Simple aplication

I want to automate my fathers garden. Turn on/off the water and I want to be able to consider outside condition in order to water it when it necessary. My question is what should I use as controlling device? I was thinking of using the S7-200 or a Logo.

Seems like overkill. But I didn't say don't do it!!!
There are many watering controls that can be readily used . And considerably cheaper than a plc or smart relay plus the valves and all.

Do a search on rain sensors ,some can be integrated to these watering controls . here's one link rain sensor

Drewcrew6

Garden automation

Another thing to consider is a couple of plastic rainbarrels with level switches to allow makeup water from the potable supply if your water level in the catch barrels gets to low . A PICO EASY LOGO AutoDirect or any of the "almost a PLC not just a progammable brick" might be cost effective if you can repair sensors and or get some very cheaply. I am going do the same type project but include lighting the chicken house, heating, watering and other things as I can think of them and then finance them. Pops has always been an on his feet and busy guy but a triple bypass and other things can slow things way down. So this way he can still take manual control when HE WANTS to and let the equipment handle it otherwise.

I have that...

I use Rain8 ... http://www.wgldesigns.com/rain8.html

Works fine. X-10 base. Triggered via a dry contact and more.