Determination of the FAO-56 Crop Coefficients for Cotton

1
Determination of the FAO-56 Crop Coefficients for
Cotton Evapotranspiration under Deficit
Irrigation in a Humid Climate Ayman Suleiman,
University of Jordan. Cecilia Tojo Soler, The
University of Georgia. Gerrit Hoogenboom, The
University of Georgia.
The University of Jordan

• Introduction
• The use of the FAO-56 reference
  evapotranspiration (ETo) approach (Allen et al.,
  1998) for irrigation scheduling has been
  recommended worldwide because it provides the
  best results under a wide range of climatic
  conditions.
• The FAO-56 reference ETo is based on the crop ET
  (ETc), which is calculated from ETo using a
  stage-dependent crop coefficient (Kc).
• Recently, the FAO-56 Penman-Monteith (PM) has
  been found to outperform PT in humid climates
  (Suleiman and Hoogenboom, 2006).
• Objective
• The objective of this study was to evaluate the
  Kc FAO-56 procedure under different deficit drip
  irrigation treatments for cotton grown in a humid
  climate.
• Materials and Methods
• An experiment was conducted in 2005 with cotton
  grown in three automated rainout shelters located
  at the University of Georgia Campus in Griffin,
  Georgia.
• When the actual soil water content in the
  effective root zone dropped below a specific
  threshold of the available water content (AWC),
  irrigation was applied until the soil water
  reached 100 of AWC.
• The irrigation treatments were 40, 60 and 90
  of the irrigation thresholds (IT).
• The observed daily ETc was estimated from daily
  soil moisture readings.

• At the end of the growing season, cumulative ETc
  for 40 and 60 IT was 58 and 91 of that of
  the 90 IT, respectively.

Fig. 1. Cotton crop evapotranspiration (ETc)

• Results
• The length of the initial, development and mid
  stages was the same for the different treatments.
  However, the length of the late-stage for the 40
  and 60 IT was 5 and 3 days less than that for
  the 90 IT, respectively (Table 1).

Table 1. Length (days) of cotton development
stages.
Region Initial (Lini) Development (Ldev) Mid (Lmid) Late (Llate) Total Planting Date
Egypt Pakistan Californiaa 30 50 60 55 195 Mar-May
California Deserta 45 90 45 45 225 Mar
Yemena 30 50 60 55 195 Sept
Texasa 30 50 55 45 180 April
Georgia, 90 IT 30 (192)b 34 (317) 46 (497) 44 (323) 154 (1328) 17-May
Georgia, 60 IT 30 (192) 34 (317) 44 (479) 41 (326) 149 (1314) 17-May
Georgia, 40 IT 30 (192) 33 (305) 45 (490) 39 (317) 147 (1304) 17-May

• Conclusion
• The FAO-56 proved to be an accurate method for
  estimating cotton ETc under deficit irrigation.
• The values for the initial, mid and end Kc and
  the length of the different developmental stages
  of the 90 IT may be used in humid climates for
  fully irrigated cotton.
• The use of such information will help in
  effective irrigation planning and more precise
  water management.

a Values are taken from Table 11 Chapter 6,
FAO-56 paper (Allen et al. 1998). b Numbers
between parentheses are growing-degree days.

• The observed Kc for the initial stage was 0.51,
  0.9 and 0.99 for the 40, 60 and 90 IT,
  respectively. The FAO-56 Kc for this stage was
  within a few percent from the observed Kc (Table
  2).

Table 2. Observed and estimated/documented FAO-56
crop coefficient.
Treatments Observed Kcini Observed Kcini Estimated Kcini Estimated Kcini Observed Kcmid Observed Kcmidae Documented Kcmid Observed Kcend Documented Kcend
Treatments 13a 17b 13a 17b Observed Kcmid Observed Kcmidae Documented Kcmid Observed Kcend Documented Kcend
40 IT 1.11 0.51 1.12 0.49 1.2c (0.92)d 1.23 1.15-1.2 0 0.5-0.7
60 IT 1.06 0.9 1.12 0.87 1.19 1.22 1.15-1.2 0.34 0.5-0.7
90 IT 1.12 0.99 1.12 0.98 1.22 1.25 1.15-1.2 0.58 0.5-0.7
a The first 13 days of the initial stage. b The
last 17 days of the initial stage c Obtained
only for days on which irrigation took place and
days right after irrigation during the mid
stage. d Obtained for all days of the mid
stage e Adjusted to daily average minimum
relative humidity of 45 and wind speed of 2 ms-1.
View of the cotton experiments

• For the mid stage, the 60 and 90 IT had a
  similar value for Kc (about 1.2), which was
  higher than the 40 IT (0.92). The end Kc for the
  40 IT (0) was lower than for the 60 IT (0.38),
  which in turn was lower than the 90 IT (0.58)

• The cumulative ETc was lower than ETo from 20
  days after planting until the end of the growing
  season for the 40 IT as a result of the limited
  water supply. The cumulative ETc for the 60 IT
  was the same as the cumulative ETo until the end
  of the development stage, after which the
  cumulative ETc for 60 IT became higher than ETo
  until the end of the growing season. The
  cumulative ETc for the 90 IT was higher than ETo
  from 20 days after planting until the end of the
  growing season (Fig. 1).

PR2 probe for soil water content monitoring
References Allen, R.G., Pereira, L.A., Raes, D.,
and Smith, M. (1998). Crop evapotranspiration.
FAO Irrigation and Drainage Paper 56. FAO, Rome,
Italy, 293 p. Suleiman, A.A., and Hoogenboom,
G., 2006. Comparison of Priestley-Taylor and
FAO-56 Penman-Monteith for daily reference
evapotranspiration estimation in Georgia, USA. J.
Irrig. Drain. Eng. (Accepted)