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.Cottoncropevapotranspiration(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. 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:Valuesare takenfrom Table 11 Chapter 6,FAO-56paper (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). Table2. Observedand estimated/documentedFAO-56cropcoefficient. 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 forsoilwater 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)