A Rapid-Circulation Evaporation Chamber for Measuring Bulk Stomatal Resistance

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Abstract
A relatively simple method is described for determining evaporation of short vegetation at any moment of time. The method is based on the measurement of bulk stomatal resistance of the crop with an evaporation chamber. Once this quantity is known, the evaporation can be calculated with the Penman-Monteith equation. Measurements are made with an open and with a closed chamber. The air inside is circulated vigorously to ensure a low value for the transport resistance of water vapor (rr) from the leaf surfaces to the measuring device in the chamber. With this feature, the chamber differs from commonly used chambers for measuring gas exchange of a crop (e.g., Louwerse and Eikhoudt, 1975). This feature is essential because an evaporation chamber measures the sum of rr and the stomatal resistance rat, so that an accurate determination of rat requires a low value of rr. The value of rr in our chamber is found to be low (∼10 s m−1) compared with commonly observed values rat (in the order of 50–200 s m−1). Results from the open and closed chamber showed a reasonable agreement. Values of rat measured with the closed chamber agreed with stomatal resistances calculated from observed values of the evaporation combined with a model for the transport of heat and water vapor in the atmosphere, by reversing the Penman-Monteith equation.

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