Heat storage and its effect on the surface energy balance closure under advective conditions

Abstract

High quality estimates of evapotranspiration (ET) are needed for water-limited agriculture where irrigation is necessary to efficiently grow crops. Eddy covariance (EC) systems are observational tools used to measure water and heat fluxes, but a tendency to underestimate fluxes causes a lack of surface energy balance closure. Surface energy budgets are useful for verifying ET estimates, especially in advective conditions that affect energy partitioning. Here, we explored the effect of heat storage and advective conditions on surface energy balance closure for the 2014 and 2015 growing seasons in Bushland, Texas. Storage components were estimated near the center of an irrigated sorghum field using an array of soil and surface layer measurements. A comparison of EC estimated turbulent fluxes and available energy consisting of net radiation, soil heat flux, and storage was used to identify advective conditions and assess surface energy balance closure. Our results indicated daytime mean heat storage of approximately 40 W m−2 with a diurnal pattern featuring a midday peak that exceeded the evening minimum in magnitude, mostly reflective of soil heat storage but also affected by air, water, and biomass heat storage during the morning hours and photosynthesis storage during midday hours. Daytime advective conditions were associated with higher heat storage frequently under stable atmospheric conditions. The surface energy balance was more closed in 2014 than 2015; the 2014 energy balance exhibited a hysteretic pattern with a surplus of turbulent energy in the afternoon, whereas systematic underestimation was common in 2015. This finding is related to a larger proportion of data in 2014 being advective, characterized by a lower Bowen ratio and greater ET, particularly during the late afternoon. Regardless of advection classification, heat storage was demonstrated to be important for daytime energy balance in this irrigated cropland.