Farm irrigation systems are the methods of applying water to crops and are classified as surface irrigation, sprinkler irrigation, and microirrigation. The decision to select an irrigation system or convert to a more efficient irrigation system is complicated. From a water conservation standpoint the choice is simple, with water savings increasing as surface irrigation systems are changed to sprinkler systems and as sprinkler systems are changed to microirrigation systems. However, the success of an irrigation system will be highly dependent on site and situation factors as well as the level of management employed. Existing irrigation systems should be carefully evaluated before switching to alternative irrigation systems.
Surface Irrigation Systems
Surface irrigation systems are classified in order of increasing efficiency as: (1) flood irrigation,
(2) border irrigation, (3) furrow irrigation, and (4) basin irrigation. The two features that distinguish surface irrigation from other methods of irrigation are that the water flows freely in response to gravity, and the on-fi eld means of conveyance and distribution is the field surface (Walker, 1989).
Uncontrolled flooding is the application of irrigation water from field ditches whereby little attempt is made to control the flow on the field by means of levees or other methods that restrict water movement (Schwab et al., 1993). This method is frequently referred to as wild flooding. Although these systems are advantageous for their low initial cost and labor requirements, they are disadvantageous for their low efficiency and uniformity. This method is mainly used on rolling land where border, basins, and furrows are not feasible and where adequate water supply is available.
Border irrigation is the application of water to sloping, long rectangular lands, and free draining conditions at the lower end of the fi eld (Walker, 1989). Border strips are typically placed in the direction of the greatest slope, are
30 to 65 ft in width, 300 to 1300 ft in length, and have small ridges between the strips to prevent water from overtopping during irrigation (Schwab et al., 1993). Land between borders should be leveled perpendicular to the direction of flow. Border irrigation is suitable for most crops and soil types, but is favored by slow to moderate intake soils and crops that can tolerate prolonged ponding. In Colorado, basin irrigation is primarily used on closely spaced crops such as alfalfa, grass and small grains, but not row crops.
Although water covers the entire surface area of a field in other surface irrigation methods, irrigation by furrows covers one-fifth to one-half the surface. Furrows vary in size and can be placed up and down the slope or on the contour. Small, shallow furrows are called corrugations and are typically used for close growing crops such as small grains and alfalfa. Larger, deeper furrows are suitable for row crops such as corn.
Furrows provide better on-farm water management flexibility under many surface irrigation conditions. The discharge per unit width of the field is substantially reduced and can therefore be practicedon slopes as steep as 12%, if furrows are placed on the contour with the appropriate non-erosive stream size. If furrows are not placed on a contour the maximum recommended slope is 3% or less. A smaller wetted area in furrow irrigation also reduces evaporation losses. Furrows provide the irrigator with more opportunity to efficiently manage irrigations as field conditions change throughout the season.
However,furrow irrigation is not always efficient and can produce significant runoff if a constant inflow rate is maintained throughout the application period. Several methods can be used to reduce runoff such as cutback operations, surge irrigation, and reuse systems (See Information Sheet No. 5).
Basins are typically rectangular in shape, level in all directions, and are encompassed by a dyke to prevent runoff. Inflow to basins is generally undirected and uncontrolled and can be relatively efficient if high rates of flow are available to quickly cover the field (Schwab et al., 1993). There are few crops and soils not amenable to basin irrigation, but it is best suited for moderate to slow intake soils, deep-rooted, and closely spaced crops (Walker, 1989). Precision land leveling is very important to achieving high uniformity and efficiency in all surface irrigation methods (See Information Sheet No. 5).
Sprinkler Irrigation Systems
Sprinkler irrigation is a versatile means of applying water to any crop, soil, and topographic condition (Schwab et al., 1993).
Sprinkler systems can be efficient on soils and topography that is not suitable or efficient for surface irrigation methods. In general, systems are described according to the method of moving the lateral lines on which various types of sprinklers are attached.
Laterals may be solid set or rotating, the latter which can be moved by hand or mechanically. Sprinkler systems are highly efficient but there are general concerns about the labor requirements and investment costs for these systems.
Hand-move laterals have the lowest investment cost but the highest labor requirement. These systems are only suitable for low-growing crops.
The side roll lateral system uses the irrigation pipe as the axle of large diameter wheels that arespaced about 40 ft apart. These laterals are moved by a gasoline-powered motor and thus require less labor than hand-move systems. Side rolls should be used for crops that will not interfere with the movement of the lateral or sprinkler pattern.
Center pivots consist of radial pipelines that rotate around a central pivot by water pressure, electric motors, or oil hydraulic motors (Schwab et al., 1993).
A variety of nozzle types, nozzle heights, and application rates can be used in center pivot systems. Sprinkler packages should be selected according to the field conditions for the most efficient operation (See Information Sheet No. 4).
Linear move laterals use hardware similar to that of a center pivot, but move in a straight line across the field. Solid-set systems have sprinklers that are placed over the entire field, where all or some of the sprinklers may operate at the same time.
Center pivots are the most common sprinkler irrigation method used in the High Plains of
Colorado. Sprinkler packages vary greatly from older impact heads to more modern spray heads that have an assortment of application and placement modes (Howell, 2003). See Information Sheet No. 3 for more on center pivot irrigation systems.
Microirrigation is a method for delivering slow, frequent applications of water to the soil using a low pressure, low volume distribution system and special flow-control outlets (Schwab et al.,
1993). If managed properly, microirrigation can increase yields and decrease water, fertilizer, and labor requirements. Microirrigation includes: microsprinklers, drip irrigation, and subsurface drip irrigation
(SDI).Microsprinklers, often referred to as minisprayers, microsprayers, and misters, typically consist of small emitters placed on short risers above the soil surface. Water is conveyed through the air, but travels only a short distance before reaching the soil surface. The wetted area of emitters in these systems is small, can be controlled fairly easily, and has different shapes to match desired distribution patterns.
Theadvantages of microsprinkler irrigation systems are the potential for controlling frost, greater flexibility in applying water, and lower susceptibility to clogging.
Drip systems deliver water directly to the soil surface or subsurface (SDI) and allow water to dissipate under low pressure in a predetermined pattern. These systems are advantageous because water is applied directly to or just above the root zone of the plant, thereby minimizing deep percolation losses, reducing or eliminating the wetted area from which water can evaporate, and eliminating losses associated with runoff. These systems are also advantageous because they reduce water consumption by weeds, while operating at a lower pressure. Micro irrigation systems apply water on a high-frequency basis and create near optimal soil moisture conditions for the crop. Under proper management, micro irrigation saves water because only the plant’s root zone is supplied with water and little, if any, is lost to deep percolation, consumption by non-beneficial plants, or soil surface evaporation.
In addition to being highly efficient, these systems also require relatively little labor input if designed properly.
Yields of some crops have been shownto increase under these systems because the high temporal soil water level needed to meet transpiration requirements is maintained (Colaizziet al., 2003).
The major disadvantages of microirrigation systems are high initial cost and potential for system clogging, especially the emitters. In some cases, labor inputs may be quite high if rodents burrow and chew system components. Proper design, operation and maintenance can overcome many of these issues. Subsurface drip irrigation ( SDI ) is becoming more popular in the High Plains of Colorado. See information Sheet No.6 for a discussion on SDI systems.
Efficiency of Irrigation Systems
There are many efficiency terms used to describe irrigation system performance. Field orapplication efficiency is defined as:
Ef= 100 Ws/Wd
Ws= water stored in the soil root zone by irrigation
Wd= water delivered to the fi eld being irrigated
The difference between water stored in the root zone (Ws) and the amount of water delivered
to farm or fi eld (Wd) is water loss in the form of deep percolation, runoff, and evaporation. More
specifically, field efficiency includes any application losses to evaporation or seepage from surface
water channels or furrows, any leaks from sprinkler or drip pipelines, percolation beneath the root zone,
drift from sprinklers, evaporation of droplets in the air, or runoff from the fi eld (Howell, 2002). For a
discussion on the various water loss components associated with surface, sprinkler, and microirrigation
systems, see Rogers et al. (1997). The amount and type of water loss that occurs in the transfer of water
from water source to where the crop actually uses water is highly dependent on the type of irrigation
delivery and distribution system used (See Information Sheet No. 1). Table 1 and Figure 4 show potential
fieldefficiencies for the various distribution systems.
The relative difference between efficiency values of different irrigation systems is a result of
changes in the amount of runoff and deep percolation and sometimes evaporation. The difference is
not a result of changing the amount of water that crops actually consume (transpiration). For example,
changing from a gradedfurrow of 65% efficiencyto a well maintained SDI of
90%efficiency will result ina 25% water savings. Thiswater savings is primarily aresult of a reduction in therunoff and deep percolationassociated with the furrowirrigation system. The SDIsystem may also reduceevaporation because waterapplication occurs belowthe soil surface and the soilsurface remains dry, unlikethe furrow system. Thereis not a difference in theamount of water that isconsumed by a crop grownunder both systems. The Eor evaporation componentof ET (evapotranspiration)might change, but the T ortranspiration component willnot.
Irrigation System Efficiency Comparisons
When the decision is made to change the method of irrigation distribution, the water savings that
can be expected is the difference between the field efficiency values for the two methods. Increasing the field efficiency by 10% will reduce the amount of water needed to achieve the same yield under the originalsystem by 10% if the new system is operated properly. Proper design, management, and maintenance ofirrigation systems will ultimately determine achievable efficiency levels. These issues are particularlyimportant when a producer chooses to convert an existing irrigation method to a more efficient, watersaving method.
Comparison of Irrigation Methods
Changing from surface irrigation to sprinkler irrigation is one of the most common conversions
used to save water (Yonts, 2002). The reason for this conversion is that surface irrigation is inherently aless efficient and more labor intensive than sprinkler irrigation. Many factors should be considered beforeconverting from a surface to a sprinkler irrigation system including: yield response, water savings, laborsavings, energy savings, economic cost, climate conditions, and fi eld characteristics. For a more completediscussion on the conversion from surface to sprinkler, see Yonts (2002), O’Brien and Lamm (1999),Heermann (1992), Heermann (1991), O’Brien and Lamm (2000), and Rogers (1991). For a more completediscussion on the conversion from sprinkler to SDI systems, see Lamm et al. (2003) and O’Brien et al.(1998).
To choose an irrigation method, the producer must know the advantages and disadvantages of the
various methods. Unfortunately, in many cases there is no single best solution because all methods have
their advantages and disadvantages (Brouwer et al.). Table 2 provides a comparison of irrigation systemsin relation to site and situation factors (adapted from Schwab et al., 1993). This table also sets forth theadvantages and disadvantages of one irrigation system relative to another system. These issues should beconsidered before conversion to a more efficient system. If an irrigation system is not well suited for aparticular situation, it may not be any more efficient or save any more water than the original method ofirrigation.
The water savings that can be expected by changing to a different irrigation method is the difference between the filed efficiency values foe the two method.