Irrigation Systems and Land Treatment Practices
Onfarm Water Conveyance Systems
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 pipelinegated-pipedistributes
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
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. Corrugationsor small, closely
spaced furrowsmay be used for close-growing field
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.
is a gravity-flood system without constructed ridges,
relying on natural slope only to distribute water across
Improved Gravity-Flow Systems
Field leveling involves grading and earthmoving
to eliminate variation in field gradientsmoothing
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).
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
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
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.
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.
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.
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 fixedusually
below the soil surfaceand 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.
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.
(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.
irrigation systemsincluding drip and trickleuse
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-sprinklersa variation of low-flow systemsuse
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
Water Use Terms
of water covering an acre of land to a depth of 1 foot,
or 325,851 gallons.
of withdrawn water lost to the immediate water environment
through evaporation, plant transpiration, incorporation
in products or crops, or consumption by humans and livestock.
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
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
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.
LossWater that is lost to the supply, at
the point of measurement, from a nonproductive use, including
evaporation from surface-water bodies and nonrecoverable
OverdraftingWithdrawing 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/useIncludes
the water withdrawn/consumptively used by public and private
water suppliers and by self-supplied domestic water users.
RechargeThe percolation of water from the
surface into a groundwater aquifer. The water source can
be precipitation, surface water, or irrigation.
Return flowWater that reaches a surface-water
source after release from the point of use, and thus becomes
available for use again.
Surface waterAn open
body of water such as a stream, river, or lake.
the water withdrawn/consumptively used in the generation
of electric power with fossil-fuel, nuclear, or geothermal
Irrigation water applicationThe
depth of water applied to the field. Irrigation application
quantities differ from irrigation withdrawals by the quantity
of conveyance losses.
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 efficiencybroadly 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 efficiencyis 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 efficiencyis 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
Project efficiencyis 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.