Deficit distribution methods are used when the irrigation demand exceeds the available water at the sources. In such cases, the deficit distribution methods describe how the available water should be distributed among the fields represented by the node. When the deficit distribution method is selected, the fields that are a part of the Water user need to be defined in the section Water users ® Irrigated field (cf. Section 8.5).
Three options are available:
· Equal shortage. The fields get the same percentage of the demand covered, and hence suffer the same relative shortage.
· By priority. The water is distributed according to the priority that has been specified for the field. The fields with the highest priority (lowest priority number) will receive the full demand first. If several fields have the same priority, water is distributed according to the equal shortage method. The priorities are specified in the Irrigated field section.
· By Ky. This method is also called yield stress deficit distribution, since the water is distributed according to how sensitive the crops are to soil water stress at the time of water shortage. This implies that the crop with the highest yield response factor (Ky) will be given the highest priority. In case of several crops with the same yield response factor, water is distributed among these fields according to the equal shortage method. The yield response factor of the crops is specified in the Crops module.