Introduction and Importance
Irrigating crops is an agricultural practice that goes back thousands of years in human history. Despite significant advances in technology over time, the basic purpose of irrigation is much the same: to supplement water available through rainfall for the purpose of increasing crop yields and/or crop quality. Different forage species have different water needs, both with respect to amount and to timing. The costs of irrigation must be weighed against the potential gain in yield or improved quality. Growers must ask: how much water is needed as a supplement; how easily can the water be distributed; when and how should the water be dispersed; and do I have the equipment to do so?
"As of 2010, the countries with the largest irrigated areas were India (39 million hectares), China (19 million), and the United States (17 million). The irrigation sector claims about 70 percent of the freshwater withdrawals worldwide. Irrigation can offer crop yields that are two to four times greater than is possible with rainfed farming, and it currently provides 40 percent of the world’s food from approximately 20 percent of all agricultural land.
Since the late 1970s, irrigation expansion has experienced a marked slowdown. The FAO attributes the decline in investment to the unsatisfactory performances of formal large canal systems, corruption in the construction process, and acknowledgement of the environmental impact of irrigation projects.
The increasing availability of inexpensive individual pumps and well construction methods has led to a shift from public to private investment in irrigation, and from larger to smaller-scale systems. The takeoff in individual groundwater irrigation has been concentrated in India, China, and much of Southeast Asia. The idea of affordable and effective irrigation is attractive to poor farmers worldwide, with rewards of higher outputs and incomes and better diets.
The option is often made even more appealing with offers of government subsidies for energy costs of running groundwater pumps and support prices of irrigated products. In India’s Gujarat state, for example, energy subsidies are structured so that farmers pay a flat rate, no matter how much electricity they use. But with rising numbers of farmers tapping groundwater resources, more and more aquifers are in danger of overuse.
If groundwater resources are overexploited, aquifers will be unable to recharge fast enough to keep pace with water withdrawals. It should be noted that not all aquifers are being pumped at unsustainable levels—in fact, 80 percent of aquifers worldwide could handle additional water withdrawals. One troubling aspect of groundwater withdrawals is that the world’s major agricultural producers (particularly India, China, and the United States) are also the ones responsible for the highest levels of depletion.
Another problem with pumping water from aquifers and redirecting flows for irrigation is the impact on delicate environmental balances. Salinization occurs when water moves past plant roots to the water table due to inefficient irrigation and drainage systems; as the water table rises, it brings salts to the base of plant roots.Plants take in the water, and the salts are left behind, degrading soil quality and therefore the potential for growth.
A potentially better alternative is drip irrigation, a form of micro-irrigation that waters plants slowly and in small amounts either on the soil surface or subsurface, closer to the roots.Using these techniques has the potential to reduce water use by as much as 70 percent while increasing output by 20–90 percent. Subsurface irrigation can approach 100% water use efficiency. Within the last two decades, the area irrigated using drip and other micro-irrigation methods has increased 6.4-fold, from 1.6 million hectares (3.95 million acres) to over 10.3 million hectares (25.45 million acres).
With predictions of a global population exceeding 9 billion by 2050, demand for higher agricultural output will put more strain on already fragile water reserves. Even without the effects of climate change, water withdrawals for irrigation will need to rise by 11 percent in the next three decades to meet crop production demands.Reconciling increasing food demands with decreasing water security requires efficient systems that produce more food with less water and that minimize water waste. Intelligent water management is crucial especially in the face of climate change, which will force the agriculture industry to compete with the environment for water."
Source: Worldwatch Institute (slightly edited)
Total irrigated acres is down less than 1% from 2008 to 2013. The total number of irrigated acres in the U.S. is 55,319,417.
However, the amount of water usage for irrigation decreased 3.7% during this same time period. Estimated water use fell about 100 million acre-feet.
Top barriers to investing in more efficient irrigation included the inability to finance such improvements, concerns about return on investment, and the uncertainty about future availability of water.
Source: AgWeb (2015)
In the more arid parts of the U.S., profitable forage production would be impossible without irrigation. In much of the arid to semi-arid west, rainfall is less than 18 inches (45.7 centimeters) a year and most parts of this region experience summer dry periods. Without supplemental water, forage stands would not produce an economic yield.
Pacific Northwest Situation
Washington Irrigated Land
Total: 1,689,377 acres - 14,887 farms
Land that was irrigated at least once in the last 5 years – 1,850,488 acres – 16,799 farms
All Irrigated Forage: 446,363 acres – 4,292 farms
Idaho Irrigated Land
Total: 3,398,266 acres – 15,597 farms
Land irrigated at least once in the last 5 years: 3,712,802 acres – 17,022 farms
All Irrigated Forage: 1,142,122 acres – 9,191 farms
Oregon Irrigated Land
Total: 1,664,921 acres - 6,291 farms
Land in Oregon that has been irrigated at least once in the last 5 years (2017-2013) is 2,021,566 acres (19,562 farms)
In 2017, 1,664,921 acres on 16,291 farms were irrigated. Approximately 43% of all farms in Oregon have irrigated acreage. There are 1,326,112 irrigated acres that are raising crops and 338,809 acres are used for irrigated pasture and other crops.
All forage: 662,708 acres – 5,520 farms
Alfalfa: 344,064 acres – 2,422 farms
Oher dry hay: 280,747 – 3,238 farms
Haylage or green chop from alfalfa or alfalfa mixtures: 16,877 acres – 136 farms
All other haylage or green chop or grass silage: 47,576 acres – 795 farms
Irrigated Forage Land (hay, haylage, green chop, grass silage)
Total: 723,134 irrigated acres - 6,269 farms
All Hay: 659,587 acres – 5,481 farms
Alfalfa Hay: 360,140 acres – 2,556 farms
Other Dry Hay: 299,447 acres – 3,662 farms
All Haylage, Green Chop, Grass Silage: 69,602 acres – 1,035 farms
Alfalfa or Alfalfa Mixtures Haylage or Green Chop: 17,022 acres – 149 farms
All Other Haylage, Green Chop or Grass Silage: 52,580 acres – 907 farms
Corn for Silage or Green Chop: 33,902 acres – 183 farms
Sorghum for Silage or Green Chop: 154 acres – 4 farms
Source: Census of Ag 2017: https://www.nass.usda.gov/AgCensus/index.php (two lists for Oregon don't match)
The main goal for irrigation is to provide plants with the proper amount of water at the best time. Adequate soil water will influence the entire growth process from seedbed preparation, germination, root growth, nutrient utilization, plant growth and regrowth, yield, and quality.
Developing Effective Irrigation Systems
Irrigation systems refer to the equipment required to provide water to plants.
The key to optimizing irrigation efforts is uniformity. The land manager has control over how much water to supply and when to apply it, but the irrigation system has the control in providing water uniformly. Determining which irrigation systems is best for your operation requires a knowledge of which system will provide uniform distribution for your land. Available labor, capital outlay, and your desired lifestyle will dictate the system you choose.
Irrigation systems should encourage plant growth while minimizing salt imbalances, leaf burn, soil erosion, and water loss. Losses will occur above and below the plant due to evaporation, wind drift, and run-off across the soil surface and percolation of water below roots.
Designing an effective and efficient irrigation system requires answering four questions: how much water is required; when will water be distributed; what materials are needed; and what is the most efficient and economical design?
Major Types of Irrigation Systems
Irrigation systems are gravity-driven or pressurized. The gravity-driven options can be called surface irrigation while pressurized systems are sprinklers or drip systems.
Surface Irrigation Systems
Surface irrigation systems consist of water being applied in furrows, basins, gated pipe, or flooded over the surface. These systems can be useful if water is sufficient. They require labor, but little equipment. Surface irrigation has a lot of water loss and poor uniformity, although uniformity can be improved with various strategies. Uniform water distribution is easier to achieve in soils with higher clay content, compared with sandy soils. Soil depth will also affect water distribution.
Sprinkler Irrigation Systems
Sprinkler systems are the most common pressurized version. They are less labor intensive, with the exception of hand-line and K-line irripod systems, but require specialized equipment and and energy for pumping and water delivery. The benefits include greater control over application rates, amounts, and timing. This control reduces water loss and soil erosion, can be used to reduce frost damage, and achieves better uniformity for better plant growth. Sprinklers can be in-ground, stationary, portable, semiportable, solid, or mobile. The stationary or solid sets cost more in initial investment by require less operational labor. The balance between cost for equipment and labor must be evaluated. The various options can prove versatile for use on various topographies. Sprinkler irrigation systems include center-pivots, side-rolls, traveling guns, boom sprinklers, and traveling lateral systems (linears). Different sizes of these systems can be matched with specific needs.
Drip Irrigation Systems
Drip irrigation systems are designed to deliver water directly in the soil surrounding roots on a frequent basis. This type of system is productive but costly since it requires extensive pipes and/or hoses and filter systems to apply the water. Drip systems can provide the best uniformity and water-use efficiency (up to 100%), but require the most capital outlay. Drip systems have been tried on a limited basis in large acreage forage plantings.
Factors Affecting Efficiency
Efficient irrigation depends on a balance between atmospheric and soil conditions. Atmospheric precipitation and evaporation play important roles as does the water-holding characteristic of the soil. The selected forage species also affects water utilization and efficiency, including root depth through transpiration.
The most important part of a successful irrigation system is management based on knowledge and observations that are acted upon quickly. Crop yields are maximized when soil moisture meets plant needs consistently throughout the growing season. Stressed plants use water less efficiently than unstressed plants. Thus, the goal is to schedule irrigation to maintain a non-stressed soil-water environment throughout the growing season while preventing water losses, leaching of salts and fertilizers, and loss of root aeration. Forage crop irrigation scheduling must match environmental conditions and plant requirements. The manager must match plant needs with soil moisture conditions.
The decision of when to irrigate forages is fundamental to optimizing forage yield and quality. Under-irrigation stresses plants, sacrifices yield, and can increase antiquality components (e.g. nitrate and prussic acid). Over-irrigating results in wasted water, wasted energy, and increased production costs, leaching of nutrients, water-logged or oxygen deprived roots, and inncreased wear on the system. Irrigation scheduling can optimize water use.
Scheduling irrigation involves making a decision of how much water to apply and when. Three factors enter into the decision: plant water needs, water availability, and soil water storage capacity. When and how much to irrigate must be driven by plant water needs, not by the calendar, and not by the manager's convenience. Water availability also plays a role if water is not available at all times. Understanding how much water the plants can store near their root systems will help managers balance plant needs with water availability.
Water is continuously used by plants, but the rate of use varies depending on atmospheric conditions, species characteristics, season, and growth and development stage. Irrigation provides water to be stored in the soil for later use. The amount of water that a soil can store is determined by the soil type, texture, organic matter, and depth. Sandy soils store less water than loam and clay loam soils. For plant growth, air is also needed in the soil. Soil that is flooded with water results in forcing air from the soil. Most forage species cannot tolerate long periods of flooding. Water needs to be provided periodically to meet plant growth needs.
Some plants exhibit physical characteristics when needing water, such as color changes (alfalfa turns dark green) or wilting (sugar beets, grains). But not all plants respond so obviously and appearance changes may be the result of nutrient deficiencies. In severe water stress, leaf tips start burning.
Seasonal water needs and availability
Species selection should reflect the local availability of water. Alfalfa, wheat, and oats are species that require early-season water. Corn and other warm-season annual species need water later in the season. Species with shallow roots will require more frequent irrigation than plants with deep taproots. Knowing the needs of species you are growing will improve your ability to optimize irrigation scheduling.
Seasonal distribution of precipitation is also important. If soils in your production region become very dry during the summer and fall; and winter precipitation is not sufficient to fill the soil profile; moderate fall irrigation can be helpful to prepare the soil for better spring seedbed preparation and seed germination. If water is available in winter in areas where freezing does not occur, irrigation during this time can be effective.
For annual forages, spring irrigation may be needed for germination and initial growth. If water is more available in spring, consider irrigating to fill the soil profile to 3.3 to 6.6 feet (1-2 meters). However, in areas where winter precipitation is sufficient to refill the soil profile, additional spring irrigation may still be needed with light irrigations sets to germinate seed. It is always a good practice to have a full soil profile of moisture beneath the seed.
Summer irrigation is most common. How often and how much to irrigate is the essential question. Frequency of irrigation is dependent on crop water needs to maintain unstressed plant growth. It is easy to over irrigate in the spring and fall.
Plant growth/development stage
Prior to planting, always preirrigate and fill the soil profile. For newly established stands, frequent light irrigation should be provided. As seedlings develop and the canopy closes, less frequent, greater amounts of water can be provided. During vegetative growth stages, sufficient soil moisture should be available for continuous growth and root development. Roots do not grow into or through dry soil. As plants mature and approach hay or silage harvest, irrigation is decreased to facilitate faster drying and haying operations. You want the soil surface to be dry when laying down the crop to dry.
Compared with shallow-rooted species, established perennials with deep taproots, can benefit from less frequent and deeper irrigation sets. However, if summer temperatures are extreme, more frequent irrigation may be needed.
When water is moved and distributed, water losses occur. This type of loss, water conveyance, can be minimized by good system design. Canals and lateral ditches lose less water when lined or are piped.
When water is applied, there is additional inefficiency. Some plants receive too much (those around the sprinklers) while others receive too little (those farthet from the nozzle for that set). Much evaporation loss can occur during irrigation. If water loss is coming from surface runoff and deep percolation (water passing beyond the root depth), then the management needs to change.
Although overwatering is an obvious waste of water, energy, and money, underwatering is also inefficient, in not providing sufficient water to stimulate improved yield and quality. Irrigation is only efficient when it provides sufficient water. Similarly, water must be provided to the desired plants and not lost to weeds or surface evaporation resulting from poor stands. Soil permeability, plant spacing, ridges, and timing of irrigation will influence how much of the applied water is used by the desired plants.
Guiding principle: Place sufficient water where it can be used most efficiently by the plant roots and prevent it from being lost, in the most economic way.
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