where, besides variables defined for DU, Ks is the hydraulic conductivity of the soil, SW represents soil water conditions at the time of irrigation, ti is the duration of the irrigation, and A ti is the time interval between irrigations. The irrigator's control basically is reduced to the timing of irrigation and the volumes and frequency of applications.
Variable Ks expresses a potential for deep percolation losses. For very permeable soils, emitter spacing must be small, irrigation must be very frequent, and applied volumes must be small to avoid percolation below the root zone for highly permeable soils. For soils with very low hydraulic conductivity, spacings can be larger, irrigation can be less frequent, and emitter discharge can be smaller. These conditions have to be set at design when selecting the emitters and lateral layout.
Soil water conditions at the time of irrigation (SW) play a major role in defining the irrigation timing and volume in humid climates, when rainfall meets part of the water requirements, and in irrigation of deep-rooted crops, mainly in orchards, where the soil water reserve can be large. Under arid conditions and most applications when fertigation with a nutrient solution is applied, light, frequent irrigations are used because the soil water reserve plays a minor role under these circumstances. SW also is considered as a variable related to soil cracking that occurs in many soils when the soil dries. Soil cracking favors percolation of water and solutes through preferential flow. Irrigation always must start when the soil is wet enough to avoid cracking. This condition also corresponds to the need to maintain soil water above the stress threshold.
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