Sap Flow Gauge Redux: How 3D Printing, DIY Electronics, and New Theory Could Transform Measurements of Transpiration (Invited Presentation)

Sap flow measurements, when deployed at sufficient spatial scale, provide estimates of canopy transpiration (T) and latent heat flux from vegetation. These data can be used to partition T and soil evaporation, evaluate water use among species in mixed canopies, and evaluate physiological responses to water stress. Unfortunately, inherent variation among plants and across the landscape requires large numbers of sensors. Thus, economic costs and logistics have limited the utility of sap flow technology in micrometeorology. A new approach to sap flow measurement was developed that takes advantage of recent breakthroughs in desktop fabrication (3D printers), low-cost electronics, and sensor networking. A new theoretical approach to the heat-pulse method also was developed to determine sap velocity using a combination of downstream and tangential temperature probes. The prototype system was calibrated on agriculture crops in the greenhouse and then used under field conditions to estimate canopy T from irrigated corn. Results show the system measures sap flow to within 10% and can operate for long periods on a small power source. The flexibility provided by 3D printing allows the same sensor package (i.e., heat pulse probes) to be reused on a wide range of stem sizes without the need to manufacture or buy new gauges as plants grow. The gauge design and software are completely open-source with the intention that others can replicate and improve the technology over time.