To determine water use in agriculture, it is necessary to know the efficiency of its use (measured with several quality of irrigation parameters) and the real-time use of water, associated with its availability. Irrigation quality is a measure of how water requirements in crops and orchards have been met. Water used in irrigation should: (1) meet the demand, (2) be uniformly distributed in the irrigated area, (3) prevent excess of percolation, and
(4) minimize soil structure erosion and deterioration. Irrigation quality is affected by the: (1) velocity of the advance of the waterfront in surface irrigation, (2) soil infiltration rate, (3) water discharge, (4) irrigation time,
(5) recession time, (6) soil water content prior to irrigation, (7) soil's spatial variability, and (8) pressure and spacing of pressurized irrigation emitters.
Water use efficiency in agriculture is low. The principal causes for the low efficiency in water use is due to deep percolation and surface runoff, where the former is associated with lack of knowledge of crop requirements and irrigation times and the latter with soil infiltration. The main factor affecting irrigation time is the user's lack of knowledge of the real effect of water in crop production.
The relation between yield and applied water is an important aspect in determining how much water to apply to the crop or orchard. The relationship is a function of the type of product, quality of product, and climatic conditions. This function increases monotoni-cally to a certain point, then decreases due to excess of water. In contrast, the increment of yield per unit of applied water is called the yield reduction ratio and is characteristic of the type of crop and orchard as well as the variety of the crop. For Mediterranean climatic conditions, the commercial range of water application is 600-1200 mm per season in orchards and 400-1500 mm per season in crops.
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