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Thornthwaite Monthly Water Balance Model



The Thornthwaite water balance (Thornthwaite, 1948; Mather, 1978; 1979) uses an accounting procedure to analyze the allocation of water among various components of the hydrologic system. Computations of monthly water-balance components of the hydrologic cycle are made for a specified location. The model can be used as a research tool, an assessment tool, and as a tool for classroom instruction. It also demonstrates basic principles about modeling with OMS3, specifically temporal advancement within a model.

Figure 1: Waterbalance Model Schematic

Inputs to the model are monthly temperature and precipitation. Outputs include monthly potential and actual evapotranspiration, soil moisture storage, snow storage, surplus, and runoff.


The schematics below shows the components and connectivity of the Thornthwaite water balance model. The boxes represent the components, the arrows show the data-flow between components.

Figure 2: Thornthwaite Component Schematic

The model processes monthly climate input (precipitation, temperature) and produces output for runoff and other state variables. It has only three parameter: latitude, soil moisture capacity, and a runoff factor.

Running the Thornthwaite Model step by step

1. Get the project.
Downlad the zip package; unpack it to the local hard drive. It contains all files to run Thorthwaite,
2. Start the OMS3 modeling console and set the 'Working Directory'
First, start the console. In the console set the work directory to the folder <INSTALL>/oms3.prj.thornthwaite. Click the blue folder icon at the top right corner of the window, navigate to the folder <INSTALL>/oms3.prj.thornthwaite and accept. The file tw.sim should be displayed in the console.

3. Run the Simulation. The model can be run by pressing the run Button in the console.

4. View the Results. The Simulation is providing basic graphical timeseries plot that can be shown by pressing the ?? icon in the console. You should see a graph like:


  • Thornthwaite, C.W., 1948, An approach toward a rational classification of climate: Geographical Review, v. 38, p. 55–94.
  • Mather, J.R., 1978, The climatic water balance in environmental analysis: Lexington, Mass., D.C. Heath and Company, 239 p.
  • Mather, J.R., 1979, Use of the climatic water budget to estimate streamflow, in Mather, J.R., ed., Use of the climatic water budget in selected environmental water problems: Elmer, N.J., C.W. Thornthwaite Associates, Laboratory of Climatology, Publications in Climatology, v. 32, no. 1, p. 1–52.
  • McCabe, G.J., and Markstrom, S.L., 2007, A monthly water-balance model driven by a graphical user interface: U.S. Geological Survey Open-File report 2007-1088, 6 pp.