How about a counter weighted cup? You could use a battery as the weight. Much like the dipping bird novelties with an added flag and optical sensor. The output shuts off power to the valves after the timer so that the time settings are unchanged by a "rain delay". The rain water would cause a tip, evaporation a reset. The sliding weight would combine "how wet do you want it?" with some hysteresis. I realize that this lacks an 8051 but I challenge any micro based solution to have longer battery life :) Terry