Irrigation Scheduling Using the Water Balance

Abstract

The simplest and most robust method for irrigation scheduling, i.e. deciding the dates and amounts of irrigation, is based on the water balance. The 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. The 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 later 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.