V2 x2 V212

where N is the number of emitters per plant, Vp is the coefficient of variation due to emitter plugging, Vk is the coefficient of variation of the emitter discharge coefficient Ke [Eq. (5.158)], x is the characteristic exponent [Eq. (5.158)], and Vh is the coefficient of variation of the hydraulic pressure.

The distribution uniformity DU [Eq. (5.119)], based on the low-quarter application relative to the average application; also can be used for microirrigation. It is practical when based on field measurements for complete systems and is useful to predict irrigation requirements based on meeting low-quarter needs.

Application Depths and System Capacity

As with other irrigation systems, the microirrigation system must be designed with sufficient capacity to meet the water requirements of the crops to be irrigated, plus allowances for leaching salts, nonuniform water application, and system downtime for repairs and cultural practices. Crop water requirements can be determined as indicated in Section 5.1. For design purposes, they should be based on peak monthly water-use values. The basal-crop-coefficient approach (Section 5.1.5) should be used because it appropriately takes into consideration the effects of wetting only a portion of the soil surface on the soil evaporation component. Once the daily net irrigation requirement In (mmday-1), is determined for the local climate and expected crop mix, it must be adjusted for other requirements before calculating system capacity.

For moderate to low water salinity values, leaching usually can be applied early or late in the season when it will not impact system capacity requirements. For high-salinity waters, a continuous leaching fraction (LF) between 5% and 20% may be required.

To take into consideration the unavoidable percolation, a transmission coefficient Tr must be considered [11] for shallow, coarse-textured soils. The daily gross application depth D (mm day-1) is then

where In is the daily net irrigation requirement (mmday-1), EU is the emission uniformity (%), LF is the leaching fraction (0.05-0.20), and Tr is the transmission coefficient (1.0-1.15).

Because microirrigation is based on frequent irrigations, and the entire root zone often is not wetted, a substantial allowance must be made for equipment failure or other potential downtime. A 25% downtime allowance DTis reasonable. With this information, the required system capacity Q (m3 h-1) can be calculated as

where DT is the downtime allowance (%), D is the daily application depth (mm day-1), and A is the irrigated area (ha).

Once the system capacity is set, irrigation times for the various crops are determined from their individual daily water requirements and the application rates of the emitters selected. Irrigation frequency for drip systems is often much higher than for other systems in order to maintain constant soil water contents. Frequency typically varies from daily to twice per week.

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