17.2: Irrigation

There are different types of irrigation systems, depending on water source, size of the system and water application method. Two main water sources exist: surface water and groundwater. On smaller scales, recycled wastewater and even desalinized seawater are used in densely populated dry areas. Irrigation systems run from small on-farm arrangements using a local water supply to vast, regional schemes that involve thousands of farms and that are controlled by governmental authorities. Conventional water application involves flood (or furrow) irrigation, which is done by gravity flow and is still the most common method around the world. Sprinkler and drip irrigation systems have also become widely adopted due to the increased availability of pumps.

First Consider Soil Improvement

Healthy soils with good and stable aggregation, enhanced organic matter levels and limited or no compaction go a long way toward “drought proofing” your farm. In addition, reduced tillage with residues on the surface also helps to enhance water infiltration and to reduce evaporation losses from the soil. Cover crops, while using water for their growth, can act as a water-conserving surface mulch once they are suppressed. But, of course, water is needed to grow crops, from 19 gallons to hundreds or more gallons of water for each pound of plant or animal product (Table 17.1). (This represents literally hundreds of pounds of water to well over 1,000 pounds of water to produce one pound of food.) And if it doesn’t rain for a few weeks, crops on even the best soils will start to show drought stress. Even in humid regions there can be stretches of dry weather that cause stress and reduce crop yield or quality. Irrigation, therefore, is an essential part of growing crops in many regions of the world. But the healthier your soil, the less irrigation water you will need.

Figure 17.1. A farm pond (left) is used as a water source for a traveling overhead sprinkler system (right) on a vegetable farm.

Surface Water Sources

Streams, rivers and lakes have traditionally been the main source of irrigation supplies. Historical efforts to use these waters for irrigation involved the diversion of rivers and then the development of storage ponds. Small-scale systems, like those used by the Anazasi in the southwestern United States and those by the Nabateans in what is now Jordan, involved cisterns and retention ponds that were filled by stream diversions.

Figure 17.2.The Ataturk Dam, part of the GAP project in Turkey, diverts water from the Euphrates River (left). The main canal (middle) conveys water to the Harran Plain for distribution to individual fields (right).

Small-scale irrigation systems nowadays tend to pump water directly out of streams or farm ponds (Figure 17.1). These water sources are generally sufficient for cases in which supplemental irrigation is used: in more humid regions where only limited amounts of additional water may be needed for good yields or high-quality crops. Such systems, generally managed by a single farm, have limited environmental impacts. Most states require permits for such water diversions to ensure against excessive impacts on local water resources. Large-scale irrigation schemes have been developed around the world with strong involvement by governments. In the 1930s the U.S. government invested $3 billion to create the intricate Central Valley project in California that has provided a hundredfold return on investment. The Imperial Irrigation District, located in the dry desert of Southern California, was developed in the 1940s with the diversion of water from the Colorado River. Even today, large-scale irrigation systems, like the GAP project in southeastern Turkey (Figure 17.2), are being initiated. Such projects often drive major economic development efforts in the region and function as a source for national or international food or fiber production. On the other hand, large dams also frequently have the detrimental effects of displacing people and flooding productive cropland or important wetlands.

Figure 17.3.Left: Satellite image of southwest Kansas showing crop circles from center-pivot irrigation systems. Photo by NASA. Right: Overhead sprinkler irrigation system, Iowa.

Groundwater

Figure 17.4. Small pump used for groundwater irrigation in northern India.

When good aquifers are present, groundwater is a relatively inexpensive source of irrigation water. A significant advantage is that it can be pumped locally and does not require large government-sponsored investments in dams and canals. It also has less impact on regional hydrology and ecosystems, although pumping water from deep aquifers requires significant amounts of energy. And deep groundwater generally contains more dissolved minerals and has greater potential to cause salt buildup after many years. Center-pivot overhead sprinklers (Figure 17.3, right) are often used, and individual systems, irrigating 120–500 acres, typically draw from their own well. A good source of groundwater is critical for the success of such systems, and low salt levels are especially critical to prevent the buildup of soil salinity. Most of the western U.S. Great Plains—much of it part of the former Dust Bowl area—uses center-pivot irrigation systems supported by the large (174,000-square-mile) Ogallala aquifer, which is a relatively shallow and accessible water source (Figure 17.3, left). It is, however, being used faster than it is recharging from rainfall, which is clearly an unsustainable practice. Deeper wells that require more energy and more expense to pump water will make this mining of water an increasingly questionable practice. There are reports of fields that used to be irrigated by single wells now requiring as much as five wells. Similar concerns about excessive groundwater withdrawals exist in other regions, notably in the Upper Gangetic Plain in India, where crop production is very intensive (Figure 17.4).

Recycled Wastewater and Desalinized Seawater

Figure 17.5. Recycled wastewater from the City of Adelaide, Australia, is pumped into an irrigation pond for a vegetable farm. Wastewater-conveying pipes are painted purple to distinguish them from freshwater conduits.

In recent years, water scarcity has forced governments and farmers to look for alternative sources of irrigation water. Since agricultural water does not require the same quality as drinking water, recycled wastewater is a good alternative. It is being used in regions where 1) densely populated areas generate significant quantities of wastewater and are close to irrigation districts, and 2) where surface or groundwater sources are very limited or need to be transported over long distances. Several irrigation districts in the United States are working with municipalities to provide safe recycled wastewater, although some concerns still exist about long-term effects. Other nations with advanced agriculture and critical water shortages, notably Israel and Australia, have also implemented wastewater recycling systems for irrigation purposes (Figure 17.5).

In a few areas of the world, water scarcity has become so critical that seawater is desalinized using reverse osmosis to create fresh water. Most is used for direct human consumption, but increasingly it is also used in high-value specialty crop production. The technology is energy intensive but improvements are making it more efficient and cost effective.

Irrigation Methods

Flood (or furrow) irrigation is the historical approach and remains widely used around the world. It basically involves the simple flooding of a field for a limited amount of time, allowing the water to infiltrate. If the field has been shaped into ridges and furrows, the water is applied through the furrows and infiltrates down and laterally into the ridges (Figure 17.6). Such systems mainly use gravity flow and require nearly flat fields. These systems are by far the least expensive to install and use, but their water application rates are very inexact and typically uneven. Also, these systems are most associated with salinization concerns, as they can easily raise groundwater tables. Flood irrigation is also used in rice production systems, in which dikes are used to keep the water ponded.

Figure 17.6.Furrow irrigation is generally inexpensive but also inefficient with respect to water use (left: irrigation canal with irrigated hay field in background; right: excess tailwater is discharged at the other end of the field. Imperial Valley, California).

Main Types of Irrigation

• Flood (or furrow)
• Sprinkler
• Drip (or trickle)
• Manual

Figure 17.7. Portable sprinkler irrigation system commonly used with horticultural crops.

Sprinkler irrigation systems apply water through pressurized sprinkler heads and require conduits (pipes) and pumps. Common systems include stationary sprinklers on risers (Figure 17.7) and traveling overhead sprinklers (center-pivot and lateral; figures 17.1 and 17.3). These systems allow for more precise water application rates than flooding systems and more efficient water use. But they require larger up-front investments, and the pumps use energy. Large, traveling gun sprayers can efficiently apply water to large areas and are also used to apply liquid manure.

Localized irrigation, especially useful for tree crops or landscaping in dry areas, can often be accomplished using micro-sprinklers that use small-diameter “spaghetti tubing” and relatively small pumps (Figure 17.8), making the system comparatively inexpensive.

Figure 17.8. Drip irrigation. Left: A water emitter allows for slow release under line pressure; right: installed on grapevines. Photo by the University of California, Davis.

Drip (or trickle) irrigation systems also use flexible or spaghetti tubing combined with small emitters. They are mostly used in bedded or tree crops using a line source with many regularly spaced emitters or are applied directly near the plant through a point-source emitter (figures 17.8 and 17.9). The main advantage of drip irrigation is the parsimonious use of water and the high level of control.

Drip irrigation systems are relatively inexpensive when used with high-value crops but are not economical for large-scale grain or forage crop production. They can be installed easily, use low pressure and have low energy consumption. In small-scale systems like market gardens, pressure may be applied through a gravity hydraulic head from a water container on a small platform or even through a human-powered treadle pump. In subsurface drip irrigation systems, lines and emitters are semi-permanently buried to allow field operations. Such systems require attention to the placement of the tubing and emitters; they need to be close to the plant roots, as lateral water flow from the trickle line through the soil is limited.

Manual irrigation involves watering cans, buckets, garden hoses, inverted soda bottles, etc. Although it doesn’t fit with large-scale agriculture, it is still widely used in gardens and in small-scale agriculture in underdeveloped countries.

Fertigation is an efficient method to apply fertilizer to plants through pumped systems like sprinklers and drip irrigation. The fertilizer source is mixed with the irrigation water to provide low doses of liquid fertilizer that are readily absorbed by the crop. This also allows for “spoon feeding” fertilizers to the crop through multiple small applications, which would otherwise be a logistical challenge. The fertilizer material needs to be of very high quality with drip systems so that water emitters don’t clog.

Figure 17.9.Drip irrigation for bean plants. Lateral movement of water to reach plant roots may be limited with drip systems (left), unless each crop row has its own drip line or the spacing between rows is decreased by using narrow twin rows (right). Note: The apparent leaf discoloration is due to a low sun angle.

Environmental Concerns and Management Practices

Irrigation has numerous advantages, but significant concerns exist as well. The main threat to soil health in dry regions is the accumulation of salts, and in some cases also sodium. As salt accumulation increases in the soil, crops have more difficulty getting the water that’s there. When sodium accumulates, aggregates break down and soils become dense and impossible to work (Chapter 6). Over the centuries, many irrigated areas have been abandoned due to salt accumulation, and it is still a major threat in several areas in the United States and elsewhere (Figure 17.10). Salinization is the result of the evaporation of irrigation water, which leaves salts behind. It is especially prevalent with flood irrigation systems, which tend to over-apply water and can raise saline groundwater tables. Once the water table gets close to the surface, capillary water movement transports soil water to the surface, where it evaporates and leaves salts behind. When improperly managed, this can render soils unproductive within a matter of years. Salt accumulation can also occur with other irrigation practices, even with drip systems, especially when the climate is so dry that leaching of salts does not occur through natural precipitation.

Figure 17.10.Over-irrigation raised groundwater tables (visible at bottom of pit, left) in the Harran Plain, Turkey. Surface evaporation of water traveling upward through soil capillaries (very small channels) from the shallow groundwater causes salt accumulation (right).

The removal of salts is difficult, especially when lower soil horizons are also saline. Irrigation systems in arid regions should be designed to supply water and also to remove water, implying that irrigation should be combined with drainage. This may seem paradoxical, but salts need to be removed by application of extra water to dissolve the salts, leach them out of the soil and subsequently remove the leachate through drains or ditches, where the drain water may still create concerns for downstream areas due to its high salt content. The lower Nile Valley, one of the long-term success stories of irrigated agriculture, provided irrigation during the river’s flood stage in the fall and natural drainage after it subsided to lower levels in the winter and spring. In some cases, deep-rooted trees are used to lower regional water tables, which is the approach used in the highly salinized plains of the Murray Darling Basin in southeastern Australia. Several large-scale irrigation projects around the world were designed only for the water supply component, and funds were not allocated for drainage systems, ultimately causing salinization.

Concerns with Irrigation

• accumulation of salts and/or sodium in the soil
• energy use
• increased potential for nutrient and pesticide loss
• water use diverted from natural systems
• displacement of people by large dams and possible flooding of productive cropland, wetlands or archaeological sites
• competing users: urban areas and downstream communities

The removal of sodium from the soil can be accomplished by exchanging it with another cation, calcium, which is typically done through the application of gypsum. In general, salinity and sodicity are best prevented through good water management. (See Chapter 20 for a discussion of reclaiming saline and sodic soils.)

Salt accumulation is generally not an issue in humid regions, but over-irrigation raises concerns about nutrient and pesticide leaching losses in these areas. High application rates and amounts can push nitrates and pesticides past the root zone and increase groundwater contamination. Soil saturation from high application rates can also generate denitrification losses. A bigger issue with irrigation, especially at regional and global scales, is the high water consumption levels and competing interests. Agriculture consumes approximately 70% of the global water withdrawals. Humans use less than a gallon of water per day for direct consumption, but about 150 gallons are needed to produce a pound of wheat and 1,800 gallons are needed for a pound of beef or almonds (Table 17.1). According to the U.S. Geological Survey, 68% of high-quality groundwater withdrawals in the United States are used for irrigation. Is this sustainable? The famous Ogallala Aquifer mostly holds “ancient” water that accumulated during previous wetter climates. As mentioned earlier, withdrawals are currently larger than the recharge rates, and this limited resource is therefore slowly being mined. In the case of the Aral Sea—formerly the fourth largest inland freshwater body in the world—the diversion of rivers for irrigated cotton farming in the former Soviet Union resulted in a 90% decrease in the area of the sea. It also became severely contaminated with drainage water from agricultural fields.

Good Irrigation Management

• Build soil to be more resistant to crusting and drought by increasing organic matter contents, aggregation and rooting volume.
• Use water conservatively: consider deficit irrigation scheduling.
• Monitor soil, plants and the weather to precisely estimate irrigation needs.
• Use precise water application rates; do not over-irrigate.
• Use water storage systems to accumulate rainfall when feasible.
• Use good-quality recycled wastewater when available.
• Reduce tillage and leave surface residues.
• Use mulches to reduce surface evaporation.
• Integrate water and fertilizer management to reduce losses.
• Prevent salt or sodium accumulation by applying basic principles of salinity management: regularly test the soil and irrigation water; calculate the leaching requirements; leach out the salts beyond the root zone; reduce sodium contents through gypsum application; and in some cases, grow salt-tolerant crops.

Several large irrigation systems affect international relations. The high withdrawal rates from the Colorado River diminish it to a trickle by the time it reaches the U.S.-Mexico border and the estuary in the Gulf of California. Similarly, Turkey’s decision to promote agricultural development through the diversion of Euphrates waters has created tensions with the downstream countries, Syria and Iraq, and has contributed to their political turmoil.

Irrigation Management at the Farm Level

Sustainable irrigation management and preventing salt and sodium accumulation require solid planning, appropriate equipment and monitoring. A first step is to build the soil so it optimizes water use by the crop. As we discussed in chapters 5 and 6, soils that are low in organic matter and high in sodium have low infiltration capacities due to surface sealing and crusting from low aggregate stability. Overhead irrigation systems often apply water as “hard rain,” creating further problems with surface sealing and crusting.

Healthy soils have more water supply capacity than soils that are compacted and depleted of organic matter. It is estimated that for every 1% loss in organic matter content in the surface foot, soil can hold 16,500 gallons less of plant-available water per acre. Additionally, surface compaction creates lower root health and density, and hard subsoils limit rooting volume. These processes are captured by the concept of the optimum water range (which we discussed in Chapter 6) where the combination of compaction and lower plant-available water retention capacity limits the soil water range for healthy plant growth. Such soils therefore have less efficient crop water use and require additional applications of irrigation water. In fact, it is believed that many farms in humid climates have started to use supplemental irrigation because their soils have become compacted and depleted of organic matter. As we discussed before, poor soil management is often compensated for by increased inputs.

Reducing tillage, adding organic amendments and preventing compaction can increase water storage. A long-term experiment showed that reducing tillage and using crop rotations increased plant-available water capacity in the surface horizon by up to 34% (Table 17.2). When adding organic matter, consider stable sources that are mostly composed of “very dead” materials such as composts. They are more persistent in soil and are a primary contributor to soil water retention. But don’t forget fresh residues (the “dead”) that help form new and stable aggregates. Increasing rooting depth greatly increases plant water availability by extending the volume of soil available for roots to explore. When distinct plow pans are present, ripping through them makes subsoil water accessible to roots. Practices like strip-till can increase rooting depth and also result in long-term increases in organic matter and water storage capacity.

Table 17.2 Plant-Available Water Capacity in Long-Term Tillage and Rotation Experiments in New York

Tillage experiments		Plant-available water capacity (%)
	Plow till	No till	% increase
Silt loam (33 years)	24.4	28.5	17%
Silt loam (13 years)	14.9	19.9	34%
Clay loam (13 years)	16	20.2	26%
Rotation Experiment	Continuous corn	Corn after grass	% increase
Loamy sand (12 years)	14.5	15.4	6%
Sandy clay (12 years)	17.5	21.3	22%
Source: Moebius et al. (2008)

These practices have the most significant impact in humid regions, where supplemental irrigation is used to reduce drought stress during dry periods between rainfall events. Building a healthier soil will reduce irrigation needs and conserve water because increased plant water availability extends the time until the onset of drought stress and greatly reduces the probability of stress. For example, let’s assume that a degraded soil with a plow pan (A) can provide adequate water to a crop for eight days without irrigation, and a healthy soil with deep rooting (B) allows for 12 days. A 12-day continuous drought, however, is much less likely. Based on climate data for the northeastern United States, the probability of such an event in the month of July is 1 in 100 (1%), while the probability for an 8-day dry period is 1 in 20 (5%). The crops growing on soil A would run out of water and suffer stress in July in 5% of years, while the crops on soil B would be stressed in only 1% of years. A healthy soil would reduce or eliminate the need for irrigation in many cases.

Increasing surface cover, especially with heavy mulch, significantly reduces evaporation from the soil surface. Cover crops can increase soil organic matter and provide surface mulch, but caution should be used with cover crops because when growing, they can consume considerable amounts of water that may be needed to leach salts or to supply the cash crop.

Figure 17.11. Tensiometers used for soil moisture sensing in irrigation management. Photo courtesy of the Irrometer Company.

Conservative water use prevents many of the problems that we discussed above. This can be accomplished by monitoring the soil, the plants or weather indicators and applying water only when needed. Soil sensors like tensiometers (Figure 17.11), moisture blocks, TDR (time-domain reflectometry) and capacitance probes can evaluate soil moisture conditions. When the soil moisture levels become critically low, irrigation systems can be turned on and water applications can be made to meet the crop’s needs without excess. The crop itself can also be monitored, as water stress results in increased leaf temperatures that can be detected with thermal or near infrared imaging.

Another approach involves using weather information from either government weather services or small, on-farm weather stations to estimate the balance between natural rainfall and evapotranspiration. Electronic equipment is available for continuously measuring weather indicators, and they can be read from a distance using wireless or phone communication. Computer technology and site-specific water and fertilizer application equipment, now available with large modern sprinkler systems, allow farmers to tailor irrigation to sub-acre-scale localized water and fertilizer needs. Researchers have also demonstrated that deficit irrigation—water applications that are less than 100% of evapotranspiration—can provide equal yields with reduced water consumption and promote greater reliance on stored soil water. Deficit irrigation is used purposely with grapevines that need a modest amount of water stress to enrich quality-enhancing constituents like anthocyanins.

Figure 17.12. No-till irrigated vegetables grown on beds with cover crop mulch. Drip irrigation lines are placed at 1–2 inches depth in the beds (not visible).

Many of these practices can be effectively combined. For example, a vegetable grower in Australia plants on beds with subsurface drip irrigation and uses controlled traffic (Figure 17.12). A sorghum-sudan cover crop is planted during the wet season and mulched down after maturing, leaving a dense mulch. The subsurface drip irrigation is installed in the beds and stays in place for five or more years (in contrast, annual removal and reinstallation are necessary with tilled systems). No tillage is performed, and vegetable crops are planted using highly accurate GPS technology to ensure that they are within a couple of inches of the drip emitters.