Is your overhead irrigation system working properly

Overhead sprinklers and irrigation systems are labor savers for irrigating crops in the greenhouse. However, depending on the system, its age and the effects of mineral deposits in the emitter, it may not necessarily apply water evenly or in the same amount to each container within an irrigation zone.

For example, overhead sprinklers apply water in circular patterns; but crops are generally arranged in rectangular patterns.  To ensure all plants in a section or block receive water, the circular water application patterns have to overlap (Figure 2).  Unfortunately these circular application patterns are less likely to overlap on the square edges of a bench or block, so less water is applied and these plants dry faster than the rest of the crop.  In comparison, where two or more sprinklers overlap there is a large volume of water applied and these plants receive more water to remain wet longer (Figure 2).


Figure 2.  Notice that the overlapping patterns of water emitted from six sprinkler heads.  In the middle of this growing area three to four times the volume of water may be applied versus the outer edge of the growing area.

How does this variation in water application affect crops? 
First, each plant will dry out at different rates, causing uneven drying.  Generally the driest plants receive water when needed and grow best.  Plants that are in the one-time pattern stay too dry, or plants in the four-times pattern stay too wet. This causes water stress and plants will be shorter in height and less vigorous.  In addition, plants that remain too wet run the risk of getting root disease.

Second, if fertilizer is applied periodically through the overhead irrigation system, the containers that receive excessive quantities of clear water between fertilizations (three times and four times) will be leached of fertilizer.  These plants can become nutrient deficient and chlorotic.  If fertilizer application rates are high, as is the case with periodic feeding, the underwatered plants (one time) may have fertilizer salts that concentrate on the roots when the pots dry out, leading to damage and potential Pythium problems.

Quick test for determining water distribution
To determine how much water is distributed over a growing section, place several measuring cups strategically throughout the crop to catch and measure the irrigation water. Run your irrigation system for the normal amount of time needed to irrigate the crop.  If all measuring cups have the same level of water, your irrigation system is adequate.  However, if there is a large variation in the volume of water collected in the measuring cups, this is a clear indication that the water is applied unevenly and your irrigation system will need some adjustments. 


Overhead irrigation systems save labor, but periodic monitoring is needed to be sure they are functioning properly.

Corrective actions
One option to lessen the effects of overlapping would be to reduce the time the overhead system runs so less water is applied.  For the plants that dry out quickly, you can hand water, but this can be labor intensive and defeats the purpose of automated irrigation. Two other options are to add more irrigation heads to increase overlapping or replace your irrigation system with a new updated system.  While the later may be the most expensive option, it will provide the most uniformity for irrigation to your crops.  Before doing this, figure a ‘cost vs benefit’ to determine if this option is right for you.  For a malfunctioning irrigation system, it may be a matter of replacing clogged or damaged heads. Then check water volume again to be sure the system is delivering adequate water volumes.

Regardless, if water distribution is uneven, crop problems will occur.  Even the best installed systems can change over time due to mineral deposits in water. Monitoring and measuring irrigation water volume(s) will ensure that problems are found before they turn into crop problems.