When irrigation is available, the crop's performance can be predicted more accurately. Figure 86 shows the amount of water needed during each month of the growing season for the crop to perform fully in average weather conditions. In particularly warm, sunny and windy conditions, the need will be greater. The water needed each month of the growing season for a crop to grow at the potential rate in average weather conditions. The first sign of stress is that leaves grow more slowly.
Photosynthesis is reduced because entry of carbon dioxide into the leaves is restricted.
As the stress builds up, some leaves wilt, roots grow more slowly, old leaves die and yield is lost. An early drought which delays attainment of full leaf cover can reduce yield more than a late drought. Irrigation given to ensure full leaf cover as early as possible is particularly valuable. Consistent production of contract tonnage Yield loss due to drought can be avoided with irrigation so the area of crop needed to fulfil contract tonnage can be calculated more precisely. This is one of the main advantages of irrigation.