An Open-Source Graphical User Interface for Estimating Heat-Pulse Based Sap Flux

Heat-pulsed based Sap flow sensors are widely used to quantify plant water uptake by employing heat as a tracer. These methods estimate sap flow using the temperature breakthrough curves measured down and/or upstream of the heat source following a heat pulse. Several established methods, such as Marshall, Tmax, Compensation Heat Pulse (CHP), and Heat Ratio (HR) methods, rely on simple analytical formulations that can be implemented using spreadsheets or basic computing tools. In contrast, physically based inverse modelling approaches such as the Sum of Squares Error (SSE)  or Sapflow+ method estimate heat pulse velocity by fitting analytical solutions of the heat transport equation to measured temperature responses using nonlinear optimization. This involves  more complex computing tools that may not be easy to implement in spreadsheets. This computational complexity has limited the broader adoption of these methods. To address this, here we develop an interactive, Python-based open-source Graphical User Interface (GUI) for estimating sap flow using the SSE method. The GUI can be accessed through a web browser making it platform-independent. Additionally, users have the option to deploy the computational platform locally on their computers/tablets/mobile devices or access the cloud computing services we have enabled. The tool enables users to i) upload temperature data, ii) apply data filtering and wound correction, iii) perform automated parameter estimation, and iv) visualize heat pulse velocity and sap flow rates through interactive plots with downloadable outputs. Users also have the option to compare sap flux estimates from different methods. By providing a user-friendly interface for different sapflow methods, including the computationally intensive SSE method, our GUI facilitates more consistent and reliable sap flow analysis across research studies.