A method of controlling the duration of irrigation is described, comprising compiling a database that includes information relating to historical evapo-transpiration rates for a plurality of sectors (preferably identified by zip code) located within a geographical area. Period Reduction Factors for each sector are derived. Parameters are entered into the controller including a Maximum Irrigation Duration, a sector identifier; and the current date. The Maximum Irrigation Duration is adjusted by multiplying the Maximum Irrigation Duration by a Period Reduction Factor for the current period associated with the sector that has been identified by the user, to obtain an Actual Irrigation Duration. The irrigation system is caused to irrigate for the Actual Irrigation Duration during the current period.