SOIL / IRRIGATION MANAGEMENT
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.
- irrigation scheduling
- soil moisture
- evapotranspiration
- water budget
Start with the decision, not a single reading
IWM-010: Use the root-zone water budget, representative soil-moisture readings, and field observations as one decision record. Irrigate now when the operation's documented, site-specific irrigation trigger is reached or projected to be reached within the available application-capacity window, and the planned application fits the field conditions. Wait when that trigger is not reached or projected within that window. Adjust the planned depth to address the root-zone deficit within application constraints. Investigate before changing the plan when calculated and observed deficits differ substantially. These are guide actions that apply documented inputs, not universal timing or depth thresholds.[1][2]
IWM-001: An irrigation schedule may use crop ET, soil-moisture monitoring, a computerized water balance, plant monitoring, or a combination. The method needs to fit the managed irrigation zone and its decision. The October 2024 NRCS Notice of Proposed Changes and Proposed Full Text for Conservation Practice Standard 449 is a redline comparison, not an eligibility determination, current universal standard, or prescription.[1]
Build a root-zone water budget
IWM-003: A root-zone balance tracks crop-water use against rainfall and irrigation. It may also account for runoff, drainage, and shallow groundwater. The balance is a working estimate of the root-zone deficit, not a promise that all measured or applied water stayed available to the crop.[2]
IWM-006: Reference ET becomes crop ET only after the appropriate crop and growth-stage coefficients are applied. Use coefficients and weather data that fit the crop, stage, method, and local guidance. Do not substitute a coefficient from another crop, season, or source as a universal value.[3]
Make the soil-moisture reading represent the decision
IWM-005: A sensor plan needs stated objectives and locations and depths that support the irrigation decision. The October 2024 NRCS proposed text calls for documenting how the sensor locations and depths inform the decision.[1]
IWM-008: UMN Extension recommends placing sensors at several depths and locations in the field, preferably in representative soil, and monitoring and managing different soil types separately when they occur in a field. Interpret each sensor record for the management zone it was selected to represent.[4]
Set depth from the field, system, and planned result
IWM-002: Irrigation depth depends on root-zone water-holding capacity, allowable depletion, current moisture, crop stage, distribution uniformity, and field constraints. Application capacity and weather also determine whether the planned depth can be applied without creating runoff, drainage, or another mismatch. These are site-specific inputs, so this guide does not set a universal depletion, rooting-depth, sensor, or depth threshold.[1]
IWM-009: The CSU water-balance method assumes water additions enter the soil surface. When precipitation or irrigation exceeds the soil infiltration rate, runoff can make the calculated deficit or net irrigation requirement too low. Compare the root-zone estimate with the planned application and its field conditions before using it as the next decision input.[2]
Reconcile conflicting signals before changing the plan
IWM-004: Compare the modeled deficit with field soil-water readings to check the modeled status against measured status. There is no universal check interval. Use the comparison when the decision needs it and after conditions that could make the estimate less reliable.[2]
IWM-007: CSU Extension recommends checking the calculated deficit against the actual field deficit from soil-water readings. If they differ substantially, use the observed deficit as the next day's deficit value to compensate for uncertainty in the water-balance variables.[2]
Keep a decision record that can be checked
For this guide, retain the management zone, crop stage, weather and ET source, water-budget inputs, soil-moisture readings with locations and depths, field observations, planned and applied water, and unresolved uncertainty in the decision record. This is the record used to investigate a conflict before changing the next plan.
Representative management zones are also useful for other soil decisions. For a separate guide to building representative soil samples for fertility work, see Before You Fertilize, Make the Soil Sample Mean Something.
Research record
Sources
Every factual claim in this note is tied to the numbered evidence below.
- government[1] USDA Natural Resources Conservation Service, Notice of Proposed Changes: Conservation Practice Standard 449, Irrigation Water Management (Proposed Full Text, October 2024). IWM-001, IWM-002, IWM-005, and IWM-010: proposed scheduling methods, management-allowed depletion and managed root-zone depth by irrigation zone, site-specific depth inputs, documentation of soil-moisture monitoring objectives, locations, and depths, and the guide's non-universal decision framework. This October 2024 proposed text is a redline comparison, not current eligibility guidance.
- university extension[2] Colorado State University Extension, Irrigation Scheduling: The Water Balance Approach. IWM-003, IWM-004, IWM-007, IWM-009, and IWM-010: root-zone water accounting, the locally specified allowable-depletion timing point and application-capacity condition, comparison of calculated and measured soil-water status, and the infiltration-rate/runoff condition used in the guide's decision framework.
- standards body[3] Food and Agriculture Organization of the United Nations, Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements. IWM-006: the crop-coefficient approach that converts reference ET to crop ET.
- university extension[4] University of Minnesota Extension, Soil moisture sensors for irrigation scheduling. IWM-008: sensor placement at several depths and locations, representative soil selection, and separate monitoring and management of different soil types in a field.
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Related operating questions
Before You Fertilize, Make the Soil Sample Mean Something
A field guide to separating management units, collecting representative cores, and reading a soil test without turning one number into a prescription.
From SOP to Proof at Shift Change
Turn procedures into observable work with triggers, owners, evidence, exceptions, and handoff notes that help the next person make a defensible decision.