Automation of the Atmometer (ETgage) recording by pressure transducers sensors

Abstract. In precision irrigation, it has become imperative to accurately evaluate the crop irrigation needs and return the right amount of water. Concerning to the application of thermodynamic-based models, the crop transpiration  can be determined with the original Penman-Monteith equation (Monteith, 1965) through the so-called "big leaf" approach. According to what was suggested by Jarvis and McNaughton (1986), for its evaluation through sensors, one valid option is the atmometer. This instrument is used to measure the quantity of water evapotranspired in a reference system (ET0), and the actual transpiration (Tc act), is calculated according to the weather-based approach (Allen et al., 1998). The ET0 in the atmometer is evaluated from the variation in the water level of the distilled water source placed inside the instrument tank, hydraulically connected to a porous ceramic capsule  covered with a green fabric (green canvas) which simulates the radiative and resistive behaviour of the reference culture. The most advanced model at present is the model-E with an electronic component for the automatic measurement of ET0 measures. In this model, the evaporated water is based on the emptying of a glass ampoule, with a capacity of 0.25 mm of water, filled automatically through a solenoid valve. Each emptying corresponds to 0.25 mm of evaporated and generates an electrical signal (count) detected by the data logger.

In our study, the atmometer (ETgage) was modified in the device for measuring the relative water level. The modification of the atmometer consists in the insertion of an RS-828-5708 piezoresistive pressure transducer. The pressure transducer returns an analog output in the 4-20 domain (mA) as a function of the hydrostatic head H (cm). The sensor was calibrated on the test bench of the DiSAAA-a AgrHySMo laboratory with paired measurements of hydrostatic head H(cm) and electrical signal read by the datalogger (mV). Therefore, the linear calibration equation between the two measurements was obtained with a slope of 0.3029 m/mV and an intercept of 10.804 m. Finally, the data series were improved thanks to a smoothing process, performed using a 3rd-4th order polynomial function (Savitzky and Golay, 1964) on data clusters equal to 17 points. The improved water level measurement system allows flow measurement at the sub-hourly scale. In open filed, the temporal dynamics of the atmometer were compared with the reference evapotranspiration calculated with the Penman-Monteith. The atmometer measurement showed an improvement compared to the respective estimated with the mathematical analogy, reducing the RMSE from 1.65 to 0.30 mm/day. The first results have demonstrated an accurate performance of the modified atmometer in estimating hourly reference evapotranspiration and its ability for precise irrigation planning based on hourly water consumption.

Keywords. Atmometer, field-instrumentation, sensor and model design, crop water status, precision irrigation.