Irrigation Chapter 10 - Scheduling Irrigations
Scheduling irrigations can positively or negatively affect irrigation costs, crop stress, environmental factors and ultimately yield. The primary goal of scheduling is to supply water at the correct time to prevent crop water stress that can reduce yield. Secondary goals are to limit runoff and deep percolation to improve efficiency, and to maximize use of any precipitation during and after the growing season.
Author: Chuck Burr, University of Nebraska Lincoln Extension Educator in Phelps and Gosper counties.
No-Later-Than Date
The first calculation that irrigators use to schedule irrigations is called the no-later-than (NLT) date. The premise of this calculation is to not allow the water balance (see Chapter 9) to drop below some minimum balance. This calculation is sensitive to meeting crop water needs. The no-later-than date is calculated as follows:
Equation 10.1 NLT = (WB - MB)
ETC
Where:
WB = Current water balance
MB = Minimum allowable balance
ETC = Forecasted crop water use
Basically the no-later-than date looks at how much water is available in the soil above the minimum balance. If you divide this amount by the forecasted crop evapotranspiration rate, the no-later-than date projects the number of days until the crop will begin to experience water stress. Irrigation must be completed before that date.
Example 10.1
Field corn is being grown on a silt loam soil. Checking soil water content indicates that 70 percent of available water capacity is remaining. The minimum allowable balance is 50 percent of available water capacity. The soil will hold 2.0 in. per foot and the root depth is 3 ft. Calculate the no-later-than date assuming the forecast ETc for corn is 0.30 in/day.
Using Equation 10.1
Water balance = 70% remaining x 2.0 in/ft x 3 ft
Water balance = 4.2 in
Minimum balance = 50% x 2.0 in/ft x 3 ft
Minimum balance = 3.0 inches
NLT = (WB - MB) ET
NLT = (4.2 - 3.0) in. 0.30 in/day
NLT = 4 days
If the crop is irrigated in four days, water stress will be avoided. Often a no-later-than date will be calculated for the beginning and end of a given irrigation sequence. The water balance may vary within a field, leading to different no-later-than dates. For example, the water content would be different at the beginning and end of an irrigation event.
Also consider the amount of time it takes to complete an irrigation sequence. If it takes three days for a pivot to turn a circle, and the no-later-than date for the end of the circle is four days, you should begin irrigating in one day so the pivot reaches the ending point before crop stress occurs.
Example 10.2
Soybeans are being grown on a sandy soil. Checking soil water content indicates that 60 percent of the water is remaining at the start position and 90 percent at the stop position on a pivot. Minimum allowable balance is 50 percent of available water capacity. The soil will hold 1.2 in/ft and the crop root depth is 2.5 ft. Calculate the no-later-than date for both start and stop positions assuming forecast ETc is 0.20 in/day. If it takes three days to reach the stop position, indicate when to start irrigating.
Using Equation 10.1
Start water balance = 60% remaining x 1.2 in/ft x 2.5 ft
Start water balance = 1.8 inches
Stop water balance = 90% remaining x 1.2 in/ft x 2.5 ft
Stop water balance = 2.7 inches
Minimum balance = 50% x 1.2 in/ft x 2.5 ft
Minimum balance = 1.5 inches
Start no-later-than date = (1.8 - 1.5) in
0.20 in/day
Start no-later-than date = 1.5 days
Stop no-later-than date = (2.7 - 1.5) in
0.20 in/day
Stop no-later-than date = 6 days
To prevent crop stress at the start position, you would need to start irrigating in one day. Since it takes three days to complete the circle, you would need to start irrigating in three days to prevent crop stress at the stop position. In this case the start position takes precedence and irrigation should begin no later than one day. You may wish to begin irrigation immediately in case the system has a breakdown.
No-Sooner-Than Date
Another calculation useful for irrigators is the no-sooner-than (NST) date. This calculation ensures that net infiltration does not exceed what the soil can hold, greatly reducing deep percolation. This calculation also accounts for anticipated rainfall to ensure there is always room to store rainfall if received. Potential rainfall is estimated, accounting for soil type, time of year, and average annual rainfall. For example, rainfall is more likely to occur early and late in the irrigation season and more likely in southeast Nebraska than in the Panhandle. Irrigators with silt loam soils can plan for a higher allowance since this soil type holds more water than those with sandy soils.
There is also an allowance for the planned effective irrigation depth (dep). For example, if you have a pivot that applies 1 inch per event and the system is 85 percent efficient, the planned effective irrigation depth would be 0.85 inches. This allowance makes sure there is room in the soil profile for the irrigation application without causing deep percolation. The NST formula is:
Equation 10.2 NST = ra + dep – (TAWC – WB) ETC
Where:
ra = rainfall allowance
dep = planned effective irrigation depth
TAWC = total available water capacity
WB = water balance
ETC = forecast crop water use
Example 10.3
Dry edible beans are being grown on a sandy loam soil with an available water capacity of 1.44 in/ft. In this area an allowance should be made for a 0.4-in. rainfall and a planned effective irrigation depth of 0.8 in. Checking soil water content indicated that 60 percent was present at the start and 90 percent was present at the stop position. Calculate a no-sooner-than date for each position assuming a forecast ETC of 0.20 in/day and a crop rooting depth of 2.5 ft.
Using Equation 10.2
Total available water capacity = 1.44 in/ft x 2.5 ft
Total available water capacity = 3.6 in.
Start water balance = 60% x 1.44 in/ft x 2.5 ft
Start water balance = 2.2 in.
Stop water balance = 90% x 1.44 in/ft x 2.5 ft
Stop water balance = 3.2 in.
Start no-sooner-than= 0.4 in + 0.8 in – (3.6 in – 2.2 in)
0.20 in/day
Start no-sooner-than= – 1 day
Stop no-sooner-than = 0.4 in + 0.8 in – (3.6 in – 3.2 in)
0.20 in/day
Stop no-sooner-than = 4 days
In this example the no-sooner-than date at the start position is minus one day. A negative no-sooner-than date indicates that irrigation could have started one day ago without contributing to deep percolation. At the stop position, irrigation must wait four days to prevent deep percolation. In this example you could delay 1 more day to begin irrigation, if it takes 3 days to turn a circle. This would amount to a total of 4 days to reach the stop position and deep percolation at the stop position would be avoided.
Irrigation scheduling using both the “no-sooner- than” and “no-later-than” dates for the start and stop positions will prevent crop water stress, and make maximum use of rainfall and irrigation applications. The following example shows how to use these dates.
Example 10.4
A furrow irrigated field requires 10 days to irrigate. The following no-sooner-than and no-later-than dates have been calculated. When should irrigation begin?
Start NST = – 1 NLT = 5
Stop NST = 6 NLT = 14
From the start position no-sooner-than date, we see that we can start irrigating now and have enough storage space for rainfall and irrigation. Since it will take 10 days to reach the stop position, irrigation can begin. According to the start position no-later-than date, we could delay irrigation five days without causing crop water stress. At the stop position, the no-later-than date indicates we can only wait four days, since it will take 10 days to reach this part of the field. Since the irrigation could start immediately and not cause deep percolation, the irrigator may choose to begin irrigation now and build in some insurance for potential system problems.
Scheduling the Last Irrigation
To make maximum use of off-season precipitation, irrigators can calculate when best to schedule their last irrigation. This process involves determining the crop growth stage and estimating how much water the crop needs to reach maturity. Historical data in table format are useful in predicting crop water needs. For more information on scheduling the last irrigation see Predicting the Last Irrigation of the Season at the UNL Extension publications website.
When calculating when to schedule the last irrigation, it is safe to use a 4-foot rooting depth for most crops. Probe the soil to the 4-foot depth, determine the percent water remaining, and calculate the current water balance. A minimum balance of 40 percent can be used in scheduling the last irrigation without affecting yield.
Example 10.5
Schedule the last irrigation for a field of soybeans which are at the R6 Stage (Full Seed Fill). The soil type is a silt loam which holds 2.0 inches per foot and has 65 percent water remaining. Calculate the amount of water needed for the crop to reach maturity.
Answer:
The NebGuide G1871, Predicting the Last Irrigation of the Season, indicates that soybeans at the R6 stage will mature in 18 days and require 3.5 in. of water.
Water balance = 65% x 2.0 in/ft x 4 ft
Water balance = 5.2 in.
Minimum balance = 40% x 2.0 in/ft x 4 ft
Minimum balance = 3.2 in.
Remaining usable water = 5.2 in. - 3.2 in.
Remaining useable water = 2.0 in.
The crop requires 3.5 in., minus useable water of 2.0 in. The crop will need an additional 1.5 in. of water to reach maturity. Since the crop will mature in approximately 18 days, the irrigator may elect to delay irrigation in the hope of rainfall. This would be especially tempting if the producer has a furrow irrigation system that can apply no less than 3 inches of water per event. The irrigator also must consider how long it will take to irrigate the field. If it takes four and one-half days, irrigation should be started at least five days before expected maturity.
In mid to late August, hot windy weather often occurs and the irrigator is tempted to add an irrigation because the crop water use rates have risen substantially. The daily rate may have increased but the crop will still only need 3.5 inches of water to reach maturity. Since the crop is using water at a faster rate, it will reach maturity sooner.