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The Long-Term Hydrological Impact Assessment (L-THIA) model is widely used to study direct runoff changes with respect to different land-use conditions. L-THIA was designed to assess the long-term impacts on the hydrology of a watershed for users who want to determine the relative change in runoff from one land-use condition to another. Some users, however, are interested in results that match observed stream-flow data, which includes both direct runoff and baseflow. A simple method of calibration of the L-THIA using linear regression of L-THIA predicted direct runoff and USGS-observed direct runoff values derived from hydrograph separation was developed and tested. The calibration model has been verified using three tests in the Little Eagle Creek watershed in Indiana. Results also raise additional questions regarding the factors that control runoff production and systematic underprediction of direct runoff by L-THIA as compared to actual observed direct runoff data.

This calibration approach is field-verified and can be used with any simple rainfall-runoff model, if there are observational data available. Interestingly, calibration and verification test results for the Little Eagle Creek watershed in Indiana show the usefulness of this approach in general and at the same time raise new questions about the sensitivity of L-THIA model predictions to land-use changes, precipitation, and selection of CN values.