Supplemental depth and irrigation volume
Net depth stops at zero when effective rainfall meets the target. Delivered volume is larger than root-zone demand when efficiency is below 100%. The tool calculates a specified water budget rather than diagnosing when plants need irrigation.
Once the water quantity is known, the Drip Irrigation Runtime Calculator can convert a specified emitter flow and water target into an operating interval.
The formula
One square foot is 144 square inches and one US gallon is 231 cubic inches. Multiplying area by a water depth gives volume. Rain offsets the target only for the same time interval, and efficiency accounts for delivery losses.
Worked example
For 100 ft² with a 1 in target and 0.25 in of effective rain, the remaining depth is 0.75 in. That equals 46.753 US gal at the root zone. At 90% application efficiency, supply about 51.948 US gal, or 196.645 L.
How to use this calculator
- Enter garden area, target water depth for this period using the units shown.
- Set effective rain for the same period, application efficiency.
- Calculate and review water to deliver alongside the supporting quantities.
Keep the target and rain period consistent
A weekly target requires rainfall from the same week; an individual watering event needs a matching event budget. Effective rain means water that contributes to the intended root-zone supply, which may be less than a rain-gauge total after runoff or poor infiltration. Local crop, soil, season, and moisture observations guide the target.
Area and delivery efficiency
Measure the planted area actually receiving water. An irrigation efficiency below 100% allows for water that does not contribute to the root-zone volume. It does not model where losses occur. Monitor the soil and application pattern, especially around slopes and containers. Excess water can drain below roots, so a volume estimate should be checked against actual conditions.
Assumptions & limitations
What this calculation assumes
- Water depth is distributed uniformly over the measured area.
What to keep in mind
- Does not forecast evapotranspiration, soil infiltration, or plant water requirements.
Common questions
What if rain exceeds the target?
Supplemental irrigation is zero for that budget. Excess rainfall is not carried into a later period automatically.
Does one inch mean wet soil one inch deep?
No. It is a volume per surface area. How far the water wets soil depends on soil properties and prior moisture.
Does this tell me when to water?
No. It converts an entered water budget into volume. Soil moisture, crop conditions, and weather guide the actual irrigation timing.
Sources & further reading
Reviewed on