Irrigation and Water Use: Glossary

Irrigation Systems and Land Treatment Practices

Onfarm Water Conveyance Systems

Open-ditch conveyance systems may be earthen, although improved systems are typically lined with concrete or other less permeable materials to reduce seepage loss. Water is delivered to gravity-flow fields by siphon tubes, portals or ditch gates, or pumped directly for certain pressurized systems.

Pipeline systems are often installed to reduce labor and maintenance costs, as well as water losses to seepage, evaporation, spills, and noncrop vegetative consumption. Underground pipeline constructed of steel, plastic, or concrete is permanently installed; above-ground pipeline generally consists of lightweight, portable aluminum, plastic, or flexible rubber-based hose. One form of above-ground pipeline—gated-pipe—distributes water to gravity-flow systems from individual gates (valves) along the pipe. Pipeline systems are the predominant means of water conveyance for pressurized application systems.

Gravity-Flow Application Systems

Furrow systems, the dominant gravity application system, are distinguished by small, shallow channels used to guide water downslope across the field. Furrows are generally straight, although they may be curved to follow the land contour on steeply sloping fields. Row crops are typically grown on the ridge or bed between the furrows, spaced from 2 to 4 feet apart. Corrugations—or small, closely spaced furrows—may be used for close-growing field crops.

Border (or flood) systems divide the field into strips, separated by parallel ridges. Water flows downslope as a sheet, guided by ridges 10 to 100 feet apart. On steeply sloping lands, ridges are more closely spaced and may be curved to follow the land contour. Border systems are suited to orchards and vineyards, and close-growing field crops such as alfalfa, pasture, and small grains.

Uncontrolled flooding is a gravity-flood system without constructed ridges, relying on natural slope only to distribute water across the field.

Improved Gravity-Flow Systems and Practices

Field leveling involves grading and earthmoving to eliminate variation in field gradient—smoothing the field surface and often reducing field slope. Field leveling helps to control water advance and improve uniformity of soil saturation under gravity-flow systems. Precision leveling is generally undertaken with a laser-guided system.

Level basin (or dead-level) systems differ from traditional border systems in that field slope is level and field ends are closed. Water is applied at high volumes to achieve an even, rapid ponding of the desired application depth within basins. Higher application efficiencies reflect uniform infiltration rates across the field and elimination of surface runoff. Precision laser leveling is required to achieve level fields suitable for this method.

Shortened water runs reduce the length of furrow (or basin) to increase uniformity of applied water across the field. Reduced water runs are most effective on coarse soils with high soil-water infiltration rates. Water runs of one-half to one mile in length may be reduced to one-quarter mile or less (with an appropriate reorganization of the onfarm conveyance system).

Surge flow is an adaptation of gated-pipe systems in which water is delivered to the furrow in timed releases controlled by a valve. Furrows are alternately wetted and allowed to dry. As the soil dries, the soil surface forms a water seal permitting the next surge of water to travel further down the furrow with less upslope deep percolation. This technique significantly reduces the time needed for irrigation water to be distributed the full length of the field, thereby reducing deep percolation and increasing application efficiency.

Cablegation is a gated-pipe system in which a moveable plug is allowed to slowly pass through a long section of gated pipe, with the rate of movement controlled by a cable and brake. Due to the oversizing and required slope of the pipe, water flow will gradually cease flowing into the first rows irrigated after the plug has progressed sufficiently far down the pipe. Improved water management is achieved by varying the speed of the plug, which controls the timing of water flows into each furrow.

Alternate furrow irrigations involve wetting every second furrow only. This technique limits deep percolation losses by encouraging lateral moisture movement. Applied water and time required to irrigate each time may be significantly less than under full furrow systems, but more irrigations may be required to supply crop needs. This technique is very effective when the desired strategy is to irrigate to a “less than field capacity” level in order to more fully utilize rainfall.

Special furrows have been employed to enhance water management. Wide-spaced furrowsfunction much like alternative-row irrigation, except that every row is irrigated but rows are further apart. Compacted furrows involve compacting the soil in the bottom of the furrow to provide a smooth, firm surface to speed water advance. Furrow diking places dikes in the furrows to capture additional rainfall, thereby eliminating runoff and reducing irrigation requirements. Furrow diking is typically used on irrigated fields in combination with alternative furrow irrigations (in the non-irrigated row) or low pressure sprinklers on fine textured soils.

Tailwater reuse systems recover irrigation runoff in pits below the field and pump the water to the head of the field for reuse.

Pressurized Application Systems

Center-pivot sprinklers are the dominant pressure technology. A center-pivot sprinkler is a self-propelled system in which a single pipeline supported by a row of mobile A-frame towers is suspended 6 to 12 feet above the field. Water is pumped into the pipe at the center of the field as towers rotate slowly around the pivot point, irrigating a large circular area.

Sprinkler nozzles mounted on or suspended from the pipeline distribute water under pressure as the pipeline rotates. The nozzles are graduated small to large so that the faster moving outer circle receives the same amount of water as the slower moving inside.

Typical center-pivot sprinklers are one-quarter mile long and irrigate 128- to 132-acre circular fields. Center pivots have proven to be very flexible and can accommodate a variety of crops, soils, and topography with minimal modification.

Hand move is a portable sprinkler system in which lightweight pipeline sections are moved manually for successive irrigation sets of 40 to 60 feet. Lateral pipelines are connected to a mainline, which may be portable or buried. Handmove systems are often used for small, irregular fields. Handmove systems are not suited to tall-growing field crops due to difficulty in repositioning laterals. Labor requirements are higher than for all other sprinklers.

Solid set refers to a stationary sprinkler system. Water-supply pipelines are generally fixed—usually below the soil surface—and sprinkler nozzles are elevated above the surface. In some cases, handmove systems may be installed prior to the crop season and removed after harvest, effectively serving as solid set. Solid-set systems are commonly used in orchards and vineyards for frost protection and crop cooling. Solid-set systems are also widely used on turf and in landscaping.

Big gun systems use a large sprinkler mounted on a wheeled cart or trailer, fed by a flexible rubber hose. The machine may be self-propelled while applying water, traveling in a lane guided by a cable. Other systems may require successive moves to travel through the field. Big guns require high operating pressures, with 100 psi not uncommon. These systems have been adapted to spread livestock waste in many locations.

Side-roll wheel-move systems have large-diameter wheels mounted on a pipeline, enabling the line to be rolled as a unit to successive positions across the field. A gasoline engine generally powers the system movement. This system is roughly analogous to a handmove system on wheels. Crop type is an important consideration for this system since the pipeline is roughly 3 feet above the ground.

Improved Pressurized Systems and Practices

Improved center pivots have been developed that reduce both water application losses and energy requirements . Older center pivots, with the sprinklers attached directly to the pipe, operate at relatively high pressure (60-80 psi), with wide water-spray patterns. Newer center pivots usually locate the sprinklers on tubes below the pipe and operate at lower pressures (15-45 psi). Many existing center pivots have been retrofitted with system innovations to reduce losses and energy needs.

Linear or lateral-move systems are similar to center-pivot systems, except that the lateral line and towers move in a continuous straight path across a rectangular field. Water may be supplied by a flexible hose or pressurized from a concrete-lined ditch or along the field edge.

LEPA (Low-energy precision application) is an adaptation of center pivot (or lateral-move) systems that uses droptubes extending down from the pipeline to apply water at low pressure below the plant canopy, usually on the ground or only a few inches above the ground. Applying the water close to the ground cuts water loss from evaporation and wind and increases application uniformity. On fine-textured soils with slower infiltration rates, furrow dikes may be necessary to avoid runoff.

Low-flow irrigation systems—including drip and trickle—use small-diameter tubes placed above or below the field’s surface. Frequent, slow applications of water are applied to soil through small holes or emitters. The emitters are supplied by a network of main, submain, and lateral lines. Water is dispensed directly to the root zone, precluding runoff or deep percolation and minimizing evaporation. Micro-sprinklers—a variation of low-flow systems—use the same type of supply system, with low-volume sprinkler heads located about 1 foot above the ground. (Micro-sprinklers are used in place of multiple drip emitters when wetting an area or perimeter is needed). Low-flow systems are generally reserved for perennial crops, such as orchard products and vineyards, or other high-valued vegetable crops.

Water Use Terms

Acre-foot—A volume of water covering an acre of land to a depth of 1 foot, or 325,851 gallons.

Consumptive use—Amount of withdrawn water lost to the immediate water environment through evaporation, plant transpiration, incorporation in products or crops, or consumption by humans and livestock.

Ground water—Generally all subsurface water as opposed to surface water. Specifically, water from the saturated subsurface zone (zone where all spaces between soil or rock particles are filled with water).

Industrial withdrawals/use (other than thermoelectric)—Includes the water withdrawn/consumptively used in facilities that manufacture products (including use for processing, washing, and cooling) and in mining (including use for dewatering and milling).

Irrigation withdrawals/use—Includes the water withdrawn/consumptively used in artificially applying water to farm and horticultural crops. Some data sources include water to irrigate recreational areas such as parks and golf courses.

Loss—Water that is lost to the supply, at the point of measurement, from a nonproductive use, including evaporation from surface-water bodies and nonrecoverable deep percolation.

Overdrafting—Withdrawing ground water at a rate greater than aquifer recharge, resulting in lowering of ground-water levels. Also referred to as aquifer mining.

Public and rural domestic withdrawals/use—Includes the water withdrawn/consumptively used by public and private water suppliers and by self-supplied domestic water users.

Recharge—The percolation of water from the surface into a groundwater aquifer. The water source can be precipitation, surface water, or irrigation.

Return flow—Water that reaches a surface-water source after release from the point of use, and thus becomes available for use again.

Surface water—An open body of water such as a stream, river, or lake.

Thermoelectric withdrawals/use—Includes the water withdrawn/consumptively used in the generation of electric power with fossil-fuel, nuclear, or geothermal energy.

Irrigation water application—The depth of water applied to the field. Irrigation application quantities differ from irrigation withdrawals by the quantity of conveyance losses.

Withdrawal—Amount of water diverted from a surface-water source or extracted from a groundwater source.

Irrigation Water-Use Efficiency Terms

Water-use efficiency measures are commonly used to characterize the water-conserving potentials of alternative irrigation systems. However, “efficiency” may be measured at various points within the hydrologic cycle, with somewhat different implications for water savings.

Irrigation efficiency—broadly defined at the field level, is the ratio of the average depth of irrigation water beneficially used (consumptive use plus leaching requirement) to the average depth applied, expressed as a percentage.

Application efficiency—is the ratio of the average depth of irrigation water stored in the root zone for crop consumptive use to the average depth applied, expressed as a percentage. Crop-water consumption includes stored water used by the plant and field surfaces. Leaching requirement, which accounts for the major difference between irrigation efficiency and application efficiency, is the quantity of water required to flush soil salts below the plant root zone. Field-level losses include surface runoff at the end of the field, deep percolation below the crop-root zone (not used for leaching), and excess evaporation from soil and water surfaces.

Conveyance efficiency—is the ratio of total water delivered to the total water diverted or pumped into an open channel or pipeline, expressed as a percentage. Conveyance efficiency may be computed at the farm, project, or basin level. Conveyance losses include evaporation, ditch seepage, operational spills, and water lost to noncrop vegetative consumption.

Project efficiency—is calculated based on onfarm irrigation efficiency and both on- and off-farm conveyance efficiency, and is adjusted for drainage reuse within the service area. Project efficiency may not consider all runoff and deep percolation a loss since some of the water may be available for reuse within the project.