A high-resolution profile technique for CO$_{2}$ and H$_{2}$O at crop sites and implications for their flux source partitioning

We present an elevator-based facility for measuring CO₂ and water vapour concentration profiles between the soil surface and the roughness sublayer above crops and similar plant canopies. The end of a tube connected to a closed-path gas analyser is continuously moving up and down over the profile height (in our case, 2 m) while concentrations are logged at a frequency of 20 sˉ¹, collecting approximately 50 profiles during 30 minutes. Using campaign measurements in winter wheat during different stages of crop development and different times of the day, we demonstrate a simple approach to correct for time lags, and the resulting profiles of half-hourly mean concentrations and turbulent variances over height increments of 2.5 cm. During conditions favouring photosynthesis, a clear local minimum of CO₂ concentration in the canopy can be found. With more moderate effort and success, we also added measurements of temperature (based on a finewire thermocouple) and wind speed (based on hotwire anemometry). Post-harvest measurements over bare soil were analysed in the framework of Monin-Obukhov similarity theory to check the validity of the measurement and raw data processing approach, to derive CO₂ and latent heat fluxes, and to compare them to eddy-covariance and chamber measurements. We also tentatively apply an approach to derive source partitioning information for net fluxes of CO₂ (soil respiration vs. aboveground net primary production) and H₂O (evaporation vs. transpiration) from profiles measured across the green crop canopy, discuss its limitations and needed improvements