The evaluation of operating systems can play a major role in improving their performance ([10, 11, 98] and ASAE EP458 ). Observations include
• discharges qi of emitters along various laterals in the irrigation unit;
• pressure at several locations within the unit, and thus P and A P;
• the area wetted by an emitter, around a tree or along a crop row;
• the soil water conditions before and after irrigation;
• working conditions of filters and control devices; and
• the fertigation strategies being applied.
This information allows calculation ofthe distribution uniformity [Eq. (5.119)] determination of the fraction of area wetted, assessment of the quality of design, evaluation of the actual conditions of functioning system, strategies for irrigation scheduling, and the maintenance conditions of the system. The information can be used to advise the farmer about improvements needed in the system, irrigation and fertigation scheduling, and in the maintenance of filters, controllers, and fertigation devices.
Microirrigation systems should be used to achieve the highest returns and yields while optimizing the use of water and other production inputs. Microirrigation systems may use less water when not all of the area is irrigated and when system and application losses are minimized, but they should not be managed with the sole intent of saving water; instead, they should be managed to supply the amount of water required by the crop with high frequency.
When very frequent irrigations are applied, the essential information for irrigation scheduling is a forecast of the crop water use. An estimation of ETc may be sufficient, either using meteorological information (see Section 5.1) or specific sensors. An example using automated pan evaporation is given by DeTar et al. . For tree crops, the use of soil and/or plant sensors may be useful . Minimizing percolation losses should be a main objective of scheduling .
Microirrigation systems allow precise control of flow rate, duration, and frequency of irrigations. However, in collective pressurized systems, unexpected pressure variations may occur in the supply system. These variations can be high during periods of peak demand. The application duration has to be increased when pressure drops. Flowmeters are critical to show that the desired amount of water has in fact been applied. These problems are reduced when pressure regulators or pressure-compensating emitters are used.
Fertigation is discussed by many authors [98, 122, 124, 128]. It is recommended that fertigation not be limited to the application of primary fertilizers (N, P, and K) but include other nutrients according to the soil conditions and plant requirements. Fertigation may cause contamination of soils and groundwater when excessive quantities of chemical ions are applied. Thus the composition of the nutrient solution has to be based on soil analysis and nutrient requirements. Concentration and composition of the nutrient solution should change during the season as the nutrient requirements change with the crop development.
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