Measuring sap flow with the heat balance approach using constant and variable heat inputs

Abstract

The performance of a heat balance-based sap flow gauge design, capable of alternating variable heat input (VHI) and constant heat input (CHI), was analyzed. The gauge employs differentially wired thermocouples to monitor differences in stem temperature and radial heat losses. The VHI method employs a control circuit to maintain a constant temperature difference between a thermocouple placed upstream from the gauge heater and a thermocouple at the heated portion of the plant stem. The CHI method applies a constant voltage to the gauge heater with no control circuit. Simultaneously, on the same plants, the effect of the placement of thermocouples, either inserted into the stem structure or placed on the surface, was also investigated. Sets of 15 min and daily integrated measurements were compared with gravimetric water losses determined by lysimetry. The evaluation included three plant species: sunflower ( Helianthus), maize ( Zea maize) and potato ( Solanum tuberosum). The results showed that the VHI gauges tended to underestimate flows for both the 15 min and daily sap flows. The CHI gauges gave generally better results and were easier to implement and monitor. No consistent differences in the performance of the inserted and surface-mounted thermocouples were found; any differences that were noted were generally small and of no practical significance. An exception was the case of sunflower CHI at large flow rates, when the inserted thermocouples outperformed the surface-mounted ones. Important departures between gauge sap flows and lysimetric plant water losses at low flow rates were observed. Gauge performance was better for daily integrated flows (relative absolute error, RAE ≈ 10% for combined data) than for 15 min interval average flows (RAE = 32–36%v for combined data). However, the performance of the short-time-interval gauge improved dramatically when low flows (< 10 g h −1) were not included in the analysis.