There are several different types of irrigation systems, depending on water source, size of the system, and water application method. Three main water sources exist: surface water, groundwater, and recycled wastewater. Irrigation systems run from small on-farm arrangements—using a local water supply—to vast regional schemes that involve thousands of farms and are controlled by governmental authorities. Water application methods include conventional flood, or furrow, irrigation—which depends on gravity flow—and pumped water for sprinkler and drip irrigation systems.
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 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). 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 the soil you have, the less irrigation water that will be needed because natural rainfall will be used more efficiently.
Surface Water Sources
Streams, rivers, and lakes have traditionally been the main source of irrigation supplies. Historical efforts involved the diversion of river waters and then the development of storage ponds. Small-scale systems—like those used by the Anazasi in the southwestern U.S. and the Nabateans in what is now Jordan—involved cisterns that were filled by small stream diversions.
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 humid regions where rainfall and snowmelt supply most of the crop water needs but 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 of state and federal governments. 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 major source for national or international food or fiber production. On the other hand, large dams also frequently have detrimental effects of displacing people and flooding productive cropland or important wetlands.
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 energy. Center-pivot overhead sprinklers (figure 17.3, right) are often used, and individual systems, irrigating from 120 to 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—clearly an unsustainable practice. Deeper wells that require more energy—plus, more expensive energy—to pump water will make this mining of water an increasingly questionable practice.
|Table 17.1: Approximate Amounts of Water Needed for Food Production|
|Product||Gallons of Water per Pound|
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) surface or groundwater sources are very limited or need to be transported over long distances. Several irrigation districts in the U.S. 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 17.4).
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 17.5). Such systems mainly use gravity flow and require nearly flat fields. These systems are by far the cheapest 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.
Sprinkler irrigation systems apply water through pressurized sprinkler heads and require conduits (pipes) and pumps. Common systems include stationary sprinklers on risers (figure 17.6) and traveling overhead sprinklers (center-pivot and lateral; figures 17.3 and 17.1). 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—can often be accomplished using small sprinklers (figure 17.7) that are connected using small-diameter “spaghetti tubing” and relatively small pumps, making the system comparatively inexpensive.
MAIN TYPES OF IRRIGATION
- Flood, or furrow, irrigation
- Sprinkler irrigation
- Drip, or trickle, irrigation
- Manual irrigation
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 applied directly near the plant through a point-source emitter (figure 17.8). The main advantage of drip irrigation is the parsimonious use of water and the high level of control. Drip irrigation systems are relatively inexpensive, 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 the small platform. Subsurface drip irrigation systems, in which the lines and emitters are semipermanently buried to allow field operations, are now also coming into use. 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 small-scale agriculture in underdeveloped countries.
Fertigation is an efficient method to apply fertilizer to plants through pumped systems like sprinkler 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” of fertilizer to the crop through multiple small applications, which would otherwise be a logistical challenge.
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 U.S. and elsewhere (figure 17.9). 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.
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 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 additional 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. One of the long-term success stories of irrigated agriculture— the lower Nile Valley—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.
The removal of sodium can be accomplished by exchange with calcium on the soil exchange complex, 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 discussion of reclaiming saline and sodic soils.)
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, plant, and weather for precise estimation of 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: Leach salt through drainage, and reduce sodium contents through gypsum application.
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 (table 17.1). According to the U.S. Geological Survey, 68% of high-quality groundwater withdrawals in the U.S. are used for irrigation. Is this sustainable? The famous Ogallala aquifer mostly holds “ancient” water that accumulated during previous wetter climates. As mentioned above, withdrawals are currently larger than the recharge rates, and this limited resource is therefore slowly being mined.
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.
Irrigation Management at the Farm Level
Sustainable irrigation management and prevention of 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, preventing compaction, and using perennialcrops in rotations can increase water storage. A longterm 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 zone tillage increase rooting depth and also result in long-term increases in organic matter and water storage capacity.
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 8 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 U.S., 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 supply the cash crop.
Conservative water use prevents many of the problems that we discussed above. This can be accomplished by monitoring the soil, the plant, or weather indicators and applying water only when needed. Soil sensors—like tensiometers (figure 17.10), moisture blocks, and new TDR or capacitance probes— can evaluate soil moisture conditions. When the soil moisture levels become critical, 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 the use of 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 continuous measuring of 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 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 limited water stress to enrich quality-enhancing constituents like anthocyanins.
Many of these practices can be effectively combined. For example, a vegetable grower in Australia uses beds with controlled traffic (figure 17.11). A sorghum-sudan cover crop is planted during the wet season and mulched down after maturing, leaving a dense mulch. Subsurface trickle 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 from the drip emitters.