To get started I’ve written a Python program to get the 24 hour irrigation required value from the WeatherReach site and output a 100% watering adjustment. My plan is to use this Python program as a way to tinker and test various data sources and formulas for computing the weather adjusted watering amount and/or percent the current program should be adjust (up or down). Feel free to contribute, branch or start your own programs. The code is here:
The program uses the BeautifulSoup Python module to parse the web page into a structured tree. This makes it much easier to pick out the needed data from a web page, once you learn BeautifulSoup. I recommend using it. The documentation is good – http://www.crummy.com/software/BeautifulSoup/bs4/doc/
The best/easiest way to adjust for weather is ET data since if you know how much water your lawn (or other crop/plants) has lost you know how much you need to replace via irrigation. So next steps are to add more data sources for ET (or similar) data.
The next step down is to use weather to compute ET. I plan to work on this too, but welcome others to work on it also.