A low-cost, open-path water vapor analyzer for eddy covariance measurement of evapotranspiration

Among various measurement techniques, eddy covariance (EC) is the most direct one for measuring evapotranspiration (ET) fluxes at field to ecosystem scales (Aubinet et al., 2000). In the past two decades, EC flux towers around the world, particularly those within the FLUXNET, have served as a worldwide network of calibration and validation for surface-atmosphere energy and ET flux data obtained from remote sensing-based models or hydrological process-based models (Wang and Dickinson, 2012). 

One of the major challenges in model-data benchmarking is the spatial mismatch issue. For example, the grid cell size of around 10⁶ – 10⁸ m² in typical Earth system regional modeling cases is often several orders of magnitude larger than the EC flux footprints of around 10³–10⁷ m². Since most flux tower sites are located in more-or-less heterogeneous landscapes, multiple measurement units for spatially adequate sampling and representative fluxes are of interest for capturing the fine-scale spatial variation. However, the deployment of higher density sampling points was mainly limited by the costs of conventional analyzers. Therefore, there is increasing demand in the development of low-cost water vapor analyzers specifically for more spatial representative terrestrial ET flux footprints measurements based on EC methods.

In recent years, laser-based gas spectrometers have shown good reliability and effectiveness in the high-frequency and high-sensitivity measurement of various atmospheric trace gases. In this work, we have developed an open-path analyzer (HT1800, HealthyPhoton Co., Ltd.) for fast and sensitive measurements of atmospheric water vapor density. The analyzer employs a low-power vertical cavity surface emitting laser (VCSEL) and a near-infrared Indium Galinide Arsenide (InGaAs) photodetector. An open-path configuration with 0.5 m effective optical path length is used for selective and sensitive detection of the single spectral transition of H₂O at 1392 nm, which has been extensively studied in the field of spectroscopic analysis. Using this spectral line to realize the single-component measurement of water vapor density can avoid the complex cross-calibration process due to the H₂O-CO₂ spectral interference as happened in traditional nondispersive infrared (NDIR) analyzers. On the other hand, the semiconductor nature of lasers and detectors can borrow the mature optical communication industry fabrication process, so that the cost of the core optoelectronic devices is expected to be reduced in mass production.

The analyzer has a precision (1σ noise level) of 15 μmol mol−1 (ppmv) at a sampling frequency of 10 Hz. Due to its open-path configuration, there is no delay or high-frequency damping due to surface adsorption. The analyzer head has a weight of ~2.8 kg and dimensions of 46 cm (length) and 9.5 cm (diameter). It can be powered by solar cells, with a total power consumption of as low as 10 W under normal operations. With good performance in terms of response time and precision, this instrument is an ideal tool for ET flux measurements based on the EC technique. An EC flux tower was built based on the open-path analyzer, which also included an integrated CO₂ and H₂O open-path gas analyzer and 3-D sonic anemometer (IRGASON, Campbell Scientific) for comparison of ET flux measurement.