Energy and evapotranspiration partitioning in a desert vineyard

The challenge of partitioning energy and evapotranspiration (ET) components was addressed over a season (bud break till harvest) in a wine grape vineyard located in an extreme arid region. A below canopy energy balance approach was applied to continuously estimate evaporation from the soil (E) while system ET was measured using eddy covariance. Below canopy energy balance was assessed at the dry midrow position as well as the wet irrigated position directly underneath the vine row, with E calculated as the residual of measured net radiation, soil heat flux, and computed sensible heat flux. The variables used to compute sensible heat flux included soil surface temperature measured using infrared thermometers and below-canopy wind speed in a soil resistance formulation that required a modified wind factor. The E derived from below canopy energy balance was reasonable at daily intervals although it underestimated micro-lysimeter E measurements, suggesting there may have been advected energy from the midrow to the below-vine position. Seasonal partitioning indicated that total E amounted to 9-11% of ET. In addition, empirical functions from the literature relating crop coefficients (K_cb) to plant size, appeared to give reasonable results under full canopy, albeit with some day to day variation, but underestimated K_cb during the growing period.