Cotton Crop Coefficients ( Kc) for Northeast Louisiana Using Weighing Lysimeters

Water use in cotton is influenced by growth stages and atmospheric demand. The effect of both of these components can be calculated in terms of crop coefficients (K_c values), which are ratios of actual evapotranspiration (ET_c) of the cotton crop at a given stage of growth to reference evapotranspiration (ET_o) calculated from weather measurements made over a reference surface. An experiment was conducted to calculate Kc values at Northeast Research Station, Saint Joseph, LA in 2009.Cotton plants were transplanted onto paired weighing lysimeters. Changes in mass (kg) on daily basis for both lysimeters were recorded using CR 3000 data logger from 5^th of August onward. These changes in mass then were converted to equivalent water depth (mm) and represent ET_c. For measurement of ET_o, a reference weather station was completed. Bermuda grass (Cynodon dactylon L.) was planted in April as a reference surface. An area of 336 ft by 336 ft was demarcated and divided into 3 bays each of 112-ft width to allow intermittent shallow flood irrigation. A tower of 10 m height was instrumented to allow an array of measurements including wind speed, solar radiation, temperature and relative humidity as required to calculate ET_o from the ASCE Standardized Reference Evapotranspiration Equation. These weather parameters were recorded using a CR 3000 data logger. Preliminary assessments show ET_c and ET_o in the range of 4.54_mm to 6.66_mm and 3.17_mm to 4.91_mm respectively during the period from the 7^th of August to the 9^th of September. ET_c and ET_o will be used in calculation of preliminary K_c values for the period in question