For a start you would need to measure:

- Local precipitation, to have a first estimate of how much water might naturally irrigate your field.

- Sunlight intensity and air temperature, to evaluate how much of that water is lost to evaporation and plant transpiration. Measuring air moisture would help also.

- Wind speed, again to estimate how much more is lost to evaporation, or how much of your irrigation will be blowing away from your fields

- Soil conductivity could also give you an estimate of its moisture, but for that you'd also need to measure the salinity and other chemical properties of the rain and irrigation waters (separately)

- Of course you would need to know what time it is, and instead of trying to have an accurate clock on the Arduino I'd have the Android device send it the time at regular intervals (in order to accomodate for clock drift and potential resets from the Arduino)

Try to see how your model would compare to what an actual farmer would do, and the outcomes in terms of plant health, and adjust from there!

Marc, for that matter what kind of sensor would I use or for that matter do you have any suggestions on how I would develop a radiation sensor because that seems to be of a head banger.

- For the sunlight intensity I guess a simple solar cell could do, or a photoresistor

- For temperature, a simple temperature sensor could do.

- For wind speed, you could build or retrofit an anemometer

- For moisture, I don't really know...

- Soil conductivity : Two nails driven into the ground - the problem will be calibration

Actually if you retrofit a hobby-grade weather station that'd be a good start for seeing how things are done, and build up from there ?

photo resistor....I thought it only worked within the band of visible light? will it yield good results?