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Irrigation Efficiency Workshop
Irrigation Requirements and Timing BasicsSalim Bawazir, Ph.D.
Civil Engineering, NMSU
For Teaching Purposes Only
Crop Coefficient
NOTE: Kc could be a function of DOY, Cumulative GDD, Percent Cover etc
ETc Kc ETo
ETcKcETo
= ×
=
For Teaching Purposes Only
Assumptions:
• The climate data are accurate• Actual ET was measured correctly• Crop was not under stress• Farming practices and crop variety have not
changed• Calculations and Units are correct!!• Climates are similar
For Teaching Purposes Only
ASCE Standardized Reference ET Equation
Penman Reference ET Equation
( ) 26.43(1 0.53 )( )oo z zE Rn G u e eγλ
γ γ ∆
= − + + − ∆ + ∆ +
For Teaching Purposes Only
0
2
4
6
8
10
12Ev
apotr
ansp
iratio
n, mm
/day
100 125 150 175 200 225 250 275 300Day of Year
Lysimeter ASCE P-M
Kimberly, Idaho 1969
Full cover alfalfa - Data from Dr. J.L. Wright
From Dr. Rick Allen, IdahoFor Teaching Purposes Only
Standardized ReferenceEvapotranspiration Equation (ETsz)
“reference evapotranspiration is defined as the ET rate from a uniform surface of dense, actively growing vegetation having specified height and surface resistance, not short of soil water, and representing an expanse of at least 100 m of the same or similar vegetation.”
For Teaching Purposes Only
Required data
The calculation of ETsz requires measurements or estimates for:
• air temperature• humidity• solar radiation• and wind speed.
For Teaching Purposes Only
For Teaching Purposes Only
Kc Vs. DOY (PECAN)
1 29 57 85 113 141 169 197 225 253 281 309 337 365Day of Year 2003
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Cro
p C
oeffi
cien
t, Kc
FAO56 Penman Penman-Sammis
For Teaching Purposes Only
Kc Vs. Percent Cover
Calculated Pecan Crop Coefficient
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
1 29 57 85 113 141 169 197 225 253 281 309 337 365Day of Year, 2003
Kc
ETszETo_HS
For Teaching Purposes Only
Net Radiation
Short Wave Long Wave
Rs Rs(α) RL_d RL_up
For Teaching Purposes Only
For Teaching Purposes Only
What if Kc and actual ET are not Available ? Use Rn
Pecan ET- 2003
0
5
10
15
20
25
1 29 57 85 113 141 169 197 225 253 281 309 337 365Day of Year, 2003
ET/R
n (M
J/m
²)
ET MeasuredRn
For Teaching Purposes Only
For Teaching Purposes Only
For Teaching Purposes Only
For Teaching Purposes Only
Irrigation 68 inchesRain = 9 inchesET = 56 inches
For Teaching Purposes Only
Daily Measured ET of Alfalfa (Willie’s Farm) including the Cutting Dates
ET of alfalfa = 4.6 ft (1403 mm)
Precipitation = 11 in or 0.92 ft (277 mm)
Irrigation Applied = 5.67 ft (1729 mm)
N= 271 days of data collection
Recharge to Groundwater: 1.0 - 1.5 ft
For Teaching Purposes Only
Evapotranspiration (ET) Measurements
Eddy Covariance Flux and Microclimate MeasurementsIn the Central Mesilla Valley, New Mexico
For Teaching Purposes Only
