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Engineering a sap flow sensor for irrigators

Skinner, A.
Acta horticulturae 2013 no.991 pp. 67-75
Vitis, ambient temperature, energy, engineering, growers, growing season, heat, irrigation scheduling, radio, researchers, sap flow, soil profiles, soil water, thermal conductivity, thermal diffusivity, vapor pressure, vineyards, water content, water stress, xylem, South Australia
Sap flow sensors designed for irrigators – as distinct from scientific researchers – must be inexpensive and robust, simple to install and interpret, and able to advise a grower when the crop is becoming water stressed. Modern growers also expect data to be delivered to them wherever they may be and whenever they want it, which impacts directly on the sensor energy budget and the costs of transmitting data via radio and web. These stringent demands have prevented sap flow sensors reaching the hands of growers in Australia as a standard irrigation scheduling tool. This paper describes a new approach to using sap flow measurements to allow plant-based scheduling of irrigation. The sensor described does not attempt to make quantitative measurements of sap flow, but relies instead on the fact that is sensitive enough to detect the daily decline of expected sap flow under atmospheric loading as indicated by in-canopy vapour pressure deficit (VPD) measurements. A thermistor probe is inserted into the xylem tissue and forced to dissipate a very precise amount of power (25 mW ± 25 μW), creating a heat field sensitive to both thermal conductivity (enhanced by sap flow) and local heat capacity (affected by cellular water content). The rate of temperature rise monitored by the self-heated thermistor at the centre of this heat field yields a measurement of thermal diffusivity (TD). This low-power transient measurement technique rejects common-mode ambient temperature effects while reporting thermal diffusivity with a resolution of 0.0001 mm2 s-1. Field data collected over three growing seasons in a commercial vineyard in South Australia showed that, in well-irrigated grapevines, morning sap flow enhances thermal conductivity – and hence thermal diffusivity. By contrast, in grapevines under water stress, this sap flow enhanced thermal conductivity disappears. A single ‘crop water stress’ value, measured in hours, was obtained from the daily phase shift between TD and VPD. This phase-shift declined with increasing soil moisture tension down through the soil profile.