The irrigation water requirement for a complete growing season is computed as
where Pe is the effective precipitation, defined as gross precipitation less all runoff and deep percolation losses; GWis upward flow of groundwater into the root zone during the growing season; AS is the change in soil water storage in the root zone during the nongrowing season, the difference between 6 at planting and at harvesting; and LR is the leaching requirement. Operational estimates of IWR are better done through crop-water simulation models.
The leaching requirement LR is estimated for noncracking soils as
5 ECe ECiw where EC;w is the electrical conductivity of the irrigation water and ECe is the desired average root zone salinity expressed as the electrical conductivity of the saturated soil extract. Values for ECe are available in Ayers and Westcot . Problems relative to salinity management are discussed in Sections 5.5-5.8.
The gross irrigation water requirement is computed as
where Eff is the efficiency of the irrigation system. The value for Eff depends on the spatial scale for GIWR. For a single field, the Eff is the application efficiency of the field irrigation system. If GIWR is for a farm, Eff includes the product of application efficiencies and conveyence efficiencies within the farm. If GIWR is for an irrigation project or scheme, then Eff includes all system losses both on and off the farm. Note that a substantial portion of these losses eventually will return to the water resources system for reuse by other users. Values for Eff for application systems are described in Section 5.4.
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