5 Modeling evapotranspiration in a spring wheat from thermal radiometry: crop coefficients and E/T partitioning

Wheat is one of the crops occupying the largest areas in the world (218 million ha in 2013). Understanding the land-atmosphere energy exchanges over these croplands becomes important not only for agronomy but also for climatic and meteorological aspects. This study continues previous work on the estimation of actual evapotranspiration (ET) and the assessment of crop coefficients of sorghum, sunflower, or canola. Two variations of a simple two-source energy balance (STSEB) approach were used in combination with land surface temperature measurements to calculate hourly and daily values of surface fluxes and actual ET. An experiment was carried out during the spring season of 2014 in Las Tiesas experimental farm in Barrax, Spain. Soil and canopy temperature components together with meteorological variables and biophysical parameters were measured from planting to senescence. Comparison to lysimeter measurements showed calculation errors of +/- 0.11 mm h(-1) and +/- 0.8 mm day(-1) for hourly and daily ET values, respectively, whereas an underestimation no > 4 % resulted from the entire campaign. Partition between soil and canopy components yielded a ratio of evaporation (E) to transpiration (T) of 36-64 %, respectively, for the total growing season. Dual crop coefficients were also calculated and compared to those proposed by FAO-56. Although separate E and T measurements were not available, similar results between the STSEB and FAO-56 models demonstrate the utility of the STSEB for investigating management strategies aimed at increasing crop water use efficiency.