Water Balance Toolset (v.3) for ArcGIS

James Dyer
Professor of Geography


 

A water balance (or budget) explores the interactive relationship between energy and moisture at a place, by modeling moisture demand (potential evapotranspiration) and supply (precipitation and soil moisture storage).  The Water Balance Tool consists of a suite of ArcGIS Toolboxes that generate all monthly water balance grids for a study site, enabling a “pixel-by-pixel” assessment across the study area.  It can be run for a single year, or any number of consecutive years (provided climate and radiation data are available).  This current version was created with ArcGIS Python Toolbox, and is compatible with ArcGIS v. 10.2 and later.  A python toolbox is a single python file with the .pyt extension, containing one or more tools that can interact with each other.  The user can run individual tools independently, or run an automated tool that sequentially executes the different steps (including the downloading of input data).  All tools include a simple user interface.

The water balance model is presented in the following article:
Dyer, J.M. 2019. A GIS-based water balance approach using a LiDAR-derived DEM captures fine-scale vegetation patterns.  Remote Sensing 11(20): 2385. https://doi.org/10.3390/rs11202385

Archived web pages are still available for version 1 and version 2 of the model.

Downloads (refresh your browser to see any updates):

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. 

Harpers Ferry AWC

Available Water Capacity (AWC), which represents soil moisture storage


The following grids compare two years that experienced very different June-July-August values of the Palmer Drought Severity Index (PDSI Source: NOAA)

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.
PET Harpers Ferry

Actual evapotranspiration refers to water loss from a vegetated surface given water availability (that is, precipitation and soil moisture storage).
AET
        Harpers Ferry

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.
Deficit Harpers Ferry

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.
Sample sites, Harpers
        Ferry
Sample sites AWC, Harpers
        Ferry
WB graphed

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 patternsTo 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:
PET Adjustment Sample


Last updated October 2019

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