Evapotranspiration, soil depletion, and runtime

Demand is a water balance

Evapotranspiration, or ET, combines evaporation and plant transpiration. Reference ET describes an assumed reference surface under the weather conditions; a plant or landscape factor adjusts it for the selected vegetation. The factor and method must match the reference data and plant assumptions. Do not apply a turf coefficient to a sparse shrub planting without review.

Effective rainfall is the portion retained where roots can use it. Rain that runs off, drains below the roots, or falls after the soil reservoir is full does not all replace irrigation demand. A rain gauge measures depth but not effective storage. Soil observation and the scheduling method determine the useful contribution.

For a supplied weekly reference ET of 1.40 inches and a plant factor of 0.70, estimated plant ET is 0.98 inch. If effective rain is 0.28 inch and the soil-water accounting calls for full replacement over that period, net irrigation need is 0.70 inch. The factor is an example, not a universal Oregon lawn coefficient. Do not subtract the rain before applying the plant factor unless the particular method specifies that operation.

Storage determines interval

Available water depends on soil and effective root depth. Suppose the root zone holds 1.2 inches of available water and the selected management allowable depletion is fifty percent. The refill trigger is 0.60 inch depleted. With estimated use of 0.15 inch per day and no effective rain, that threshold is reached in four days. A smaller root zone would reach it sooner.

Do not let the depletion calculation become a rigid calendar rule. Forecast rain, seasonal demand, root stress, and observed moisture can justify adjustment. New stock can have a smaller root reservoir than established plants, while a compacted layer can reduce the actual depth. The schedule must use the root zone that exists, not the one hoped for in a plant description.

Overwatering after every small rain can fill the reservoir and create runoff or deep drainage. Track the balance rather than irrigate simply because the controller's weekday arrived. Soil-moisture sensors and ET controllers can support this decision only when correctly installed and configured.

Net depth, gross depth, and runtime

Net depth is the water intended to remain available in the root zone. Gross depth is what the system applies when the chosen efficiency assumption accounts for losses. If net need is 0.60 inch and assumed application efficiency is 0.80, gross depth is 0.60 divided by 0.80, or 0.75 inch. Multiplying by 0.80 would understate the required application.

At measured precipitation rate 1.5 inches per hour, 0.75 inch requires half an hour, or thirty minutes. The formula is runtime in minutes equals depth divided by precipitation rate times sixty. Use inches with inches per hour, or millimeters with millimeters per hour. Gallons delivered to one tree cannot be inserted directly into a depth equation without a wetted-area conversion.

Distribution uniformity can require a different scheduling approach to ensure the drier portion receives enough water. Do not blindly divide by both an efficiency and a uniformity value if the adopted method already combines the same losses. Identify the method and assumptions, then verify that extra runtime is not causing runoff in the wetter areas. Repair poor distribution before compensating with unlimited time.

Cycle and soak

If thirty minutes is the required total runtime but runoff begins after ten continuous minutes, divide the application into three ten-minute cycles with enough soak time between them. The total remains thirty minutes. Three thirty-minute cycles would triple the intended depth. Soak duration depends on infiltration, slope, soil, and equipment; it is not a universal twenty-minute interval.

The controller may interleave other zones during soak time. Check that the watering window and source constraints support the complete sequence. Several start times can accidentally repeat an entire program, so verify the controller's programming model. Use a rain or soil-moisture interruption correctly and explain manual overrides.

Verify and adjust

Inspect the wetted depth, plant condition, runoff, and meter or flow readings after scheduling. A calculated thirty-minute runtime with a clogged head does not produce the intended distribution. Recheck nozzle type and pressure after repairs, because precipitation can change. Reduce demand assumptions and runtime seasonally where warranted rather than leave the hottest-week program year round.

Record ET source, plant factor, effective rainfall, root storage, allowable depletion, precipitation, and any efficiency adjustment. This makes a schedule reviewable and lets a technician explain why it changed. A smart controller still needs correct inputs and maintenance; automation cannot infer every soil, plant, or equipment defect.

Reference table