Irrigation scheduling can be used to determine exact amounts of water needed by the crop. Table 2 shows a hypothetical example where different parts of a field hold different amounts of water in the root zone. Different management strategies can be used to determine amount of water added, which results in different efficiencies. The average root zone capacity is 11 cm of water. If this amount is added, there will be leaching in half the field, and the other half will not reach capacity.
Efficiency is defined as the proportion of the water applied that is stored in the root zone.
If 14 cm of water is added, the amount needed to fill the entire field to root zone capacity, leaching occurs in three quarters of the field. This is true even for a uniform soil, and increases as length of furrow increases. The cost of water influences the irrigation efficiency achieved in practice. Table 3 Effect of water cost on amount of irrigation water used. For a crop of white potatoes.