Climate change is increasing the importance of irrigation for reliable access to water for crops. With erratic and changing weather patterns, like droughts and excessive rainfall, crops can wilt in the fields without access to irrigation.
According to IWMI, only 5% of farmed land in sub-Saharan Africa uses irrigation. This low irrigation use reduces overall crop yields and quality. With one rainfall season (usually between November and March in Zimbabwe) farms are generally unproductive for most of the year.
Irrigation enables you to grow high-value vegetables year round by improving your crops' access to water. Growing crops in the winter (off-season) lets you sell when markets are not saturated and prices are higher. Most high-value vegetables for export such as peas are grown in autumn/winter when there is no rainfall.
Choosing an Irrigation System
A common question that comes up from emerging farmers is:
"Which irrigation system should I choose?"
The most common types of irrigation systems used in Zimbabwe are drip (micro-irrigation), sprinkler (e.g. centre pivot) and flood irrigation (surface or furrow).
This post in our irrigation system covers drip irrigation. But before we discuss drip irrigation it is important to understand how to select an irrigation system that is appropriate for your field or crop.
Irrigation is a high capital investment and must be planned and chosen carefully. It is there important that understand irrigation basics.
Here are some factors to consider when choosing an irrigation system for your fields:
- Soil Type. The soil type affects the irrigation method. Clay soils absorb more water and sandy soils less. You need low application rates on clay soils and higher rates on sandy soils.
- Topography and Shape. The topography (slope), size and shape of the land affects the irrigation system you use. Hilly or sloping land are not suitable for centre pivots. Drip irrigation is more adaptable and a good option for sloping and irregularly shaped land. Overhead irrigation systems such as centre pivots are better suited to circular and square land.
- Crops Grown. The type of crop you want to grow affects your irrigation choice. Certain crops such as tomatoes need to be watered at their roots, not their leaves to prevent leaf diseases and to stay healthy. It is not however suitable for growing rice. Also, the high upfront costs of drip irrigation make it suitable only for high-value crops (fruits and vegetables) in order to get a good return on your investment.
- Local Weather Patterns. Strong winds are not good for sprinkler systems. High heat areas need an irrigation option that does not evaporate quickly.
- Water Quality. Drip irrigation requires some type of filtration not required for overhead systems.
- Water Supply and Source. Is water in short supply or abundant? What is the water source? Where water supplies are limited or costly it is better to target water application than to cover the whole field with water.
What is Drip Irrigation?
Drip irrigation is a low pressure, low-volume watering system. irrigation system that drips water slowly (2-20l/hr) directly to the plant roots.
It is the most efficient irrigation system, with an efficiency of 90% and above. The high efficiency of drip irrigation results from two primary factors:
First, water seeps directly into the soil before it can evaporate or run off.
Second, the water is only applied where it is needed (at the plant’s roots) rather than sprayed all over the field.
While it has many benefits especially in conservation farming, it is not always appropriate for every situation.
While it has many benefits especially in conservation farming, it is not always appropriate for every situation.
Why Drip Irrigation?
You may be asking yourself why drip irrigation?
Here are some of the benefits of drip irrigation:
- Uses water efficiently. Drip irrigation delivers water right to the soil and does not waste it by spreading it all over the field. It uses 80% less water than overhead watering systems. It is more efficient than flood irrigation systems and loses less water through evaporation and runoff. Water efficiency is critical in Zimbabwe because of high water costs and water shortages in most parts of the country. By applying water only where it is most needed it reduces erosion, leaching, compaction and weed problems.
- Increases yields. By maintaining even and consistent soil moisture.
- Reduces fertiliser waste. Fertiliser can be used in the drip irrigation system, a process called fertigation. This precise fertiliser application reduces waste and volumes needed.
- Reduces plant disease. It reduces plants diseases by keeping moisture off plant leaves and directly watering roots.
- Saves time, labour and energy. Hand watering is time-consuming and requires a lot of labour and energy. Drip irrigation requires less labour and time to operate. Automatic timers can be attached to the system which control when the system is turned on or off.
While drip irrigation is adept at saving water. It still has some limitations:
- High initial costs. Drip irrigation has very high upfront costs. Locally a drip irrigation system costs about $1,800 per acre which excludes the pump cost (Drip-Tech). Without access to low-interest financing or credit with extended payback periods, the price is simply too expensive. The crop grown needs to have a high economic value.
- Machine movement. It limits the movement of agricultural machines in the field since the laterals can be easily damaged or disturbed.
- Needs diligent oversight and maintenance. You need to regularly check that your drip system is working properly. The biggest problem is that drip systems get clogged. You need to learn about unclogging emitters, upkeep and setting up irrigation for new crops. The on-going upkeep costs are higher than for other irrigation systems.
Recommended drip irrigation companies:
If you are looking to purchase review their irrigation design, installation and after sales support. Some of the reputable local dealers and manufacturers offering drip irrigation systems are:
Prices vary. Look out for promotions, including sales to save on costs.
Components of a Drip Irrigation System
Before you start shopping, it is good to know some of the drip irrigation components and basics. The basic components of a drip irrigation system include a pump unit, a backflow prevention device, a filter, a water distribution system, and some devices for controlling the volume of water and pressure in the system.
Pumping Set:
- Consists of the pump and prime mover. The role of the pumping set is to lift water from the water source and push it through the distribution system, It also pushes water to a tank or reservoir, where it flows to the field using gravity.
- It may also be used to boost the water in the existing system so as to force it through the system at the desired pressure.
- It is important that the pump is designed to lift the required amount of water from the source of supply to the highest point in the field and maintain an adequate operating pressure.
- Pumps convert kinetic energy to pressure energy.
- Submersible pump, especially when the water source is a borehole.
- Centrifugal pump.
- Prime movers or power to drive the pump are diesel engine, petrol engine, electricity, solar or wind. Solar and wind pumps are important when they are intermittent power cuts.
- Solar and wind power are mainly used on a system with a storage tank or reservoir.
The water distribution network constitutes of the main line, sub-mains line and laterals with drippers and other accessories.
Mainline
The mainline transports water within the field and distribute it to sub-mains. The mainline is made of rigid PVC or High-Density Polyethylene (HDPE). Pipelines of 65 mm diameter and above with a pressure rating 4 to 6 kg/cm2 are used for main line pipes.
Sub-Mains
Sub-mains distribute water evenly to a number of lateral lines. For sub-main pipes, rigid PVC, HDPE or LDPE (Low-Density Polyethylene) of diameter ranging from 32 mm to 75 mm with a pressure rating of 2.5 kg/cm2 are used.
Laterals
Laterals distribute the water uniformly along their length by means of drippers or emitters. These are normally manufactured from LDPE and LLDPE. Generally, pipes have a 10, 12 and 16 mm internal diameter with wall thickness varying from 1 to 3 mm are used as laterals.
Emission Devices
- The actual application of water in a drip or micro- irrigation system is an emitter.
- The emitter is a metering device made from plastic that delivers a small but precise discharge.
- The quantity of water delivered from these emitters is usually expressed in litres per hour (Lh-1).
- These emitters dissipate water pressure through the use of long-paths, small orifices or diaphragms.
- Some emitters are pressure compensating meaning they discharge water at a constant rate over a range of pressures and are suitable on sloping lands.
- Emitters apply water in drip mode. In drip mode, water is applied as droplets or trickles.
- Emitters for drip mode are available in several discharge increments.
- Some emitters are adapted to apply water to closely spaced crops planted in rows.
- Other emitters are used to irrigate several plants at once.
- There are emitters that apply water to a single plant.
They function as energy dissipaters, reducing the inlet pressure head (0.5 to 1.5 atmospheres) to zero atmospheres at the outlet.
The commonly used drippers are online pressure compensating or online non-pressure compensating, in-line dripper, adjustable discharge type drippers, vortex type drippers and micro tubing of 1 to 4 mm diameter. These are manufactured from Poly- propylene or LLDPE.
Online pressure compensating drippers:
A pressure compensating type dripper supplies water uniformly on long rows and on uneven slopes. These are manufactured with high-quality flexible rubber diaphragm or disc inside the emitter that it changes shape according to operating pressure and delivers uniform discharge. These are most suitable on slopes and difficult topographic terrains.
Online non-pressure compensating drippers:
With this type of drippers, discharge tends to vary with operating pressure. They have simple thread type, labyrinth type, zigzag path, vortex type flow path or have float type arrangement to dissipate energy. However, they are cheap and available in affordable price.
With this type of drippers, discharge tends to vary with operating pressure. They have simple thread type, labyrinth type, zigzag path, vortex type flow path or have float type arrangement to dissipate energy. However, they are cheap and available in affordable price.
Point source emitters
Point source emitters are typically installed on the outside of the distribution line. Point source emitters dissipate water pressure through a long narrow path and a vortex chamber or a small orifice before discharging into the air. The emitters can take a predetermined water pressure at its inlet and reduce it to almost zero as the water exits. Some can be taken apart and manually cleaned. The typical flow rates range from 2 to 8 Lh-1.
Line source emitter
Line source emitters are suitable for closely spaced row crops in fields and gardens. Line source emitters are available in two variations:
• Thin wall drip line,
• Thick wall drip hose.
A thin walled drip line has internal emitters moulded or glued together at set distances within a thin plastic distribution line. The drip line is available in a wide range of diameters, wall thickness, and emitter spacing and flow rates. The emitter spacing is selected to closely fit plant spacing for most row crops. The flow rate is typically expressed in gallons per minute (GPM) along with a 100-foot section.
Drip lines are either buried below the ground or laid on the surface. Burying drip lines is preferred to avoid degradation from heat and ultraviolet rays and displacement from strong winds. However, some specialised equipment is required to install and extract the thin drip distribution line.
Drip lines are either buried below the ground or laid on the surface. Burying drip lines is preferred to avoid degradation from heat and ultraviolet rays and displacement from strong winds. However, some specialised equipment is required to install and extract the thin drip distribution line.
Emission device selection:
- Your selection of an emission device involves choosing the type of device to use and then determining the capacity of the device.
- The type of emission device depends on the crop to be irrigated, filtration requirements, the need for a cover crop and/or frost protection, cost and preference.
- Line-source emitters are especially well suited for row crops, although closely spaced point-source emitters, bubblers and micro sprinklers can also be used. In situations where filtration requirements are high, bubblers and micro sprinklers may be the most viable alternatives.
- On sloping land the crop rows must be grown along the contour lines so that the water supply pipes (laterals) can be laid along the contour lines, this helps with reducing pressure variation.
- Avoid daily application of water to crops in order to promote the development of a deep root system that can sustain the crop in case of a system breakdown.
- Apply water slowly on clay to avoid surface ponding and runoff, and on sandy soils, higher emitter discharge rates must be used to ensure adequate lateral wetting of the soil.
- Cover your drip lines with mulch to extend their longevity.
You now have a basic understanding of drip irrigation. You understand why and where to use drip irrigation, the components of a drip system, companies that sell and install drip irrigation kits and the downsides of a drip system. Consult an AREX irrigation expert or a reputable supplier and do your research to help get you started.
Are you currently irrigating on your farm? What system are you using? Let us know in the comments below.
Irrigation 101: Guide to Drip Irrigation in Zimbabwe
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