**Nebraska Cooperative Extension NF93-118**

Dean Eisenhauer, Associate Professor, Biological Systems Engineering

[] [] []

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*.

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 Ends | 0.85 | 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 gpm | Q = __________ gpm |

flow length, L = 1320 ft | L = __________ ft |

wetted furrow spacing, W = 2.5 ft | W = __________ ft |

set time, T = 12 hours | T = __________ hrs |

desired application, D = 3.5 in | D = __________ in |

loamy soil with re-use |

N = | 900 × 12 × 96.25_________________________ | = 90 gates |

3.5 × 2.5 × 1320 |

N = | × | × | = ______ gates | |||

________________________________________ | ||||||

× | × |

Z = | Q × 96.25 _________________ |

D × W × L |

Z = | 900 × 96.25_________________ | = 7.5 |

3.5 × 2.5 × 1320 |

Z = | × | 96.25 | = _______ | |||

_______________ | ||||||

× | × |

CR* = 0.35 | CR* = _____ |

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 | observed90 |
observed12.0 |
observed8.0 |
C2/B28/12=0.67 |
CR* × B20.35×12=4.2 |
E2/C24.2/8.0=0.53 |
A3(E2/C2)^{0.5}90(4.2/8.0)0.5 = 65 |

3 | G265 |
A3/Z65/7.5=8.7 |
E24.2 |
C3/B34.2/8.7=0.48 |
CR* × B30.35 × 8.7=3.0 |
E3/C33.0/4.2=0.72 |
A3(E3/C3)^{0.5}65(3.0/4.2)^{0.5} = 56 |

4 | G356 |
A4/Z56/7.5=7.4 |
E33.0 |
C4/B43.0/7.4=0.41 |
CR* × B40.35×7.4=2.6 |
E4/C42.6/3.0=0.85 |
A4(E4/C4)^{0.5}56(2.6/3.0)^{0.5} = 51 |

5 | G451 |
A5/Z51/7.5=6.8 |
E42.6 |
C5/B52.6/6.8=0.38 |
CR* × B50.35×6.8=2.4 |
E5/C52.4/2.6=0.92 |
A5(E5/C5)^{0.5}51(2.4/2.6)^{0.5} = 49 |

6 | G549 |
A6/Z49/7.5=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.

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. *