Crop yield response to water - Cra

For training systems on a vertical plane, such as many trellis systems used for winegrapes,pears and apples, a different relation from that of Figure 11 between percent ground coveror intercepted radiation at noon and percent of ET would apply. This is because, as thesecanopies expand, they grow vertically with little increase in percent ground cover or noonintercepted radiation but their Tr increases nevertheless. This is because of the vertical growth(wall) intercepting more radiation in the morning and afternoon and also to more advectiveenergy transfer to the trellis rows. The apple and pear Sections describe specific relationsbetween intercepted radiation and percent mature ET for such crops, which should be usedfor hedgerows and other types of canopies.Another aspect specific to young plantations is that their canopies grow and expand as timeadvances until close to leaf fall. Therefore, the seasonal evolution of K c is different in youngorchards than in mature orchards. Thus, estimates of ground cover should be updated on amonthly or bimonthly basis in young orchards/vineyards to adjust the estimation of ET and theresulting irrigation rates.Variations in the E and Tr of orchards and vineyardsSite specificity affects the ET c of orchards and vineyards more than that of herbaceous crops.In perennial crops, tree or vine canopy size and leaf area density determine the Tr rate, whilerainfall and irrigation frequency determine the E rate. In arboriculture, canopy size may bemanipulated by pruning, and hence Tr can vary depending on pruning practices. In intensiveproduction systems, however, pruning is kept to a minimum and Tr is not subjected to wideyear-to-year variations, other than those caused by changes in evaporative demand or byinternal tree controls, often related to crop load.The E losses from an orchard or vineyard are somewhat easier to manipulate. Evaporation fromsoil is minimized when irrigation applications are as infrequent as possible (without causingtree-water deficits). If trees have small canopies and a significant fraction of the soil is exposedto direct solar radiation, E can be an important ET component, in particular if the irrigationmethod wets a significant portion of the soil surface. In these cases, irrigation frequencyshould be managed to minimize E loss. Under microirrigation, E losses are comparatively muchless, because the wetted areas of soil are smaller and normally located under the canopyshade. Nevertheless, high (daily) irrigation frequency is common for drip systems, and theareas wetted by the emitters always stay wet. If the number of emitters per tree is high andthe wetted areas are exposed, significant E losses may occur from these spots. In situationswhere water is in short supply, microirrigation frequency should be decreased to the longestinterval compatible with having an optimal soil water regime, such as a week or even more inextreme cases of very low supply. In these cases, subsurface drip systems that eliminate E fromirrigation applications should be considered.Since Tr and E, do not occur independently, it is important to understand their interactionsthat are related to the energy balance of the orchard. Adjective energy transfer from thehot, dry soil surfaces in the rows towards the trees will increase Tr. On the other hand, Tr willdecrease when E is high following an irrigation or rain. These interactions have not been fullydocumented, and thus, cannot be included in current procedures for calculating orchard orvineyard ET. However, they need to be considered at least qualitatively.Yield Response to Water of Fruit Trees and Vines 267