Development of a regional specific crop coefficient (Kc) for castor (Ricinus communis L.) in Florida, USA by using the sap flow method

Although castor (Ricinus communis L.) oil has high commercial value for industrial uses as well as a feedstock for biodiesel, the United States has relied on imported castor oil since 1972. In Florida, the crop has been considered as an alternative crop to the more traditional agronomic row crops and would likely be produced under center pivot or other irrigation systems to maximize yield and fit with existing cropping systems. To develop an appropriate Kc value for the central Florida region, research was conducted at the University of Florida’s Plant Science Research and Education Unit in Citra, FL. Season-long crop transpiration was measured for the castor cultivar ‘Brigham’ in 2011 and 2012 and soil evaporation rates were quantified in 2012. Crop transpiration was obtained by sap flow analysis when the plots had reached complete canopy cover, were fully irrigated, and when daily solar radiation levels were at or above a historical radiation average to accurately reflect optimum growing conditions. Daily average plant transpiration (57,199 and 56,560 L ha−1 in 2011 and 2012, respectively) were predominantly greater than daily soil evaporation (38 L ha−1 in 2012), and as a result, the season long plant transpiration was the main factor included for the calculation of crop evapotranspiration (ETc). The ETc was evaluated against the reference evapotranspiration (ETo) from site specific meteorological data to calculate daily Kc values. The 10-day Kc averages were compared to published Kc values from the Food and Agriculture Organization (FAO) for castor. The crop developmental timeframes in this study were roughly aligned with the FAO timeframes when making these comparisons. The mid-season Kc values were similar to the FAO reference (1.19, Florida vs. 1.15 FAO ref.), but the late season Kc values for castor in Florida were higher as compared to the FAO reference (1.50 vs. 0.55, respectively). The increase in water use during this late season developmental period could be explained by a lack of physiological decline in this sub-tropical climate and continued irrigation application for this trial during the latter part of the season. These results provide appropriate Kc values for central Florida and will aid growers in irrigation decisions when producing castor in this region.