Irrigation management strategies

In the spring, always make sure the soil in the germination and early-growth root zone is moist when planting. If necessary, irrigate to wet this zone.

As the plant grows, moist soil is necessary for proper root development, as roots will not grow through a dry layer of soil. A dry layer will result in a shallower rooting depth than desirable.

For corn, experience shows that the soil water deficit can be as high as 60 to 65 percent in the early vegetative growth stage (germinating to 10th leaf) without affecting plant development.

Root zone at this time may only be half to two-thirds of the crop's potential. Holding back on irrigating during the early vegetative growth stages promotes deeper root growth, increases the opportunity to store rainfall when it occurs and decreases the risk for leaching valuable nutrients.

As the crop nears its critical growth period or its usual peak water-use period, reduce the selected allowable soil water deficit to minimize the risks of not meeting the crop's water needs and causing yield losses.

For most crops, this may mean changing to a 30 to 40 percent soil water deficit limit before entering the critical growth stage – such as the 10th to 12th leaf stage for corn, or early flower for soybeans. During these critical periods of high water use, regularly project the next two to three days of water needs to plan ahead and avoid stressing any part of the field before it’s irrigated.

For example, when using a center pivot, which takes three days to cover the field, project what the water deficit will be after three days and use this to determine when to start irrigating. To reduce the leaching potential of a rainfall event, always consider the weather forecast when scheduling the next irrigation.

As the crop nears maturity, you can generally increase the soil water deficit to greater limits without causing stress to the crop. For example, after corn kernels have begun to dent, research has shown that allowing the soil water deficit to increase to 60 to 70 percent should not reduce yields in most years.

Table 1 shows field test results which support this early cutoff strategy. These results are from a 1989-1990 Agricultural Utilization Research Institute (AURI)-supported research/demonstration project conducted in west central Minnesota on a Renshaw sandy loam soil with 3.5 inches available water holding capacity (AWC).

Source: Westgate, Olness and Wright (1991).

Generally, a corn crop will need about 2 to 2.5 inches of evapotranspiration (ET) after first dent to come to full maturity, depending on emergence date. For soils holding at least 3.5 inches of available water at this time, you shouldn’t need additional irrigation if air temperatures remain at or below normal. A heavier soil may tolerate an even earlier cutoff, but a lighter soil may need one or two more irrigations.

Determining the amount and timing of the last few irrigations of the season is one of the most critical water management decisions. Discontinuing too early in the season to save water or reduce pumping cost could mean a much greater reduction in yield returns than the cost of pumping. On the other hand, irrigating right up to crop maturity may mean using 1 to 3 inches more irrigation water than necessary and increasing operating costs $3 to $15 per acre depending on power source. Here we present some guidelines for predicting the last irrigation for corn and soybeans when irrigation water supplies are adequate.

Basic requirements for estimating the last irrigation of the season

The two basic irrigation water management strategies that an operator should keep in mind when predicting the last irrigation are:

1. To provide adequate soil moisture in the root zone to carry the crop to maturity without reducing yields.
2. To deplete the soil moisture farther than normal (i.e., 60-70% of available water can be depleted at maturity) when nearing maturity. This will minimize irrigation water supply needs, fuel and labor for the season and allow the off-season precipitation to recharge the soil profile.

These requirements may appear to be conflicting, but the problem can be solved rather easily if adequate field information is available or is predictable. The following field information is necessary to predict the date of the last irrigation:

• Current crop growth stage and predicted crop maturity date.
• Predicted rate of water use by the crop to maturity.
• Remaining useable water in the root zone.
• The probability of significant amounts of rainfall before crop maturity.

Information on the probability of rainfall will not be discussed in this article. But, the latest weather forecast at the time of predicting the last irrigation should be considered in the decision. Also, if you are located in an area having some level of drought, the ongoing drought situation can be monitored at the University of Minnesota climate web site: www.climate.umn.edu.

Steps to schedule the last irrigation

1. Record the date, field, crop, soil type, and the crop growth stage (Table 2). The soil type information can be obtained from the NRCS Web Soil Survey and Tables 3-4 can be used to determine the crop growth stage.
2. Determine the water use by the crop to reach maturity (WUCM). Table 5 presents the approximate water use at particular growth stage to reach maturity.
3. Determine the allowable soil moisture deficit (ASMD) or management allowable depletion (MAD) for the soil listed in step 1. The ASMD values for different soil types are presented in Table 6.
4. Measure the current soil moisture deficit (CSMD).
5. Calculate the remaining useable soil moisture (RUSM) in the rooting zone by subtracting CSMD from step 4 from ASMD found in step 3.
6. Calculate the irrigation water requirement (IWR) by subtracting the remaining usable soil moisture found in step 5 from water use to crop maturity found in step 2. If the remaining usable water (step 5) is greater than the water use to crop maturity (step 2), then no irrigation is required.

*Note: If line 5 is greater than or equal to line 2, no additional irrigation is needed.

Source: Corn growth and development, PMR 1009, Iowa State University, 2011.

Source: Soybean growth and development, PM 1945, Iowa State University, 2014

*Water capacity in the top 3 feet or less for soils having root restrictive layer, like coarse gravel.

Step 1. Crop growth stage 

Corn and soybean plants require some moisture right up to the time of maturity. However, with shorter and cooler days towards the end of the season, the crop is using less water per day than during the rest of the season. 

Since some of the required moisture near the end of the season can be obtained from the soil moisture reservoir, the last irrigation can usually be applied two to three weeks prior to physiological maturity depending on the soil's water holding capacity. To estimate the number of days left to reach maturity and water use by crop until maturity, knowledge of crop growth stage is very important.

Maturity of a crop is defined as the time when the kernels or seeds have reached maximum dry weight. For corn, a black layer formation at the tip of the kernel is the normal indication of physiological maturity. This occurs approximately 7 days after the kernel has reached the ¼ milk line. For soybeans, beginning maturity is generally identified when one normal pod on the main stem has reached its mature yellow or brown color. Tables 3 and 4 of this article describe the reproductive growth stages of corn and soybeans. More detailed information about crop growth can be found in the source provided at the end of each table.

Step 2. Water use to maturity (WUCM) 

Estimating crop water use until maturity involves summing the daily estimates of crop evapotranspiration (ET) from the growth stage of interest until maturity. ET is the combination of evaporation from the soil surface and transpiration from the plants. It is the total amount of water used by the crop.

Evapotranspiration is affected by many factors including weather (radiation, air temperature, humidity and winds speed), crop factors such as crop type, variety and development stage, and management conditions. The evapotranspiration from a reference surface (grass and alfalfa) not short of water is called reference evapotranspiration (ET_ref). To estimate the crop ET for a particular crop at a particular growing stage, reference ET must be multiplied by a crop coefficient (Kc):

Crop ET = ET_ref x Kc

Table 5 shows the estimated water requirements (ET) between a given growth stage and maturity for corn and soybeans for central Minnesota under normal climatic conditions. These estimates were calculated by using normal crop development rates for 95 RM corn and central soybean maturity zone and normal water use patterns for central Minnesota.

Step 3. Determining available soil moisture deficit (ASMD) or management allowable depletion (MAD) 

The available water holding capacity (AWC) of different soils varies. It is a function of soil texture, soil structure and organic matter of the soil. It is the soil moisture (water) that can be extracted and used by plants. Available water holding capacity (AWC) is the amount of water the soil holds between the upper limit, i.e., field capacity, and the lower limit, i.e., permanent wilting point. To determine the total available water (TAW) in the root zone depth, the AWC is multiplied by root zone depth (RZD).

TAW = AWC x RZD

Table 6 presents the TAW of different soil types in top 3 feet or less. The AWC for other soil types can be obtained from NRCS web soil survey https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

Since most fields have several types of soils, the lowest water holding capacity soil covering at least 25 percent of the field should be used in the above calculations. Lower water holding soils found on ridges or hill tops should not be used to plan the next irrigation. Research shows that 60 to 70 percent of the TAW can be depleted at crop maturity without reducing the grain yield. Therefore, the ASMD in top 3 feet or less can be calculated by the following equation:

ASMD = 0.60 x TAW

Step 4. Measuring current soil moisture deficit (CSMD)

The current soil moisture deficit is the difference between the TAW and the actual soil moisture status in the root zone depth in field. There are many methods available to measure the current soil moisture deficit. These methods include measuring soil moisture electronically using neutron gauge or resistance blocks, measuring by physical methods like tensiometers, estimating using traditional hand feel or appearance method, and using irrigation scheduling methods such as checkbook method which uses ET data.

More information on estimating the current soil moisture deficit can be found on the University of Minnesota Extension irrigation website.

Steps 5-6. Remaining usable water (RUSM) and irrigation water requirement (IWR) 

Once we determined the ASMD and the CSMD, the remaining usable soil moisture (RUSM) in the root zone can be calculated by subtracting the current soil moisture deficit (CSMD) from the allowable soil moisture deficit (ASMD) at maturity.

RUSM = ASMD - CSMD

Irrigation water requirement (IWR) can be calculated by subtracting the RUSM from the WUCM:

IWR = WUCM – RUSM

Note: If IWR is negative that means no irrigation is needed.

Tools

Several tools can estimate daily crop water use, such as:

• Central Minnesota Ag Weather Network - includes a crop water use calculator

• NDAWN Crop water use (NW Minnesota and North Dakota)

• Daily ET estimates (for SE Minnesota and Wisconsin)

• Crop water use (ET) tables in the MN-ND Checkbook irrigation scheduling spreadsheet.

Crop water use tables

The checkbook spreadsheet will estimate daily ET and crop water use automatically.

The spreadsheet includes tables with estimated crop water use values for various maximum temperature ranges at different growth stages for several commonly irrigated crops in  Minnesota.

Prior to full canopy, reduce the ET estimate by a crop correction value between 0.2-1.0, depending on growth stage.

North Dakota State University originally developed these crop water use tables, but agricultural engineering researchers at the University of Minnesota recalibrated them to central Minnesota average climatic conditions.

Manual calculations

If you want to calculate this manually, you can estimate daily crop water use from these ET tables by observing the:

• Maximum daily temperature. Get these from local weather broadcasting stations or an on-farm max-min thermometer.

• Crop growth stage.

• Weeks after emergence.

If a season’s climatic conditions cause the crop to grow more slowly or more quickly than normal, use the crop growth stages listed in the table instead of the week after emergence to select the appropriate ET estimation.

Pumping capacity can be expressed in terms of either the:

• Pumping rate in gallons per minute (gpm) divided by the number of acres irrigated (gpm per acre).  For example, the pumping capacity of a traveling gun covering 100 acres and pumping 500 gpm is 500 divided by 100, or 5 gpm per acre.

• Average daily application amount (inch per day).

Pumping rate: How to measure

To accurately measure the pumping rate and monitor for changes, install a water meter.

Average application amount: How to measure

You can determine the average application amount based on a 24-hour pumping day from Table 7, which shows various pumping capacities and application efficiencies.

Because sprinkler irrigation isn’t 100 percent efficient, the calculated average application rate (inches per day) needs to reflect losses from evaporation, wind drift and system uniformity. Different system types give different application efficiencies depending on operation method and time of day.

Center pivots and linear movement systems generally have between 80 to 90 percent application efficiency. Traveling guns are 65 to 75 percent efficient. If the average daily pumping time is less than 24 hours, proportionately reduce the application rate.

Video: Irrigation uniformity testing

Example

To interpret a system’s capability, let’s assume a center pivot with a pumping capacity of 5 gpm per acre with an application efficiency of 85 percent.

• As shown in Table 4, this system will give a net daily application amount of .23 inches per day.

• If it’s set to make a revolution in three and a half days, the system will apply a total of .80 inches (3.5 days x .23 inches per day = .80 inches per revolution).

• In mid-July, we know daily crop water use may be as high as .25 to .30 inches per day.

Because the daily application amount in the example is slightly lower than the peak, this tells the manager that it may be wise to start irrigating earlier in the year to avoid getting behind in meeting the crop’s water needs.

For irrigation systems with limited or underdesigned pumping capacities for a specific crop and soil type, there are limited water management strategy alternatives for reducing the risk of moisture stress.

For example, research on irrigated corn in west central Minnesota has shown that producers should set the allowable deficit to no more than .75 inches to reduce the risk of stress with an underdesigned system. This deficit should start in mid-vegetative stage (about 10th leaf) and continue until late dent.