Crop Coefficients for Colorado: The Rocky Ford Lysimeters

Allan AndalesAssistant Professor and Extension Specialist

Soil and Crop Sciences DepartmentColorado State University

Estimation of crop evapotranspiration (ETc)

where

ETrs = reference crop ET (tall reference like alfalfa) = the ET rate from a uniform surface of dense, actively

growing vegetation (hypothetical crop) having specified height (50 cm or 20 inches for alfalfa) and surface resistance (to vapor transport), not short of soil water, and representing an expanse of at least 100 m (328 ft) of the same or similar vegetation (ASCE-Standardized Reference ET equation)Kcr = crop coefficient based on tall reference

rs

c

ETET

ETc = ETrs x Kcr

Crop lysimeter and instrumentationPrecipitationRadiation (Rs, Rn, PAR)Temperature (air, canopy, soil)Wind (speed, direction)Humidity (ea)

Atmospheric pressureSoil water content (Neutron probe)Soil heat fluxET(atmometer)

Lysimeter project at Rocky Ford

• One reference lysimeter (1.5 m x 1.5 m x 2.4 m) to measure ETr (completed in 2009)

• One crop lysimeter (3.0 m x 3.0 m x 2.4 m) to measure ETc (completed in 2006)

• Network of 12 weather stations along the Arkansas Valley (part of COAgMet)

• Lysimeters located at CSU-Arkansas Valley Research Center (AVRC), Rocky Ford, Colorado

1,274 m (4179 ft) above MSL300 mm (11.8 in) avg. annual precip.

Rocky Ford clay loam soil

Example lysimeter load cell outputJune 7, 2008

Alfalfa hay raw Kcr curve in 2008(cutting cycles 2, 3, and 4)

160 180 200 220 240 260 280 3000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

Day of year

Kcr

Evaporation from surface wetted by rainfall

Alfalfa hay raw Kcr curve in 2009(4 cutting cycles)

80 100 120 140 160 180 200 220 240 260 2800.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Day of year

Kcr

Fitted Kcr curve for 2nd alfalfa cutting in 2008

y = -0.00176x2 + 0.66938x - 62.47402R² = 0.95

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

160 170 180 190 200 210

Kcr

Day of year

0

2

4

6

8

10

12

14

160 170 180 190 200 210

Refe

renc

e ET

(mm

/day

)

Day of year

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

160 170 180 190 200 210

Kc

Day of year

0

2

4

6

8

10

12

14

160 170 180 190 200 210Cr

op E

T (m

m/d

ay)

Day of year

ETrs x Kcr = ETc

Acknowledgements• Funding provided by the Colorado Water Conservation Board,

the Colorado Agricultural Experiment Station, Colorado Water Institute, and the Colorado Division of Water Resources

• Technical assistance provided by USDA-ARS (Fort Collins, CO and Bushland, TX)

• Collaborators: Lane Simmons1, Mike Bartolo1, Dale Straw2, Tom Ley2, Abdel Berrada1, Lee Sommers1, Reagan Waskom1, Frank Johnson1, Hamdan Al Wahaibi1, José Chávez1, Tom Trout3

1Colorado State University2Colorado Division of Water Resources3USDA-ARS