Irrigation scheduling checkbook method
With irrigation scheduling, you can plan when and how much water to apply for maintaining healthy plant growth during the growing season. It’s an essential daily management practice for a farm manager growing irrigated crops.
On this webpage, we’ll describe how to monitor a field's daily soil water balance using what’s commonly known as the checkbook method. This can be used to plan the next irrigation.
Basics of irrigation scheduling
Properly timing irrigation water applications is a crucial decision for a farm manager to:
Meet the water needs of the crop to prevent yield loss due to water stress.
Maximize the irrigation water use efficiency, resulting in beneficial use and conservation of the local water resources.
Minimize the leaching potential of nitrates and certain pesticides that may impact groundwater quality.
Effective irrigation is possible only by regularly monitoring soil water and crop development conditions in the field, and by forecasting future crop water needs.
Delaying irrigation until crop stress is evident or applying too little water can result in substantial yield loss. Applying too much water leads to extra pumping costs, wasted water and increased risk for leaching valuable agrichemicals below the rooting zone and possibly into the groundwater.
In addition to the checkbook method, other tools to assist with irrigation scheduling include soil probes, soil moisture sensors, in-field weather stations, crop water use estimators, daily soil water balance checkbook worksheets, computerized daily soil water balance accounting programs and private consultants.
Checkbook scheduling: How it works
The checkbook method of scheduling enables irrigation farm managers to monitor a field's daily soil water balance (in terms of inches of soil water deficit), which can be used to plan the next irrigation.
Download the spreadsheet
Following is the spreadsheet version of the North Dakota-Minnesota checkbook method, as well as a user manual:
Keep each field's soil water balance in individual spreadsheets or spreadsheet tabs because of the differences in soil, crop, planting date, rainfall and plant growth rates.
What you need to do
This method requires that you:
Monitor the crop’s growth.
Know your soil texture(s) in the rooting zone
Observe and log the maximum air temperature each day.
Measure and log the rainfall or irrigation applied to the field.
The checkbook spreadsheet will automatically estimate evapotranspiration and soil water deficits.
Effectiveness of the checkbook method depends on the accuracy and regularity of the in-field observations and measurements. This is because crop water use estimates are influenced by more climatic factors than considered in this method.
To be successful, visit the field every three to seven days to determine if field conditions agree with the estimated soil water deficits predicted in the spreadsheet. If they don’t agree, the estimated soil water deficit can be adjusted.
The best time to make the daily update is early morning, after measuring the in-field rain gauges.
Using the checkbook method
Operate the spreadsheet just like a checkbook. Each day, log the maximum temperature and rainfall or irrigation amounts. To set up and operate an effective soil water accounting system like the checkbook method, you need to understand how field characteristics and soil-water-plant factors interrelate.
The soil water deficit is the amount of water needed to fill the soil profile to field capacity. With the checkbook spreadsheet, this is calculated automatically. It forms the basis for making irrigation decisions.
The general formula for calculating the daily water balance is expressed in rainfall depth equivalents (in):
SWD(today) =SWD(yesterday) - Rainfall - irrigation + evapotranspiration + water loss (percolation or runoff)
Where SWD = soil water deficit
When the soil water deficit exceeds the maximum allowable deficit, then irrigation should be scheduled.
Measuring soil water inputs
Accurately measuring rainfall and irrigation water are essential in estimating a field’s daily soil water deficit.
Locate at least two rain gauges within the field to give representative values of the net water received from either rainfall or irrigation. If you have soil moisture sensing devices in the field, placing a rain gauge at each site would be a good choice.
Read rain gauges within a day after a precipitation event. To be fairly accurate, rain gauges should have an opening of at least 2 inches and be positioned near the top of the crop canopy in the irrigated field.
Irrigation system pumping capacity
You can also estimate the net irrigation amount from Table 1 by knowing the irrigation system's pumping capacity, application efficiency and operating time. However, in-field measurements of irrigation water applied is more accurate.
Table 1: Average daily net application depths for various pumping capacities
|Pumping capacity||65% average application efficiency||75% average application efficiency||85% average application efficiency|
|4 gallons per minute (gpm) per acre||0.14 net inches per day||0.16 net inches per day||0.18 net inches per day|
|5 gpm per acre||0.17 net inches per day||0.20 net inches per day||0.23 net inches per day|
|6 gpm per acre||0.21 net inches per day||0.24 net inches per day||0.27 net inches per day|
|7 gpm per acre||0.24 net inches per day||0.28 net inches per day||0.32 net inches per day|
|8 gpm per acre||0.28 net inches per day||0.32 net inches per day||0.36 net inches per day|
|9 gpm per acre||0.31 net inches per day||0.36 net inches per day||0.41 net inches per day|
If a daily rainfall or irrigation event minus the daily crop water use is greater than the current deficit, consider most of the excess to be lost due to deep percolation below the rooting zone. The new deficit balance is calculated by the checkbook spreadsheet and generally set to zero.
However, for most soils, some of the excess water is still available to the plant during deep percolation. This period of excess soil water may last from one day on sandy soils to more than two days on a heavy-textured soil. During this period or until crop water use (ET) consumes the excess water, the soil water deficit will be equal to zero.
Handling differences between predictions and in-field estimates
If figures differ, change the checkbook spreadsheet prediction to the in-field estimation.
If you stop updating the spreadsheet
If the checkbook spreadsheet is interrupted and a period of time elapses, you can easily restart the spreadsheet anytime to assist in scheduling future irrigations.
To decide when to start irrigating, compare the latest soil water deficit balance in relationship to the:
Selected irrigation water management strategy for that crop.
Crop's projected water needs.
An irrigation water management strategy outlines the manager's plans for irrigating, including the manager's selected allowable soil water deficit limits for different growth stages of the crop. Maintain the crop’s soil water balance within the set deficit limits by either rainfall or irrigation.
When possible, the amount of applied irrigation water should be somewhat less than the soil water deficit to provide some soil water storage reserve for rainfall.
For most soils, the net irrigation application during early plant growth and the last few weeks before maturity should be only 30 to 50 percent of the soil water deficit. This practice will increase the opportunity to store more rainfall and reduce the potential for leaching from normal rainfall events.
On most sandy soils, the irrigation depth should be 80 to 100 percent of the soil water deficit during the crop’s critical growth period. On medium- to fine-textured soils, irrigation application depth should be 50 to 100 percent of the soil water deficit depending on the irrigation system's pumping capacity.
Irrigation management strategy
Deciding when to irrigate to optimize production is a daily judgment call that requires you to consider several factors. Many of these factors change as the crop develops.
Below are some general guidelines to consider when developing a water management plan and setting allowable soil water deficit limits.
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
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
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 2 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.
The table shows how early irrigation cutoff impacts corn grown in west central Minnesota on a Renshaw soil with 3.5 inches of available water capacity. Rainfall amounts are between cutoff stage and corn maturity. Source: Westgate, Olness and Wright (1991).
Table 2: Impact of early irrigation cutoff
|Growth stage at cutoff||1989: Yield||1989: Irrigation||1989: Rain||1990: Yield||1990: Irrigation||1990: Rain|
|Late dent||199 bushels per acre||8.2 inches||3.7 inches||148 bushels per acre||9.0 inches||1.9 inches|
|First dent||202 bushels per acre||7.5 inches||5.8 inches||144 bushels per acre||8.3 inches||3.5 inches|
|Dough||202 bushels per acre||6.2 inches||6.5 inches||141 bushels per acre||7.6 inches||3.7 inches|
|Blister||204 bushels per acre||4.2 inches||6.7 inches||122 bushels per acre||5.3 inches||3.8 inches|
|LSD (.05)||8.0 bushels per acre||--||--||5.2 bushels per acre||--||--|
Generally, a corn crop will need about 2 to 2.5 inches of 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.
Tables 3 and 4 list estimated ET requirements between several crop stages of growth and maturity for corn and soybeans under normal weather conditions in central Minnesota. Managing a larger soil water deficit near maturity may reduce the irrigation water needs by 1 to 3 inches per acre, which saves pumping costs and conserves the irrigation water supply.
Table 3: Corn evapotranspiration (ET) requirements
|Stage of growth||Days to maturity||Water use (ET) to maturity|
|Blister||45 to 50 days||7.0 to 7.5 inches|
|Milk||38 to 42 days||4.8 to 5.3 inches|
|Dough||30 to 35 days||3.2 to 3.6 inches|
|First dent||23 to 27 days||2.1 to 2.4 inches|
|Full dent||19 to 21 days||1.6 to 1.8 inches|
|1/2 milk line||12 to 14 days||0.9 to 1.2 inches|
|1/4 milk line||6 to 8 days||0.4 to 0.6 inches|
Table 4: Soybean ET requirements
|Stage of growth||Days to maturity||Water use (ET) to maturity|
|Full flower: R2||48 ro 54 days||6.8 to 7.6 inches|
|Full pod: R4||35 to 39 days||4.0 to 4.8 inches|
|Begin seed fill: R5||27 to 31 days||2.7 to 3.3 inches|
|Begin maturity: R7||9 to 11 days||0.4 to 0.7 inches|
Another possible irrigation water management plan is to set the allowable soil water deficit equal to, or slightly greater than, the irrigation system’s normal net application amount.
For example, if the typical application is .75 inches net, then choose a planning deficit limit of .75 to 1 inch. If this exceeds 50 percent of the available water capacity in the root zone, make the amount smaller – especially during the critical stages of crop growth – to reduce the risk of moisture stress.
This strategy will require more irrigation applications than the variable deficit strategy described earlier
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.
For each irrigated field, establish an allowable soil water deficit for various crop growth stages. This planning limit specifies the maximum amount of soil water you choose to use from the rooting zone before scheduling the next irrigation event. This reduces the probability of incurring crop moisture stress.
Allowable soil water deficit is expressed in either inches of soil water or a percentage of the total available water in the rooting zone.
To express the percentage deficit in terms of inches of water, multiply the set percentage deficit by the available water capacity in the root zone. For example, if you want a 30 percent deficit limit for soil that’s holding 3.50 inches of water, the deficit level in inches of soil water would be 1.05 inches (.30 x 3.50 = 1.05).
In the past, irrigations were planned to prevent the soil water deficit from becoming greater than 50 percent of the total available water capacity in the rooting zone. This was a general guideline, but today’s research suggests it should vary depending on the crop, growth stage, soil water capacity and the irrigation system's pumping capacity.
This variable deficit plan will better help optimize the field’s production and minimize any impact to the local water supply.
Find more guidance on setting these limits in the above irrigation management strategy section.
Soil water deficit is the amount of water required at a given time to restore a soil profile’s active crop rooting zone to field capacity. Any additional water will percolate through the soil profile within 24 to 48 hours.
Several in-field tools can help you estimate the current soil water deficit in the rooting zone. These are detailed in the next section, “Soil water measurements”.
Regularly updating and reviewing the soil water deficit in a field every two to three days can provide useful information for planning the next irrigation. In the checkbook method, adjust the predicted soil water deficit on the balance sheet to the actual in-field soil water deficit, if these figures are different.
Soil water measurements
Two common ways to estimate the soil water deficit are by the feel/appearance method or using soil water sensors.
Collect soil samples in the root zone with a soil probe or spade.
Estimate the water deficit for each sample by feeling the soil and judging the soil moisture as outlined in Table 5. Take soil samples at several depths in the root zone and at several places in the field.
Use these estimated deficits to estimate the total soil water deficit in the root zone.
This method requires frequent use to develop consistent estimates.
Table 5: Guide for judging soil water deficit based on soil feel and appearance for several soil textures
|Moisture deficiency||Soil texture classification: Coarse (loamy sand)||Soil texture classification: Sandy (sandy loam)||Soil texture classification: Medium (loam)||Soil texture classification: Fine (clay loam)|
|0.0 inches per foot (field capacity)||Leaves wet outline on hand when squeezed||Appears very dark, leaves wet outline on hand, makes a short ribbon||Appears very dark, leaves wet outline on hand, will ribbon out about one inch||Appears very dark, leaves slight moisture on hands when squeezed, will ribbon out about two inches|
|0.2 inches per foot||Appears moist, makes a weak ball||Quite dark color, makes a hard ball||--||--|
|0.4 inches per foot||Appears moist, makes a weak ball||Quite dark color, makes a hard ball||Dark color, forms a plastic ball, slicks when rubbed||Dark color, will slick and ribbons easily|
|0.6 inches per foot||Appears slightly moist, slightly sticks together||Fairly dark color, makes a good ball||--||--|
|0.8 inches per foot||Appears to be dry, will not form a ball under pressure||Slightly darker color, makes a weak ball||Quite dark, forms a hard ball||Quite dark, will make a thick ribbon, may slick when rubbed|
|1.0 inches per foot||Appears to be dry, will not form a ball under pressure||Lightly colored by moisture, will not ball||Fairly dark, forms a good ball||Fairly dark, makes a good ball|
|1.2 inches per foot||Dry, loose, single grains flow through fingers (wilting point)||--||Slightly dark, forms weak ball||Will ball, small clods will flatten out rather than crumble|
|1.4 inches per foot||--||Very slight color due to moisture, loose, flows through fingers (wilting point)||Lightly colored, small clods crumbles fairly easily||Will ball, small clods will flatten out rather than crumble|
|1.6 inches per foot||--||Very slight color due to moisture, loose, flows through fingers (wilting point)||--||Slightly dark, clods crumble|
|1.8 inches per foot||--||--||Slight color due to moisture, powdery, dry, sometimes slightly crusted but easily broken down in powdery condition (wilting point)||Some darkness due to unavailable moisture, hard baked, cracked, sometimes has loose crumbs on surface (wilting point)|
|2.0 inches per foot||--||--||--||Some darkness due to unavailable moisture, hard baked, cracked, sometimes has loose crumbs on surface (wilting point)|
Soil water sensors
Soil tension is a measurement usually expressed in centibars, and describes how tightly the water is held in the soil profile. It can be measured at any point in the soil profile by using sensors such as tensiometers or electrical resistance blocks.
If you know the soil texture, you can use Table 6 to estimate the amount of soil water deficit for a given tension reading.
Table 6: Soil water deficit estimates in inches per foot for various soil tensions
|Soil texture||Soil tension: 10 centibars||Soil tension: 30 centibars||Soil tension: 50 centibars||Soil tension: 70 centibars||Soil tension: 100 centibars||Soil tension: 200 centibars||Soil tension: 1,500* centibars|
|Coarse sand||0 inches per foot||0.1 inches per foot||0.2 inches per foot||0.3 inches per foot||0.4 inches per foot||0.6 inches per foot||0.7 inches per foot|
|Fine sand||0 inches per foot||0.3 inches per foot||0.4 inches per foot||0.6 inches per foot||0.7 inches per foot||0.9 inches per foot||1.1 inches per foot|
|Loamy sand||0 inches per foot||0.4 inches per foot||0.5 inches per foot||0.8 inches per foot||0.9 inches per foot||1.1 inches per foot||1.4 inches per foot|
|Sandy loam||0 inches per foot||0.5 inches per foot||0.7 inches per foot||0.9 inches per foot||1.0 inches per foot||1.3 inches per foot||1.7 inches per foot|
|Clay loams||0 inches per foot||0.2 inches per foot||0.5 inches per foot||0.8 inches per foot||1.0 inches per foot||1.6 inches per foot||2.4 inches per foot|
*1,500 centibars is permanent wilting point. The soil water deficit value is equal to the soil's total available water-holding capacity
Tensiometers and resistance blocks
Tensiometers directly read soil tension between 0 and 80 centibars and work best in sandy loam or lighter-textured soils. Resistance blocks work in a wider soil texture range; some models work as well in lighter textured soils as tensiometers.
To convert electrical resistance block readings to soil tension values, you need a calibration curve from the manufacturer. It should be provided with the meter.
To get representative soil tension readings with any sensors, leave them installed throughout the irrigation season and preferably at two or more locations in the field (Figure 1; see above “Estimating soil water deficit” section). Two depths are generally desired at each location. These depths should be about one-third and two-thirds of the active root zone (Figure 2).
Soil moisture sensor installation
To install electrical resistance type soil water sensors, follow the steps below along with the instructions given by the sensor manufacturer. Sensors should be placed in a field within two weeks after plant emergence.
Soak each sensor in clean water for one to two hours to remove the air and then allow drying for four to six hours. Repeat this step two times more. Prior to placing sensors into the soil soak at least five minutes. If the sensors were used in a previous season, evaluate for damage to the wire leads or the sensor surface and discard the sensor if surface looks plugged with soil or damaged.
Select two or more locations for the sensors in the first third of the irrigated field and one location in the last third of the field (Figure 3). All sensors should be located in a representative soil type in the field. Under a center pivot, the sensors should not be installed near pivot wheel tracks or other high traffic areas. The sensors should be placed somewhere between the second tower and the last tower. Each sensor should be positioned within the plant row near a healthy plant in a location with a normal plant population. At each site a sensor should be set 4 to 6 inches below the ground surface and another set at 9 to 12 inches below the surface.
To install a sensor in the soil, first make a hole with a soil probe or auger to a depth a little deeper than desired. To get good sensor contact with the soil, pour a little dry soil and water into the new hole. To position the sensor into the hole, draw the lead wire through the soil probe tube and hold the sensor on the end (Figure 4). Push the probe and sensor into the hole to the desired depth until set firmly. Fill the hole by adding some dry soil and a little water at a time and firm with a tamping stick until the hole is filled.
Mark each sensor site with a colored flag or stake to locate the site easily. Wrap the extra lead wire around the stake. Mark each sensor’s wire lead to indicate its depth with a tag or creating one or more knots near the wire end to indicate depth position (for example one knot means shallow and two knots means a deep sensor). At one or two of the locations near the start-up position of the irrigation cycle include a rain gauge with a 2-inch diameter or greater. Also place a marker at the end of each row or along the road to indicate the entrance path to the sensor site.
Wait to start taking readings until one to two days after installation to allow the added water to equalize with the surrounding soil moisture. Sensors should be read every two to three days and recorded in a notebook or spreadsheet to track the soil water changes in the soil profile throughout the growing season.
Remove sensors before harvest using a shovel. Clean soil from sensor surfaces with only water pressure and hang up to dry for use next year.
Crop water use is the amount of water given up to the atmosphere by a crop due to evaporation from the soil surface and transpiration through the plant leaves. Crop water use is also called evapotranspiration (ET).
Daily crop water use changes throughout the growing season due to weather variation and crop development. The checkbook method needs daily ET estimations to update the soil water deficit balance.
Crop water use depends on many factors including:
Climatic conditions; parameters that have a major effect on a crop's daily water use include maximum and minimum temperatures, solar radiation, humidity and wind.
Several tools can estimate daily crop water use, such as:
The evaporation pan.
Potential ET equation.
Local crop ET hotline services.
Daily ET estimates (for Wisconsin)
Crop water use (ET) tables.
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.
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.
Available water capacity in the root zone is defined as the total amount of water capable of being held by the soil that’s available for plant use. Soil texture and crop rooting depth are the primary governing factors.
The total available water in the soil root zone for a specific crop is equal to the crop’s rooting depth multiplied by the available water-holding capacity per unit depth of the soil. However, the plant can only readily use a portion (40 to 60 percent) of this total amount without developing crop water stress.
In most fields, there are many soil types. Generally, the soil with the lowest water-holding capacity should be managed for irrigation as long as it exists in a significant (30 to 50 percent) part of the field.
County soil surveys identify soil types in most fields. County and area personnel from the Natural Resource Conservation Service (NRCS), Soil Water Conservation District (SWCD) or University of Minnesota Extension can assist with determining a field’s water storage characteristics.
Crop rooting depth
Table 7 lists suggested rooting depths for irrigation water management of commonly irrigated crops in Minnesota. Each crop can potentially develop a greater rooting depth.
However, because most of a plant’s roots are located in the upper portion of the root zone, irrigation water applications are generally managed to a shallower depth than the crop’s full rooting depth. For annual crops, this rooting depth isn’t usually achieved until 30 to 50 days after planting.
Note that local soil and climatic condition may reduce these values.
Table 7: Range of maximum crop rooting zones for irrigation water management at mid-season
|Alfalfa (established)||24 to 36 inches|
|Corn, sugarbeet||24 to 36 inches|
|Asparagus, small grain||24 to 36 inches|
|Potato, sweet corn||18 to 24 inches|
|Soybean, field bean||24 to 30 inches|
|Tomato, muskmelon||12 to 24 inches|
|Broccoli, cauliflower||12 to 18 inches|
|Blueberry, strawberry||9 to 18 inches|
If the field's soil profile is shallower because of a restrictive layer such as coarse sand and gravel, which prevents deeper root penetration, reduce the irrigation management depth accordingly.
Table 8 shows an example of a typical irrigated soil’s water-holding characteristics. This can be identified for any soil in your field. In the example, note there’s a root restriction layer of sand and gravel starting at 18 inches, which limits the available water capacity to 3.5 inches for any crop with greater rooting potential.
Table 8: Available waterholding capacity (AWC) characteristics of a Renshaw soil series
|Profile depth||Texture class||AWC per inch||AWC per zone||Cumulative AWC|
|0 to 12 inches||Loam||0.21 inches||2.52 inches||2.52 inches|
|12 to 18 inches||Sandy loam||0.16 inches||0.96 inches||3.48 inches|
|18 to 24 inches||Sand and gravel||0.04 inches||0.24 inches||3.72 inches|
|24 to 60 inches||Sand and gravel||0.03 inches||1.08 inches||4.80 inches|
You can get AWC info for others soil series from county soil surveys or SWCD and NRCS offices.
A system’s pumping capacity defines the ability of the irrigation system to refill the soil profile with water. Knowing this capacity enables you to better judge when to start an irrigation in order to complete an irrigation before any part of the field exceeds the allowable soil water deficit.
Measuring pumping capacity
Ways to measure
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)
Average daily application amount (inch per day). 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.
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 1, 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.
For 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 1, 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.
Bergsrud, F., Wright, J., Werner, H., & Spoden, G. (1982). Irrigation system design capacities for west central Minnesota as related to the available water-holding capacity and irrigation management (American Society for Agricultural Engineers paper NCR 82-101). St. Joseph, Mich.: American Society for Agricultural Engineers.
Duke, H.R. et al. (1987). Scheduling irrigations: A guide for improved water management through proper timing and amount of water application. Fort Collins, Colo.: USDA-ARS and the Soil Conservation Service, Cooperative Extension Service-Colorado State University.
Killen, M. (1984). Modification of the checkbook method of irrigation scheduling for use in Minnesota (design project). University of Minnesota.
Laboski, C., Lamb, J., Baker, J., Dowdy, R., & Wright, J. (2001). Irrigation scheduling using mobile frequency domain reflectometry with checkbook method. Journal of Soil & Water Conservation, 56 (2).
Lundstrom, D. & Stegman, E. (1977). Checkbook method of irrigation scheduling (American Society for Agricultural Engineers paper NCR 77-1001). St. Joseph, Mich.: American Society for Agricultural Engineers.
Seeley, M. & Spoden, G. (1982). Part 2: Background of crop water use models (Special Report
Soil Conservation Service. (1976). Irrigation guide for Minnesota. St. Paul, Minn.: United States Department of Agriculture.
Steel, D., Scherer, T. & Wright, J. (2000). Proceeding from American Society for Agricultural Engineers National Irrigation Symposium: Irrigation scheduling by the checkbook method: A spreadsheet version. Arizona.
Stegman, E.C. (1988). Chapter V: Water Management. In Best Management Practices Manual for Oakes Irrigation Area. North Dakota State University.
Agricultural Utilization Research Institute (AURI)/Greater Minnesota Corporation. (1991). Final report of energy conserving irrigation management: Impact of early irrigation cutoff on corn (Project # EP106). Westgate, M., Olness, A. & Wright, J.
Wright, J. 2018. Irrigation water management consideration for sandy soils in Minnesota.
Reviewed in 2018