Part 12, Additional Uses of Irrigation
By Ed Butts, PE, CPI
This series has concentrated so far on irrigation design procedures and methods strictly for crop production and aesthetic purposes. Although it has outlined the most common methods of irrigation, there are a few other uses that do not directly involve using irrigation for crop germination, growth, or maintenance.
These include using irrigation for dust suppression, frost control (crop survival), hydraulic mining, turf cooling and cleaning, barn and livestock cooling, and wastewater disposal.
This month, as the next to last insert of this series, we will depart from irrigation solely for crop production and examine several of these other ways irrigation is used.
Dust Suppression and Control
Figure 1 illustrates how airborne dust in coal mines, feedlots, horse arenas, parking areas, and other industrial and commercial locations can be controlled by using stationary, portable, or traveling automatic, manual, or rotary sprinklers.
U.S. federal laws require dust control on areas such as vacant lots, unpaved parking lots, construction sites, and unpaved roads.
Dust suppression works by causing the dust to be too heavy to become airborne. This can be accomplished by coating the dust particles or causing them to bind together.
Using water to suppress dust is one of the most used methods of control. Dust control is generally considered as the suppression of solid particles with diameters less than 500 micrometers. Dust suppression is often most effective when performed using a stationary or traveling Big-Gun type of sprinkler with a 40°-45° nozzle trajectory along with enough pressure to ensure adequate stream breakup and coverage.
In some cases, dual sprinklers with smaller nozzle sizes are used to optimize the covered area.
When applying water for dust control, the primary objective is to apply water over the largest area, but at the lowest application rate to avoid runoff or ponding. A longer throw increases the dust-controlled area and reduces the rate at which the water is applied, thus achieving the objective of the control in an excellent way.
Frost Control or Crop Protection
Several diverse methods—solid set, traveling, and portable sprinklers; micro-sprinklers (misters, foggers, spray heads); and surface (flood and furrow) irrigation—are all used for frost control on many different types of crops. Figure 2a shows overhead impact sprinklers being used for frost control of blueberries.
Frost/freeze damage occurs when ice crystals form inside of and destroy the plant cells. Therefore, frost protection is a matter of providing appropriate heat balance to the crop. As long as the plant surfaces are kept wet, even if ice does form, the plant tissue temperature will not fall appreciably below freezing (32°F), and frost damage likely will not occur.
Figure 2b demonstrates applying irrigation to ice encased over strawberry plants. The heat released by irrigation water is used to keep crops warm enough to prevent the formation of ice inside the plant tissue.
Protection from possible crop damage due to freezing is a critical need for many regions of the United States as more economic losses occur due to freeze damage than caused from any other weather-related hazard. Consequently, considerable effort and expense to reduce potential crop damage is often expended.
Cost-effectiveness depends on the frequency and severity of frost occurrence, cost of the protection method, and the cash value of the crop.
Frost protection, in passive or active forms, is used most often in southern regions with year-round crop or orchard production—such as California, Florida, Texas, and Arizona—and is used on such diverse crops as strawberries, fruit, and citrus trees.
Proper site selection is the single most important element of passive freeze protection. Because cold air is denser than warm air, it flows downhill and accumulates in the lower areas of the field. This can create a region where frost protection methods may be totally ineffective. Generally, passive freeze protection is easily justified. This includes steps and practices conducted in advance of a potential freeze that reduces the potential for damage.
Active frost protection includes energy-intensive practices such as heaters, sprinklers, wind machines, or helicopters that are used during the freeze to replace thermodynamic energy losses. Active protection, however, is sometimes not cost-effective for marginal freezes.
Important considerations of active protection include critical bud/plant temperatures, humidity, presence or absence of an inversion layer, wind, wet-bulb and dew-point temperatures, and soil moisture content.
Humidity is an important factor in freeze protection because of the phase changes which convert sensible heat to latent heat—resulting in condensation—or latent heat to sensible heat—resulting in evaporation—and because moist air absorbs more radiant energy than dry air.
Two common causes of frost events are advection and radiation. An advection frost occurs when cold air blows into an area to replace the warmer air that was present before the change of weather occurred. It is associated with moderate to strong winds, without a temperature inversion, and low humidity. Oftentimes, temperatures will drop below 32°F and remain there all day.
Radiation frost events are characterized by clear skies, calm winds, and temperature inversions and occur because of heat losses in the form of radiant energy. Under a clear nighttime sky, more heat is radiated away from an orchard than it receives; thus, the local temperature drops. The temperature falls faster near the radiating surface, causing a temperature inversion to form.
The mechanics of how water works for frost control involves a complex change with the chemical state of water. This chemical reaction consists of one oxygen atom bound to two hydrogen atoms to form a water molecule. The molecules chemically join by forming hydrogen bonds to make liquid water and ice.
When in a liquid state, the molecules clump together, but the groups of molecules can move as a fluid. When the water freezes to ice, the groups of water molecules form a crystalline structure.
This phase changes water vapor to liquid water and liquid water to ice. This means the chemical energy (latent heat) is converted to sensible heat, which is heat that can be measured with a thermometer. The total heat content of the air is the sum of the sensible and latent heat.
Latent heat is the chemical energy stored in the bonds that join water molecules together and sensible heat is what is measured with a thermometer. When latent heat is changed to sensible heat, the air temperature rises. When water condenses, cools, or freezes, the temperature of the immediate environment around the water rises because the latent heat is changed to sensible heat.
Frost control, although technically the same as crop irrigation, is performed for the purpose of heat transfer rather than crop growth. The decision about when to start and stop the sprinklers for frost protection should be based on both temperature and humidity in the orchard.
When a sprinkler system is first started, the air temperature in the sprinkled area will fall to the wet-bulb temperature. Of course, this initial drop will be followed by an increase in temperature as the water freezes on the ground and plants start to release heat and warm the air.
However, if the dew-point temperature is low, the wet-bulb temperature can be considerably lower than the air temperature, and the initial temperature drop can lead to plant damage.
How water is to be applied is an important design consideration of frost control. Both over-crop and under-canopy irrigation systems are used for frost protection. With either one, water must supply enough heat to compensate for the losses from radiation, convection, and evaporation.
Over-crop (or over-canopy) sprinkler irrigation applies water to the upper crop surfaces. Most of the heat is released as the water freezes on the plant. That is, the latent heat of fusion releases 1200 BTUs per gallon or 144 BTUs per pound of freezing water.
In contrast, under-canopy irrigation applies water beneath the canopy of the crops, heating the air primarily due to the applied water is warmer than the cold surrounding air. That is, the sensible heat releases only 8.4 BTUs per gallon per degree Fahrenheit or 1 BTU per pound per 1°F of cooling water.
Water will release the same amount of heat when it freezes as it will when it cools by 144°F. Optimum frost protection is obtained when irrigation is cycled for short periods of time to take advantage of the latent heat contained in the water.
Starting and stopping sprinklers for frost protection should always occur when the wet-bulb temperature is above the crop’s critical damage temperature. Adequate levels of protection for over-crop and under-canopy frost protection in still air requires an application rate of 0.10 to 0.20 inch per hour (50 to 90 GPM) per acre of water or roughly 10-12 times the rate needed for crop production to the entire field for the duration of the frost event.
A rule of thumb is to apply at least one-tenth of an inch of water per hour (about 50 GPM/acre) and to begin irrigating when the field’s ambient temperature is about 36°F. This can add as much heat as 4 million BTUs of heat per acre each hour; about 4 million BTUs is released when one-tenth of an acre-inch of water (2715 gallons) is cooled from 60°F and forms ice at 32°F.
Frost protection using irrigation methods requires consideration of many factors and should only be performed by individuals experienced in this branch of irrigation design. Table 1 provides guidance on the selection of an application rate for various wind speeds and air temperatures with under-canopy plants.
This method is an old and established—though destructive—practice of conducting mining on open surfaced beds of aggregate, minerals, and precious metals. Hydraulic mining is conducted with water delivered to a site and shot through a highly pressurized water cannon nozzle directly onto the face of an exposed cliff, thereby eroding the binding soil and washing away tons of boulders, gravel, and dirt along with whatever is being mined, such as gold or silver.
The method has also been used in the underground mining of coal to break up the coal seam and wash the resulting coal slurry toward a collection point.
Although a primitive form was used by Roman engineers during the first centuries, the first use of this method in the United States is credited to Edward Mattison in 1853 as a part of the California Gold Rush. He supplied his water through a rawhide hose to a nozzle he carved from wood. Miners later upgraded their hoses to metal or the more desirable canvas with nozzles made of iron.
Technological advances made the hose and nozzle connections more flexible and allowed greater rotational and site movement. Flow rates often exceeded 2000 GPM and outlet pressures between 90 to 150 psi were typical.
Hydraulic mining was extensively used in the United States, primarily in the West, during the mid to latter 19th century. It rapidly fell out of favor as thousands of acres of potentially productive farmland along riverbanks and numerous open pits, rivers, and other waterways were eroded, flooded, and in many cases ruined forever by the sediments and destruction associated with this action.
Although this method is presently limited by regulatory oversight and environmental laws in the United States, it is still extensively used in other regions of the world. Due to the erosive force needed for extracting materials, hydraulic mining is currently performed mainly using large, adjustable trajectory Big Gun sprinklers (Figure 3) or one of the water cannons from earlier days.
Turf Cooling and Cleaning
Not to be confused with using irrigation for natural turfgrass, irrigation used for artificial turf cooling and cleansing (Figure 4) is an additional but uncommon application.
Because artificial turf is manufactured from synthetic materials primarily made from plastic (polyethylene) fibers, high temperatures and radiation from direct sunlight can degrade, soften, and eventually destroy its surface if not adequately cooled.
The Synthetic Turf Council recommends periodic watering using irrigation sprinklers to reduce heat (through evaporative cooling) and improve sanitation and overall use in sporting activities. These recommendations are pertinent to either long-fiber infill systems such as for football and soccer, and short-fiber systems such as those used for field hockey.
Temperatures on typical synthetic turf using rubber infill can reach as high as 170°F since the infill material acts like an insulator and allows little heat to be absorbed to the underlying soil. It is important to note a high volume of water associated with high pressure is required for adequately cooling synthetic turf.
In order to obtain any of the protection expected from irrigating synthetic turf for cooling, a large volume of water must be applied evenly over the entire field, but this dramatically increases the required flow rate in a short period of time.
If not equipped with underground cooling loops, artificial turf can be cooled through the use of sprinkler irrigation with the most common methods consisting of an automated or manual sprinkler (impact or rotor) irrigation system, travelers, hand-move laterals, or portable Big Guns.
Automated systems consist of sprinkler heads that can be snapped into quick-connect (QC) couplings or pop-up heads in the field operated electrically from a controller in the case of an existing system originally installed for natural turf.
A traveler generally consists of a turbine-driven or cable-pull hard or soft hose portable machine. The traveler is set at one end of the field and works across it rapidly at a high output rate to apply the needed cooling water. A manual system can be as simple as the installation of QC couplings around a field’s perimeter and dragging a hose around to cool or clean the field.
Where adequate water and pressure are available, another popular system consists of large Big Guns that are operated manually. One at each end of the field or at the corners will generally do the job. These guns can shoot water nearly 25% to 50% of the field’s length. However, they must be physically moved to get total coverage.
Another advantage of turf irrigation is cleaning the field as the system can aid in properly grooming a synthetic field to maintain consistent cleanliness, performance, and safety. Many of the irrigation systems for this purpose use a municipal or fire protection water supply with high potential flow rates, but pressures generally limited to 60-100 psi unless a booster pump is added.
Livestock and Barn Cooling
Keeping cows, hogs, horses, poultry, and other livestock cool and safe from excessive heat, plus providing a measure of fresh air, is important for sustaining animals’ good health.
This is particularly true for dairy livestock, since reducing thermal stress to maintain milk production and reproduction during the summer months is a key issue in a sustained dairy operation. Sweltering summer heat can dramatically impact a livestock’s feed intake, daily weight gain or loss, reproductive performance, and ultimately milk production.
A temperature humidity index is generally used to evaluate the thermal stress of the barn’s environment. This index combines the relative humidity and temperature into a single value to estimate the potential environmental heat stress on animals. An environment is generally considered stressful for cattle when the temperature humidity index exceeds a value of 72.
Cooling systems can consist of air conditioning, fans, water-supplied misting and fogging, and sprinklers or a mixture of these.
Cooling systems can be designed to operate automatically or manually. Due to the higher cost and operating expense, air conditioning is generally not a viable solution, except in smaller contained areas where considerable human activity takes place such as milking parlors.
Circulating fans with a diameter of 2 to 24 feet can be used as permanent or portable devices moving air to maintain airflow beginning at one end, going through the structure, and out the other end to lower the air temperature as much as 10°F.
Fans are generally designed to move high volumes of air at a low blade speed and can be set in gables, roofs, or suspended within the barn.
When combined with a water misting or fogging system, temperatures can be lowered as much as 30°F. Misting and fogging water systems often combine the advantages of cool water with fan-driven airflow to distribute the mist throughout a barn.
By introducing a steady volume of moisture into the area, barn misting systems are a popular way to improve the health of livestock. The effectiveness of a misting system depends highly on balancing the airflow and water sent into the barn with the humidity because misting as a cooling solution relies on evaporation. Thus, excessive humidity will result in condensation, wetting of animals, and corrosion of metallic materials. Therefore, the lower the airflow, the less evaporation will occur.
Not only can excessive humidity limit its effectiveness for cooling, it can also leave the barn damp. This can result in higher incidents of animal illness and respiratory problems.
When properly used, mists do more than just cool the area. The added moisture helps reduce airborne particles like dust, which can help prevent respiratory issues. In dry regions, the added moisture in the air is also helpful to make the animals more comfortable. Lastly, the misting acts as a deterrent for small insects and helps keep them away from the livestock.
Impact sprinklers set over the roof are often used to reduce the heat within the barn underneath, especially when a metal roof is present.
Sprinklers are generally set up for pulse operation of 1 to 2 minutes followed by a preset off period. The on and off cycling periods depend on the actual heat index existing within the barn at the time.
The specific type of cooling system used is highly dependent on the region and its unique values of temperature, humidity, and degree of required temperature reduction. This type of system should only be designed by a person knowledgeable and experienced in heat transfer and the application of these systems.
This is the second most common use of irrigation technology behind only crop production. Wastewater land disposal can consist of domestic, municipal, commercial, industrial, and agricultural (dairy, swine, and other confined animals) applications and will usually be conducted for effluent or sludge disposal.
In the proper situation, virtually all methods of irrigation can be used for wastewater disposal, including impact and spray sprinkler, Big Gun (portable, stationary, or traveler), pivots and linears, surface applied methods (border, basin, and furrow), and drip/micro.
When used for crop production, the previously stated requirements of using freshwater irrigation for crop production are equally applicable when the source of irrigation water is treated wastewater. The primary goal here is to apply enough water and nutrients for crop use and uptake without applying excessive nutrients that could overload the soil or result in contaminating underlying groundwater.
Nutrients in municipal and dairy wastewater and treated effluents provide a greater percentage of these sources over most conventional irrigation water sources, and supplemental fertilizers are sometimes not necessary. However, added setbacks from property lines, stringent environmental and health requirements, and following the proper design and operation to prevent soil loading buildup and percolation into groundwater of excessive nitrates, phosphorus, and heavy metals must also be considered when municipal or agricultural wastewater is the source of irrigation water.
Although conventional pumping and irrigation equipment can be used for most effluent water disposal, the equipment required for land disposal of sludge is drastically different. While most effluent possess roughly the same consistency, weight, and solids content of clean to dirty (turbid) water, sludge is heavier. The specific gravity of sludge varies between 1.05 for effluent and dairy wastewater to 1.30 for dewatered sludge, contains more solids, and requires greater horsepower and head to pump at comparable flow rates.
In addition, sludge has a greater tendency to resist movement in a pipeline. Therefore, velocities must be maintained above 2.5 to 3 feet per second to avoid settling. Pumps used for wastewater disposal are typically classified as solids-handling pumps, with effluent pumps requiring a ½-inch to ¾-inch solids capability and sludge-handling pumps up to 3 inches spherical solids-handling ability.
When hard and soft hose travelers are used, engine-driven hydraulic systems (Figure 6) must be used for the power source, as turbine and bellows drives will not properly function and will plug with most wastewater.
Since plugging of sprinkler nozzles is always a concern, the use of Big Gun nozzles 1 inch in bore diameter or larger are usually selected over smaller impact sprinkler or drip irrigation systems.
Wastewater land application systems also have tighter controls placed on application rates to prevent overland runoff to adjacent properties, as well as greater setbacks from roadways and neighboring properties to prevent overspray or wind drift onto adjacent land.
Irrigation systems for wastewater disposal usually require advance design and review as well as restrictive EPA and state operating permits. Many permits require specific reporting information of applied flows and volume, ongoing soil conditions, and other operational data. The guidelines detailed in Table 2 can be used when considering using wastewater.
This concludes this edition of Engineering Your Business. Next month, we will wrap up this series with an overview on selecting the proper system,a review of the use of smart irrigation technology, and a summary about irrigation efficiency and ways to maintain it.
Until then, work safe and smart.
Ed Butts, PE, CPI, is the chief engineer at 4B Engineering & Consulting, Salem, Oregon. He has more than 40 years of experience in the water well business, specializing in engineering and business management. He can be reached at email@example.com.