Micro-Irrigation for Greenhouse Crops
Water Quality for Micro-Irrigation Systems
Water quality and its chemistry are directly related to the clogging of micro-irrigation emitters. When the chemistry of irrigation water is not adequately considered, clogging can be one of the major problems affecting this method of irrigation. Clogging can be caused by physical, chemical, and biological contaminants or a combination of these. Clogging can be caused by inorganic solids (e.g., silt and sand), organic solids (e.g., algae, bacteria, and slime), and dissolved solids (e.g., calcium, iron, and manganese). Because there are so many variables involved in the clogging of emitters, there are no foolproof quantitative methods for predicting the amount and rate of clogging.
Collecting a Water Sample
As general guidance, when collecting a sample from a well, first allow the pump to run for at least 15 minutes (even longer is preferable). A quart sample volume is usually ample for chemical constituent analyses. Make sure the sample bottle is clean and rinsed. Collect the sample as close to the well as possible, but do not collect samples too close to a chemical injection point since there may be insufficient mixing.
Water Quality Analysis
A water quality analysis can give the grower a heads up on potential trouble areas for the micro-irrigation system. This test should be accomplished before the final design of the system to ensure that proper components are installed to address any problem areas. The analysis should include testing for pH, dissolved solids, manganese, iron, hydrogen sulfide, carbonate, and bicarbonates. The quantity and size of particulate matter should also be known as this will determine the aperture size of any screen filters. Table 17.1 lists the common water quality issues and what values can affect micro-irrigation systems.
Physical Clogging
Suspended solids in the water supply include soil particles ranging in size from coarse sands to fine clays, living organisms including algae and bacteria, and a wide variety of miscellaneous waterborne matter. Suspended solids can serve as carriers for organic compounds.
Chemical Clogging
Chemical clogging usually results from precipitation of one or more of the following minerals: calcium, magnesium, iron, or manganese. The minerals precipitate from the solution and form encrustations (scale) that may partially or completely block the flow of water through the dripper. Water containing significant amounts of these minerals and having a pH greater than 7 has the potential to plug drippers. The precipitation of minerals in irrigation water is usually a problem only with groundwater sources. Particularly common is the precipitation of calcium carbonates, which is temperature and pH dependent.
Biological Clogging
Growth of algae within the irrigation system is seldom a problem since most algae require sunlight to grow, and virtually all system components are made of opaque materials. However, if surface water is used to irrigate, algae often exist in the water supply. A micro-irrigation system can provide a favorable environment for bacterial growth, resulting in slime buildup. This slime can combine with mineral particles in the water and form aggregates large enough to plug drippers. Certain bacteria can cause enough precipitation of manganese, sulfur, and iron compounds to cause dripper plugging. In addition, algae can be transported into the irrigation system from the water source and create conditions that may promote the formation of aggregates.
System Evaluation
In addition to testing water quality, potential clogging problems can be identified and diagnosed by monitoring the amount of water being discharged from the emitters, the uniformity of this discharge, and the presence of contamination in system flush water. Procedures for evaluating system discharge uniformity vary based on the type of micro-irrigation system. Ideally, water discharge is measured at multiple emitters, along with flow pressure along lateral lines. System evaluations can provide information on the location and severity of a clogging problem. This can be very useful in identifying clogging before the problem becomes so severe that it impacts crop health or becomes irreparable.
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