Calculate crop ETc, irrigation depth, acre-inches, and source-water gallons for one field using direct ETc or daily ETo and Kc.
Crop water use
Rainfall, system, and field
Table of contents
How to use our Crop Evapotranspiration and Irrigation Requirement Calculator
- Choose how you want to enter crop water use: enter an accumulated Crop ETc for this period, or calculate it from daily Reference ETo and Crop coefficient Kc.
- Enter Irrigation period in days, making sure your crop water use and Effective rainfall during this period cover the same dates.
- Enter Effective rainfall during this period, Irrigation system efficiency, and the Irrigated area served by this irrigation set or zone.
- Click Calculate to see the Gross irrigation depth to apply and Source water needed for the field.
- Sanity-check the result: Gross irrigation depth to apply should be higher than Net irrigation depth needed at the root zone when efficiency is below 100 percent, and zero when effective rainfall meets or exceeds crop ETc.
Definitions
Crop ETc: Crop evapotranspiration, meaning the water used by a specific crop during the selected period. It is measured here in inches.
Reference ETo: Weather-based evapotranspiration from a reference surface. The calculator uses a daily value in inches per day.
Crop coefficient Kc: A unitless multiplier that adjusts Reference ETo for the crop and its growth stage.
Effective rainfall: Rainfall that is available to the crop root zone during the selected period. It can be less than measured rainfall.
Net irrigation depth: The root-zone water shortage after effective rainfall is credited against crop ETc.
Gross irrigation depth: The application depth needed after allowing for irrigation-system losses.
Acre-inch: The volume that covers one acre of land one inch deep. One acre-inch equals about 27,154.29 US gallons.
Common mistakes and quick fixes
Mistake: Entering a daily number in Crop ETc for this period.
Fix: Enter the accumulated inches for the full Irrigation period, or choose the Reference ETo and Crop coefficient Kc path for a daily ETo value.
Mistake: Using rainfall from a different date range than Irrigation period.
Fix: Make Effective rainfall during this period cover the same days as crop water use.
Mistake: Entering 0.80 for Irrigation system efficiency.
Fix: Enter a whole percent such as 80. The label uses percent, not a decimal fraction.
Mistake: Treating Crop coefficient Kc as a percent.
Fix: Enter the unitless multiplier, such as 0.8, not 80.
Mistake: Using the whole farm for Irrigated area when only one zone will run.
Fix: Enter the acres actually covered by the irrigation set or zone being planned.
Mistake: Crediting all measured rain as Effective rainfall during this period.
Fix: Enter only the rain expected to remain available in the crop root zone after runoff and deep drainage.
Limitations & Key Assumptions / Boundary Conditions
- This is a period water-balance estimate. Crop ETc, rainfall, and the Irrigation period must use the same date span.
- The ETo and Kc path assumes crop ETc equals daily Reference ETo times Crop coefficient Kc times days. Use a locally appropriate Kc for the crop and growth stage.
- Effective rainfall is entered by the user. The calculator does not estimate runoff, soil storage, deep drainage, or rainfall timing.
- Irrigation system efficiency is a planning assumption. Actual delivery can change with pressure, wind, maintenance, distribution uniformity, slope, and operation.
- The result is a field depth and source-water volume, not a runtime. Pump flow, sprinkler application rate, soil moisture, irrigation restrictions, and equipment capacity are not included.
- If effective rainfall equals or exceeds crop ETc, the calculator reports zero irrigation required for the entered period. It does not decide whether irrigation is needed later.
Methodology
Calculation method
The calculator uses one of two paths for crop water use. Direct mode uses the entered Crop ETc for this period. The other path multiplies daily Reference ETo by Crop coefficient Kc and Irrigation period. These paths are alternatives, so they are never added. The ETo and Kc approach is commonly used to estimate crop water use from reference evapotranspiration and a crop coefficient. [1]
period crop ETc = direct crop ETc, or Reference ETo x Crop coefficient Kc x Irrigation period
Effective rainfall is limited to the period crop ETc. This prevents rainfall from creating a negative irrigation requirement.
effective rainfall used = smaller of effective rainfall and period crop ETc
net irrigation depth = period crop ETc - effective rainfall used
The net root-zone shortage is divided by efficiency written as a fraction. For example, 80 percent becomes 0.80.
gross irrigation depth = net irrigation depth / (irrigation efficiency / 100)
The calculator then multiplies gross depth by irrigated acres to get acre-inches. It converts acre-inches to US gallons using 27,154.2857142857 US gallons per acre-inch.
gross field water volume = gross irrigation depth x irrigated area
source water needed = gross field water volume x 27,154.2857142857
average crop ETc per day = period crop ETc / irrigation period
Worked example
For 1.4 inches of period crop ETc, 0.25 inches of effective rainfall, 80 percent efficiency, and 40 acres:
net irrigation depth is 1.15 inches. Gross irrigation depth is 1.15 / 0.80 = 1.4375 inches. The field volume is 1.4375 x 40 = 57.5 acre-inches, or about 1,561,371 US gallons.
Calculation boundaries
Values below zero are not allowed for water depths, ETo, or Kc. Irrigation period and Irrigated area must be greater than zero. Irrigation system efficiency must be greater than 0 and no more than 100 percent. If effective rainfall is greater than crop ETc, irrigation depth and volume are set to zero.