Kris,
That link you provided is to the paper by Samani and is what the current Hargreaves-Samani calculations are based on. It uses min and max temperature plus latitude. They estimate solar radiation from temp and location data, then use that to calculate ET.
What I am planning to do is use ET data directly when available, then Penman-Monteith if solar radiation data is available, then Hargreaves-Samani if limited weather data is available, and as a last resort, use historical weather data.
Ideally the weather data input should be automatic such as by accessing WeatherUnderground on a daily basis. WU provides data for many parts of the world and is probably a good starting point using Hargreaves-Samani.
In some locations such as California there is a specialized network which provides free access to irrigation oriented weather data:
http://wwwcimis.water.ca.gov
I believe Australia has a similar network and probably more will come online in the future.
One of my goals is to make the system useable world wide as much as possible. It will require being able to acquire weather data from several different sources. This is the tricky part and will need to be developed over time.
As semag mentioned in an earlier post, some jurisdictions are providing home owner oriented irrigation recommendations to help conserve water. This should also be taken into consideration and implemented in the software where possible.
There is another factor that needs to be included in the calculations in order to arrive at a reasonable irrigation schedule. That is the type of plants being irrigated be it lawn, shrubs, vegetables, etc and including growth stage. This is known as “crop factor” or “Landscape factor”.
There is an excellent publication that discusses the application of these things in relation to landscaping:
http://www.water.ca.gov/pubs/conservation/a_guide_to_estimating_irrigation_water_needs_of_landscape_plantings_in_california__wucols/wucols00.pdf
This is just a starting point and I look forward to any input others have to offer.
Dan