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Calcimator

Crop Coefficient Calculator

Calculate growth-stage Kc values and crop ET using FAO-56 methodology for irrigation scheduling.

Inputs

Results

Current Kc

1.15

Crop ET (in/day)

0.29

Growth stage (1-4)2
Crop ET (mm/day)7.3
Season avg Kc0.79
Seasonal ETc (in)24.6
Total season (days)125
Daily water (gal/acre)7,807
Stage NameDevelopment
How to Use This Calculator
  1. Enter reference ET (ETo) in inches per day from a local weather station or Blaney-Criddle calculation.
  2. Input Kc initial, mid-season, and end values for your crop from FAO-56 tables.
  3. Set growth stage durations: initial, development, mid-season, and late season days.
  4. Enter the current day of season to get today's interpolated Kc and crop ET.
  5. Read Crop ET (in/day), Season average Kc, and Seasonal ETc to plan irrigation scheduling.

How the result changes with Kc mid-season

Kc mid-seasonCurrent KcCrop ET (in/day)
0.240.240.06
0.590.590.15
1.011.010.25
1.361.360.34

What each input means

Reference ET (in/day)
Daily reference evapotranspiration from weather data or Blaney-Criddle.
Kc initial
Crop coefficient at planting. Bare soil: 0.15-0.4.
Kc mid-season
Peak Kc at full canopy. Corn=1.2, wheat=1.15, turf=0.85.
Kc end
Kc at harvest or end of season.
Initial stage (days)
Days from planting to 10% ground cover.
Development stage (days)
Days from 10% cover to full canopy.
Mid-season stage (days)
Days at full canopy before senescence.
Late season (days)
Days from senescence to harvest.
Current day of season
Day number since planting to calculate current Kc.

What each result means

Current Kc
Interpolated crop coefficient for the current day.
Growth stage (1-4)
1=Initial, 2=Development, 3=Mid-season, 4=Late season.
Crop ET (in/day)
Current daily crop evapotranspiration = ETo × Kc.
Crop ET (mm/day)
Current crop ET in millimeters per day.
Season avg Kc
Weighted average Kc over the full growing season.
Seasonal ETc (in)
Total crop ET over the growing season.
Total season (days)
Total growing season length.
Daily water (gal/acre)
Current daily water use in gallons per acre.

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Reference ET (in/day) = 0.25, Kc initial = 0.3, Kc mid-season = 1.15, Kc end = 0.4 = 9 input(s) provided
  2. Calculate Current Kc
    Current Kc
    1.15 = 1.15
  3. Calculate Crop ET
    Crop ET = eToInPerDay * currentKc
    0.288 = 0.288
  4. Calculate Growth stage
    Growth stage = currentDay <= iniDays ? 1 :
    2 = 2
  5. Calculate Crop ET
    Crop ET = eTcInPerDay * 25.4
    7.3 = 7.3

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