drip Irrigation for high density orchards
Establishing Drip Irrigation
Irrigation improves plant establishment, nutrient use, bearing area and plant health. It also improves fruit size and quality.
Drip irrigation is based on the concept of preventing rather relieving moisture stress. The crop response to this approach is positive. Some advantages of drip irrigation include:
- easily automated,
- water can be applied on windy days or during spraying operations,
- foliage is not wetted – reduces disease problems and does not remove crop protection materials from leaf canopy or maturing fruit,
- water does not come into contact with the produce and therefore the food safety risk associated with lower quality water is reduced,
- well suited for fertigation (uniform water distribution).
How it works
A drip irrigation system supplies a small amount of water (0.5 – 2 US gal/hr [2-8 L/hr]) near the base of each tree. (Figure 1) The system components can be downsized because water is delivered on a more continuous basis (usually on a daily basis when needed) and only the rooting areas are watered (not between the rows). As compared to an overhead irrigation system, the pumps are smaller, less power is required, less energy is used and the water conveyance lines are smaller (Figure 2).
In a high density orchard one to two lines of drip hose (Figure 3) will supply each row of trees. The emitters (equivalent to sprinklers in other systems) may be evenly spaced along the lines or clustered near each tree.
Drip systems require filtration units to provide clean water and avoid emitter plugging (Figure 4). Systems also require pressure regulators at the head of each sub-main or other appropriate locations.
Design
A high level of design is imperative for this system to operate properly, especially on rolling terrain. Seek a professional drip irrigation designer.
Choose high quality drip hose (not thin-walled tape) with a minimum expected performance of 15 years. Choose a product where the manufacturer’s coefficient of variation (Cv) for the emitter type is less than 0.07. A Cv of less than 0.03 is excellent. Choose an emitter with a low x exponent of 0 to 0.5 (x=0 is a fully pressure compensating emitter). The cost of pressure compensating emitters (lowest x exponent) is higher than non-pressure compensating. A system using emitters with higher x exponent (such as 0.5) may be satisfactorily offset by a skilled system designer.
Filters
There are three main types of filters: screen, disk (Figure 5) and sand The choice of filter type should be made in consultation with the irrigation designer and is based on the irrigation water quality and the emitter orifice size. Filters must be back-flushed to keep them clean and operating properly – this is preferably automated and based on both a schedule and a pressure differential trigger. Filters are classed by the size of the openings of their mesh, which is called “mesh equivalent”. A minimum of 80 mesh screen must be used for any drip irrigation system. (Figure 6).
Maintenance
Flush lateral lines at the beginning and end of the irrigation season. Begin by flushing one lateral to observe the amount of debris that has accumulated. If the water runs dirty for 5 seconds or more, all the laterals should be flushed. Emitter plugging can be monitored by measuring the flow rate for each zone. If the flow rate begins to decrease over the season, the emitters may be starting to plug. Inspect emitters and make a note if the clogging is caused by a build up of organic material or chemical precipitate. Begin at the end of the lines as these emitters are generally the first to experience plugging. Also check that the filtration system is operating correctly as this may be the reason for the reduced flow rate.
Scheduling
A crop’s water requirement can be expressed as evapotranspiration (ET). This is the amount of water transpired by the plant and evaporated from the soil surface. ET may be expressed as millimetres or inches of water used per day and is affected by temperature, light intensity, wind, humidity, crop cover and crop growth stage. For optimum crop production, the water use (ET) must be replenished by irrigation or rainfall. With a drip irrigation system, irrigation (water use replenishment) should be done daily, on alternating days or every third day (as needed).
An irrigation schedule (the daily amount of water to apply) may be developed by tracking water entering and leaving the root zone – this is called a water budget. By measuring or calculating the ET and measuring the rainfall the water deficit (or irrigation demand) can be determined.
Soil moisture levels can be monitored and used as the primary method of scheduling irrigation or as a periodic verification of the water budgeting method.
Resources
Irrigation Management, Best Management Practices (2004). AAFC-OMAFRA
www.omafra.gov.on.ca/english/engineer/irrigation.htm
B.C. Trickle Irrigation Manual (1999). B.C. Ministry of Agriculture and Food and Irrigation Industry Association of B.C.
www.irrigationbc.com
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