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 tool - Day Length Tools for ArcGIS

Day length turns out to be one of the critical calculations for calculating potential evapotranspiration in the Climatic Water Deficit Toolbox .

In order to improve the ease of use of the Climatic Water Deficit Toolbox I've decided to automate the calculation of day length for any given latitude for any given Julian day. The result is the Day Length Tools for ArcGIS . There is also a version of the tool that calculates day length for all months using the 15th day of each month. I can imagine that there is potential for this tool beyond just climatic water deficit. For example, maybe this tool could be used by people studying plant phenology to map areas where day length exceeds a certain number of days.

One question that people may have is "where can I get a latitude raster?". I created one in ArcGIS using lines of latitude and then performing a second order trend analysis using the Trend tool in Spatial Analyst (ArcToolbox - Spatial Analyst Tools - Interplotation - Trend).