craigmw,
Being able to use ET data for irrigation management was actually my goal when I started this project.
Its pretty complicated. There are basically three levels of detail you can use for irrigation management.
1. Monthly adjustments based on historical weather data.
2. Daily adjustments using minimal weather data and the Hargreaves-Samani equation
3. Daily adjustment using additional weather data and the Penman-Monteith equation
All these methods take into account the type of plants and growth stage (crop factor), along with such things as soil type and efficiency of the irrigation system. You can read all about it at:
http://www.fao.org/docrep/x0490e/x0490e00.htm#Contents
Here in California we have a network of specialized weather stations specifically for the purpose of providing Evapotranspiration data. I already have Python code for accessing this data as well as code for both Hargreaves-Samani and Penman-Monteith calculations. This is as yet untested.
These techniques are mostly used in agriculture but there is development of landscape related applications. A very informative free publication can be found at:
http://www.water.ca.gov/pubs/conservation/a_guide_to_estimating_irrigation_water_needs_of_landscape_plantings_in_california__wucols/wucols00.pdf
This is focused on California landscapes but the principles can be applied anywhere.
Ideally it should be possible to develop a system that can be used anywhere in the world.
Dan