U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2008–3004

February 2008

Introduction

The GIS Weasel is based on the 

ArcInfo Workstation GIS (ESRI, Inc., 

2001). The only required input is a raster 

elevation data set, commonly referred to as 

a digital elevation model (DEM), stored in 

an ESRI GRID format. The GIS Weasel 

creates a variety of products, most notably 

digital spatial data sets and text parameter 

files. All interfaces to the GIS Weasel 

tools are menu and map driven. The user 

does not need to have any knowledge of 

the command line operating procedures 

of a GIS. The system has three major 

processing phases: setup, delineation, and 

parameterization.

The GIS Weasel helps modelers develop maps of many kinds of geographic features.

An Overview of the GIS Weasel

By Roland J. Viger

The GIS Weasel is a product of the Modeling of Watershed Systems (MoWS) project (http://wwwbrr.

cr.usgs.gov/projects/SW_MoWS/), a part of the USGS National Research Program. This project seeks 

better understanding of hydrologic processes, their spatial and temporal variability, and their interactions 

across scales. These goals are met through the development of more robust computational models, as well 

as parameter estimation techniques.

The Setup Phase

In the setup phase, a variety of topographic surfaces are derived from the user-supplied 

DEM. The most important of these products are 

1. a version of the DEM useful for routing hydrologic flow, 

2. a surface of flow direction values,

3. a surface of flow accumulation values (measuring upstream contributing area), and

4. the area of interest (AOI).

The AOI represents the geographic extent of the modeling domain and serves to spatially 

limit almost subsequent outputs. The AOI is usually a watershed, although it can represent 

a groundwater system, an ecological unit, or any arbitrary land feature. The setup phase of 

a GIS Weasel processing session establishes many of the most commonly needed maps for 

delineating geographic features relevant to most ESMs.

This fact sheet provides a highlevel description of the GIS Weasel, a 

software system designed to aid users 

in preparing spatial information as 

input to lumped and distributed parameter environmental simulation models 

(ESMs). The GIS Weasel provides 

geographic information system (GIS) 

tools to help create maps of geographic 

features relevant to the application of a 

user’s ESM and to generate parameters 

from those maps. The operation of the 

GIS Weasel does not require a user to 

be a GIS expert, only that a user has an 

understanding of the spatial information requirements of the model. The GIS 

Weasel software system provides a GISbased graphical user interface (GUI), 

C programming language executables, 

and general utility scripts. The software 

will run on any computing platform 

where ArcInfo Workstation (version 8.1 

or later) and the GRID extension are 

accessible. The user controls the GIS 

Weasel by interacting with menus, maps, 

and tables.
The Delineation Phase

Once the setup phase is completed, 

the GIS Weasel provides the Tool Panel to 

allow the user to delineate different kinds of 

geographic features within the AOI. Here, 

the GIS Weasel functions like a toolbox; 

the user is able to select and control tools 

to create maps of features needed for their 

ESM. The user is able to control which geographic data sets are used as input to each 

tool, and thus has the option of integrating 

previously derived data into an ongoing 

GIS Weasel processing session. When a 

tool requires user input, the interaction has 

been simplified and is clearly explained 

through contextual menus.

The GIS Weasel is a general toolset 

for the application of ESMs. This means the 

user is responsible for correctly applying a 

tool or sequence of tools. The GIS Weasel 

has no ESM-specific knowledge, and therefore it will not force the usage of a specific 

geoprocessing methodology.

There are several tools that have been 

developed in support of specific ESMs, 

such as the Precipitation Runoff Modeling 

System (PRMS) (Leavesley and others, 

1983) and TOPMODEL (Beven and others, 

1995) watershed models. “Plug-ins” are 

available for more comprehensive support 

of selected ESMs. Plug-ins range in content 

from ESM-specific instructions to whole 

sets of software that can be used in conjunction with the GIS Weasel. The development and use of plug-ins are documented 

in the GIS Weasel user’s manual (Viger and 

Leavesley, 2007).

The Parameterization Phase

After the user has created maps of 

the different geographic features to be 

modeled, information can be generated 

about each map to create a set of ESM 

parameters. The GIS Weasel contains a 

library of over two hundred parameterization routines. The user can apply any of the 

routines to most raster maps, although the 

relevancy of the output from a routine for 

a particular ESM needs to be determined 

by the user. While the delineation phase of 

a GIS Weasel processing session is highly 

interactive, the parameterization phase is 

highly automated. Once the user specifies 

the list of parameterization methods and the 

maps to which the methods will be applied, 

almost no further user input is required. The 

GIS Weasel will produce “parameter maps” 

showing the derived information. These 

maps are then processed by the GIS Weasel 

to produce simple ASCII files containing all 

the requested parameter information. There 

is an option in this process to produce an 

additional file that is specifically formatted 

for use with the Modular Modeling System 

(Leavesley and others, 1996; http://wwwbrr.

cr.usgs.gov/mms). Even if these formats are 

incompatible with the ESM, the ASCII files 

have been designed for easy reformatting 

by external tools such as Microsoft Excel, 

Microsoft Access, Python, AWK, or Perl.

Availability

The GIS Weasel is available from 

the web page http://wwwbrr.cr.usgs.gov/

weasel, along with a user’s manual, online 

training, email support, and other resources.

References Cited

Beven, K.J., Quinn, P.F., Romanowicz, R.J., 

Freer, J., Fisher, J.I., and Lamb, R., 1995, 

TOPMODEL and GRIDATB—A user’s 

guide to the distribution versions (95.02): 

Lancaster, United Kingdom, Lancaster 

University, Institute of Environmental 

and Biological Sciences, Centre for 

Research on Environmental Systems and 

Statistics, 31 p.

The GIS Weasel allows modelers to integrate different kinds of spatial data to generate 

many kinds of parameters.

ESRI, Inc., 2001, ArcInfo Workstation, Version 8.1 [software].

Leavesley, G.H., Lichty, R.W., Troutman, 

B.M., and Saindon, L.G., 1983, Precipitation-runoff modeling system—User’s 

manual: U.S. Geological Survey WaterResources Investigations 83-4238, 207 p.

Leavesley, G.H., Restrepo, P.J., Markstrom, 

S.L., Dixon, M., and Stannard, L.G., 

1996, The modular modeling system 

(MMS)—User’s manual (ver. 1.1): U.S. 

Geological Survey Open-File Report 

96-151, 142 p.

Viger, R.J., and Leavesley, G.H., 2007, The 

GIS Weasel user’s manual: U.S. Geological Survey Techniques and Methods, 

book 6, chap. B4, 201 p., http://pubs.

usgs.gov/tm/2007/06B04/.

Contact Information

Modeling of Watershed Systems Project

U.S. Geological Survey

MS 412, Box 25046 Denver Federal Center

Denver, CO 80225 

MoWS_help@usgs.gov