Comparison of Flux Footprint Models to a Mixed Fetch Heterogeneous Cropland System

Abstract

Flux footprint models delineate the source area of micrometeorological fluxes as measured by the eddy covariance (EC) system. An  accurate knowledge of flux footprint is crucial for effective interpretation of measured fluxes and upscaling to a regional scale with the help of satellite data. Flux footprint models are derived based on the assumption that “measured fluxes are originated from a homogeneous cropland system.” For small and fragmented land-holdings of the Indian agricultural system, this assumption is often violated, hence questioning the applicability of existing models. The objective of this paper is to evaluate the performance of various footprint models to a mixed fetch (heterogeneous) cropland system. Field-scale experiments were conducted in continuous cotton (C3) sugarcane (C4) plots located in Nandikandi village, Telangana, India. Two low height EC towers capturing homogeneous fluxes from individual fields and one tall EC tower capturing heterogeneous fluxes from the combinations were used in the study. Half hourly fluxes are calculated as the covariance between vertical wind velocity and scalar concentration of interest (water vapor, air temperature, or carbon dioxide). Three analytical models (viz. Hsieh, Schuepp, and Kormann and Meixner) were applied along with the Lagrangian particle dispersion model, considering unstable (daytime) and stable (night time) flux measurements. Our results conclude that, for homogeneous croplands, the Schuepp model performs best during unstable atmospheric condition and Hsieh model performs best in stable atmospheric conditions. Hseih model is suitable in representing the source area fluxes in a mixed fetch condition for all atmospheric stability conditions.