Irrigation Scheduling Using Water Balance

The simplest and most robust method for irrigation scheduling, i.e., deciding the dates and amounts of irrigation, is based on water balance. Soil water deficit (depth of water required to bring the soil to field capacity) is calculated using ET and rainfall data, and rules are defined for calculating the dates and depths of irrigation. Rules are based on the critical SWD, the amount of water that the crop can extract in the rooting depth before water stress occurs. The critical SWD is the product of root depth, soil available water, and allowable depletion. The latter depends on evaporative demand when we want to prevent reductions in expansive growth. Otherwise, most crops can use around 70% of stored soil water before stomatal closure occurs. In arid areas, we can calculate mean irrigation calendars for planning purposes. Irrigation scheduling of high-frequency irrigation systems is very simple as it focuses only on irrigating with an amount equal to actual crop ET since the last irrigation while ignoring water storage in the soil.