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 and user's manual of the North Dakota-Minnesota checkbook method from NDSU.
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.
- Two or more rain gauges.
- Max-min thermometer or access to local temperature reports.
- Checkbook spreadsheet
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.
Rain gauges
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 |
Excess water
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.
-
Weather forecast.
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.
Irrigation amount
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.
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.
Acknowledgements
The author wishes to thank former University of Minnesota colleagues Hal Werner, Extension engineer at South Dakota State University, and Fred Bergsrud, retired Extension engineer, for their previous development efforts in earlier iterations of this content. Also, thank you to the original developers of this scheduling tool, Darnell Lundstrom and Earl Stegman, Agricultural Engineering Department, North Dakota State University.
Reviewed in 2018