AN AUTOMATED RECORDING ATMOMETER .1. CALIBRATION AND TESTING

A newly designed automated atmometer (porous-surface evaporimeter) capable of short-interval recording of potential evaporation is calibrated and tested. The rate of evaporation from a carborundum stone surface is measured with the use of a view tube of the type used in medical intravenous systems and an IR light-emitting diode (LED)-photodetector pair to count water drops. A resolution of approximately 0.66 mum pan evaporation equivalent is obtained with the new design. Simultaneous measurements using the new atmometer, a manual atmometer of similar design, and a US Class A pan evaporimeter are used in the calibration. Evaporation estimates from four Penman-type models are also compared with estimates from the new atmometer. At the 1 h interval, 95% of the variance in atmometer evaporation is explained by the Penman-Monteith model. In comparison with other evaporimeters, the new instrument offers improvements in resolution, accuracy, response time, short-interval measurement capability, unattended operation, and portability.