Water Balance Toolset (v.3) for ArcGIS

James Dyer
Professor of Geography

• User Manual, PDF format. Note: "Step 1" of the Automatic Run tool automatically downloads and prepares input grids, but changes to server configuration since the code was written have affected this ability.  Click here for Manual workarounds to Prepare Input Data. 
  
• Water Balance Toolset for ArcGIS v.3, Zipped file. NOTE: As explained in the User Manual (Section 3.2), ArcGIS v. 10.5.1 crashes when computing PET, since this version is unable to perform a necessary calculation on solar grids stored in multi-band tif format.  The Water Balance Tool successfully executes on ArcGIS v. 10.2, 10.3, 10.4, 10.6 and 10.7 (i.e., all other versions as of April 2019).
• Note: Changes on the Web Soil Survey server have apparently resulted in errors in the soil AWC grid downloaded by the Water Balance Tool (soils_awc.tif having only “0” values, and soils.tif having astronomically large Low/High values). Users can follow these steps to create the necessary soils AWC grid to run the Water Balance Tool.
  
• Demo, Zipped file containing instructions and shapefiles for Fernow Experimental Forest, WV.  Extract the folder "Fernow" to your computer's hard drive.  This folder contains the file "WB_Demo.pdf" with instructions for running the demo. You will also need to download the Water Balance Toolset.  
• Demo version 2, with input data downloads already completed, Zipped file containing instructions and shapefiles for Fernow Experimental Forest, WV, as well as all input data from running "Automatic Run Step 1."  Extract the folder "Fernow" to your computer's hard drive.  This folder contains the file "WB_Demo.pdf" with instructions for running the demo. You will also need to download the Water Balance Toolset. 

Version History:
3.0.2    June 2019   Fully implemented radiation parameter change from v.3.0.1; tool descriptions reinserted.
3.0.1    May 2019    Solar Radiation parameter changed from default "Uniform_Sky" to "Standard_Overcast_Sky."

Jump to: Adjusting monthly PET grids created by the Water Balance Toolset to account for diurnal temperature patterns. 

Potential Evapotranspiration represents moisture demand, and is a function of heat and radiation.  It is the evaporative water loss from a vegetated surface in which water is not a limiting factor.

Deficit represents moisture stress, and occurs when evaporative demand is not met by available water.  In other words, it is the difference between potential and actual evapotranspiration.

The tool enables extraction of monthly water balance values to points.  Note the difference in values at a site between years, and the difference between nearby points within the same year. 
Site 1 and 2 are 3.6 km apart.  Site 1 is on a southeast-facing ridge, available water capacity = 47 mm.  Site 2 is situated in a northwest-facing valley, available water capacity = 83 mm.




The water balance tool computes monthly potential evapotranspiration using the Turc method. The tool then computes soil moisture storage, actual evapotranspiration, soil moisture deficit, and soil moisture surplus for every grid cell within a DEM, using the Thornthwaite-Mather approach. Data needs for performing a water balance using GIS are few: a digital elevation model (DEM), soil available water capacity (AWC), and monthly temperature, precipitation, and solar radiation.  In “automatic run” mode, the tool can download and prepare all input data, but user-provided data or grids may also be used.

If you use this model in your research, please acknowledge the source and cite the original publication (provided above).  The Water Balance Tool was developed by James Dyer; Steve Porter automated the tool using Python scripting.  Funding to automate the tool was provided by the National Park Service under Cooperative Agreement P14AC00882.  The User Manual and Demo were written by James Dyer.  Suggestions, corrections, or comments can be directed to dyer@ohio.edu.

Adjusting monthly PET grids created by the Water Balance Toolset to account for diurnal temperature patterns.  To model moisture demand (potential evapotranspiration), the Water Balance Tool considers temperature and radiation using a monthly time step.  Since diurnal variations in temperature are not considered, maximum demand occurs on southern exposures (in the northern hemisphere), since that is where maximum insolation occurs (and potential evapotranspiration is symmetrical about the N-S axis).  Yet typically, higher PET is associated with more westerly aspects because temperatures are warmer in the afternoon when these aspects are receiving the most direct radiation.  For this reason, an (optional) method has been developed to create “PET adjustment coefficients.”  Essentially, daily PET is calculated for a representative day each month using an hourly time step, and this is compared to daily PET derived using average daily temperature.  Differences in the two approaches allow a pixel-by-pixel adjustment to PET values created with the monthly time step.  By factoring in diurnal temperature differences, higher monthly PET values can occur on western aspects compared to their eastern-aspect counterparts. (See sample images below.)  However, the approach has not been fully automated, and is time-consuming.  It is presented here for users who wish to explore these adjustments. Instructions and necessary ArcGIS toolbox available here (zipped file).

Differences in Adjusted vs. Unadjusted PET, Annual Values:



Last updated April 2021

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