Gravity Irrigation Systems and Practices Open-ditch conveyance systems have been the traditional means to supplying gravity irrigation systems. Open ditches 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 through siphon tubes, portals, or ditch gates. 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) application 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 to distribute water. Improved System and Practices: Pipeline conveyance 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. 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 systems differ from traditional border application 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 and elimination of surface runoff. 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 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. Initial water surges travel partway down the furrow, and all standing water is allowed to infiltrate. The wetted soil surface forms a water seal permitting successive surges to travel further down the furrow with less upslope deep percolation. This technique significantly reduces the time needed for water to be distributed the full length of the field, thereby increasing application efficiency. Cablegation is a gated-pipe system in which a moveable plug passes slowly 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 will gradually cease flowing into the first rows irrigated as the plug progresses 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 irrigation involves wetting every second furrow only. This technique limits deep percolation losses by encouraging lateral moisture movement. Applied water and time required per irrigation 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 furrows function much like alternative furrow irrigation, except that every row is irrigated with rows spaced further apart. Compacted furrows involve packing the soil within the furrow to provide a smooth, firm surface to speed water advance. Furrow diking places dikes in the furrows to capture additional rainfall, eliminating runoff and reducing irrigation needs. Furrow diking on gravity-irrigated fields is typically used in combination with alternate furrow irrigation. Tailwater reuse systems recover irrigation runoff in pits below the field and pump it to the head of the field for reuse. Pressurized Irrigation Systems and Practices Pipeline conveyance is most often used to deliver water to fields with pressurized systems. Water, once under pressure, requires a pipeline for conveyance. Pipelines may be above or below ground. 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 with sprinkler nozzles elevated above the surface. In some cases, handmove systems may be installed prior to the crop season and removed at or after harvest, effectively serving as solid set. Solid-set systems are commonly used in orchards and vineyards for frost protection and crop cooling, and are widely used in turf production and landscaping. Big gun systems use a large sprinkler mounted on a wheeled cart or trailer, fed by a flexible hose. The sprinkler is usually self-propelled while applying water. The system may require successive moves to irrigate 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 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 water 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 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 only a few inches above the ground. Applying water close to the ground cuts water loss from evaporation and wind and increases application uniformity. On soils with slower infiltration rates, furrow dikes are often used to avoid runoff. Low-flow irrigation systems include drip/trickle and micro-sprinkler systems. Drip and trickle systems use small-diameter tubes placed on 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 use a similar 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 a broader area or perimeter.) Low-flow systems are generally reserved for perennial crops, such as orchard products and vineyards, or high-valued vegetable crops. the Plains States. Alternate furrow irrigation is practiced on over 20 percent of gravity-flow acres, with special furrows (widespaced, compacted, or diked) applied on more than 10 percent of acres. Roughly 5 percent of FRIS respondents indicated that water runs had been shortened to facilitate water management, primarily in the Southwest (Arizona, California) and Southern Plains. About 12 percent of all irrigated acres have been precision laser-leveled, predominantly on gravity-flow systems in the Southwest, Delta, and Southeast regions. Highefficiency level-basin systems are concentrated in the Southwest. Deficit irrigation techniques-such as reduced irrigation set-times, partial-field irrigation, and reduced irrigations are practiced on roughly 10 percent of gravity-flow acres, with highest acreage concentrations in the Northwest (Washington, Oregon, Idaho). Tailwater reuse systems-which recirculate runoff water on the field-have been installed on over 20 percent of gravity-system acreage nationwide. Tailwater reuse systems are disbursed throughout the major gravity-irrigated States, with California leading both in total acreage (1.9 million) and share of gravity acres (38 percent) with tailwater systems. Pressurized Systems. The decline in gravity-flow acreage has been accompanied by an increase in acreage under pressurized systems. Pressurized systems including sprinkler and low-flow irrigation systems-use pressure to distribute water. With rare exceptions, the pressure to distribute water involves pumping, which requires energy. Acreage in pressurized systems expanded from 19 million acres (37 percent of total irrigated acreage) in 1979 to 23 million acres (50 percent) in 1994 (table 4.6.2). Sprinkler systems-in which water is sprayed over the field surface, usually from above-ground piping-accounted for 46 percent of irrigated acreage in 1994 (table 4.6.3). Concentrations of sprinkler acreage are highest in the Northern Pacific, Northern Plains, and Northern Mountain States. Sprinkler systems are also used extensively for supplemental irrigation and specialty-crop irrigation in the humid eastern States. Sprinkler irrigation has been adopted in many areas as a water-conserving alternative to gravity-flow systems. Field application efficiencies typically range from 60 to 85 percent under proper management (Negri and Hanchar, 1989). Sprinklers may be operated on moderately sloping or rolling terrain unsuited to gravity systems, and are well suited to coarser soils with higher water infiltration rates. Sprinkler design is important, and careful consideration of soil type, wetting area per spray nozzle, operating pressure, and the rate of sprinkler movement are required to avoid plant stress from too little water and excess runoff from too much water. Capital costs for sprinkler systems are higher than for gravity-flow systems, although gravity-system installation often requires greater expenditures for land preparation. Operating costs for sprinkler systems are often higher than for gravity systems as they require more energy and more sophisticated technical and management capability. Labor costs are typically lower under sprinkler systems, particularly with self-propelled systems. Sprinkler technologies include a wide range of adaptations, with significant shifts in technology shares in recent years. The development of self-propelled center-pivot systems in the 1960's greatly expanded the acreage suitable for irrigation, and accounted for much of the growth in acreage irrigated during the 1970's. Acres irrigated with center pivots increased by 6.2 million acres from 1979 to 1994, with about half of the increase attributable to net increases in irrigated area under sprinkler and about half from the net replacement of other sprinkler types with center pivot (table 4.6.2). Center-pivot systems accounted for nearly 70 percent of sprinkler acreage in 1994, or 32 percent of total irrigated acreage (table 4.6.3). Largest acreage concentrations under center-pivot are in the Northern Plains, Southern Plains, and Delta regions. Sprinkler systems other than center pivot-including hand move, mechanical move, and solid set-made up about 31 percent of total sprinkler acreage in 1994, down from 53 percent in 1979. Acreage in handmove systems has declined by nearly one-half since 1979; mechanical-move systems have declined by more than 25 percent (table 4.6.2). Center-pivot technology serves as the foundation for many technological innovations—such as lowpressure center pivot, linear-move, and low-energy precision application (LEPA) systems—which combine high application efficiencies with reduced energy and labor requirements. Approximately 40 percent of center pivot acres in 1994 were operated under low pressure (below 30 pounds per square inch (psi)), with just 22 percent operating at high pressure (above 60 psi). (Forty-two percent of center pivot acres were high-pressure systems as recently as 1988.) Adoption of low-pressure systems has been particularly strong in the Southern Plains, reflecting higher-cost groundwater pumping in much of the region. Current advances in sprinkler technology focus on location of spray heads and low-pressure sprinklers and nozzles; the trend is toward energyand water-conserving nozzles located closer to the soil. In addition, advances are being made in remote control of sprinklers and individual nozzle control for precision agriculture. Low-flow irrigation systems are a form of pressurized system in which water is applied in small, controlled quantities near or below ground level. Low-flow irrigation systems-including drip, trickle, and micro-sprinklers-comprise 4 percent of irrigated cropland acreage (table 4.6.3), up more than four-fold since 1979 (table 4.6.2). Low-flow systems are most commonly used for production of vegetables and perennial crops such as orchards and vineyards, although experimentation and limited commercial applications are occurring with certain row and field crops. Low-flow irrigation systems are located primarily in California and Florida, reflecting large acreages in specialty produce and orchard production. Field application efficiency of 95 percent or greater can be achieved under low-flow systems, although proper design is required to avoid moisture stress and soil-salinity accumulation. High capital costs and short lifespan of components characterize most systems. Filtration of the water supply and careful system maintenance may be required to prevent clogging of small orifices. Advances in low-flow technology focus on field depth and spacing of tubing, emitter spacing, durability of materials, and reduced costs. Water Management Practices Determining when and how much irrigation water to apply is an important part of the irrigation management process. Well-informed decisions increase the likelihood that water is applied according to crop needs, with minimal water loss. Improved management practices are often more cost-effective than structural improvements, although structural upgrades may be required to achieve highest management potential. Irrigation scheduling involves the application of irrigation water based on a systematic monitoring of crop soil-moisture requirements. Sophisticated scheduling methods-based on sensors, microprocessors, and computer-aided decision tools may be used to determine the optimal timing and depth of irrigation to meet changing crop needs over the production season. Various methods are available to assess crop water needs. Crop water requirements can be indirectly estimated through climate variables. Local weather-station data-including temperature, humidity, wind speed, and solar radiation-are applied in formulas to calculate crop water needs for a wide range of crops and locales. Soil moisture available for plant growth may also be measured directly through periodic soil testing. Soil probes are used to obtain soil samples at various depths for "feel and visual" evaluation. More sophisticated devices such as tensiometers, neutron probes, and various electrical conductivity devices-can be used to accurately quantify the amount of water removed from the soil profile. Finally, plant moisture monitors may be used to detect crop water availability and stress in plant tissue. In separate Farm and Ranch Surveys for years 1984 and 1994, irrigators were asked to indicate all methods used in deciding when to irrigate (USDC, 1986 and 1996). Survey results suggest that a slightly larger share of irrigators are using advanced, information-intensive methods to schedule irrigation, but that current levels indicate potential for much improvement. In the 1994 FRIS, 10 percent of irrigators used soil moisture-sensing devices (up from 8 percent in 1984), 5 percent used commercial scheduling (up from 3 percent), 4 percent used media reports on plant water requirements (down 1 percent), and 2 percent used computer simulations (not asked in 1984). Water flow measurement is an important component of water management at the farm level. Measurement of water flows through the onfarm conveyance system ensures optimal water deliveries to the field, as determined by irrigation scheduling methods. Measuring devices-often installed in conjunction with conveyance system upgrades-include weirs, flumes, and in-canal flow meters for open ditches, and external and internal meters for pipe. Irrigation Drainage Systems The collection and disposal of drainage flows from irrigation and precipitation is an important management consideration in many irrigated areas. Irrigation drainage includes surface runoff and deep percolation from water applied to meet crop consumptive needs. In some areas, periodic flooding of fields may also be required to leach soil salts from the crop root zone, often increasing the need for drainage systems. Irrigation drainage is often collected and reused in irrigated production. Tailwater systems recover drainage flows below the field (or in low-lying areas of the farm), recirculating the water to the top of the field for reuse. Drainage flows may also be used as irrigation supplies downslope, both onfarm and off-farm. In some cases, drainage systems may be used to drain excess water during wet periods as well as "subirrigate" during dry periods by regulating underlying water tables. In many cases, drainage flows of poor quality become a disposal issue. Primary disposal methods include onfarm evaporation ponds, direct discharge to off-farm surface water bodies through drainage canals, and reuse in salt-tolerant crop and tree production. Other Practices Affecting Irrigation Other practices-while not water-management practices per se-can be important components of an irrigated farming system. Such practices, in combination with improved irrigation systems, may enhance returns to irrigated production while reducing offsite environmental impacts. Nutrient and Pest Management. Irrigation affects the optimal timing and application rate of chemical applications for nutrient and pest management. Fertilizer use is typically greater for high-valued, high-yielding irrigated production. Weed and pest conditions may also increase under irrigated field conditions, necessitating increased use of pesticides, herbicides, and fungicides. Careful nutrient and pest management increases the effectiveness of water and applied chemicals, while reducing offsite impacts. Chemigation or the application of fertilizers, pesticides, and other chemicals through irrigation water-permits controlled applications when used in conjunction with highly efficient irrigation systems. Chemigation can reduce the costs of applying chemicals, while avoiding equipment use and soil compaction. Chemigation is used on all major crops, with the largest treated acreages in orchard crops, hay, and corn-and the greatest concentration of use in potato, rice, and sugarbeet production (USDC, 1996). Erosion Control. Soil erosion can be a serious problem for less efficient irrigation systems on sloping fields. Soil erosion creates barriers to even water flow in furrows, reduces long-term field productivity, and contributes to offsite water-quality problems. Irrigation-induced erosion is particularly severe in areas of the Northern Pacific, Southern Pacific, and Mountain regions (USDA, 1992). Measures to improve uniformity of applied irrigation water can help control soil loss. Gravity-flow systems may be modified to reduce flow velocity or field slope in accordance with soil-water infiltration rates. Soil erosion may also be a problem with sprinkler systems, particular on steeply sloping fields and under outer spans of center-pivot systems where water application rates are higher. System adjustments to reduce erosion include reduced water applications per irrigation set, larger pattern sprinkler heads, and booms to increase sprinkler head spacing. Other practices may also limit soil erosion on irrigated fields. Crop residue management to maintain vegetative material on the soil surface increases infiltration while protecting the soil from erosive water flow. In some cases, deep tillage can reduce runoff through increased infiltration. Land treatment measures may be installed to slow runoff and trap sediment on the farm. These include furrow dikes in the field, vegetative filter strips below the field, mini-basins in tailwater ditches, larger sediment ponds constructed in drainage ditches, and tailwater reuse systems. A promising new soil amendment-Polyacrylamide, more commonly known as PAM-may be added to irrigation water to stabilize soil and water-borne sediment. Under experimental field-trial conditions, proper application of PAM with the first irrigation has substantially reduced soil erosion in furrow systems. Potential benefits include reduced topsoil loss, enhanced water infiltration, improved uptake of nutrients and pesticides, reduced furrow-reshaping operations, and reduced sediment-control requirements below the field. An estimated 50,000 irrigated acres were treated with PAM after just 1 year on the market, including 30,000 acres in the Pacific Northwest. Research is underway to determine the best PAM formulations and application techniques (Sojka and Lentz, 1996). Irrigation Technology and Environmental Adoption of improved irrigation technology has been advanced as a means to reduce offsite water quantity and quality problems. The effectiveness of technology in achieving environmental goals has important implications for regional water policy. Water Conservation Improved irrigation and conveyance technologies may substantially increase onfarm water-use efficiency. Whether technology adoption can achieve significant |