A novel sap flow system to measure maize transpiration using a heat pulse method

Finding ways to irrigate more efficiently is crucial for long term agricultural sustainability around the globe. Assessing plant water consumption using sap flow gauges (SFG) can quantify irrigation water requirements and identify ways to improve irrigation efficiency. We developed a novel sap flow sensor that uses a new heat pulse technique to measure the flow of water through plant stems. These SFGs were made using a desktop 3D printer, low-cost components, open-source electronics and cellular-based Internet-of-Things (IoT) technology. Sensors were calibrated and validated in a greenhouse and evaluated in a well-irrigated maize (Zea mays L.) field in northern Colorado, USA for two years. Greenhouse validation studies showed that the SFGs could measure daily water use (g day−1) to within 5.9 % and 9.4 % of gravimetric measurements, for 2020 and 2019 respectively. In the field, comparison with commercially available SFGs showed a difference of 8 % for daily transpiration (Tr) estimation. Comparison with maize Tr calculated from the ASCE reference evapotranspiration (ETr) and basal crop coefficients (Kcb) showed average differences of 4.4 % and 2 %, over periods of 12–17 days, for 2020 and 2019 respectively. Results demonstrated that the combination of the sensor design, model for heat velocity and calibration factor gave an acceptable performance. It was shown to be a reliable approach to measure maize transpiration at the individual plant or field scale in well-watered conditions and is therefore a potential tool to develop or verify Kcb curves, validate transpiration or canopy conductance models or estimate plant water stress.