As irrigation costs climb and weather grows more erratic, farmers are under pressure to make every inch of water count. One of the simplest, most practical tools they can use this season won’t require new hardware on the pivot — just a different way of thinking about soil moisture.
North Dakota State University associate professor and irrigation engineer Dean Steele encourages farmers to manage soil water like their checkbook: track deposits and withdrawals, and don’t let the account get overdrawn. That mindset, he says, is the foundation of better irrigation timing and improved efficiency.
“The soil is our bank account. We’ve got withdrawals and deposits,” he notes. “Your deposits are the rain and irrigation. Your withdrawals are the crop water use and things like the deep percolation and maybe some runoff.”
The soil profile starts each growing season with a certain balance of water. Every day, evapotranspiration (ET) — the combined effect of evaporation from the soil and transpiration from the crop — pulls moisture out. Rain and irrigation add it back in.
Just as with a financial account, it’s not enough to know how much “money” moves in and out over a year. What also matters is when it moves — especially during critical periods like tasseling or grain fill, Steele says.
Why Seasonal Totals Can Mislead
Steele uses a favorite classroom trick question to show why irrigation timing is so important.
He asks students: If a crop needs 18 inches of ET over a season and the farm receives 12 inches of rain, how much irrigation is required?
The obvious answer is six inches. But that is incorrect.
“If all the rain of that 12 inches comes on May 1, and you get nothing the rest of the season, then you still need 18 inches,” Steele explains.
In that scenario, early-season rain may fill the soil profile, but if it’s not replenished as the crop draws water in July and August, the soil account will be overdrawn exactly when the plant is most sensitive to stress.
The lesson, Steele says, is that seasonal totals hide risk. Farmers need to track the running balance in the soil, not just the sum of rainfall and irrigation on a yearly chart.
Build A Simple Water-Balance Ledger
Steele says growers can implement a practical water-balance approach with tools many already have: a rain gauge, basic ET information and records of irrigation events, often available in their spreadsheet.
A basic checkbook-style water balance would include these four elements:
1. Starting balance: Estimate available water in the rooting zone at planting (for example, after pre-watering or spring recharge).
2. Daily withdrawals: Use ET estimates (from local weather networks, Extension tools or ET calculators) to subtract crop water use.
3. Daily deposits:
- Add effective rainfall (total rain minus runoff or obvious losses).
- Add irrigation applied (inches per pass or per revolution).
4. Running balance: Track how much water remains in the effective root zone relative to field capacity and a chosen depletion limit.
Steele compares ET and side losses to an unavoidable set of expenses — “groceries… housing and taxes” — that must be paid out of the account every day. If those outflows consistently exceed deposits, the crop will eventually experience stress long before the calendar suggests it should.
Adapting The Method To Different Climates
The same accounting framework applies whether you farm in the upper Midwest or the High Plains, but the numbers in the ledger will look very different.
In North Dakota, Steele notes, seasonal ET is relatively modest and summer rainfall sometimes helps “catch up,” meaning there can be more opportunities to pause or reduce irrigation when rainstorms arrive.
In the central and southern High Plains the withdrawals are much larger, according to Brian Arnall, a precision nutrient management Extension specialist at Oklahoma State University.
“Our max ETs can easily hit three‑quarters of an inch a day; our normal ET is half an inch,” Arnall says.
With 100-degree days, 30% humidity and rapidly growing corn, the soil account in the High Plains empties fast. That’s why, in many of those systems, pivots rarely shut off once they’re started, notes Arnall.
“By the end of our cropping season, we’ll probably be right at neutral, if not negative, as far as total ET and application,” he says.
For growers in Arnall’s area, the checkbook model confirms that almost constant deposits are required just to keep pace — and it can help reveal when small interruptions in irrigation might tip the balance into stress.
Match Irrigation To Crop Root Depth And Soil Type
Steele emphasizes that the size of a farmer’s “bank account” also depends on crop rooting depth and soil characteristics. Deep‑rooted corn on heavier soils can draw from a larger reservoir; potatoes on sandy ground with shallow roots cannot, he notes.
“With a corn crop… two‑thirds of an inch, that’s not a lot of water,” Steele says. In potato ground, by contrast, “if you’re managing 12 inches or 18 inches of root zone depth, that’s maybe what you’ve got to work with, so you’ve got to be around the circle more frequently.”
For farmers, that means:
- In deep profiles with good water-holding capacity, the starting moisture balance is higher, and the system can tolerate larger withdrawals between irrigations.
- In shallow or sandy profiles, the usable balance is small, so even modest daily ET can rapidly overdraw the account unless irrigations are more frequent.
Using The Ledger To Time Irrigation
Once a farmer has a running soil water balance, the irrigation decision can become a more disciplined approach. Steele advises growers to:
1. Irrigate when the projected balance approaches a chosen depletion threshold, not just when the soil surface looks dry.
2. Adjust application depth so that deposits match likely withdrawals over the next several days, considering forecast ET and possible rainfall.
3. Plan ahead for long pivot runs or “wipers,” where the time needed to complete a pass can allow the far end of the field to spend down its account before the irrigation system returns to that point in the field.
Steele says that on complex systems like windshield‑wiper pivots, he would pay special attention to water balance at both the starting and ending points.
“If I had a windshield wiper, I’d want to keep track of the starting and ending points and see how I’m doing, to make sure… you get back to that starting point in time,” he explains.
In practice, this might mean increasing application depth on certain passes, slowing the pivot at critical growth stages or strategically skipping lower‑risk areas where the account is still healthy.
Adding Sensors And ET Tools To The Checkbook
While Steele’s checkbook analogy can be implemented with simple records, it also provides a framework for using more advanced tech tools. Soil moisture sensors can serve as “bank statements,” verifying that the modeled balance matches reality. ET models and remote sensing can sharpen estimates of daily withdrawals, especially as researchers develop radar‑ and satellite‑based crop water use tools.
“There are people using satellite imagery as part of developing an integrated irrigation management system ... they’re keeping track of weather and soils and doing some estimation of crop water needs, and trying to estimate when the crop is going to need water, and then actually run the irrigation system,” Steele says.
In addition, local irrigation dealers and irrigation equipment manufacturers have apps and tools for managing water in the field, including variable rate irrigation. These tools are typically integrated into phone or desktop apps linked to the control panel of the irrigation system.
He suggests all of these technologies should feed into answering the same core questions: What is my soil water balance today, and what will it be if I do — or don’t — irrigate?
Manage Water Like Money
Behind the math and models, Steele’s message is that farmers who manage soil water like their money are better positioned to use irrigation when it delivers the highest return.
By tracking deposits and withdrawals, recognizing that “when” matters as much as “how much,” and understanding how soil and climate shape their account size, growers can head into this season with a clearer picture of where every inch of water is going — and whether it’s truly helping their crop.
Listen to more of Steele and Arnall’s recommendations on The Crop Podcast Show here.
