Model help:TopoFlow-Infiltration-Green-Ampt

TopoFlow-Infiltration-Green-Ampt

This module is the infiltration process component (Green-Ampt method) for a D8-based, spatial hydrologic model

Model introduction

This process component is part of a spatially-distributed hydrologic model called TopoFlow, but it can now be used as a stand-alone model.

Model parameters

Parameter	Description	Unit
Component status	Enabled / Disabled	[-]
Input directory	The location of the input files	[-]
Output directory	The location for the output files	[-]
Site prefix		[-]
Case prefix		[-]
Number of steps	Number of simulation steps	[-]
Number of soil layers	number of soil layers	[-]
Time step	timestep for infiltration process	[sec]

Parameter	Description	Unit
K_sat type	Allowed input types: Scalar / Grid /Time_Series /Grid_Sequence	[-]
K_sat	sat. hydraulic conductivity	[m / s]
K_init type	Allowed input types: Scalar / Grid /Time_Series /Grid_Sequence	[-]
K_init	init. hydraulic conductivity	[m / s]
theta_sat type	Allowed input types: Scalar / Grid /Time_Series /Grid_Sequence	[-]
theta_sat	sat. soil water content	[-]
theta_init type	Allowed input types: Scalar / Grid /Time_Series /Grid_Sequence	[-]
theta_init	init. soil water content	[deg_C]
G type	Allowed input types: Scalar / Grid /Time_Series /Grid_Sequence	[-]
G	capillary length scale	[-]
Closest soil_type	closest standard soil type	[-]

Parameter	Description	Unit
Save grid timestep	time interval between saved grids	[sec]
Save v0 grids toggle	option to save grids of infil. rate at surf	[-]
Save v0 grids file	filename for grid stack of v0	[m / s]
Save I grids toggle	option to save grids of cumul. infil. depth	[-]
Save I grids file	file name for grid stack of I	[m ]
Save pixels timestep	time interval between time series values	[sec]
Save v0 pixels toggle	option to save time series of infil. rate	[-]
Save v0 pixels file	file name for time series of v0	[m / s]
Save I pixels toggle	option to save time series of cumul.infil. rate	[-]
Save I pixels file	file name for time series of I	[m]

Uses ports

• Meteorology
• Snow (Snowmelt)
• Evap (Evaporation)
• Satzone (Subsurface flow in saturated zone)
• Channels (surface water flow in a network of channels)

Provides ports

• Infil (Infiltration)
• Configure (tabbed dialog GUI to change settings)
• Run (only if used as the Driver)

Main equations

Infiltrability (max infiltration rate)

[math]\displaystyle{ \begin{align} f_{c} &= K_{i} + [\left (K_{s} - K_{i} \right) \ast \left ( F + J \right) / F ] \\ &= K_{s} + \left ( J / F \right) \ast \left ( K_{s} - K_{i}\right) \end{align} }[/math]

(1)

[math]\displaystyle{ J = G \ast \left ( \theta_{s} - \theta_{i} \right) }[/math]

(2)

Infiltration rate at surface

[math]\displaystyle{ v_{0}=\left\{\begin{matrix} min \left (\left ( P + M \right), f_{c} \right) & K_{s} \lt \left ( P + M \right ) \\ \left ( P + M \right) & K_{s} \gt \left ( P + M \right ) \end{matrix}\right. }[/math]

(3)

Cumulative infiltration depth from times 0 to t (vs. I' Smith (2002))

[math]\displaystyle{ F= \int v_{0}\left ( t \right) dt }[/math]

(4)

Nomenclature

Symbol	Description	Unit
K_s	saturated hydraulic conductivity	m / s
K_i	initial hydraulic conductivity (typically much less than K_s)	m / s
θ_s	soil water content at ψ = 0 (typically set to the porosity, ψ)	-
θ_i	initial soil water content	-
G	capillary length scale	m
P	precipitation rate	mm / sec
M	snowmelt rate	mm / sec
f_c	infiltrability or max infiltration rate	mm / sec
F	cumulative infiltration depth	m
v0	infiltration rate at surface	mm / sec
t	time	s
J	a user-defined variable	m

Notes

Notes on Input Parameters

For a detailed discussion of these variables and infiltration theory, see the References below.

For each variable, you may choose from the droplist of data types. For the "Scalar" data type, enter a numeric value with the units indicated in the dialog. For the other data types, enter a filename. Values in files must also use the indicated units.

Single grids and grid sequences are assumed to be stored as RTG and RTS files, respectively. Time series are assumed to be stored as text files, with one value per line. For a time series or grid sequence, the time between values must coincide with the timestep provided.

Choosing an entry from the droplist labeled "Closest standard soil type" will change the values in the dialog to tabulated values for the selected soil type. However, these values were determined from plot-scale measurements and are unlikely to be appropriate for large grid cells. For large grid cells, some type of upscaling is typically required.

Notes on the Equations

All variables and their units can be seen by expanding the Nomenclature section above.

t_p = time of ponding [minutes] = the time when the soil becomes saturated at the surface, after which v₀=f_c or v₀=0 (after surface inputs stop). If (P + M) < K_s, then ponding cannot occur.

The equation for v₀ implies that v₀ = 0 whenever (P + M) = 0, since f_c > 0.

If (P + M) > K_s, then after a sufficiently long time F will become large, (F+J)/F will approach a value of 1 and f_c will decrease asymptotically to K_s.

The definition of F implies that dF/dt = v₀. Here, F is the quantity that Smith (2002) refers to as I', but that doesn't display well in HTML.

The current implementation is meant for single events only since F is only reset to 0 at the start of each model run.

In the case where (P + M) is uniform in time, it is possible to compute the surface soil moisture using a relationship of the form, K = K(θ). This type of relationship, found empirically, is called a soil characteristic function and is also used in conjunction with the 1D Richards' equation method of infiltration. See Smith (2002, pp. 81-85) for details.

The Green-Ampt and Smith-Parlange methods for modeling infiltration are based on the infiltrability-depth approximation or IDA, which uses the cumulative infiltrated depth as a "replacement" for time. For details, see Smith (2002, pp. 71-73). These methods are not well-suited to modeling redistribution between events or drying of surface layers by evaporation. They are best used for single events.

If there is standing water of depth, d, at the surface, then G can/should be replaced by (G + d) in the equation for J. This isn't done in the current version, since channels typically only occupy a small fraction of a grid cell and water depth on hillslopes is typically very small.

This model can be viewed as a special case of the Smith-Parlange 3-parameter model in which γ=0.

More information on how soil is modeled in TopoFlow along with published soil property tables can be found on the soil properties page.

Examples

An example run with input parameters, BLD files, as well as a figure / movie of the output

Follow the next steps to include images / movies of simulations:

Upload file: https://csdms.colorado.edu/wiki/Special:Upload
Create link to the file on your page: [[Image:<file name>]].

Developer(s)

Scott Peckham

References

Smith, R.E. (2002) Infiltration Theory for Hydrologic Applications, Water Resources Monograph 15, AGU.

Links

Related Help Pages

Model Metadata