Long-Term Hydrological Impact Analysis (L-THIA)
the Model’s Purpose and Capabilities?
L-THIA was developed as a
straightforward analysis tool to provide estimates of changes in runoff,
recharge and nonpoint source pollution resulting from past or proposed land use
changes. It gives long-term average annual runoff for a land use configuration,
based on actual long-term climate data for that area. By using many years of
climate data in the analysis, L-THIA focuses on the average impact, rather than
an extreme year or storm.
L-THIA results are intended
to provide insight into the relative hydrologic impacts of different land use
scenarios. The results can be used to generate community awareness of potential
long-term problems and to support physical planning aimed at minimizing
disturbance of critical areas. It is an ideal tool to assist in the evaluation
of potential effects of land use change and to identify the best location of a
particular land use so as to have minimum impact on the natural environment of
the area. Recent concern over urban
sprawl has focused on several land use change issues, including the failure to
account for hydrologic aspects of land use change that can result in flooding,
stream degradation, erosion, and loss of groundwater supply. L-THIA was
developed to provide a quick, accessible tool to use in assessing the long-term
impacts of land use change.
L-THIA results can be used to aid land use planners in a variety of ways. For instance, a planner may decide to change the land use based on soil type, to minimize impact in a given area. That is because the same land use located on different hydrologic soil types has different impacts. Also, since the amount of runoff generated by different land uses is a function of the hydrologic soil type and the land use, relocating land uses based on the hydrologic soil type can in some cases significantly reduce the long-term impact of the development.
L-THIA is currently
available in three forms:
WWW is a spreadsheet version that models runoff and NPS pollution changes;
GIS is a set of Avenue scripts that automate the process of runoff impact modeling
within ArcView; and
GIS WWW allows interactive mapping of an area of interest with a custom java
interface within a web browser.
Are the Model’s Key Features?
accessible tool to use in assessing the long-term impacts of land use change;
on computations of daily runoff obtained from long term climate records, soil
data, curve number (CN) value and land use of the area;
surface runoff averages;
nonpoint source pollution;
a numeric description of runoff, as well as charts ands maps for output; and
hydrologic impact of land usage.
Are the Model’s Strengths and Limitations?
not require detailed data input –
required data is readily available in most planning settings;
available in GIS version; and
friendly and Internet-accessible;
only to areas where the curve number (CN) method is used;
not analyze parameters such as political, economical, and infrastructural;
permafrost, variation in moisture conditions neglected in calculations; and
is not intended for storm water drainage system planning.
Environmental Effects Can This Model Address in Terms of Changing Land-Use
Water Quality (ground and surface)
Are the Primary Land Uses Addressed by This Model?
(high or low density for basic run and by 1/8, 1/4, 1/3, ˝ or 2 acre parcels
for detailed run)
Types of Policy Questions Might Be Answered By This Model?
is the best place, in terms of impacts to water quality, to site a commercial
facility; in the urban core or in an urban fringe location?
will the impact daily runoff be over a 30-year time series in a given land
a wetland area is to be turned into a residential neighborhood, what effect
will this development have on the hydrologic regime of the area?
Model Accommodate a Community the Size of Mine and My Data?
The model is best able to model areas
that range from a few acres in size to tens or even hundreds of square miles in
size. Note however, that the model estimates surface runoff and associated
nonpoint source pollutants. For larger areas processes such as groundwater
contribution to stream flow can become a significant portion of the total flow
in streams. The L-THIA model would not estimate this portion of stream flow but
only that portion attributed to surface runoff.
L-THIA has ben applied throughout the
continental US. Data requirements are shown in a section below.
How Far into the Future Will
I Be Able To Forecast Using this Model?
The model estimates runoff and nonpoint source pollution for land
use change data provided by the user. Therefore, there is no limitation on how
far into the future the model can forecast. Note that 30 years of historical
daily rainfall data is used in such analyses. If a long-term change in rainfall
is anticipated, the model is not applicable in its current form.
What Types of
Information/Data Is Needed to Run This Model?
Choose county and state from pull down pick lists
Land Use Data
Choose from pull down pick list (see categories listed above)
Hydrologic soil groups
Choose from pull down pick list; General soil group maps by state are provided at the L-THIA web site
Area of land (numeric)
Sample Input Screen for L-THIA
What Types of Information/Data Does This Model
The outputs of L-THIA
provide the user with a broad array of information in both table and graphic
form. To determine the best possible
land use scenario, the user can compare outputs from several runs of L-THIA, in
addition to comparing results to constraints or limitations set forth by local,
state, and/or federal requirements.
L-THIA provides ample
information to help the user make an informed decision on how to best utilize a
given land area, based on estimates of changes in runoff, recharge and nonpoint
Average annual runoff volumes
Average annual runoff depth
Average annual runoff volume
Average annual runoff depth
Land use area
Average annual runoff volume
Annual variations in runoff
Time series graph
% of exceedence for runoff
Time series graph
Non-point source pollution for the following
- nitrogen -
- suspended solids -
- copper -
- cadmium -
- nickel -
- COD -
oil and grease
- fecal coliform - fecal streps
NPS pollutants map, if applicable
Output from L-THIA
How Can I Obtain This Model?
L-THIA is available in
three versions on the web site:
1. LTHIA WWW in a spreadsheet version
that models runoff and NPS pollution changes
2. L-THIA GIS is available as an ArcView
download; also requires Spatial Analyst extension.
3. L-THIA GIS WWW allows interactive
mapping of the area of interest with a custom java interface within a web
Must I Know to Install and Use This Model?
Required Computer Skills:
utilize the WWW version, one needs basic understanding of the Internet. To utilize the GIS versions of L-THIA, one
must be well-versed in using ArcView.
Required Technical Expertise:
knowledge of hydrologic data is useful.
Availability of Technical Assistance:
Technical assistance is available electronically by contacting Professor
Bernard Engel at Purdue University – .
Type of Equipment/Software Will I Need to Run This Model?
Type of Computer:
Minimum CPU (MHz):
Minimum Disk Space/RAM:
Program Compiler Needed?
Data Management Tools:
Statistical Software Needed?
GIS Software Needed?
PC for Internet access
Color monitor useful
ArcView for L-THIA GIS (only for the GIS version)
Will It Cost To Obtain and Maintain This Model?
Purchase Costs: 0
Operating Costs: 0
Maintenance Costs: 0
Training Costs: 0
Can I Find More Information on This Model?
web site includes a large amount of information on background information, how
to run the model, technical information, interpreting model results,
documentation, case studies, and more.
Documentation is available
on the web site. Selected references are listed below:
Bhaduri, B., 1998. A GIS-based model to assess
the long-term impacts of land use change on hydrology and nonpoint source
pollution. Unpublished Ph.D. Thesis. West Lafayette, IN: Purdue University,
Department of Earth and Atmospheric Sciences.
Engel, B.A., (2001 Update), L-THIA NPS (Long Term
Hydrologic Impact Assessment and Non Point Source Pollutant Model, Version
2.1A), Purdue University and U.S. Environmental Protection Agency.
Grove, M. 1997.
Development and Application of a GIS-Based Model For Assessing the Long-Term
Hydrologic Impacts of Land Use Change. Unpublished M.S. Thesis. West Lafayette,
IN: Purdue University, Department of Earth and Atmospheric Sciences.
Lim, K. J., Engel, B. A., Kim, Y.,
and Harbor, J. (1999). "Development of the Long Term Hydrologic Impact
Assessment (L-THIA) WWW Systems." 10th International Soil Conservation,
Purdue University, West Lafayette, Indiana. L-THIA WWW System.
http://danpatch.ecn.purdue.edu/~sprawl/LTHIA2 last accessed July 28, 1999.
McClintock, K. A.,
Harbor, J. M. and Wilson, T. P. 1995. Assessing the Hydrological Impact of
Land-Use in Wetland Watersheds: A Case Study from Northern Ohio, USA. In
Geomorphology and Land Management in a Changing Environment, eds. D. F. M.
McGregor and D. A. Thompson. Chichester, UK: John Wiley and Sons, 107-119.
Additional References/Case Studies: (All are available on the web site)
Impact Analysis of Two Land Use Change Scenarios in the Cuppy McClure
Watershed, West Lafayette, Indiana.
Comparative Soil Conservation Service Curve Number (SCS CN) and L-THIA Based
Method Estimation of Runoff for Historical Land Use Changes.
Assessing the Long-Term Hydrologic Impacts of Land Use Change in the
Wildcat Creek watershed using web-based GIS and spreadsheet version of L-THIA :
a users perspective.
Selected application sites
are listed below. Additional
information is available through the documentation listed above and on the web
Ohio: Assessing the hydrological impact of land use change on groundwater
recharge and of suburbanization on runoff into a wetland.
Town planning and coastal management.
Indiana: Examining the hydrologic implications of future land use change for an
# Indianapolis: Examining the impact of historical land use change, using remote-sensing based land use maps, focusing on spatial patterns of change within the watershed.
The curve number (CN) is developed from real-world data by the United States Department of Agriculture, Soil Conservation Service. The CN is used in an empirically based formula to determine how much of a given rainfall event becomes surface runoff. See Engel, L-THIA NPS Version 2.1A for additional information.