A heat-pulse method for measuring sap flow in corn and sunflower using 3D-printed sensor bodies and low-cost electronics

Sap-flow (SF) measurements provide unique and valuable data for studying plant water relations and crop water use. In this study we utilize new developments in heat pulse theory, low-cost electronics, and 3D-printing to fabricate, calibrate, and field test an affordable research-grade sap flow instrument. Each gauge included three needle probes that were inserted into the stem. A central needle contained a resistance heater for applying the heat pulse, while two additional needles measured the resulting temperature increases at positions downstream and to the side of the heater. Time series data following a heat pulse were used to calculate heat velocity using two techniques. The Tmax method used the time to temperature maximum while a novel temperature ratio method (TmRatio) used the ratio of the temperature maxima at the downstream and side probes. Data acquisition systems were built from low-cost Arduino microcontrollers. Prototype SF gauges were tested and calibrated for corn and sunflower in the greenhouse. Once calibrated for a specific gauge design and species, the gauges tracked gravimetric measurements of transpiration rate to within 10%. The Tmax method performed well under high rates of sap flow (i.e., up to 300g hr−1) in both sunflower and corn, but overestimated flow at low transpiration rates. The new TmRatio method accurately tracked sap flow at rates near 150g hr−1 and also performed well during nighttime flows as low as 3g hr−1 in corn. However, both theory and observation suggest the TmRatio approach may fail at very high flow rates. The gauges and data acquisition systems were deployed in the field on irrigated corn. Sap flow was calculated using the Tmax method, the TmRatio method, and a hybrid approach that used the TmRatio method for flow rates <130g h−1 and the Tmax method for flow rates >130g h−1. Estimates of canopy transpiration over a two-week period were, on average, within 5% of calculated reference crop evapotranspiration. The do-it-yourself simplicity and low cost of the approach make it possible to deploy large numbers of gauges in the field to capture spatial variability, compare water use among agronomic plots, or scale-up sap flow to measure canopy transpiration.