Regulated Deficit Irrigation

With RDI, trees are kept short of water when fruit growth is slow or after harvest but are given ample water during the time of rapid growth of fruit. This reduces the growth of shoots. If RDI is properly managed, there is no reduction in the size of fruit or yield; in fact, both may increase - such results have been achieved. The reason why the above technique works relates to the growth pattern of shoots and fruit. On most deciduous fruit trees, the shoots grow rapidly early in the season, and their growth slows down as the fruit begins to grow rapidly. In contrast, early in the season the fruit grows slowly. Water stress at this time will reduce the growth of shoots without markedly affecting the growth of fruit. With RDI, the irrigation season can be divided into four periods. The duration of these periods is determined by both weather and the relationship between vegetative growth and the growth of fruit. Period 1 In this period, the trees are not irrigated, which allows the soil to dry out. With most crops this period follows flowering, however, with peaches there is initial rapid fruit growth following flowering when water stress must be avoided. In the Goulburn Valley trees are not irrigated until evaporation exceeds rainfall by 100 mm. If low rainfall during winter and early spring occurs or in environments dissimilar from the Goulburn Valley (for example, trees growing in lighter or deeper soil types), soil moisture must be measured. Irrigation should commence when the soil has dried out to 100 kPa in a sand or 400 kPa in a clay loam. In the Goulburn Valley this could be as late as mid-November in a wet spring or late October in a dry spring.

Irrigation Tube and tensiometers.

Period 2
Once irrigation commences, the trees are watered, but with greatly reduced volumes of water compared to that which would normally be applied. Irrigation replacements of pan evaporation of less than 30% are recommended. Period 2 commences at the initial irrigation and continues until six weeks before harvest for early-maturing fruits (that is, before mid-January), and eight weeks before harvest for later maturing fruits. Soil moisture in the middle of the wetted fibrous root zone should not exceed 100 kPa in sand or 400 kPa in clay loams.

Period 3 (from the end of period 2 until harvest)
In this period, the fruit is growing rapidly and the tree now needs ample water to maintain this growth. Water stress must not occur during this final period of fruit growth. Irrigation replacements of pan evaporation of 80 to 100% are recommended. Soil moisture in the middle of the wetted fibrous root zone should not exceed 40 kPa in sand or 60 kPa in clay loams.

Period 4 (post harvest)
After harvest a similar strategy as during period 2 can be implemented. In early maturing varieties and species (for example, cherries and apricots) there is considerable shoot growth after harvest which should be kept in check to maintain fruitfulness and even cropping within the canopy. Irrigation replacements of pan evaporation of less than 30% are recommended. Soil moisture in the middle of the wetted fibrous root zone should not exceed 100 kPa in sand or 400 kPa in clay loams.

(Source: Information Note AG0299: Irrigation scheduling for regulated deficit irrigation (RDI).