Estimating low and reverse sap flux density with the temperature ratio heat pulse (TRHP) method

The compensation heat pulse (CHP) method is widely-used for monitoring sap flux density, but it has a limited measurement range and often underestimates daily transpiration in plants. Here we present a temperature ratio heat pulse (TRHP) algorithm for extending the measurement range of the CHP method to low and reverse sap flows. With a similar probe configuration to CHP, TRHP utilizes the ratio of temperature changes associated with downstream and upstream probes (ΔT_down/ΔT_up) for sap flow measurements. We verified the method through sand column and field employment. Sand column experiments show that the TRHP method successfully measured low and reverse flows, and the measured reverse sap flux density can be as low as −10 cm³ cm^-2 h⁻¹. Field deployments show that TRHP can monitor nocturnal sap flow more accurately than CHP method. This study thereby extends the capabilities of the CHP method for measuring low and reverse sap flows, which has wide applications in monitoring plant species where low and reverse sap flows are prevalent.