

• infil (mm/int) - depth of infiltration in time step, Dt – expressed as an average depth of water on an HRU.
• cuminfil (mm) - cumulative infiltration - expressed as an average depth of water on an HRU.
• runoff (mm/int) - amount of daily runoff – expressed as an equivalent depth (m³/m²).
• cumrunoff (mm) - cummulative runoff.
• meltrunoff (mm/int) - amount of interval melt runoff. .
• cummeltrunoff (mm) - cumulative interval melt runoff.
• snowinfil (mm/int) - amount of interval infiltration – expressed as an equivalent depth (m³/m²).
• cumsnowinfil (mm) - cumulative interval infiltration

Parameters

• basin_area (km^2) - area of watershed.
• hru_area(km^2) - area of HRUs.
• soiltype () -  0 through 12 for water/sand/loamsand/sandloam/loam/siltloam/sasclloam/clayloam/siclloam/sandclay/siltclay/clay/pavement.
• soil_moist_max (mm) - maximum available water holding capacity of soil profile.
• soil_moist_init (mm) - initial value of available water in soil profile.

Variable Inputs

• net_rain (mm/int) (*) - depth of rain received by the ground surface in time step, Dt. Variable available from module intcp,  etc..
• snowmeltD (mm/d) (*)
• soil_moist (mm) - from module Soil, smbal, etc.

Notes on Green-Ampt.

The module is used when unlimited infiltration is not possible.

When the surface ponding is negligible,

f (mm/hr)  =  k*(y*Dq/F + 1),    (eq. 1.)

where:

k (mm/hr) = saturated hydraulic conductivity.

y (mm)  = capillary suction at the wetting front.

Dq = (qs -  qi). change in volumetric soil moisture content.

When ponded,

F1 (mm) = k*Dt + y*Dq * ln( (F1 + y*Dq)/(F0 + y*Dq)).       (eq. 2.)

where F0 (mm) = cumulative infiltation at the beginning of the time interval i.e. t,

f0 (mm/hr) =  infiltration rate at the beginning of the time interval i.e. t,

F1 (mm) = cumulative infiltration at the end of the time interval i.e. t+Dt,

f1(mm/hr) =  infiltration rate at the end of the time interval i.e. t+Dt,

It (mm/hr) = rainfall intensity during the interval.

The cumulative infiltration (Fp) at instant of ponding time is

Fp = k*y*Dq/(It - k).     (eq. 3.)

The portion of the interval, after which ponding occurs is Dt',

Dt' = (Fp - F0)/It.    (eq. 4.)

Flow Chart

Calculate the maximum infiltration rates,  f0 and f1 at the beginning and end of the time step using equation 1. using F1 = F0+Dt*It.

Case 1:  When f1 is >= rainfall intensity,  all the rainfall infiltrates with F1 = F0+Dt*It.

Case 2:  When the rainfall intensity > f0,  all the rainfall ponds and F is calculated using an iterative solution of equation 2.

Case 3:  When the rainfall intensity <= f0 but the rainfall intensity > f1,   ponding occurs during the interval and the solution consists of  solving for Fp using equation 3, then using equation 4 to solve the time into the step during which ponding occurs. Next, using equation 2 with Fp as F0 the cumulative infiltration F1 after the ponding time of (Dt - Dt') is calculated.

Assigned Soil Properties

All, except fcap are specified at 0% saturation from Chow pg 115.

Value for wilt is from Chow.

Values for por, fcap is from Floods in Canada

Soil curves of form K = Ks / (A*Psi^3 + 1 )