evapotranspiration measurements at the landscape scale using a Micro-Wave scintillometer prototype: Evaluation from two field campaigns.

Quantification of evapotranspiration over complex terrain is still challenging because all the known methods are indirect and rely on strong assumptions. The bichromatic scintillometry method, combining optical/near-infrared and microwave scintillometers, is probably one of the closest methods to the turbulent theoretical framework as it measures turbulent parameters for temperature and moisture fluctuations. However, since its description in the 90s, very few works have been published using this method, mainly because of the availability of manufactured instruments but also because of technical and methodological issues.

In this study we present evapotranspiration series from two different campaigns with the combination of two scintillometers operating, one in the near infra-red domain and the other in the radiofrequency domain (94GHz), a prototype developed in collaboration with the Rutherford Appleton Laboratory (UK). The first 18-month time series has been measured over a crop mosaic These instruments have been installed in the Critical Zone observatory Oracle, located east of Paris in the Seine Catchment, and have run continuously since May 2016 to the end of year 2017 on a 4.5km pathlength. The data processing has been developed from raw received intensity data logged at 1kHz for both scintillometers. Turbulent fluxes have been processed from Cn² measurements using the bichromatic method. The data processing toolbox has been fully developed at IGE. Fluxes are then compared with aggregated fluxes from Eddy-Covariance stations representative of the different land cover within the footprint. Results are very encouraging with very good energy balance closure on short periods. However an underestimation of fluxes during summer times suggests some possible saturation impacts. To address this issue we installed a renewed scintillometry setup on a shorter 600m path length at the P2OA facility site near Lannemezan (South of France). The presentation will focus on these new results.