Methods and devices are provided to automatically determine plant water requirementsand adjust irrigation in order to make efficient use of water. In one implementation,an irrigation control unit (100) comprises a memory (104) storing historicalvalues of a plurality of variables used at least in part in calculating plant waterrequirements, and at least one input (106) adapted to receive signals correspondingto current values of one or more of the plurality of variables. The unit (100) alsocomprises a processor (102) coupled to the at least one input and the memory, theprocessor adapted to determine the plant water requirements at least in partusing, for each of the plurality of variables, a current value in the event thecurrent value is available and at least in part using, for each of the pluralityof variables, a stored historical value in the event the current value is not available.
