Radial variability in sap flux density represents a major source of uncertainty in tree- and stand-level transpiration estimates. Despite its importance, the temporal dynamics of radial sap flux heterogeneity, particularly under drought conditions, remain poorly constrained in temperate deciduous forests. This knowledge gap is critical because transpiration constitutes a major outgoing flux of the forest water balance during droughts. This project aims to improve ecohydrological estimates of tree water use by monitoring temporal changes in radial variability in sap flux density and assessing the key environmental and physiological drivers of this variability. The study was conducted at the private temperate forest reserve Kenauk (45°42′N, 74°53′W) in southwestern Quebec, Canada. At each of two plots (~50 m apart), three mature sugar maple (Acer saccharum Marsh., DBH 19.7–36.9 cm) trees were selected. High-resolution sap flux density was measured at 15-minute intervals using multi-depth heat-ratio sensors with five thermistors spanning the full sapwood profile. The study period, from July to September 2025, coincided with a particularly dry summer for the region (summer precipitation anomaly = -59.5 mm, -21.6%). Using a suite of statistical approaches, non-linear radial patterns in sap flux density were quantified to assess how these patterns vary diurnally, seasonally, and in response to environmental drivers and physiological controls. This research provides new insights into the internal regulation of water transport in sugar maple, the environmental and physiological drivers of radial variability in sap flux density, and key methodological considerations for scaling sap flux density measurements to whole-tree and stand levels.