Crop Evapotranspiration Advection Derived from Multiple Eddy Covariance Systems and Lysimeter

A basic assumption of eddy covariance (EC) flux measurements is that there is no horizontal divergence of scalars in the control volume of interest. However, large eddies can transport drier air into this volume, resulting in a top-down diffusion of heat, especially in semi-arid conditions. This advection may enhance crop evapotranspiration (ET) and result in a discrepancy between ET from EC systems and a large weighing lysimeter. A spectral analysis could produce better understanding of ET discrepancies resolved by eddy sizes between EC and lysimeter. We examined frequencies in both the large eddy sizes and in the inertial subrange for the lysimeter and three different heights of EC systems located in the same irrigated field in Bushland, Texas, during the 2014 and 2015 growing seasons. Our results will show that total water vapor flux from EC should include advection contributions for achieving reliable estimates of crop ET.