Abstract: Flux footprint models are often used to explain the sources of flux data from flux tower measurements, which is helpful for estimating the position and size of surface source areas and the relative contribution of passive scalar sources to the measured fluxes. In this study, we analyzed the variations of CO₂ flux on different time scales based on the observed data from eddy covariance system. The Kljun footprint model was used to analyze the flux source of the observation data of a rice-wheat rotation cropland ecosystem in Shouxian County, Anhui Province from November 1, 2020 to October 30, 2021. The flux source of different atmospheric conditions and different growth stages of crops were examined. We found that the CO₂ flux had obvious temporal variations. The annual variation of CO₂ flux featured a W-typed bimodal absorption peak, and the annual average CO₂ flux was -0.81 μmol·m^-2·s^-1. The daily mean CO₂ flux (-3.7 μmol·m^-2·s^-1) in vegetative growth and reproductive stages of rice was the smallest, showing the strongest carbon sink capacity, while the daily mean CO₂ flux (1.03 μmol·m^-2·s^-1) of the wheat vegetative growth period was the largest, serving as a carbon source. The southwest wind prevailed in this region, followed by southeast wind. The maximum length of the flux source area was thus mainly distributed in the southwest and southeast. When the contribution rate of flux was 80%, the maximum length of annual flux source was 158.17 m. The range of flux source area under atmospheric stable state was larger than that under atmospheric unstable state. The flux source area at different growth stages of crops was significantly different. The flux source area of winter wheat vegetative growth stage was the largest, while the source area of winter wheat vegetative growth and reproductive stage was the smallest. The accurate simulation of flux source area of the rice-wheat rotation agroecosystem in Shouxian County plays a vital role for the upscaling exercises of flux data from single site flux measurements to regional scale. Understanding the operation of the flux source area model on cropland underlying surfaces in Huaihe River Basin is also of great practical significance to improve the accuracy of greenhouse gas budget in this area.

Key words: Huaihe River Basin, rice-wheat rotation agroecosystem, eddy covariance, Kljun model, flux source