What is a soil water balance (checkbook method)?
North Dakota State University Extension's Checkbook Method of irrigation scheduling tracks soil moisture the way you'd track a bank account: crop water use (ETc) is a withdrawal, rain and irrigation are deposits, and the running total is the soil moisture deficit, how much water the root zone has used since it was last full. Unlike a single fixed-interval schedule, this ledger updates period by period, so it reflects what's actually happened, or what a specific forecast would do to the deficit.
This calculator flags the first period where the deficit reaches your management allowed depletion (MAD), the share of total available water NDSU Extension's method allows to be used up before the next irrigation, commonly 50%.
If you just need a quick lawn answer without setting up a full ledger, the lawn water deficit calculator covers that simpler case. For a projected schedule from a single set of inputs rather than a period-by-period ledger, see the irrigation scheduling calculator.
How this soil water balance calculator works
1. Total available water (TAW)
University of California Agriculture and Natural Resources gives typical available water capacity (AWC) ranges of about 0.5-1.0 in/ft for sand, 1.0-1.5 in/ft for sandy loam, 1.5-2.0 in/ft for silt or clay loam, and 2.0-2.5 in/ft for clay. This calculator uses the middle of the range for the texture you choose, or your own entered value.
TAW
TAW = Root zone depth × (AWC ÷ 12)
2. Allowed depletion (MAD)
MAD depth
MAD depth = TAW × MAD%
3. The running ledger
For each period, the deficit carries forward from the previous period, adds that period's ETc, and subtracts rain and irrigation, following NDSU Extension's Checkbook Method.
Deficit for period i
Deficit(i) = Deficit(i−1) + ETc − Rain(i) − Irrigation(i)
Held between 0 (full) and TAW (fully depleted); water beyond either bound is treated as lost rather than carried over.
4. Flagging MAD
Each period's deficit is compared against the MAD depth from step 2; a period is flagged once the deficit meets or exceeds it.
When this calculator isn't the right tool
Use a different approach for:
- A simple lawn dry-spell check, without building a full ledger. See the lawn water deficit calculator.
- A projected fixed schedule from one set of inputs rather than period-by-period tracking. See the irrigation scheduling calculator.
- Working out ETc itself before you can fill in the ledger. See the reference ET calculator or the vegetable garden water calculator.
Soil water balance calculation example
Sandy loam soil (AWC 1.25 in/ft), a 12 in root zone, 50% MAD, starting at 0 deficit, 0.2 in/day ETc, daily periods:
- TAW: 12 × (1.25 ÷ 12) = 1.25 in. MAD depth: 1.25 × 50% = 0.625 in.
- Day 1: 0 + 0.2 − 0 − 0 = 0.2 in.
- Day 2: 0.2 + 0.2 = 0.4 in.
- Day 3 (0.1 in rain): 0.4 + 0.2 − 0.1 = 0.5 in.
- Day 4: 0.5 + 0.2 = 0.7 in → 0.7 ≥ 0.625, MAD reached on Day 4.
- Day 5 (0.8 in irrigation applied): 0.7 + 0.2 − 0.8 = 0.1 in.
The same idea in metric
Silt/clay loam soil (AWC 1.75 in/ft), a 30 cm root zone, 50% MAD, starting deficit 1 cm (10 mm), 25 mm/week ETc, weekly periods:
- TAW: 30 × (1.75 ÷ 12) = 4.375 cm = 43.75 mm. MAD depth: 21.875 mm.
- Week 1 (10 mm rain): 10 + 25 − 10 = 25 mm → 25 ≥ 21.875, MAD reached in Week 1.
- Week 2 (30 mm irrigation): 25 + 25 − 30 = 20 mm.
Both examples use the same formula the calculator runs; enter your own site's data above for your ledger.
Practical guidance
Only count effective rainfall, the portion that actually infiltrates the root zone, as a deposit; a light shower that mostly evaporates from the surface contributes far less than its measured depth. A rain gauge near the site is more reliable than a regional report for this purpose.
Update ETc through the season rather than reusing one figure throughout the ledger; ET rises in hot, dry, windy weather and drops in cool, calm conditions. If your actual weather varies a lot period to period, running the ledger again with an updated ETc, or splitting it into shorter daily periods instead of weekly ones, will track reality more closely than one constant rate.
Check local watering rules. Many water utilities set watering days, hours or restrictions, especially during drought. Where those apply, they take priority over any calculated ledger.
What changes the ledger
| Factor | Effect | What to check |
|---|---|---|
| Soil texture / AWC | Coarser soils store less water, so the deficit climbs to MAD faster. | Use a soil test or Web Soil Survey value where possible. |
| Root zone depth | A deeper effective root zone stores more water and delays MAD. | Use the plant's real rooting depth, not a maximum potential depth. |
| Starting deficit | Starting already partly depleted brings MAD closer, or reaches it immediately. | Base it on a recent soil check where possible, not a guess. |
| ETc | A higher rate depletes the profile faster each period. | Update ETc for current weather and growth stage rather than one seasonal figure. |
| Rain and irrigation timing | The same total spread differently changes which period first reaches MAD. | Enter amounts in the actual period they occurred or are planned. |
| MAD | A lower allowed depletion reaches the flag sooner. | Extension guidance commonly uses 50% as a general default; lower it for sensitive plantings. |
Frequently asked questions
What is a soil water balance or checkbook method?
North Dakota State University Extension's Checkbook Method tracks soil moisture like a bank account: crop water use (ETc) is a withdrawal, and rain and irrigation are deposits. The running total, the soil moisture deficit, tells you how much water the root zone has lost since it was last full, period by period.
What is management allowed depletion (MAD), and what happens when it's reached?
Management allowed depletion is the share of total available water (TAW) that's allowed to be used up before the next irrigation. North Dakota State University Extension's Checkbook Method starts irrigation once the deficit reaches that point, commonly set at 50% of TAW; this calculator flags each period where the running deficit meets or exceeds that threshold.
How is total available water (TAW) calculated?
TAW is available water capacity (AWC), typically inches of water per foot of soil, multiplied by the root zone depth. University of California Agriculture and Natural Resources gives typical AWC ranges from about 0.5-1.0 in/ft for sandy soils up to 2.0-2.5 in/ft for clay.
Why does the deficit sometimes stay the same even though ETc keeps using water?
The deficit can't exceed total available water (TAW); once the root zone has used up all of its available water, this calculator holds the deficit at TAW rather than letting it climb further, similar to how a soil can't hold a debt beyond what it was capable of storing in the first place.
Can the deficit go negative?
No. If rain and irrigation in a period exceed the existing deficit plus that period's ETc, the extra water is treated as lost to runoff or deep percolation beyond the root zone, per the checkbook method's approach, and the deficit is held at zero rather than going negative.
Should I use daily or weekly periods?
Daily periods track conditions more precisely and catch short dry spells that a weekly total might smooth over, which matters more for shallow-rooted or sandy-soil situations. Weekly periods are simpler to maintain and work reasonably well for deeper-rooted plantings on soils that hold more water.
How is this different from the irrigation scheduling calculator?
The irrigation scheduling calculator projects a fixed net depth, interval and gross depth forward at a constant rate from a single set of inputs. This calculator instead runs an actual period-by-period ledger, letting rain and irrigation vary by period, which suits tracking what's really happened or planning around an uneven forecast.
See the irrigation scheduling calculator.
Is this the right tool for a quick lawn dry-spell check?
Not if you just want a simple lawn answer without setting up a full ledger. The lawn water deficit calculator covers that simpler case directly.
See the lawn water deficit calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- The TAW, MAD, and running-deficit math were verified by hand across multiple periods, and both worked examples were traced against the code.
- AWC and MAD figures are checked against the extension sources below.
- The 0/TAW clamping and MAD-flagging logic were tested against cases that hit each boundary, so no period can show a negative deficit or one above TAW.
Sources
- North Dakota State University Extension. Scherer, T.F. & Steele, D.D. Irrigation Scheduling by the Checkbook Method (AE792).
- University of California Agriculture and Natural Resources. Why Is Soil Texture Important? (AWC by soil texture).
- USDA Natural Resources Conservation Service. National Engineering Handbook, Part 652, Irrigation Guide (standard TAW, MAD and soil-water-budget terminology).