A Water Balance Based Version of the
Soil & Water Assessment
SWAT-WB is a slightly modified version of the
USDA's Soil & Water Assessment Tool watershed model. Instead
of using the traditional Curve Number method to model surface
runoff, SWAT-WB uses a physically based soil water balance.
This new method results in a model that predicts runoff that
is generated from saturated areas, contrary to the traditional
The impetus for creating a version of SWAT free of the Curve
Number methodology was born out of the Soil and Water Lab's work in
two very different watersheds: New York City's drinking water
supply watersheds in the Catskill Mountains, and the Blue Nile
watershed in the Ethiopian Highlands.
Differences between SWAT and SWAT-WB
The main difference between the original version
of SWAT and this new version is the fact that SWAT-WB does not rely
upon the Curve Number method to determine daily surface runoff
volumes. There are numerous justifications for using a soil
water balance, rather than the Curve Number, to determine runoff
The Curve Number method, although widely used,
was not designed for use in temporal watershed models.
While it can be successfully used as a calibration paramter for
models such as SWAT, the original methodology was developed for
use in determining streamflow from single events, not on a
The Curve Number method can be
modified to account for different runoff generating processes,
however, as it was designed (and implemented into SWAT) the
method is simply a way to match observed rainfall data to
predicted streamflows. The Curve Number method does not
indicate how the water in the stream originated within the
our descripition of runoff generating processes for a full description).
By using a soil water balance, SWAT-WB determines how much
rainfall is required to saturate portions of the watershed, at
which point surface runoff will be generated.
The original version of SWAT does already
offer a Curve Number alternative, the Green-Ampt routine.
Green-Ampt is a physically-based method, however it requires
hard to come by data (sub-daily precipitation records) and
models runoff strictly from infiltration-excess conditions (i.e.
rain is falling faster than it can be absorbed into the soil).
While more physically based than the Curve Number, Green-Ampt is
not appropriate for use in watersheds that have significant
portions that are saturated. Furthermore, the required
sub-daily precipitation data presents a large obstacle in many
minimally instrumented watersheds. By using SWAT-WB, no
extra data is required (whatever is needed for the Curve Number
works for SWAT-WB), and runoff is modeled for saturation-excess
Although it has been used to model
watersheds all over the world, the Curve Number method was
developed using only data from a very small portion of the
United States. On the other hand, physically-based water
balance models have proven to be simple, yet accurate, tools in
predicting runoff and streamflows in virtually every climate
around the world. Using these time-tested water balance
models within the SWAT framework results in a version of SWAT
that can be successfully applied anywhere in the world,
particularly areas in which saturation processes tend to govern
surface runoff flows.
As can be seen from the above points, the novelty
of SWAT-WB is that it models saturation-excess generated surface
runoff with minimal changes to the SWAT data input requirements.
SWAT-WB, being based on a widely applicable soil water balance, is
more appropriate for use in regions around the world where the Curve
Number might not be the best choice.
With more and more researchers around the
world developing SWAT models, we are confident that SWAT-WB provides
an easy to use, physically based, accurate alternative to the Curve
Number based version of SWAT.
SWAT-WB would not be a very compelling
alternative to SWAT2005 if it did not produce better results.
However, that is not the case; SWAT-WB returned noticeably more
accurate streamflow predictions for the two watersheds that we
tested it on. Fsull description of these test watersheds
and the model results are provided in the official SWAT-WB paper
(available in the downloads section of this site), but the following
tables summarize the improvements in model statistics when SWAT-WB
is compared to SWAT2005.
Model Results for the Gumera Watershed in
the Ethiopian Highlands
Model Results for the Townbrook Watershed
in New York State
How to Use SWAT-WB
Once a SWAT project has been developed by the
user (either using the GIS interface or by creating the text input
files by hand), very few changes must be made in order to use
SWAT-WB. A new one line text file must be created for each soil type
present in the watershed. These text files will be placed,
along with the swatWB.exe executable file, into the project folder
which contains all text files developed by the GIS interface.
The program, when run from this folder, will produce the same output
files as the original SWAT program.
Resources and Downloads
At the top right of this page you will find files available for download. The first file contains everything required to run SWAT-WB, whereas the other files contain help documents as well as the source code which was used to compile SWAT-WB.
This file is
an example of the text file that must be created in order to use
SWAT-WB. This text file will be read by the program and is
similar to the other text files that are required to run the
original version of SWAT. This file should be opened with
a text editing program, such as Notepad.
describes exactly how to use SWAT-WB
A brief discussion of the equations used. For a full discussion
(and a complete list of references), please download the
official SWAT-WB paper.
There is no need to download the Fortran code in
order for SWAT-WB to run, however if you are working with your own
customized SWAT program, feel free to examine our code and
incorporate it into your own work if you so desire.
For information regarding the GIS interface for
SWAT, information regarding formatting of other text input files,
or general information regarding the equations used within SWAT,
please refer to the official
Any questions or problems regarding SWAT should
first be directed to the
User Group. If the question is specifically regarding
SWAT-WB please contact someone from the Soil and Water Lab listed
Any questions or comments can be directed to
Dan Fuka, or