Released Date: Feb. 2013
Page edited May 2017

The tool was reprogrammed in ArcPy and can be found at

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. A GIS-based L-THIA (ArcL-THIA) was been developed in the ArcGIS 10.x 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. The author is Dr. Youn Shik Park, who can be contacted at 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)

Developed by Dr. Park while at the Agricultural and Biological Engineering Department of Purdue University
Copyright © 2017, 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.