On the correlation of water vapor and CO2: Application to flux partitioning of evapotranspiration

The partitioning of evapotranspiration (ET) between plant transpiration (Et) and direct evaporation (Ed) presents one of the most important and challenging problems for characterizing ecohydrological processes. The exchange of water vapor (q) and CO2(c) are closely coupled in ecosystem processes and knowledge of their controls can be gained through joint investigation of qand c. In this study we examine the correlation of water vapor and CO2(Rqc) through analyses of high-frequency time series derived from eddy covariance measurements collected over a suburban grass field in Princeton, NJ during a 2 year period (2011–2013). Rqcat the study site exhibits pronounced seasonal and diurnal cycles, with maximum anticorrelation in June and maximum decorrelation in January. The diurnal cycle of Rqcvaries seasonally and is characterized by a near-symmetric shape with peak anticorrelation around local noon. Wavelet and spectral analyses suggest that qand care jointly transported for most eddy scales (1–200 m), which is important for ETpartitioning methods based on flux variance similarity. The diurnal cycle of the transpiration fraction (ratio of Etto total ET) exhibits an asymmetric diurnal cycle, especially during the warm season, with peak values occurring in the afternoon. These ETpartitioning results give similar diurnal and seasonal patterns compared with numerical simulations from the Noah Land Surface Model using the Jarvis canopy resistance formulation. The correlation of water vapor and CO2exhibits pronounced diurnal and seasonal cyclesWavelet and spectral analysis show that q and c are jointly transported for most eddy scalesET partitioning results based on the q–c correlation analysis are comparable with the Noah LSM