Carbon dioxide and evapotranspiration fluxes in an urban area of Krakow, Poland

Abstract Urban areas play an essential role in the global carbon balance, being responsible for more than 70% of anthropogenic carbon emissions, entering the atmosphere mainly in the form of carbon dioxide (CO 2 ). Another crucial component of the carbon balance of the urban atmosphere is CO 2 exchange with the biosphere, expressed in both emission and absorption fluxes. The biosphere component of the net CO 2 flux is inseparably linked to the water (H 2 O) balance due to evapotranspiration. This article presents an estimation of CO 2 and H 2 O net fluxes based on eddy covariance (EC) tower measurements and in‐depth analysis of its temporal and spatial variability for a typical urban site at mid‐latitude of the northern hemisphere. The source area was divided into two sectors, one representing dense urban development with sources of CO 2 from traffic and households, and the second containing predominantly recreation and sports areas. Temporal variability analysis of evapotranspiration showed a clear seasonal cycle closely correlated with incoming radiation and air temperature, with the seasonal mean ranging from 0.4 mm·day −1 in winter to 1.6 mm·day −1 in summer. As a result of similar green coverage between the urban and green sectors, spatial variability was statistically insignificant. The net CO 2 flux over a seasonal cycle was less pronounced for the total source area with a mean of 5.0 g C·m −2 ·day −1 in summer and 7.5 g C·m −2 ·day −1 in winter. However, significant variations between urban and green sectors were observed with the highest seasonal mean difference of 4.5 g C·m −2 ·day −1 in winter. The results confirm that while on the local, short time‐scale urban vegetation has a potential to mitigate the emissions, it is not able to offset them.