Crop yield response to water - Cra

Four crop-water-production functions are shown in Figure 3a, each for a different cultivar.Three are from California and one from Spain. Each of the data sets involved field experimentson mature trees that were conducted over at least four years. First order best fit lines for allstudies show fairly strong correlations with correlation coefficients ranging from 0.68 to 0.98.The best fit linear expressions for the Frost Nucellar and the Lane Late studies in Californiahave slopes less than one; the Parent Washington study in California has a slope of almostexactly one, and the Clementine study in Spain has a slope greater than one, suggesting thatthis cultivar may be more sensitive to water deficits than navel oranges.The revenue-water-production functions (Figure 3b) generated from three of these studies (theClementine study did not include revenue data) have a much different appearance than theircompanion crop-water-production functions. With the exception of the Parent Washingtonstudy, it is obvious that the relationships between gross revenue and consumptive use are notlinear. The Lane Late study shows that revenues can be increased by about 80 percent with RDIregimes that reduce consumptive use by either 10 (late season stress) or 40 percent (seasonlongstress). On the other hand, a less successful RDI regime (midseason stress) reducedrevenue by 15 percent with a 23 percent reduction in consumptive use. This large range inrelative gross revenues for different RDI regimes clearly illustrates the importance of stresstiming in some citrus varieties.Both the Lane Late and Frost Nucellar orange studies show that gross revenue can beincreased and/or consumptive use dramatically reduced with optimal RDI regimes. The resultsfrom these cultivars differ markedly than that of Parent Washington. The reason is that bothof the former cultivars had production problems that were lessened with the successful RDIregimes — peel creasing with Frost Nucellar and excessively large fruit with Lane Late. Therewere no identifiable issues of peel appearance and/or fruit size with the Parent Washington.Nevertheless, the Parent Washington data show that consumptive use can be reduced by7 percent with no impact on gross revenue or 18 percent with only a 6 percent decrease ingross revenue (Figure 3b).Suggested RDI RegimesSoil water instrumentation has been used for many years to monitor irrigation and imposewater deficits in citrus. Hilgeman and Sharp (1970) used tensiometers in their RDI work withValencia oranges. They recommend full irrigation from March through mid-July, then irrigatingwhen tensiometers reach 75 cbar. This regime reduced irrigation by 66 percent relative to fullyirrigated trees. Yield was not reduced, sugars were higher, and the resulting smaller treeswere easier to pick. However, since fruit value depends on many factors discussed previously, itmust be emphasized that a successful RDI approach on a given cultivar in a given location willnot likely be optimal for other cropping situations. Additionally, it is dangerous to recommendsoil-based stress thresholds because of differences in soils, placement of the instrument (depthand location), irrigation methods, and varieties/cultivars.The easiest approach to imposing RDI regimes is to irrigate at given percentages of maximumET c at specific periods of the season. For the Frost Nucellar cv., it is recommended to irrigateat 50 percent ET c from mid-May through early July (Goldhamer and Salinas, 2000). Goldhamerfound that a late summer/early autumn stress was optimal with Lane Late in shifting the fruit326crop yield response to water