Grain Guide 2018

Process for buying an irrigation system Currently there is a wide variety of irrigation systems available and each one has its unique application. When choosing an irrigation system, one naturally has to consider the technical as well as the agricultural and economic factors. Decision-making process It is not an easy task reaching a decision on which irrigation system to buy. Producers who have already bought a system, can testify to this. The following needs to be considered in the decision-making process: 1 2 3 Efficiency of irrigation Irrigation system System efficiencies (net to gross) Minimum (%) Maximum (%) Drip (surface and sub-surface) 90 95 Micro-spray 80 85 Centre Pivot, Linear move 80 90 Centre Pivot LEPA 85 95 Flood: Pipe supplied 80 95 Flood: Lined canal supplied 70 90 Flood: Earth canal supplied 60 83 Sprinkler permanent 75 90 Sprinkler movable 70 83 Traveling gun 65 78 Crop choice Every crop has its own water re­ quirements and the chosen system should meet these. The water re­ quirements of the crop, soil water retention capabilities, rainfall and planting dates are all interlinked. The values vary for the different areas and a thorough investiga­ tion and planning is necessary to determine the crop water needs for a specific area. Although the design can allow for the right amount of water, the timing and quantity applied per interval should be managed correctly. In new research that looked at the principle of beneficial water use (in other words, the water used to enable crop production), the irrigation efficiency of the different systems produced the following values (table to the left). The distribution efficiency should also be taken into account with the above. Water availability and the cost of water will also influence the decision and choice of system. The higher the efficiency, the more expensive the system.  Water Quantity: Depending on the type of crop, the irrigation system and the design, approximately 3 to 5 (m 3 /h)/ha is needed. If the crop needs 500 mm in its growth cycle, a net volume of 5 000 m 3 /ha is needed. Quality: The soil and crops have certain tolerances for the building up and sensitivity to unnecessary salts, and threshold values can be obtained through testing. The system choice can also be determined by this. It is also important to determine the aggressiveness (is the water eroding or depositing in nature) – which might have an influence on the irrigation equipment. Purity: This is especially important with respect to the use of the system. Prevent drippers and nozzles from clogging through good filtering and closely monitored management programmes. 112 GRAIN GUIDE 2018 Irrigation