Differences in Evapotranspiration from Eddy Covariance and Lysimeters in Advective Conditions.

Direct measurements of evapotranspiration (ET) in irrigated agriculture are often made through either a soil water balance with lysimeters or energy balance with eddy covariance (EC). Both systems operate at a high measurement frequency that must be averaged over a more stable period, with the lysimeter affected by wind loading and EC system unable to resolve large eddies associated with sensible heat advection. In this experiment, we compared rates of ET at a typical flux averaging period of thirty minutes with that of a shorter averaging period of five minutes. During the 2015 sorghum growing season in Bushland, TX, three EC systems at 2, 4, and 8 meters above ground were evaluated against a large weighing lysimeter located a short distance upwind of the EC tower. ET was processed and compared for 19 non-consecutive days amidst a thirty-two day period when the crop was well-developed. Advective conditions were identified based on wind direction and the ratio of available energy to latent heat. We found that wind speed was important for measurement error above a threshold of around 2 m s-2 in the well mixed air above the canopy, but error magnitude is mostly very small relative to lysimeter calibration. Daytime fluxes were higher with longer averaging time, with the lowest EC system generally more accurate on both five and thirty minute timescales. Our results are mostly consistent with previous ET studies in this semi-arid location indicating systematic underestimation of ET by EC systems on a daily timescale relative to the lysimeter.