Irrigation Chapter 8 - Irrigation Efficiencies

Author: Bill Kranz, University of Nebraska Lincoln Extension Irrigation Specialist, Northeast Research and Extension Center, Norfolk, NE.

The ability to manage an irrigation system is contingent on an accurate estimate of the percentage of water pumped that becomes available for crop use. No irrigation­ system delivers water at 100 percent efficiency. Water may be lost through delivery systems or pipelines and some water may remain in the soil, but not be used by the crop. Some water may run off the soil surface into lowland areas. Still other water may be lost to evaporation in the air, or from the soil and plant surfaces. Figures 8.1a and 8.1b show the major­ losses for sprinkler and surface irrigation systems. To know how much water to pump, these losses must be totaled and added to the amount of water needed by the crop.

In most cases, the goal is to insure that all areas of the field receive a set amount of uniformly applied water. Consider the catch can test data shown in Figure­ 8.2. The cans recording application depths below­ the horizontal line are not receiving enough water — catches are less than the desired 0.85 inches. Another application will be needed to insure that the entire field receives at least 0.85 inches of water. This will require using more water and energy than is necessary­. If this pattern occurs during each irrigation, plants in the areas receiving less than 0.85 inches eventually could experience water stress. The cans record­ing­ application depths above the line receive at least 0.85 inches of water. Any extra water applied could lead to surface runoff or deep percolation.

Water application efficiency accounts for how uniformly the water is applied and can be used for other assessments. If the center pivot owner is trying to decide whether switching to a new sprinkler package would be economical, the change in water application efficiency could be a major factor. If water becomes limited, changing to a system with a higher water application efficiency will provide more useable water to the crop and reduce pumping costs.

To maximize  irrigation water use, it must be uniformly applied in the right amount and at the right time. Reaching these objectives requires knowledge of water delivery characteristics, field soils and slopes, and the expected crop water use rates.

Mathematical relationships have been developed­ to help quantify the amount of  applied water that becomes available for plant use.

Water application efficiency refers to the amount of water applied that is stored in the crop root zone. This value is determined by water distribution characteristics­, system management, soil conditions, the crop, and weather conditions. Water application efficiency pertains to an individual irrigation event.

Equation 8.1 is used to determine water application efficiency.

          Equation 8.1        E_a = [Depth of water stored in the rootzone (d_s) x 100                                                      Depth of water pumped (d_p) ]

          where:

          E_a  =   Average water application efficiency, %

          Depth of water stored in the rootzone (d_s)   =   Average depth of water stored in the rootzone, inches

          Depth of water pumped (d_p)   =   Average depth of water delivered from source, inches

Irrigation efficiency refers to the amount of water­ removed­ from the water source that is used by the crop. This value is determined by irrigation system management, water distribution characteristics, crop water use rates, weather and soil conditions. Irrigation­ efficiency pertains to the use of water for an entire­ growing season.

The depth of water stored in the root zone can be estimated based on field observation of what happens to the water during an application event. Field observation reports should note if runoff occurs and estimate the amount of runoff. With experience, you’ll begin to know where and when runoff is likely to occur­. For example, runoff from center pivot systems will likely occur first near the outside edge of the irrigated area because the water application rate is greatest there. Other factors include low infiltration rate soils, steep slopes and lack of plant residue cover.

Another more accurate method is to record the soil water content before and after an irrigation event using one of the methods discussed in Chapter 3, Soil Water. If the hand-feel method is used, the soil water content will need to be recorded at enough locations to develop accurate estimates of the water stored in the crop root zone. The depth of water applied is found by subtracting the reading taken before­ the irrigation.­

          Equation 8.2      d_s   =   [“After” reading – “Before” reading]

          Where:

          d_s = Depth of water stored in the rootzone

A center pivot irrigation system is supplied with enough water to apply 1.1 inches of water to an irrigated area. Soil water content readings recorded before­ the irrigation event showed an average water content of 3.5 inches in the top 3 feet of soil. Soil water­ content readings after the irrigation showed an average of 4.4 inches in the top 3 feet of soil. To find the average depth of water stored in the crop rootzone we subtract the before irrigation reading from the after irrigation reading.

          Using Equation 8.2      d_s   =   [“After” reading – “Before” reading]

              d_s   =   [ 4.4 inches – 3.5 inches ]

              d_s   =   0.9 inches

The depth of water pumped can be determined using the procedures presented in Chapter 7, Flow Measurements and Basic Water­ Calculations. The information needed includes an accurate estimate of the pumping rate in gallons per minute. This information can be recorded using a flow meter installed as part of the system or periodically using an attached flow meter (ultrasonic flow meter, pilot tube type meter, etc.).

The average flow rate can be determined by recording­ the accumulator reading prior to and after each irrigation event. Subtracting the reading recorded­ prior to the irrigation from the reading after the irrigation event will result in the total volume of water pumped. Taking the total volume and dividing by the irrigation time will give the average pumping rate. For this estimate to be accurate, the irrigation time must be accurate to the nearest hour if possible. A more precise record of the total irrigation time will improve the estimate of the pumping flow rate. (The hour meter on the motor or center pivot is accurate enough to estimate the pumping time.) Equations 8.3 and 8.4 are used to make these calculations. The following­ example shows how to incorporate field data into the equations.

          Equation 8.3      Pumping rate =   [ Reading 2  –  Reading 1 ] / [ Time ] 

          where:

          Pumping rate      =   Water deliver rate, gallons per minute or acre-inches per minute

          Reading 1   =   Totalizer reading before the irrigation event, gallons or acre-inches

          Reading 2   =   Totalizer reading after the irrigation event, gallons or acre-inches

          Time    =   Time required to complete the irrigation event, minutes

The meter also has an accumulator at the bottom that registers total gallons pumped. Before the irrigation event, the accumulator reading was 6,553,300 gallons,­ and after the irrigation event the meter read 10,167,500 gallons. The irrigation event required 77 hours and 15 minutes.

Using Equation 8.3

Pumping rate   =  [ Reading 2-Reading 1 ] / [ Time ]

Pumping rate      =   [ 10,167,500 - 6,553,300 ] gallons                                       [(77 hr x 60 min/hr ) + 15 min ]

Pumping rate      =   [ 3,614,200 ] gallons / [ 4620 + 15 ] minutes

Pumping rate      =   780 gallons per minute

If the accumulator records flow in acre-inches, the same process is used unless the desire is to determine the flow rate in gallons per minute. To convert acre-inches per minute to gallons per minute, multiply the result from Equation 8.3 in acre-inches per minute by 27,154 gallons per acre-inch.

To convert the flow rate in gallons per minute to the gross depth of water pumped, we use Equation 8.4.  If the result from Equation 8.3 is in acre-inches per minute, the constant 27,154 gallons per acre-inch is not used.

          Equation 8.4    d_p = [flow rate x time ] / [area irrigated x 27,154]

         where:

         d_p = Depth pumped  =   Average depth of water pumped, inches

         Flow rate   =   Average water delivery rate, gallons per minute

         Time   =   Total irrigation time, minutes

         Area irrigated  =   Total irrigated area, acres

         27,154  =   Conversion factor, gallons per acre-inch or gal / ac-in

Let’s assume that the field area for Example 8.2 was 123 acres. We calculated the flow rate at 780 gallons per minute and the total irrigation time at 4635 minutes. Using Equation 8.3:

          Depth pumped (d_p)  =   [ Flow rate x time ] /  [ Area irrigated­ x 27,154 ]

          Depth pumped (d_p)  =   [ 780 gal/min x 4635 minutes ]                                               [ 123 acres x 27154 gal / ac-in ]

          Depth pumped (d_p)  =   [ 3,615,300 ]gallons / [ 3,339,942 ] gallons / inch

          Depth pumped (d_p)  =   1.08 inches

To complete the calculation of the water application efficiency, use Equation 8.1 to compare the amount of water pumped with the increase in water stored in the crop rootzone.

From Example 8.1 we found that 0.9 inches of water­ was stored in the three-foot crop rootzone. From Exampl­e 8.3 we found that 1.08 inches of water was pumped from the water source into the center pivot. To find the application efficiency we use Equation­ 8.1.

          E_a     =   [ Depth of water stored in the rootzone (d_s) x 100                                  Depth of water pumped (d_p) ]

          E_a     =   [ 0.9 inches / 1.08 inches ] x 100

          E_a      =   83%

In these examples it was determined that only 83 percent of the water pumped from the irrigation source reached the soil and was usable by the crop. That means that 17 percent of the water was lost during­ application.

The amount of water loss due to irrigation depends­ of the type of irrigation system — sprinkler­ or surface. In addition, the magnitude of each type of loss may be different. Let’s begin by listing some major­ sources of water loss during irrigation. To keep the losses for surface and sprinkler irrigation separate, Table 8.1 lists the potential losses for each type of system.

The major losses for surface irrigation systems are deep percolation and surface runoff. These two losses could cause the water application efficiency to be reduced to less than 50 percent if the system is not managed properly. Ways to minimize these losses are discussed in Chapter 11, Furrow Irrigation Management.

Another source of water loss is in the distribution system. If the water flows across the head of the field in an open ditch, each foot of ditch loses water to soil infiltration and water surface evaporation. The best way to eliminate these losses is to transport the water through an enclosed pipeline. For many furrow irrigated fields this will require a small pumping plant to overcome the friction loss associated with forcing water through the pipeline.

Surface irrigation implies that surface evaporation will contribute to water loss. One way to limit soil evaporation loss is to wet less of the soil surface. For fields with slopes less than 1 percent, irrigating every other furrow is a viable option. This effectively cuts surface evaporation losses by nearly 50 percent without sacrificing crop production. Irrigating every other furrow also will reduce the amount of water lost to deep percolation and surface runoff.

Pipelines can have losses too. Worn gaskets or loose fitting pipeline connections could produce leaks at each joint. These losses are usually small in comparison to other losses, but by their sheer number could add up to substantial water losses. This kind of loss is the easiest to eliminate by replacing gaskets.

Sprinkler irrigation systems, especially center pivots, typically have greater water application efficiencies than surface systems. While they may have more potential sources of loss, the magnitude of each loss is generally quite low. Table 8.1 shows that sprinkler­ irrigation systems may experience loss from all six of the potential water loss sources while surface irrigation systems lose water from only four. This is because most sprinkler irrigation systems spray water­ into the air to deliver water to the entire soil surface with an upright­ crop canopy located between the sprinkler and soil.

Developments in sprinkler technology have reduced the amount of water lost between­ the sprinkler/nozzle and soil surface. The irrigation­ time or the accumulated time that water is applied to the crop canopy causes the major loss during sprinkler irrigation events. Applied water evaporates off the leaves of the crop canopy. Thus, the longer water droplets are delivered to the crop, the greater the total evaporation loss. Lowering the sprinkler/nozzle pressure reduces the wetted diameter of the sprinkler/nozzle thus reducing irrigation time and total canopy evaporation losses. In addition, lower wetted diameters reduce water evaporation losses in the air and wind drift losses.

Proper management of sprinkler irrigation systems can greatly reduce deep percolation losses. An irrigation system managed to keep the soil profile completely full at all times will experience some deep percolation losses. This is because the system does not apply water at 100 percent uniformity. Some areas­ will receive more water than others due to sprinkler pressure differences caused by soil elevation differences. Pressure regulators or flow control nozzles help insure that water delivered to the soil surface is as uniform as possible. Other portions of the field could be affected when wind distorts the water application pattern. Such distortion can be reduced­ by avoiding operation when winds exceed 10 mph.

There are two main ways to evaluate water loss during irrigation: 1) take detailed field measurements; and 2) visually estimate losses. In some cases it may be necessary to combine these methods to develop­ an accurate estimate of where losses occur and how significant they are to the system’s application efficiency. For example, to estimate water losses during irrigation, measure the flow rate of water entering­ the system with a flow meter. Visually estimate how much of the water is lost to runoff. This amount, however, will not account for other potential losses. Table 8.2 presents the potential magnitude of some of these losses for different irrigation systems. For furrow irrigation systems record how long it takes for the water to reach a certain point in the field or record flow rates into the furrow and how long it takes water to reach the end of the furrow. When coupled with soil types and furrow slopes, a computer model can be used to estimate how efficiently the water is being applied.

With the increased competition for water and the cost of irrigation, it is imperative that irrigation efficiency be considered when making management decisions.­ An application efficiency estimate is important for determining the amount of irrigation water needed. Without realizing the amount of applied water the crop can use, the field may be over or under irrigated. Knowledge of the water application efficiencies for different types of irrigation systems will help producers make informed decisions about the cost and return associated with switching from one irrigation system to another.­