Nebraska Cooperative Extension NF93-118

A University of Nebraska NebFact Publication

Fine Tuning Furrow Irrigation Systems


Joel Cahoon, Extension Water Management Engineer
Dean Eisenhauer, Associate Professor, Biological Systems Engineering

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This NebFacts shows how the set time and number of gates opened during furrow irrigation affect the total amount of water applied, and how changes in these factors can enhance the effectiveness of the system.

Many factors affect the performance of furrow irrigation systems, including soil texture and structure, furrow length, spacing between wetted furrows, crop residue and furrow slope.

Other elements that come into play have more to do with the management of the system, meaning changes can easily be made throughout the irrigation season. The set time and number of gates opened are examples of management factors, and are the primary tools that irrigators have to optimize irrigation performance given the physical features of the irrigation system.

The set time and number of gates opened not only determine the gross application (the total amount of water applied) but also affect the distribution of water along the length of the furrow. The techniques discussed here can be used for continuous flow or surge flow irrigation. Keep in mind, however, that all times discussed are on-times (water flowing into the furrow).

The number of gates opened and the set time determine the gross application. One way to think of this is in the form of a question; if I want to apply a certain amount of water and I usually run a certain set time, how many gates should I open? This can be calculated as:

(1)
N =Q × T × 96.25
_______________
D × W × L

where: N = number of gates to open,
Q = pump flowrate (gpm),
T = set time (hrs),
D = desired depth of application (inches),
W = wetted furrow spacing (ft),
L = average furrow length for the set (ft).

This equation can be rearranged to determine the average depth of application based on the set time and number of gates opened:

(2)
D =Q × T × 96.25
_______________
N × W × L

One indicator of furrow irrigation performance that also depends on the set time and the number of gates opened is the cutoff ratio, or CR, defined as:

(3)
CR =the average advance time (hrs)
_________________________
divided by the set time (hrs)

The cutoff ratio is an indicator of efficiency. Achieving the optimum CR will minimize deep percolation and runoff, distributing water as evenly as possible down the length of the furrow. The CR that results in optimum system performance depends on both soil and system characteristics. The recommended CR's are shown in Table I.

Table I. Recommended cutoff ratios.

Recommended Cutoff Ratio (CR)
System Type Sandy Soils Loamy Soils Clayey Soils
No Re-Use 0.45 0.60 0.70
With Re-Use 0.25 0.35 0.45
Blocked Ends0.85 0.80 0.75

When using the CR as a management tool, it is critical to change both the set time and number of gates. Reducing only the number of gates will result in excessive application depths - causing excessive deep percolation, runoff or both.

One strategy for fine tuning furrow irrigation systems is to choose a combination of set time and number of gates opened that results in both the desired application amount and the desired CR. The attached worksheet shows the calculations necessary to determine these settings. First, determine the number of gates to open for the desired set time and application depth from Equation 1. Use these settings on the first set. If water does not reach the furrow end, make the necessary adjustments in set time, remembering the application amount will also increase. Note the number of gates opened, the set time and the average advance time observed during the first set and record these in cells A2, B2 and C2, respectively. Calculate the value of Z, a term used repetitively in the calculations. Select the appropriate CR from Table I. Use this value for CR*, the desired cutoff ratio, in the calculations. Follow through the calculations from left to right on the first line. Repeat the calculations for each new line, bringing in the appropriate values from the previous line. The final recommendations will be in boxes A6 (recommended number of gates) and B6 (recommended set time). The settings in columns A and B in any row achieve the desired application amount, D, and as the calculations proceed downward the CR (column D) approaches CR*. The calculations in the worksheet may be easily entered into a microcomputer spreadsheet program.

One limitation of this procedure occurs with high furrow stream sizes. The flow rate in each furrow equals the pump flow rate, Q, divided by the number of gates open, N, if pipe leaks are negligible. The maximum desired furrow flowrate can be estimated by dividing 12.5 by the furrow slope, S, in percent. Stream sizes greater than this will result in excessive erosion. If the value in cell B5 results in a furrow flow rate greater than 12.5/S, scan upwards in column A until the value is greater than Q × S/12.5. Watch also for very small flow rates, as rates of less that 7 gpm will often result in inadequate soaking of the furrow width due to the small wetted perimeter.

You can work through the following example (in italics) or use the space provided to work through your own calculations.

Example:Your Calculations:
pump flowrate, Q = 900 gpmQ = __________ gpm
flow length, L = 1320 ftL = __________ ft
wetted furrow spacing, W = 2.5 ftW = __________ ft
set time, T = 12 hoursT = __________ hrs
desired application, D = 3.5 inD = __________ in
loamy soil with re-use

Use Equation 1 to determine number of gates for first set:

N =900 × 12 × 96.25
_________________________
= 90 gates
3.5 × 2.5 × 1320

N =
×
×
= ______ gates
________________________________________

×
×

Calculate the value of Z as:

Z =Q × 96.25
_________________
D × W × L

Z = 900 × 96.25
_________________
= 7.5
3.5 × 2.5 × 1320

Z =
×96.25= _______
_______________

×
×

Select the recommended cutoff ratio from Table I.

CR* = 0.35CR* = _____

Record the observed values in cells A2, B2, and C2, and complete the calculations in the worksheet.

A B C D E F G
1 Number
of Gates
Set Time
(hrs)
Advance
Time (hrs)
Cutoff
Ratio
Desired Advance
Time (hrs)
Check New Number
of Gates
2 observed
90
observed
12.0
observed
8.0
C2/B2
8/12=0.67
CR* × B2
0.35×12=4.2
E2/C2
4.2/8.0=0.53
A3(E2/C2)0.5
90(4.2/8.0)0.5 = 65
3 G2
65
A3/Z
65/7.5=8.7
E2
4.2
C3/B3
4.2/8.7=0.48
CR* × B3
0.35 × 8.7=3.0
E3/C3
3.0/4.2=0.72
A3(E3/C3)0.5
65(3.0/4.2)0.5 = 56
4 G3
56
A4/Z
56/7.5=7.4
E3
3.0
C4/B4
3.0/7.4=0.41
CR* × B4
0.35×7.4=2.6
E4/C4
2.6/3.0=0.85
A4(E4/C4)0.5
56(2.6/3.0)0.5 = 51
5 G4
51
A5/Z
51/7.5=6.8
E4
2.6
C5/B5
2.6/6.8=0.38
CR* × B5
0.35×6.8=2.4
E5/C5
2.4/2.6=0.92
A5(E5/C5)0.5
51(2.4/2.6)0.5 = 49
6 G5
49
A6/Z
49/7.5=6.6





In this example, it would be more desirable to open 49 gates for 6.6 hours, than to use the original setting of 90 gates for 12 hours. These calculations could be extended several lines if using a spreadsheet. This would bring the CR, column D, to exactly 0.35, but would not change the actual results much.



File NF118 under IRRIGATION ENGINEERING
B-2, Irrigation Operations and Management
Issued April 1993

Electronic version issued July 1995
pubs@unl.edu


Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture. Kenneth R. Bolen, Director of Cooperative Extension, University of Nebraska, Institute of Agriculture and Natural Resources.

University of Nebraska Cooperative Extension educational programs abide with the non-discrimination policies of the University of Nebraska-Lincoln and the United States Department of Agriculture.