Crop yield response to water - Cra

production recovery from severe water stress was possible because of the absence of stressinduceddisease or insect pressures. Trunk diseases such as deep bark canker that often occurin water stressed orchards were not evident in these studies.Water UseWalnut orchards have high water use rates as because of the high leaf, tall tree stature, andnear full ground cover when the trees are fully mature. Table 1 provides the crop coefficientsfor mature walnut orchards obtained from studies in California (Goldhamer, 1997).Table 1DateCrop coefficients for maturewalnut trees (Goldhamer, 1997).Cropcoefficient(K c )Mar. 16-31 0.12Apr. 1-15 0.53Apr. 16-30 0.68May 1-15 0.79May 16-31 0.86June 1-15 0.93June 16-30 1.00July 1-15 1.14July 16-31 1.14Aug. 1-15 1.14Aug. 16-31 1.14Sept. 1-15 1.08Sept. 16-30 0.97Oct. 1-15 0.88Oct. 16-31 0.51Nov. 1-15 0.28Deficit Irrigation StrategiesThe general strategy followed experimentally forreducing irrigation in walnut orchards has beento limit water deficits during early stages of treeand crop development in favour of imposingthem during mid and late season. One study innorthern California used midday stem-waterpotential to impose the ‘low’ and ‘moderate’stress treatments. The target midday stem-waterpotential values were -0.5 to -0.7 MPa and -1.2 to-1.4 MPa during the bulk of the season for thesetwo regimes with corresponding reductions inapplied water of 30 and 50 percent of potentialET c , respectively (Fulton et al., 2002). It shouldbe noted that the water potential values of fullyirrigated walnut trees are much less negative thenthe two primary nut crops, almond and pistachio.Walnut predawn leaf water potential values forfully irrigated trees range between -0.15 and -0.2MPa and midday stem-water potential between-0.40 to -0.60 MPa. After three seasons, yields inthese stress treatments had declined by 26 and40 percent, respectively, relative to fully irrigatedtrees (Lampinen et al., 2004). Full recovery wasachieved after two years of full irrigation. Acompanion study was conducted on deeper soilswith older trees with a lower tree density. Yieldreductions were appreciably lower at this site,which was attributed to the stress developmentbeing relatively slow because of the largersoil moisture reservoir and possibly the largercarbohydrate reserves of the bigger trees.To simulate a one-year drought with a watersupply of 400 mm where potential ET c was412crop yield response to water