Measuring evapotranspiration fluxes using a tunable diode laser-based open-path water vapor analyzer

Evapotranspiration (ET) is one of the essential components of the hydrological cycle of terrestrial ecosystems. Among various techniques for measuring ET, the eddy covariance (EC) is the most direct one for measuring ET fluxes at field to ecosystem scales. It has been used worldwide to monitor the biosphere-atmosphere exchanges of energy, water, and carbon, particularly in some global and regional networks (e.g., FLUXNET) for ecosystem studies.

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. We have earlier presented a cost-effective, open-path water vapor analyzer (Model: HT1800, HealthyPhoton Co., Ltd.) suitable for EC measurement of ET based on the tunable diode laser absorption spectroscopy (TDLAS) technology. The analyzer utilizes a low-power vertical cavity surface emitting laser (VCSEL) and a near-infrared Indium Galinide Arsenide (InGaAs) photodetector in an open-path design, which avoids delay or high-frequency damping due to surface adsorption. The analyzer has a precision (1σ noise level) of 10 μmol mol⁻¹ (ppmv) at a sampling frequency of 10 Hz. 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.

Recent studies have emphasized the importance of spectroscopic effect correction for EC measurement using a laser-based open-path gas analyzer. This additional correction arises from the absorption line broadening due to atmospheric water vapor, temperature, and pressure fluctuations. In this study, we prepared two HT1800 water vapor analyzers. One is equipped with an infrared laser operating near 1392 nm and the other near 1877 nm. The water vapor line near 1392 nm is one of the most used for detecting water vapor because laser and photodetector operating near this wavelength are readily available and relatively inexpensive. However, its broadening effect, mainly caused by temperature variation, is expected to be stronger than the 1877 nm line, according to theoretical analysis using the HITRAN database.

Using the two HT1800 analyzers, we conducted two EC measurement campaigns at an agricultural site in 2022. Two commercial gas analyzers, EC150 (Campbell Scientific Inc., Logan, UT, USA) and LI-7500RS (LI-COR Biosciences, Lincoln, Nebraska, USA), were also running during the campaigns to compare with HT1800. The first purpose of this study is to test the performance of HT1800 under field conditions and evaluate its applicability for ET flux measurements. The second purpose is to quantify and compare the spectroscopic effect on the ET fluxes using the 1392 nm and 1877 nm water vapor analyzers. Meanwhile, we proposed a hypothesis that the 1392 nm analyzer can provide comparable ET fluxes with LI-7500RS and EC150 after accounting for the spectroscopic effect. If it is the case, this cost-efficient water vapor analyzer will become an effective tool for water and ecological studies in the future.