Improving the stem heat balance method for determining sap-flow in wheat

A novel micro-sensor for measuring sap-flow in thin plant stems designed by Dynamax Inc. based on the heat-balance theory was applied in wheat (Triticum aestivum) grown under ambient field conditions. The sensor measures axial and radial temperature changes in a constantly heated and thermally insulated stem section. The temperatures are altered by sap-flow activity and this information is used to solve the stem energy balance equation with respect to its convective heat flow residual which indicates sap-flow. Results from four different field experiments show that the majority of heat energy input was diverted to radial heat flow, leaving only little energy partitioned to convective heat flow. Determinations of gravimetric sap-flow were extremely noisy in consequence, rendering the method unreliable for practical application. Temperature differences across the heater consistently correlated with fluctuating net-radiation however, which motivated us to establish an empirical method for determining gravimetric sap-flow based on this temperature information alone. Numerical simulations showed that gravimetric sap-flow and temperature difference are nearly linearly and positive correlated within an observed sap flow range between 0 and 1.7 g h−1, beyond which the relation became non-linear and even inverse at higher velocities. It remains to be tested whether such higher fluxes can be reached in practice and we provide a solution for these cases. Statistical noise overrode the error introduced by assuming a linear relation between sap flow and temperature difference within the range between 0 and 1.7 g h−1. The resulting factors were determined under stable sap flow conditions greater than 1 g h−1and used for generating daily cycles of sap flow using temperature information alone. The approach was successfully validated in 2011 and 2012 against independent measurements of latent heat flux conducted in closed and dense wheat fields using the eddy-covariance technique. We thereby improved the application of the new micro-sensor in wheat. Suggestions for further enhancements of the method are discussed.