What you need to know before buying/building irrigation system
By Angie Sullivan, MOSES
With the changing climate delivering hotter, drier summers, many farmers are seeking solutions by irrigating crops. Options for irrigation depend on a farm’s size, crops grown, soil type, and budget. Here’s what you should know before you invest in an irrigation system for your farm.
Benefits of Irrigation
Most often, irrigation is used to meet the water needs of the current year’s crop, giving the plants the moisture they need to reach peak flavor and quality. Irrigation significantly increases crop yields, particularly on sandy soils which have low moisture-holding capacities. In addition, irrigation can allow a farmer to double crop—plant a field with soybeans following wheat in the same year, for example.
In other situations, irrigation is viewed as insurance against occasional drought. Knowing there is a way to water crops without depending on rainfall, brings many benefits by reducing risk, helping the farmer to control crop loss and income fluctuation.
Best Management Practices
Irrigation systems can use surface or ground water. However, avoid using a water source that might be polluted by manure runoff, which could contain pathogens. If you are certified organic, your certifier might request a water test if you’re using a water source that could contain prohibited materials such as herbicides. Ground water pumped through your farm’s well might be the best water source for irrigation, especially for a small-scale system.
No matter what water source you use or what system of irrigation you choose, you should follow these best management practices:
• Minimize water use. Apply only enough water to meet crop needs. This can be determined through regular soil moisture monitoring or through a “checkbook” system to monitor water applied and crop needs.
• Use efficient systems. Choose the system that makes best use of water and results in the least waste of this precious resource.
• Apply at a rate the soil can absorb. Runoff due to excess irrigation can cause soil erosion and wastes water.
• Apply water uniformly. This reduces the chance of runoff and leaching in areas where water may pool.
• Provide good drainage. Good drainage along with irrigation minimizes soil salinization in areas of low rainfall by allowing salts to percolate down through the soil profile.
Drip irrigation targets the water supply directly to the base of plants, reducing runoff, evaporation and wetting of non-targeted areas. This kind of system nearly eliminates wet foliage, making plants less susceptible to disease. It is a popular choice for vegetable farmers, but also can be used on a larger scale for row crops.
A drip irrigation system includes a main header line with individual drip lines running from it. The fittings at the header line can be adjusted for specific watering rates, or turned off completely for rows of plants that require less water. Flow meters and pressure gauges can be added to the system to ensure valves are set for adequate flow and water pressure to prevent runoff, erosion or inadequate watering.
The distance between the holes in the drip lines can be matched to fit the plantings in a particular row. Vegetable farmers can take advantage of that fact by laying drip tape on a prepared bed and turning on the water to mark where to put transplants.
Timers allow for automatic watering. Even on a small operation, installing a timer will prevent the flooded fields that happen when someone forgets to turn off the water. To make the best use of timers, you need to know the water requirements of the plants served by the irrigation system. Overwatering or watering at the wrong time can be just as detrimental as watering too little for many plants.
Because the drip lines rest on top of the soil, they can be damaged by hand tools or cultivation equipment. Be aware and use caution when working fields that have a drip irrigation system.
Sub-surface irrigation uses tubing that is placed about 5 inches below the surface. The obvious advantage of this method is the water really goes exactly where it’s needed—the roots of the plants. This also greatly reduces evaporation, and there is no opportunity for runoff.
Having the irrigation underground and keeping the soil surface dry not only reduces evaporation, but also allows you to have your equipment in the field while you’re irrigating. Also keeping the surface of the soil dry can reduce the growth of shallow-rooted weeds and inhibit weed germination.
The main disadvantage of sub-surface irrigation is the initial cost—for both the system parts and their underground installation. To figure your cost-per-acre, determine your field size, distance from the water source, and how much automation you would like to include with the system.
The emitters and tubing in both this system and drip irrigation systems can become clogged with small rocks or soil particles. With sub-surface systems, though, fixing these problems is a bigger issue. Implementing a good filtration system will save you time and money in the long run. Rodents also can be an issue with sub-surface irrigation lines.
Surface irrigation often is used for crops that are sown, drilled or seeded. Closely spaced crops with deep roots are particularly suited to this method. There are three basic types of surface irrigation: basin, border, and furrow. Which type works best depends on the size and shape of your fields, soil characteristics, and availability of a water source or rainfall. All three types channel water by gravity flow into a field to irrigate it.
Basin irrigation is common when growing rice or wheat, and is used most often in small fields. The fields should be mostly level with loamy soil—sandy soil allows water to filter too quickly, while clay soil can become waterlogged. A soil berm surrounds the field to prevent runoff. Water flows into the field from an adjacent water source through a gap in the perimeter berm and soaks the entire field. Crops need to withstand being wet for periods longer than 24 hours.
Border irrigation is best for sloping or contoured fields. Long strips of soil berms separate strips of crops, guiding water across the field. This is the type of irrigation used in keyline design, which was explained in the Jan.|Feb. Organic Broadcaster (online at organicbroadcaster.org in Archives).
Furrows are small, parallel channels that irrigate crops grown on the ridges between them. This type of irrigation is suited to many crops such as row crops, fruit trees, and broadcast crops and is good for crops that would be damaged if water covered their stems or crowns, such as most vegetables and potatoes.
Overhead Sprinkler Irrigation
This type of irrigation mimics rainfall. Water is pumped through pipes and sprayed over crops through pressurized sprinklers. The advantage to this system is that it does not require surface shaping or leveling, and can be applied to all types of topography. Sprinkler heads offer a wide range of discharge capacity, and can be managed through automatic timers.
The main disadvantage with this system is its inefficiency. A lot of water is lost to evaporation or redirected by wind so it never reaches its target. These systems also require a high degree of maintenance to ensure they are working properly, and providing the right amount of water for the crop. Another issue to consider before purchasing a sprinkler system is that not all crops grow well with overhead watering. Some crops are particularly sensitive and may suffer leaf scorch as irrigation water evaporates and deposits salts on the leaves. Other crops are especially sensitive to fungal diseases if the leaves don’t dry in 8-12 hours.
Irrigation options, like anything else on your farm, need to be considered carefully. Keep in mind long-term plans for your operation so you can choose a system that is efficient and works not only now, but also if you decide to expand your business in the future.
Angie Sullivan is a MOSES Organic Specialist.
From the May | June 2014 Issue