Released Date: Feb. 2013

Please note- as of December 2015 this VB-based extension does not work with Windows 7 and ArcGIS 10. It is no longer available from this site. Contact the author, Dr. Youn Shik Park (below) for information on when a new Python-based version will be available.

The Long-Term Hydrologic Impact Assessment (L-THIA) model has been used to estimate the long-term impacts of direct runoff and nonpoint source (NPS) pollution. L-THIA, requiring a modest effort to prepare input data, estimates runoff by SCS-CN method and pollutant loads by pollutant coefficients (Event Mean Concentration; EMC).
L-THIA was originally implemented as a spreadsheet application and has been developed in ArcView 3 and upgraded to ArcGIS 9. A GIS-based L-THIA (ArcL-THIA) was been developed in the ArcGIS 10.1 platform to estimate long-term direct runoff and NPS loads, considering actual daily precipitation data. The easy-to-use philosophy was maintained with a user friendly interface, the model does not require profound knowledge and experience in ArcGIS. X A short training video is available here: movie.
For more details : (Updated on May 6th 2013)

Frequently Asked Questions (Updated on May 9th 2013)

Download correctly formatted precipitation data ( 1965 - 1995) by state and county here: (Updated on May 28th 2015)

Some users of newest ArcGIS have issues because the extension is created in, and ESRI is dropping support for that. Ask ESRI for a VBA license to add to your license server, you just have to specifically request it. It shows up in administrator as below.
So ArcGIS may tell you that lack the license to turn on VBA and you will need to request this from ESRI and get it installed.

Developed by the Agricultural and Biological Engineering Department at Purdue University
Copyright © 2013, Purdue University, all rights reserved.
Purdue University is an equal access/equal opportunity university.
Contact Dr. YS Park for more details if you need any help for L-THIA GIS 2013.