Long-Term Hydrological Impact Analysis (L-THIA)


Bernard Engel, Ph.D.                                                  Jon Harbor, Ph.D.

Agricultural and Biological Engineering                                 Earth and Atmospheric Sciences

ABE Building Room 309, Purdue University                CIVL Building, Purdue University

W. Lafayette, IN 47907-1146                                       W. Lafayette, IN 47907

765-494-1198                                                              765-494-9610





What Is 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:

1.                   L-THIA WWW is a spreadsheet version that models runoff and NPS pollution changes;

2.                   L-THIA GIS is a set of Avenue scripts that automate the process of runoff impact modeling within ArcView; and

3.                   L-THIA GIS WWW allows interactive mapping of an area of interest with a custom java interface within a web browser.


What Are the Model’s Key Features?

#                                            Quick, accessible tool to use in assessing the long-term impacts of land use change;

#                                            Based on computations of daily runoff obtained from long term climate records, soil data, curve number (CN) value and land use of the area;

#                                            Calculates surface runoff averages;

#                                            Calculates nonpoint source pollution;

#                                            Provides a numeric description of runoff, as well as charts ands maps for output; and

#                                            Determines hydrologic impact of land usage. 


What Are the Model’s Strengths and Limitations?



#                                            Does not require detailed data input –  required data is readily available in most planning settings;

#                                            Is available in GIS version; and

#                                            User friendly and Internet-accessible;




#                                            Applies only to areas where the curve number (CN) method[1] is used;

#                                            Does not analyze parameters such as political, economical, and infrastructural;

#                                            Snow, permafrost, variation in moisture conditions neglected in calculations; and

#                                            L-THIA is not intended for storm water drainage system planning.


Which Environmental Effects Can This Model Address in Terms of Changing Land-Use Patterns?



Water Quality (ground and surface)


Air Quality






Specific Species/Organisms






Sensitive Waterbodies





What Are the Primary Land Uses Addressed by This Model?


#                                            Commercial

#                                            Industrial

#                                            Residential (high or low density for basic run and by 1/8, 1/4, 1/3, ˝ or 2 acre parcels for detailed run)

#                                            Open Spaces

#                                            Parking/Paved

#                                            Water/Wetland

#                                            Grass/Pasture

#                                            Forest

#                                            Agriculture

#                                            Percent impervious area

#                                            Custom


What Types of Policy Questions Might Be Answered By This Model?


#                                            Where 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?

#                                            What will the impact daily runoff be over a 30-year time series in a given land area?

#                                            If a wetland area is to be turned into a residential neighborhood, what effect will this development have on the hydrologic regime of the area?  


Can the 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?




Location Data

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 Produce?



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 source pollution.




Average annual runoff volumes


Average annual runoff depth


Average annual runoff volume

Bar chart

Average annual runoff depth

Bar chart

Runoff depth

Bar chart

Land use area

Bar chart

Land use

Pie chart

Average annual runoff volume

Pie chart

Annual variations in runoff

Time series graph

% of exceedence for runoff

Time series graph

Non-point source pollution for the following pollutants:

- nitrogen                               - phosphorus

- suspended solids              - lead

- copper                                 - zinc

- cadmium                              - chromium

- nickel                                   - BOD

- COD                                     - oil and grease

- fecal coliform                      - fecal streps

NPS pollutants map, if applicable



Example 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 browser.


What Must I Know to Install and Use This Model?


Required Computer Skills:


To 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:


Some knowledge of hydrologic data is useful.


Availability of Technical Assistance: Technical assistance is available electronically by contacting Professor Bernard Engel at Purdue University –  .


What Type of Equipment/Software Will I Need to Run This Model?


Type of Computer:

Minimum CPU (MHz):

Minimum Disk Space/RAM:

Necessary Peripherals:

Operating System:

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)


What Will It Cost To Obtain and Maintain This Model?


Purchase Costs:                     0

Operating Costs:                    0

Maintenance Costs:   0

Training Costs:                       0





Where Can I Find More Information on This Model?


Web Site:


The 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.

Burges, S. J., Wigmosta, M. S., and Meena, J. M. (1998). "Hydrological Effects of Land-Use Change in a Zero-Order Catchment." J. Hydr. Engr., ASCE, 3(2), 86-97 .

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.


Engel, B. A. (1997). "GIS-based CN Runoff Estimation." Agricultural and Biological Engineering Departmental Report, Purdue University.

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.

________ and Grove, M. 1997. L-THIA: Long-Term Hydrological Impact Assessment——A Practical Approach. Ohio Environmental Education Fund / Purdue University, Manual.

Harbor, J., Grove, M., Bhaduri, B. and Minner, M., 1998, Long-Term Hydrologic Impact Assessment (L-THIA) GIS. Public Works, 129, p.52-54.

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.

Minner, M. 1998. Sensitivity analysis and advanced applications of L-THIA. Unpublished M.S. Thesis. West Lafayette, IN: Purdue University, Department of Earth and Atmospheric Sciences.


Additional References/Case Studies: (All are available on the web site)

#                                            Comparative Impact Analysis of Two Land Use Change Scenarios in the Cuppy McClure Watershed, West Lafayette, Indiana.

#                                            A 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.


Application Sites:


Selected application sites are listed below.  Additional information is available through the documentation listed above and on the web site.

#                    Northeast Ohio: Assessing the hydrological impact of land use change on groundwater recharge and of suburbanization on runoff into a wetland.

#                    Barbados: Town planning and coastal management.

#                    North-Central Indiana: Examining the hydrologic implications of future land use change for an urbanizing watershed.

#                   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.

[1]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.