>It seems like a relatively easy device to make or you could buy one
>ready-made in various levels of quality vs price.  It would be good to be
>able to measure the quantity of rainfall possibly along with other
>weather factors then have the computer use some model to decide how much
>additional water is required.
>
>

One supplier of a low cost kit:
http://www.columbia-center.org/fascinating/obs/instrmnt.html