Daily energy fluxes, evapotranspiration and crop coefficient of soybean

Evapotranspiration represents the main consumptive use of water in agricultural production and its magnitude is important for irrigation water management. Since water shortages are increasing in many areas, there is a pressing need to improve irrigation water management, for which farmers need reliable information and tools to make better irrigation decisions. There is a lack of knowledge about the water use and irrigation requirements of crops grown in different environments, especially of new crop hybrids. The overall objective of this study was to improve our understanding of the water requirements of soybean. Specific objectives were to: (1) measure and document the daily crop evapotranspiration (ETc) and other energy fluxes, (2) document the daily and seasonal behavior of crop coefficients (Kc), and (3) evaluate the impact of weather variables on alfalfa-reference (ETr) and grass-reference (ETo) evapotranspiration. Here we report results of direct ETc measurements using an eddy covariance system obtained from soybean fields at North Platte, Nebraska, during 2002, 2003, and 2005. We found considerable differences in weather conditions among seasons that affected the accumulation of growing degree days, crop development pattern, crop ETc and Kc. We found that ETr values were on average 32.3% greater than ETo, which is important when choosing Kc values for calculating crop ETc. We also found that vapor pressure deficit (VPD) explained 90 and 92% of the variability in ETo and ETr, respectively. We presented daily measurements of energy fluxes and Kc values and found that measured Kc values were quite variable and often deviated considerably from the average Kc curves given in FAO-56 due to wetting events (rain and irrigation) and crop stress. Therefore, we recommend using the dual Kc method, rather than the single Kc method, for irrigation scheduling. In addition, we found considerable differences in crop maturity among years and suggested that acceleration in maturity could be due to crop stress, especially during the reproductive period. We raised the need for accurate methods to quantify the effect of stress on crop maturity and its impact on Kc.