Drip irrigation plays an integral part in Southeastern vegetable production, and most long-time growers agree that a good system begins with a good design.
When planning a drip irrigation system for vegetable production, growers need to consider factors such as water application uniformity and water use efficiency. Such factors are especially critical when chemicals are injected through the irrigation system.
University of Florida Extension engineers recommend the following factors be considered when designing a new drip irrigation system or when converting from a sprinkler irrigation system to drip:
o The length of lateral lines should not exceed the manufacturer’s recommendations for the specific tape used. Excessive length of laterals will result in poor uniformity and uneven water application. The amount of water applied by the emitters in the last section of the lateral will be reduced significantly when compared with the amount of water applied by the emitters close to the lateral entrance. If the irrigation system uniformity is low, chemicals should not be applied through the system because the chemical application uniformity also would be low.
o All delivery lines — mains and sub-mains — should be sized to avoid excessive pressure losses and velocities. Excessive pressure losses result in a large difference in pressure at the beginning and at the end of the line. Since the flow rat of the emitters usually is a function of the pressure, the water application at the beginning of the line may be very different from the water application at the end of the line, resulting in poor application uniformity. Excessive water velocities in the lines, due to a small diameter, can create a water hammer (pressure wave) which can damage the delivery lines.
o Lateral lines should be laid following the contour lines of the land. This avoids pressure variations within the line due to a change in elevation. If significant changes in elevation along lateral lines cannot be avoided (rolling hills), tape with pressure-compensating emitters may be needed.
o The maximum size of the zones depends on the flow rate from the well. The water flow rate from an existing water pump limits the maximum size of the irrigation zone. This is especially important when converting from another type of irrigation system to drip irrigation. Flow rates should be measured when pumping against the pressure required to operate the irrigation system. The pumping rate will be reduced when pumping against pressure as compared to free discharge.
o Zones should be approximately the same size. A variation in zone sizes will reduce the efficiency of pump operation. When all zones are of the same size, pipe sizes and system costs usually will be minimal.
o Pressure regulators may be required if the pressure produced by the pump is too large or if zones vary greatly in size. If the pump was sized for a previously existing sprinkler system, it likely would operate at pressures that are excessive for components of a drip system. If the system consists of different sized zones, the pump must deliver the amount of water required by the tape used for lateral lines. If some zones are significantly smaller, the pump will produce higher pressure at the smaller discharges required by these zones. This pressure must be reduced by pressure regulators to the pressure required by the drip tape used for the lateral lines.
o Drip irrigation systems require filtration. The selection of filters depends on the water source and quality. Surface water normally requires sand media filters to trap organic materials such as algae, bacteria and other organic debris. Screen or disk filters usually are sufficient for well water.
o An irrigation system should include an injection port to allow for the injection of chlorine or other chemicals which normally will be necessary. Many drip systems experience plugging problems that reduce water application uniformity. The degree of the problem depends on water quality, the type of emitters and the quality of the water filtration. Under Florida conditions, the injection of chemicals is almost always required to avoid emitter plugging.
o The injector should be located as close as possible to the irrigation zone. When there are hundreds of thousands of feet of pipe between the injector and the zone, a large amount of water (necessary to pressurize the system at the beginning of the cycle) must be applied to the crop before the chemical can reach the zone. This can create problems with over-watering, especially for small zones. It usually is less of a problem in large fields (more than 25 acres).
o Florida state law requires that any irrigation system injecting fertilizers or toxic chemicals be equipped with proper backflow and anti-siphon equipment to prevent the chemicals from contaminating the water source.
o Flow meters and pressure should be used to help manage the system. Sudden changes in water flow rates or pressures indicate system problems. These problems usually require immediate attention. For example, an increase in flow rate may be a sign of a broken pipeline. Whereas, a gradual decrease may indicate plugging problems. Flow meters also are necessary in properly scheduling irrigation. It is important to know how much water is applied to each zone with every irrigation cycle. The duration of the cycle may not be sufficient, especially if the flow rate is decreasing gradually due to plugged emitters. Pressure changes also should be monitored. An increase in pressure can be a signal of emitter plugging or some other blockage such as the failure of a valve or pressure regulator. A drop in pressure may indicate a broken line, leaking valves or the failure of flush valves to close properly, or too many zone valves open at one time.