Methods and devices are provided to automatically determine plant water requirements and adjust irrigation in order to make efficient use of water. In one implementation, an irrigation control unit comprises a memory storing historical values of a plurality of variables used at least in part in calculating plant water requirements, and at least one input adapted to receive signals corresponding to current values of one or more of the plurality of variables. The unit also comprises a processor coupled to the at least one input and the memory, the processor adapted to determine the plant water requirements at least in part using, for each of the plurality of variables, a current value in the event the current value is available and at least in part using, for each of the plurality of variables, a stored historical value in the event the current value is not available.