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 James Dyer
Professor of Geography

Water Balance Toolbox for ArcGIS




Location of the Ohio and North Carolina
study areas in the eastern United States. 
Ohio and North Carolina study areas, showing
A) topography and location of soil moisture probes
(note difference in scale between the two study areas),
B) Annual actual evapotranspiration, and
C) Annual deficit. 
Note the topographic pattern of (A) reflected in the maps of AET and deficit

Citation:
Dyer, J.M. 2009. Assessing topographic patterns in moisture use and stress using a water balance approach, Landscape Ecology 24: 391-403.

As described in this paper, the water balance tool computes potential evapotranspiration using the Turc method for humid climates.  If relative humidity is <50%, the adjusted Turc equation (with relative humidity term) would be more appropriate.  To adjust PET for semi-arid and arid climates, multiply monthly PET grids by [1+((50-RH)/70)], where RH is the average monthly relative humidity value. The toolbox 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.
Click on the links below to download:

A GIS-Based Water Balance Tool to Quantify Ecological Variation in Complex Terrain -- poster presented at the Annual Meeting of the Association of American Geographers, April 2013.

NEW:  I am working on a new User Manual and "polished" release of the Version 2 Model.  If you would like me to notify you when I post it, please send me an e-mail to that effect.

A Version 2 of the Water Balance Toolbox is now available here: Water Balance (v. 2) beta.  The new version offers more efficient code to speed up processing, the ability to run routines for an entire year instead of a month-by-month basis, and extensive annotation of the models in case users wish to modify the routines.  The User Manual has not yet been updated, but still should provide sufficient instruction to run the routines.  (An update is forthcoming.)  Documentation for a new [optional] toolset is included in the zipped file.  The PET Adjust toolset attempts to account for temperature changes throughout the day.  With a monthly time-step, there is no diurnal variation in temperature, and so maximum PET occurs on southern exposures, since that is where maximum insolation occurs.  (PET is then symmetrical about the N-S axis.)  This toolset employs “adjustment coefficients” that either increase or decrease PET based on topographic position.


Original (version 1) Water Balance model:


User Manual for the Water Balance Toolbox [Revised August 2010, to include note about aligning all input grids]
in PDF format (click here if you need to download the Acrobat Reader.)
Water Balance Toolbox - Demo Version (98 MB)
Zipped toolbox (Water Balance.tbx) developed for ArcGIS (v.9.2) PLUS sample grids to run the model. Zipped with complete folder structure for executing the model.
Update [May 2010]: Temperature grids for months 04 & 09, and the Precipitation grid for month 10 were originally integer format; these have been converted to floating point.  (Precipitation for month 10 is zero -- this is not an error.)
Water Balance Toolbox (175 KB)
Zipped toolbox (Water Balance.tbx) developed for ArcGIS (v.9.2), without sample grids or folders.
Guidelines for Computing Monthly Radiation Grids Using the Solar Radiation Toolset in ArcGIS
in PDF format (click here if you need to download the Acrobat Reader.)

If you use this model in your research, please acknowledge the source and cite the original publication (above).  The model is distributed "as-is" but I welcome any suggestions, corrections, or comments: dyer@ohio.edu


Additional Information:
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.  Below are some potentially useful links for obtaining these data, along with some notes on their use.
 

Solar Radiation
The National Renewable Energy Laboratory provides solar radiation estimates for the U.S. at 10-km resolution.
URL: http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/

The “readme” file for the SUNY 10-km gridded data contains information about file structure and organization.  Global Horizontal Radiation data can be downloaded for a study area and used to parameterize the Diffuse Proportion (D) and Transmittivity (T) values in ArcGIS’s Solar Radiation Toolset.  Since the SUNY 10-km gridded data are provided at hourly intervals, the global horizontal radiation values can be summed to provide a monthly total, which will be directly comparable to monthly estimates derived from the Solar Radiation Toolset.  Refer to these Instructions for Processing SUNY Gridded Radiation [added August 2010].
 

Available Water Capacity
In the U.S., soil water-holding capacity is available from digitized soil surveys from the Natural Resources Conservation Service (1:12,000 to 1:63,360 mapping scales; the “SSURGO” database).
URL: http://soildatamart.nrcs.usda.gov
The polygon shapefiles can be joined with available water capacity attributes (located in the "muaggatt" table in the accompanying .mdb database; the join will be based on the "mukey" field.)  To perform water balance analysis, these polygon shapefiles can be converted to AWC grids.  In ArcGIS, this would involve using ArcToolbox – Conversion Tools – To Raster – Polygon to Raster:
The resulting grid will need to be converted to millimeters.  In ArcGIS, this would involve using ArcToolbox – Spatial Analyst Tools – Math – Times.
 
Digital Elevation Models
DEMs are available for many locations at ⅓-arc second (~10 m) resolution (or finer) from the U.S. Geological Survey’s National Elevation Dataset.
URL: http://seamless.usgs.gov/
The DEMs that you download will have elevation in meters, with a Geographic Coordinate System (NAD83) projection (units = degrees).  Thus, the x, y, and z values will be in different units.  You will need to reproject the grid to a "meter-based" projection (such as Plate Carree (world), or UTM).  An easy way to accomplish this is to set the projection of the Data Layer that contains the DEM, then right-click on the DEM and select Data – Export Data.  For Spatial Reference, select "Data Frame (Current)."  The output raster should have square cells, and be in Grid format. 
 

Climate
Monthly temperature and precipitation grids are available for the U.S. at 30-arc second (~800 m) resolution from the PRISM Group.
URL: http://www.prism.oregonstate.edu/
After downloading and unzipping the grid, change its extension to ".txt"  You can then use ArcGIS's ArcToolbox – Conversion Tools – To Raster – ASCII to Raster.  The new grid will be in Geographic Coordinate System - Spheroid-Based - GRS 80 (ArcToolbox – Data Management Tools – Projections and Transformations – Define Projection).
 

Last updated 5 April 2013


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