New versions of climatic water deficit tools and scripts

Miranda Redmond from our lab has created an R script for running PRISM climate data to generate climatic water deficit data for discrete sites. The functionality is similar to our existing spreadsheet version of the Climatic Water Deficit Toolbox for ArcGIS in that it runs on discrete sites rather than raster cells. You can view all three versions of the tools HERE as well as a whole variety of other tools. We anticipate that there will be improvements to all versions of the climatic water deficit tools in the near future so stay tuned!

New dataset - latitude

Both the Climatic Water Deficit Toolbox and the Day Length Toolbox rely on a latitude raster for some of their calculations.  I've had many people ask me where they can get a latitude raster.  Now the answer is if you're working in the western US (WA, OR, CA, NV, UT, NM, AZ, WY, CO, MT, ID, ND, SD, and TX) you can download a raster off of our lab's webpage HERE .

These data are at 1200 m cell size and are in a UTM Zone 12 NAD83 projection.

If you are working outside of this area it is still possible to create your own from lines of latitude.  First, remove the lines of longitude.  Then covert the line vertices to points taking care to ensure that the points are somewhat distributed.  Finally, a second order trend analysis in Spatial Analyst should do the trick.

New paper - Functionally relevant climate variables for arid lands: using climatic water deficit modeling to predict distribution of desert shrub vegetation

I'm pleased to announce that my paper "Functionally relevant climate variables for arid lands: using climatic water deficit modeling to predict distribution of desert shrub vegetation" got accepted in the Journal of Biogeography. The paper compares two different suites of climatic variables used for modeling the distributions (climatic envelopes, niches, habitat suitability) of eighteen shrub species in the Great Basin. The first approach uses the so-called bioclimatic variables commonly used today in species distribution modeling. The latter approach uses the climatic water deficit which uses a Thornthwaite water balance model to estimate potential and actual evapotranspiration and climatic water deficit (PET minus AET). We expanded upon the work of others by deriving new variables that more fully describe the  shape of the water balance climograph. Our study found that for these shrub species there was similar model performance using the two suites of variables. However, we believe that the climatic water deficit approach may prove to be more effective in studies that predict range shifts under climate change.

The paper compliments the Climatic Water Deficit Toolbox for ArcGIS which can be found HERE. Thank you co-authors Peter Weisberg, Jeanne Chambers, and Camie Dencker as well as those who provided critical reviews.

The paper can be downloaded HERE.

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 www.wmrs.edu/projects/cerec/pdfs/Lutz3CERECposter.pdf)

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.