12.2.3: Management Plan

The large number of variables and calculations needed to describe moved-lateral systems can be perplexing. Managing irrigation systems usually involves an existing system. Therefore, the first process is to describe the characteristics of the existing system. Second, the existing conditions should be evaluated to determine if the system is capable of efficient irrigation. The third step is to develop a plan to meet crop needs and achieve efficiency.

A management spreadsheet such as in Figure 12.7 can help alleviate confusion and facilitate development of a management plan. Users enter parameter values into the shaded boxes and then compute results for the unshaded cells. These data are critical, but they are characteristics of the system and only need be considered once unless major changes are made which would require redesign and new investment.

The Moved-Lateral Management Spreadsheet is divided into four portions. The first portion is an inventory of field characteristics. The values entered for this example are from the field shown in Figure 12.6—an actual field from western Nebraska. The second portion includes design variables which represent the considerations made when designing the system. These are choices made during design; however, these values are generally constant once a system has been installed. Designs can be modified which would result in changing parameters, but once changes were made the variables would be constant again. Thus, design variables are only defined once and are not modified routinely by managers. You can think of field characteristics and design variables as a description of the system you are called upon to manage.

The third portion represents the parameters that describe system performance based on field characteristics and design parameters. These boxes contain either calculation results or parameter values derived from earlier sections. Values in these boxes do not represent choices. The fourth portion includes management variables which can be changed annually or within the season. These values are adjusted to provide the desired performance. Conflicts can arise in selecting management values and issues must be resolved when compromise is needed.

Values for the mainline portion of the operation results section include the total inflow which equals the flow per lateral times the number of laterals. The remaining values are based on application of procedures developed in Chapter 8 for friction loss and flow velocity. One should check values against guidelines for friction loss and velocity.

The lateral information represents calculations described in Chapters 8 and 11. Note that the friction loss calculations are for the most critical lateral that runs uphill—the lateral north of the mainline. This example is for 5-inch aluminum pipe for the side roll. The Hazen-Williams C value is taken as 120 and the wall thickness for the side-roll pipe is 0.072 inches.

Figure 12.7. Example of the Moved-Lateral Management Spreadsheet.

Four-inch pipe can also be used for the side-roll lateral and the wall thickness is the same as for the 5-inch pipe. The spacing limits between sprinklers and laterals are derived from Table 11.3. The application rate is also computed. The number of sets in the field and per lateral are also shown. Completion of these sections and validation that characteristics meet established guidelines establishes the foundation for managing the system.

The fourth section of the sheet is for management choices. These parameters are frequently changed or adjusted to achieve short-term management goals for crop and system performance. The management decisions must be entered into the shaded cells. Management choices vary throughout the season and annually. The following examples illustrate the computation process for the field in Figure 12.6. The second example illustrates computation of the parameter values for the lateral information portion of the operation results in Figure 12.7. The last section in the Moved-Lateral Management Spreadsheet involves the core of managing the system. Example 12.3 illustrates the decision-making process for the management section.

The side-roll system is designed to meet crop water use during the middle of the season when water use rates are at the peak value. During other times of the year the system will have excess capacity. Early in the growing season the root depth will be less than 4 feet, so the root zone would not store a net application of 3.38 inches. When the system has excess capacity the irrigation interval can be extended through accurate scheduling. Timers can also be used to shutoff the pump for a shorter application time for the same set time. This will reduce the net depth when the root zone is swallower or when water demands are less. Once a management plan has been developed it is important to ensure that the system is operating properly—as designed.

Example 12.1

You were retained to evaluate the side-roll system shown in Figures 12.6. Measurements for the system are shown in Figure 12.7. Verify the calculations for the mailine portion of the system to determine if the system meets management guidelines.

Solution

1. Data from Figure 12.6 shows that the field is 2640 ft long and 1320 ft wide giving an area of 80 acres: Area=(2640×1320)/43560=80 acres.
2. The available water supply is about 700 gpm and the fine sandy loam soil holds about 1.8 inches of water per foot of soil.
3. Designers choose to orient the pipeline so that the mainline runs through the middle of the field which requires the least amount of mainline pipe—1320 ft. This results in little elevation change along the mainline.
4. The design included sprinklers with a 28/128 or 7/32 inch nozzle operating at 55 psi which produces a sprinkler discharge of 10.3 gpm (see Table 11.1). The lateral incorporates 32 full-circle sprinklers and two half-circle sprinklers. This results in (10.3 gpm/sprinkler x the equivalent of 33 spriklers) = 340 gpm per lateral.
5. With two laterals the flow in the mainline is 680 gpm.
6. Pressure loss due to friction is computed using Equation 8.11b. The inside diameter of 8-inch aluminum pipe is 7.898 inches and the roughness coefficient is C = 120. So the loss is:
   \(P_f=456\left(\dfrac{Q}{C}\right)^{1.852}\left(\dfrac{1}{d^{4.866}} \right)=456\left(\dfrac{680}{120} \right)^{1.852}\dfrac{1}{7.898^{4.866}}=0.49\text{ psi}/100\text{ ft}\)
7. The maximum length of the mainline is 1320 ft so the maximum friction loss is PL = 0.49 × 1320 /100 = 6.5 psi. Since there is negligible elevation change the pressure loss is 6.5 psi and inlet pressure to the mainline will need to be about 61.5 psi for the highest case.
8. The velocity of flow in the mainline is:
   v = 0.409 Q / D² =0.409×680 / 7.898² = 4.45 ft / sec

The velocity is less than the 5 ft/sec limit for mainline, thus the mainline is appropriate and the calculations are correct.

Example 12.2

The next phase in the assessment of the system in Figures 12.6 is to compute values for the lateral portion of the side-roll system using the data in Figure 12.7. The assessment should determine if calculations are correct and if the lateral satisfies management guidelines. Solution: 1. The side-roll system uses 5-inch aluminum pipe that has a wall thickness of 0.072 inches; hence, the inside diameter is 4.856 inches. 2. Pressure loss due to friction is computed using equation 8.11b. with a roughness coefficient of C = 120. So the loss is:

Solution

1. The side-roll system uses 5-inch aluminum pipe that has a wall thickness of 0.072 inches; hence, the inside diameter is 4.856 inches.
2. Pressure loss due to friction is computed using equation 8.11b. with a roughness coefficient of C = 120. So the loss is:
   \(P_f=456\left(\dfrac{Q}{C}\right)^{1.852}\left(\dfrac{1}{d^{4.866}}\right)=456\left(\dfrac{340}{120}\right)^{1.852}\dfrac{1}{4.856^{4.866}} = 1.44\text{ psi}/100\text{ ft}\)
3. The multiple outlet factor (F) for the sprinkler lateral from Table 8.3 is about 0.36; therefore, the friction loss in the lateral is 0.36 x 19 = 6.84 psi.
4. The lateral north of the field runs uphill with the change in elevation of about 8 ft which is equivalent to 8 ft / 2.31 ft/psi = 3.5 psi.
5. Then, the total pressure loss along the critical lateral is 6.84 + 3.5 = 10.3 psi.
6. The average sprinkler pressure is 55 psi from Figure 12.7, so the ratio of pressure variation to the average sprinkler pressure is 10.3/55 = 19% which is smaller than the 20% limit.
7. The average wind speed for the middle of the season is listed as 7 miles/hr. From Table 11.3, the maximum spacing of sprinklers for a rectangular sprinkler orientation is 45% of the sprinkler spacing and 60% of the lateral spacing.
8. The wetted diameter for the 7/32-inch nozzle at 55 psi is 113 ft from Table 11.2.
9. The maximum sprinkler spacing along the lateral is then 0.45 x 113 = 50.9 feet and the maximum lateral spacing is 0.6 x 113 = 67.8 feet. Both actual spacings are less than the maximum, so the spacings are acceptable.
10. The application rate for the side roll is:\(A_r=\dfrac{96.3q_s}{S_L S_m}=\dfrac{96.3 \times 10.3}{40\times 60}=0.41\text{ in/hr} \)
11. The irrigated area per lateral is: L x Sm / 43560 = 1320 x 60 / 43560 = 1.82 acres/set.
12. The number of sets is: \(N_s=\dfrac{W_f \times L_f}{S_m \times L}=\dfrac{1320 \times 2640}{60 \times 1320}=44\text{ sets and }22\text{ sets/lateral} \)
13. The flow velocity in the lateral is v = 0.409 Q/D2 = 0.409 x 340/4.8562 = 5.9 ft/sec which is less than the 7 ft/sec limitation. Note that this is the velocity of inflow to the lateral.

Results show that the calculations are accurate and the lateral conforms with management guidelines.

Example 12.3

The final assessment of the system in Figure 12.6 is to ascertain if the management parameters will achieve operational objectives.

Solution:

1. The soil and crop information must be used to compute the allowable depletion. The root depth and critical depletion for the sugar beet crop are estimated from Chapter 4 and local sources to be 4 feet and 50% respectively. This gives an allowable depletion for the fine sandy loam soil of:
   \(AD=R_d \times AWC \times f_{dc}=4 \text{ ft} \times 1.8 \text{ in/ft} \times 0.5 = 3.6 \text{ in} \)
2. The times are dependent on operator preferences. The choice was for 12-hour sets with one-hour downtime to move the lateral. It also requires one-half day to reposition the lateral back to the first set after an irrigation.
3. The irrigation interval depends on the number of sets per lateral and the number of sets per day: The irrigation interval depends on the number of sets per lateral and the number of sets per day:
   \(I_I=\dfrac{12\text{ sets/lateral}}{2\text{ sets/d}}+0.5\text{ d}=11.5\text{ d} \)
4. The gross depth of application is the product of the application rate times the application time:
   \(d_g=A_R\times T_a=0.41\text{ in/hr} \times 11\text{ hr}=4.5\text{ in} \)
5. Since the application efficiency is 75% the net depth is:
   \(d_n=AE_{LQ}\times d_g=0.75 \times 4.5=3.38\text{ in} \)
6. The net depth of application is less than the allowable depletion of 3.6 inches so deep percolation should not be a problem.
7. With an irrigation interval of 11.5 days and a net depth of 3.38 inches, the net crop water use that the system can satisfy is:
   \(C_n=\dfrac{3.38\text{ in}}{11.5\text{ d}}=0.29\text{ in/d} \)
8. The net crop water use rate should be compared to local conditions to decide if the crop water needs will be met. The value of 0.29 in/day is barely acceptable for this location but should be adequate most years.

The calculations in Figure 12.7 appear to be correct. The system and management decisions should meet crop water requirements efficiently.