Seasonal and interannual variations of evapotranspiration, energy exchange, yield and water use efficiency of castor grown under rainfed conditions in northeastern Brazil

Brazil is the world's third largest producer of castor. The crop is mainly cultivated in the northeastern region of the country, but little information is available concerning the diurnal, seasonal and interannual variability of evapotranspiration (ET) and energy exchange of castor crops grown in the region, as well as the water use efficiency and yield. To address this gap in knowledge, the Bowen ratio method was used to measure energy balance and ET in 2004, 2005 and 2007. The experiments were performed in a 4-ha area at the Centro de Ciências Agrárias, Federal University of Paraíba in Areia in the state of Paraiba, Brazil (6°58′ S, 35°41′ W, elevation 620 m). The study site had a micrometeorological tower with sensors for measuring air temperature and relative humidity located at two heights above castor canopy. Global and net radiation, rainfall, soil heat flux, and stored soil water at 0-1.0 m depth were also measured. Measurements from all of the sensors were recorded by a data logger every 60 s and mean/sum data were logged every 1800 s. Over 3 years, the net radiation (Rn) varied from 53.2 to 461.7 W m−2 and soil heat flux (G) varied from −10.5 to 58.9 W m−2. Variation in energy partitioning into latent (LE) and sensible (H) heat fluxes was mainly associated with changes in stored soil water. H values were higher in 2004 and 2005 (35% and 37% of Rn, respectively) than in 2007 (25% of Rn). Daily ET was very similar in 2004 and 2005 (2.29 and 2.34 mm day−1, respectively) but increased to 3.42 mm day−1 in 2007, mainly due to increased volume and more even rainfall distribution throughout the growing season. Total ET was 299.5, 334.3 and 656.6 mm in 2004, 2005 and 2007, respectively. Castor showed a low yield (60, 324 and 988.3 kg ha−1 in 2004, 2005 and 2007, respectively) and low water use efficiency (0.02, 0.10 and 0.15 kg m−3 in 2004, 2005 and 2007, respectively), especially in drier years, indicating that under short water supply the water use efficiency of castor plants was very low. The seed oil content varied from 33.6% to 49.2% by weight. There was also a strong correlation between castor yield and daily ET (R2 = 0.9433, RMSE = 91.1 kg ha−1), and between castor yield and rainfall (R2 = 0.9902, RMSE = 30.6 kg ha−1).