Automated chamber system to measure field evapotranspiration rates

Reliable estimation of evapotranspiration is important for many hydrological applications, including agriculture, climatology, and assessment of stormwater control measures. An automated chamber system was constructed to measure field evapotranspiration. The system consists of six chambers, sampled consecutively by means of a dynamic closed-chamber design, and is fully automated to allow replicable long-term measurements. However, the use of such a system is subject to measurement errors due to vapor adsorption along the sampling tubes and inappropriate internal ventilation. Because these issues are rarely addressed in the literature on flux chambers, a laboratory calibration was performed to estimate errors in evapotranspiration rates measured by the system. The effects of the sampling tube length and air mixing were investigated. A correction factor for evapotranspiration rates relative to the tube length was established. Appropriate air mixing induced by small fans ensured that the measured rates were representative of the internal conditions of the chambers. With appropriate internal ventilation and correction factors, the relative error of measurements was less than 5% for high evapotranspiration rates.

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