Irrigation efficiency and water recycling

13 min read

Introduction

Irrigation has helped the Great Plains spring into life, propelling the US to become one of the world’s biggest producers of grains, with a net surplus of food. The center-pivot irrigation system has been instrumental in converting land that was previously marginal—once referred to as the Great American Desert—into America’s breadbasket. The center-irrigation system draws water from the Ogallala Aquifer, and more than a third of the water is already depleted. In western Kansas, more than half the water is gone.((Geologists expect the Ogallala Aquifer to be depleted within this century and take 6,000 years to restore to its original level. In some states, as much as 2 feet is being depleted each year. Measures to save water, such as immediately reducing pumping by 30%, could help and would be possible with modern irrigation and water-conservation techniques, as well as turning to more drug-resistant crop varieties.)) Currently, the region is lush, as the figure below shows. This may soon change, as farmers run out of water.

Satellite image of farms irrigated by center-pivot systems in Kansas.

Fossil water from aquifers is a nonrenewable resource. Aquifers have been filling up for thousands of years, with little or no exploitation. In a few decades, the water has been depleted, with no hope of enough rainfall to ever replenish them. Annual precipitation in western Kansas is about 22 inches, and the rate is not any higher elsewhere across the Great Plains.((According to the National Park Service, any place that receives less than 20 inches of rain annually is regarded as a semi-desert, while those that receive less than 10 inches are deserts. At 19–22 inches, West Kansas is a semi-desert that cannot support regular rain-fed agriculture.))

In areas closer to the coast, depleted groundwater may not dry up in the same sense as Ogallala. Instead, seawater gradually intrudes on the aquifers, worsening the water quality and eventually making it unsuitable for agriculture.(( Seawater intrusion into aquifers is especially likely in coastal areas. After extensive exploitation, the “diffuse zone,” an interface wherein seawater and freshwater mix, is distorted and fails to keep out the seawater.)) To continue, farmers must desalinate the water. Farmers also must actively contend with the fact that they waste too much water by using inefficient irrigation systems, with enormous amounts of water lost through spillage, evaporation, and runoff.

Decline in the level of the Ogallala Aquifer, according to a study published by the Oklahoma Cooperative Extension Service

Little attention is paid to the main culprits behind fresh water depletion. Inefficient irrigation by itself is to blame, but so too, is the choice of the crop that makes farmers contemplate methods such as flood irrigation to sustain their farms. Alfalfa, a protein-rich crop used to feed dairy and beef cattle, accounts for more than a quarter of the water drawn from the Colorado River every year. Three times more water is used to grow this crop than is used by all those living in the area for all residential use (showering, watering lawns, laundry, etc.). Much of the water used to grow the alfalfa is wasted, through flood irrigation and other inefficient methods.((For more details see the article “The Crop That’s Sucking the Colorado River Dry” by Samuel Shaw in the Atlantic (June 2023) or the original version “Hay – yes, hay – is sucking the Colorado River dry” in the High Country News (June 2023).))

What defines irrigation efficiency?

Efficiency can be measured and defined using several methods. One measure of efficiency could easily be negated by inefficiency in another measure. One of the most important measures is conveyance efficiency,((Modern metal or plastic pipes ensure almost 100% conveyance efficiency, since seepage, runoff, evaporation, and transpiration are effectively eliminated.)) which is the amount of water that reaches the field and is, consequently, available for use by the crop. Open ditches and canals, especially in overly hot areas, are very inefficient since a significant amount of the water is lost through runoff and evaporation. Closed water pipes are, on the other hand, very efficient, since no spillage occurs unless the pipes are not operating optimally.

A canal in Egypt. Lining such canals to control seepage will likely save Egypt up to 5 billion cubic meters of water.

The primary goal of an irrigation system is to deliver water to a crop’s roots. It is possible to achieve 100% efficiency in conveying water to the field and still experience crop failure if the water is not properly applied where it is needed. Water is lost due to spillage, overapplication in some areas, and inefficient delivery. Water application efficiency((Water application efficiency refers to the ratio of water that eventually gets to a plant’s root zone, compared to the water initially applied. Many surface irrigation methods, such as the use of sprinklers, may have far less application efficiency when compared to drip tape or tubes, especially if such tools are installed below the surface.)) refers to the amount of water that reaches the plant. The water can then be taken up by the plant’s roots, runoff, or evaporate. Therefore, while it is an important measure, application efficiency does not mean that all water that reaches the plant is subsequently utilized properly.

A third measure appreciates that irrigation water is not solely applied to meet plant water needs. Salt leaching, reducing salt concentration in the soil, agrochemical application, and soil cooling are all achieved by applying water. Irrigation efficiency, therefore, assesses the amount of water delivered to the field that is used beneficially. This is a broader measure and may gloss over water losses that would otherwise be captured through other measures.

While, in some instances, water is applied to crops to reduce soil temperatures, evapotranspiration is a significant consumer of water. Variables such as solar radiation, air temperature, wind speed, and humidity influence the rate at which the water applied to an irrigated field is lost.((Potential evapotranspiration, also referred to as PET, has a positive correlation with three variables: solar radiation (sunlight), air temperature, and wind speed. It has a negative correlation with humidity. However, ET is affected by several other variables, such as plant height, soil cover, and soil characteristics.)) While evapotranspiration may help reduce the soil temperature and improve humidity to the plants’ benefit, it is important to assess the tradeoff and whether it is really necessary to use such a precious commodity. It is also necessary to consider whether other water needs can be achieved without using water or through more optimized irrigation systems.

Subsurface irrigation systems

Arid and semiarid lands, which are hot, often windy, and have very little humidity, also see a high rate of evapotranspiration. This, as we have seen, demands greater amounts of water, and, besides through evaporation, a lot of water is lost due to conveyance and application inefficiencies. Over the last three decades, technologies that enable subsurface irrigation have accelerated in scalability and versatility. These technologies are primarily designed to preserve water. More importantly, they deliver water to exactly where it is needed: the roots. This results in better yields. Since most weed seeds are near or on the soil surface, applying water below the surface helps arrest the germination and proliferation of such unwanted plants.((Subsurface drips are placed between 10 and 60 cm below the soil surface. Such a depth is enough to ensure that only minimal evaporation occurs. Additionally, the plants can easily access the water they need, significantly reducing the chances of water stress, which, if persistent, negatively affects plants’ photosynthetic capacity and, as a result, their productivity.)) Additionally, large-scale farms can irrigate their plants even close to harvest time, since the dry surface soil will enable machinery to easily move.

Diagram of a subsurface irrigation system

Subsurface irrigation is a low-pressure irrigation system that uses drip tubes or tape buried below the soil surface. The system is favored because of several advantages that it has over surface irrigation systems. For instance, efficiency is relative in surface systems, with some systems only able to efficiently deliver water to the field but not to the plant or to the plant but not the roots. Subsurface drips are placed after carefully assessing a field’s root system and soil characteristics to determine exactly how deep to place the tube or tape.((Tomatoes, in particular, do very well with subsurface irrigation. However, other crops that encourage farmers to practice crop rotation, such as corn and wheat, maybe less suited, though studies have shown that corn thrives with subsurface irrigation. Adapting the system to different root structures for different crops is necessary to include more crops.))

Besides the high efficiency in water usage, subsurface irrigation prevents crusting, a particularly persistent problem in hot and arid areas. Crusting can negatively affect yield by preventing germinating seeds from breaking through the surface and by killing off other aspects of the soil ecosystem. Many diseases may also be encouraged due to soggy surfaces, again in arid areas. The absence of excess moisture on the ground effectively deals with diseases, reducing farm operational costs and contributing to a higher yield.

Subsurface irrigation system installation.

Due to the initial cost of installation, subsurface irrigation systems are capital-intensive. They also demand higher-quality filtration systems than surface irrigation, since the risk of solid matter clogging the tubes is considerably higher and expensive to resolve. Presently, the technology has advanced well enough to ensure most plants can be irrigated in this way. However, some plants still require surface drip, sprinkler, or flood irrigation systems. Subsurface irrigation is best suited for large-scale farms, where the unit cost for installation and management is much lower than in small-scale farms. These systems are easily automatable, significantly reducing overall farming costs.

Irrigation has significant impacts on the environment. Studies show that in Nebraska, the effects of surface irrigation have led to a measurable cooling of the state in recent decades.((Nebraska has experienced a one-degree reduction in temperatures in the last few decades, a phenomenon that has been directly linked with widespread irrigation. The American Meteorological Society has established that irrigation can alter local weather patterns and regional climate, due to the way it modifies or influences latent heat through evaporation.)) It can be argued that by eliminating surface irrigation, the temperature is likely to rise again, resulting in a higher evaporation rate and inefficient water use. However, farmers cannot yet measure which avenue for water loss is greater, rising temperatures, or inefficient surface irrigation.

Evaporation barriers

Evaporation is certainly a major issue that forces farmers to use more water to irrigate their crops. As much as a third of the water applied to farms is lost through evaporation and runoff. With the subsurface irrigation method, runoff and evaporation can be effectively minimized. However, some crops are incompatible with subsurface irrigation. This necessitates the application of surface irrigation with interventions to reduce or eliminate evaporation that would jeopardize crop health.

Mulching using organic matter.

Mulching is one such irrigation intervention. Traditionally, mulching was done using dry vegetative matter primarily sourced from the remains of the previous harvest. Mulching helps the soil retain moisture. It also helps soil regenerate by providing shelter from the sun for organisms that have a synergic relationship with the soil, including worms and microbes. Mulching helps add to soil fertility as the warm, moist environment hastens the decay of organic matter into compost. However, mulch may not be an effective barrier against evaporation because it is highly permeable. Additionally, it may be difficult to effectively control pests, since, in some cases, weeds are a significant part of the mulch.

Plastic mulching.

An alternative to organic mulch is plastic mulch.((Despite the many advantages, plastic mulching may trap too much heat or moisture, defeating the logic of using it in the first place. Black plastic mulching is effective in warming the soil during the cold season, enabling farmers to do some farming before winter becomes colder. It is also more effective at suppressing weeds than clear plastic mulching.)) Basically, plastic mulch is a polyethylene cover that is used on farms to cover the root zone of crops. The plastic prevents evaporation by protecting the soil and roots, while also capturing any vapor that develops as the temperature becomes hotter. Wind-assisted evaporation is also effectively eliminated by plastic mulch. A key advantage of plastic mulch over vegetative matter is that it is highly effective in suppressing weeds. On the other hand, it does not provide the same benefits to the soil that organic mulch normally does because organisms may have a difficult time thriving under the plastic, due to a lack of air. If improperly installed, plastic mulch can, in the long term, lead to significant soil pollution, negating other benefits.((Normally, plastic generates over time due to solar radiation. If substandard plastic mulching is used, the plastic could soon degenerate and mix with the soil. Soil with plastic pollution suffers a drastic loss in productivity, as shown by tests on soil in China.))

Water recycling

According to the United States Geological Survey (USGS), the average American uses about 82 gallons (310 liters) of water per day, which translates to almost 30,000 gallons (111,600 liters) per year.((Water efficiency has been a priority of government agencies involved in water provision. Basic initiatives to conserve water can easily save more than 20% of the water. Through recycling techniques, much of this water can be recaptured and used either in domestic settings or for agriculture.)) Most of this water is used for activities that produce greywater and blackwater and is consequently carried off for disposal in sewers. A similar amount of water is needed to grow 70kg of wheat, perhaps showing the enormous potential of recycling at a time when many places around the world are experiencing water stress or soon will.

Thanks to modern technology, the UN reports that wastewater can soon be used for irrigation with only minor treatment, or even to replenish depleted aquifers, though this is a much more complicated process. Replenishing aquifers would include transport and treatment costs for a course that may not guarantee success or a quick return on investment.

Water can be treated in several ways. The main method for energy savings and scale is multi-effect distillation, which is also used for desalinating seawater. Because steam from one effect (stage) is used to heat the next effect, with pressure significantly reduced to lower the boiling point, less energy is used.((Multi-effect distillation by NeverDump (a Hall Labs project) has managed to drastically cut the energy demands associated with multi-effect distillation by using the steam from each effect to power the subsequent effect, meaning that only the first effect needs external heating.)) The process produces water that can then be used at home and for irrigation.

Pyrolysis is yet another method. First, the wastewater is heated and the steam is captured and condensed to produce clean water. The residue is then heated in the absence of oxygen at high temperatures. This process produces charcoal, which can then be used to meet energy requirements for the recycling process or as a soil amendment.

Rainwater harvesting for irrigation

Harvesting rainwater is the capture of rainwater runoff from building roofs, canopies, greenhouses, and even land for domestic use and irrigation. Rainwater is popular in many parts of the world, especially those with ample rainfall and a scarcity of developed water infrastructure. Harvested rainwater is generally clean and requires minimal or no processing before storage.

Rainwater harvesting from greenhouse roofs. The water is treated and used to irrigate crops inside the greenhouse.

Rainwater harvesting has huge potential. To illustrate, Nebraska has the largest area of land under irrigation in the US, with more than 8 million acres. On average, the state receives more than 680mm of rain annually. Power REIT, a greenhouse farm based in O’Neill, has 25 acres of greenhouse cover. To calculate the annual rainwater harvesting for this farm, we can multiply the area (25 acres = 101,171m2) by annual rainfall, arriving at a figure of 68,796,280 liters, or 68,796 cubic meters of water. Growing 1 kg of greenhouse tomatoes uses less than 8 liters of water. Using a conservative estimate of 20kgs per square meter of a greenhouse, we can assume that if Power REIT were to exclusively plant tomatoes on the 25 acres, they would need less than 20 million liters of water. The farm can therefore comfortably farm the tomatoes by relying on rainwater exclusively and also supply 170 households with all their domestic water needs.((Each person uses 300 liters of water daily. An average household has 2.6 people.)) Alternatively, the farm could store the water in anticipation of drought or use it for adjacent outdoor farming.

Stormwater can be harvested and reused for irrigation, with appropriate filtration and treatment systems.

Despite the obvious benefits of rainwater, it is not always easy to harvest this priceless yet free resource. In 2012, Gary Harrington, a farmer in Oregon, was jailed for 30 days after being found guilty of breaking a 1925 law that forbade the practice of building reservoirs without a permit.((Mr. Harrington did not simply “collect rainwater” as some simplified accounts at the time maintained. Details of his hearing and the charges from the Oregon Water Resources Department are available in this article from a local news station: “Eagle Point man jailed for illegal water reservoirs“)). This law exists because the state has large regions that receive low rainfall, and the lawmakers feared that large-scale water harvesting would harm the state’s watersheds. Similar laws exist elsewhere, where rainwater harvesting is seen as diverting a public good, and stormwater is seen as an essential part of the environment’s survival and regeneration.

Rainwater is not harvested from rooftops only. Stormwater runoff from farms can also be collected and put in reservoirs. If such water is from farms with significant agrochemical applications, the water may have negative health effects on humans. Additionally, chemicals from other human actions, such as industry, or from naturally occurring sources could pollute the water and make it unfit for domestic use or irrigation. For instance, salinity in rainwater is a concern for farmers, as scientists have shown that, after persistent application, the sodium contained in the rainwater replaces magnesium and calcium in the soil. This in turn causes soil to be less permeable and erodible and, ultimately, less productive.

In areas that experience high levels of air pollution, gases such as sulfur dioxide and nitrogen dioxide can react with other gases in the atmosphere and, upon mixing with rainwater, causing a phenomenon known as acid rain. This rain is harmful to humans and crops, especially if the acidity is high enough.

One straightforward solution to dealing with pollution is using water-treatment techniques discussed in this article, which are employed for water recycling. In other instances, simple techniques for water purification using filters can suffice, depending on the level of pollution and the end use of the water. Matters of law, however, are more complicated.

Conclusion

Efficient irrigation systems are an integral part of the push to ensure that the earth’s limited water can be stretched further. For this, there must be a congruence of intentions between authorities on how available water is optimized through harvesting of runoff, desalination, recycling, and the adoption of irrigation systems that will considerably reduce the amount of water used to grow food and possibly yield better results. Innovative ideas are an essential part of this conversation, such as Refresh Shower, which guarantees a perpetually running shower while saving 99% of the water that people currently consume, and NeverDump, which helps recycle grey and black water while creating byproducts that can be used as domestic fuel and soil additives.

Author
Mbau Tim