Assessing crop coefficients of sunflower and canola using two-source energy balance and thermal radiometry

A new technique for the local adjustments in crop coefficients is presented. This is an alternative to conventional lysimeter measurements traditionally used for improved irrigation scheduling. The method is based on the combination of a two-source energy balance model and local measurements of radiometric temperatures. Two experimental campaigns were carried out on sunflower and canola in a cropland area located in Barrax, Albacete, in the summer of 2011 and spring of 2012, respectively. Radiometric temperatures of soil and canopy were collected, together with biophysical and meteorological variables. Combining all these data in a two-source energy balance model allowed separation of both the evaporation and transpiration components of the total evapotranspiration (ET). Model results were first compared to local measurements from a lysimeter. Estimation errors around ±0.20mm/h and ±1.0mm/d were observed for both sunflower and canola crops at hourly and daily scales, besides uncertainties lower than 3% for the cumulated ET for the whole campaigns. Results were then used to assess values of the different crop coefficients for this site and the two crops. Comparison with values proposed by FAO56 showed significant discrepancies that yielded to 1–2mm/d uncertainty in terms of daily evaporation and transpiration values, and underestimations of 0.6 and 1.3mm/d, together with estimation errors of ±1.1 and ±1.7mm/d for sunflower and canola, respectively, in terms of daily ET values. Although partitioning of ET needs further study involving field data of evaporation and transpiration, these results reinforce the necessity for the local adjustment of the crop coefficients used as inputs in water balance models, and show the potential of the technique proposed to achieve this goal.