SOIL / IRRIGATION MANAGEMENT
Work Through a Daily Irrigation Water Budget
Use a simple daily root-zone deficit example to track depletion, record the assumptions, and correct the estimate when field measurements disagree.
- irrigation scheduling
- water budget
- soil moisture
- evapotranspiration
Use the budget to estimate a deficit, not to predict the field perfectly
A root-zone water budget tracks water removed by crop evapotranspiration against water added by precipitation and irrigation. Colorado State University Extension describes the resulting deficit as the difference between field capacity and the current root-zone water level. It is a decision aid, not a substitute for checking conditions in the field.[1]
Keep every term in the same depth unit for the same management zone. The example below uses inches of water over one zone. Its values are illustrative and do not set a crop, soil, rooting-depth, or irrigation threshold.
Set up the daily calculation
For a simplified daily balance, start with yesterday's deficit, add today's crop ET, then subtract precipitation and the net irrigation that infiltrated the root zone. CSU presents this form as Dc = Dp + ETc - P - Irr, with a negative result reset to zero because the simple model does not store water above field capacity in the root zone.[1]
The calculation needs a known starting root-zone water status, crop ET, precipitation, and the soil's available water capacity. Crop ET is not a universal weather number. It depends on crop development and the method used to turn reference ET into crop ET.[1][2]
A two-day worked example
Suppose a zone begins Day 1 with a 0.45-inch deficit. Crop ET for the day is 0.18 inch, measured precipitation is 0.04 inch, and no irrigation is applied. The estimated deficit is 0.45 + 0.18 - 0.04 - 0 = 0.59 inch.
On Day 2, begin with 0.59 inch. If crop ET is 0.20 inch, no effective precipitation reaches the root zone, and 0.25 inch of irrigation infiltrates, the estimate becomes 0.59 + 0.20 - 0 - 0.25 = 0.54 inch. The arithmetic shows the direction of the account. It does not prove that every applied inch entered the root zone or that 0.54 inch is an irrigation trigger.
Record the assumptions that can break the estimate
The simple balance assumes that precipitation and irrigation enter through the soil surface. When application or rainfall exceeds infiltration capacity, runoff can make the calculated deficit too small. Deep percolation, drainage, and shallow groundwater can also change the root-zone account when they matter for the site.[1]
Record the management zone, beginning deficit, ET source and crop-stage basis, precipitation, planned and applied irrigation, and any runoff, drainage, or groundwater assumption. A number without those inputs cannot be checked or compared later.
Correct the budget with representative field measurements
Compare the calculated deficit with a representative field measurement often enough for the decision and after conditions that could make the model less reliable. CSU advises using the observed deficit as the next day's value when the calculated and observed deficits differ substantially, so that the account does not carry its uncertainty forward unchanged.[1]
Before using a sensor reading to correct the balance, confirm that it represents the same management zone and active root zone. For the broader timing-and-depth decision, see Schedule Irrigation With Soil Moisture and ET. For placement, see Place soil-moisture sensors where they represent the root zone.
Research record
Sources
Every factual claim in this note is tied to the numbered evidence below.
- university extension[1] Colorado State University Extension, Irrigation Scheduling: The Water Balance Approach. Root-zone deficit accounting, the simplified daily water-balance equation, model assumptions, infiltration and runoff limits, and correction with observed field deficit.
- standards body[2] Food and Agriculture Organization of the United Nations, Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. The crop-coefficient approach for converting reference evapotranspiration into crop evapotranspiration under stated conditions.
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Related operating questions
Schedule Irrigation With Soil Moisture and ET
Use a root-zone water budget, representative soil-moisture readings, and field observations together to decide whether to irrigate, wait, investigate, or adjust depth.
Place soil-moisture sensors where they represent the root zone
Choose representative management zones, depth bands, and in-row locations before treating a soil-moisture reading as an irrigation decision input.