Chapter 14

Drip Irrigation for Greenhouse Crops

Water Quality for Drip Irrigation Systems

Water quality is probably the most serious concerns when considering drip irrigation. In order to discharge very low flow rates, the diameter of the emitter orifices must be very small. This results in the emitters being blocked very easily by even the smallest contaminants in the water supply. Of particular concern are suspended solids, such as silt and sand, minerals that precipitate out of solution, such as iron or calcium, and algae that may grow in the water. A system with poor quality water simply will not function reliably enough to warrant the maintenance requirements needed to keep it in operation.

Water Quality Analysis

A water quality analysis can give the grower a “heads up” on potential trouble areas for the drip 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.

Physical Properties

Suspended solids in the incoming water are the most common stress impinging upon the drip irrigation system and the easiest to control. These particles can be either organic such as algae or inorganic such as sand. Surface water generally contains a combination of algae, moss, aquatic animals as well as suspended sand, silt and clay particles. Filtering this mix of material is a challenge that is best accomplished using three-dimensional filtration, such as disc or sand media filters.

Chemical Properties

Chemical precipitate clogging is most often encountered in drip irrigation systems supplied by groundwater. High levels of calcium, magnesium, bicarbonate, and sulfate can result in chemical precipitation, particularly if the irrigation water is highly alkaline. Calcium carbonate or lime is the most common precipitate.

Calcium Carbonate (lime scale)

The precipitation and deposition of calcium carbonate (lime scale) in drip irrigation systems is one of the most common causes of system plugging and the associated loss of irrigation efficiency. The causes of the precipitation include the following.

Biological Properties

A drip 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 emitters. Certain bacteria can cause enough precipitation of manganese, sulfur, and iron compounds to cause emitter 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. Emitter plugging problems are common when using water that has high biological activity and high levels of iron and hydrogen sulfide. Soluble ferrous iron (Fe+2) is a primary energy source for certain iron-precipitating bacteria.

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