Wednesday, March 25, 2015

Updates to the Climatic Water Deficit Toolbox for ArcGIS 10.1

Recently I posted about a new tool for calculating day length from a raster of latitude and mentioned that this improvement would be incorporated into the Climatic Water Deficit Toolbox.  I'm happy to announce here it is.  The updated version of the Climatic Water Deficit Toolbox no longer requires the cumbersome day length lookup table, which allows it to be run on much larger areas. In fact colleagues and I have recently generated a 60 year dataset for the entire western USA.

You may ask "What is the Climatic Water Deficit Toolbox?". In a nutshell, it is a toolbox that runs in ArcGIS that calculates potential and actual evapotranspiration based on a Thornthwaite water balance approach and water supply as the sum of snowmelt and rainfall. In essence, it quantifies the demand that a plant experiences on water (potential evapotranspiration), the supply of water (water supply), and the amount of biological activity expected given the limitation of water (actual evapotranspiration). The unmet demand (difference between potential and actual evapotranspiration) is referred to as the climatic water deficit. The tool uses PRISM climate data (30 year normals or monthly rasters) plus SSURGO soils for available water supply and a DEM to incorporate the effects of  fine-scale variation in solar radiation on potential evapotranspiration. The tool accounts for snowpack and soil water storage, runs on a monthly time scale, but does not simulate movement of water through the soil (e.g. groundwater model) nor does it measure actual evapotranspiration via remote sensing. Despite these limitations, the approach is expected to provide a more proximate measure of water stress on plants compared to just using temperature and precipitation variables.

For those interested I highly recommend reading the following papers:

Lutz, J.A., van Wagtendonk, J.W., Franklin J.F. 2010. Climatic water deficit, tree species ranges, and
climate change in Yosemite National Park. Journal of Biogeography 37:936-950. (see also

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

Stephenson, N. L. 1998. Actual evapotranspiration and deficit: biologically meaningful correlates of vegetation distribution across spatial scales. Journal of Biogeography 25: 855-870.

 Here is how the tool works in ArcMap:

There are two versions of the tool. One that runs on 30 year normals (average climate over a 30 year period) and the other that runs on monthly PRISM data. There are some really basic tools for downscaling the 4 km monthly data to 800 m (the resolution of the normals).  There are seven basic steps in the calculation which can be run together in the all-in-one model or separately.

The outputs are stored in the following folders. Be forewarned, however, that these files can take up a lot of disk space.

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