Crop yield response to water - Cra

Modify horticultural practicesPruning – More severe pruning may be effective in reducing water use since Tr is related tocanopy size and leaf area density. However, the relationships between canopy size and Trare not linear (see Figure 11) and significant pruning may be needed to change Tr. When thereduction in water supply is going to be drastic (a small fraction of tree Tr), a mature plantationcan be saved for later years by heavy pruning (sometimes called ‘dehorning’ or ‘stumping’).The objective is to remove most of the canopy, leaving only short primary scaffolds. Thiseffectively eliminates any yield for that season but it permits the tree to survive the drought.Full production may not resume for several years.Fruit thinning – Heavy fruit thinning under limited water allows the grower to produce fruit ofmarketable sizes, thus increasing crop value relative to normal thinning. This practice is commonand achieves higher grower revenue, even though yields are lower. Also, because the presenceof fruit enhances Tr, heavy thinning can reduce Tr rates somewhat in many tree species anddecrease the level of water stress in the trees, leading to an acceptable commercial size for theremaining fruit. As a general conclusion, it must be said that there is not much evidence thatthese measures are very effective, relative to others discussed in this Section, except in extreme,very low water supply situations. Further, because the reduction in transpiration associated withfruit thinning is mediated by reduced stomatal conductance, this practice might increase heatdamage where water deficit and high temperatures occur at the same time.Reduce evaporation from soil – If runoff is avoided and deep percolation is minimized, theonly option left to decrease unproductive water use is to reduce or even eliminate E loss. Infull coverage systems, irrigating as infrequently as possible would also minimize E. Evaporationlosses from drip irrigation are low but they can be reduced further if the systems are notrun daily, but every few days. The optimum interval would depend on the depth of waterrequired and on soil type, as deep percolation must also be avoided. Because the depth ofapplied water is reduced under deficit, the irrigation set (duration of irrigation) should notbe changed, but the interval between applications should be expanded (irrigation frequencylowered). Having the drip lines under the canopy will contribute to the E reduction because ofboth the shade and the mulch layer of dead leaves over the wetted soil. Buried drip networkscan theoretically eliminate E. However, some surface wetting has been reported with manyburied drip systems, even with line placement 45 cm deep. The magnitude of this problemseems to depend on soil transport properties, the installation depth, the method used forline installation, and the duration of the irrigation. Moreover, these systems are relativelyexpensive and more difficult to maintain. Thus, the installation of buried drip systems for thepurpose of reducing E is generally not justified, as the savings relative to surface drip would besmall in absolute terms. In cases where the amount of water applied is very low, the relativeimportance of E increases, and the small E savings from buried drip may pay off.Deficit irrigationOnce all the measures described above have been considered and adopted as needed, the onlyoption left to cope with water scarcity is to reduce the application of irrigation water. Deficitirrigation (DI) is defined as a regime where the irrigation water applied is less than the orchardET requirements. When the irrigation rate is below the ET rate, there will be a net extraction ofwater from the soil reservoir. Two situations may then develop. In one case, if sufficient wateris stored in the soil and transpiration is not limited by soil water, the consumptive use (ET)Yield Response to Water of Fruit Trees and Vines 289