Irrigation requirements are often determined by some form of schedule, such as so many megalitres after so many days. This schedule may be based on evaporation charts, or from monitoring evaporation pans. These methods are open loop systems. Open loop systems will invariably apply too much or too little water. Fixed irrigation schedules, such as systems which operate on timers, are very inefficient because the water requirements vary dramatically.
Wind is the major factor and transpiration rates vary dramatically, even for the same temperature.
Transpiration rates will also vary dramatically with location as the wind swings around. A southerly morning breeze will give maximum evaporation at one side of the paddock in the morning, but a swing to northerly conditions will give maximum evaporation at the northern area during the afternoon. Closed loop control irrigation implies measuring the soil moisture content and controlling the irrigation to maintain some set value. Closed loop control is vastly superior because if there is any variation between the actual soil moisture content and the set point the system will adjust to correct the error. However, in any closed loop system there is always some delay between a correction and the result.