An in situ real time probe spacing correction method for multi-needle heat pulse sap flow sensors

Abstract

Probe spacing of sap flow sensors affects the quantification of sap flow in plants. The assumption of nocturnal zero flow conditions, required by most non-destructive methods for probe spacing calibration, is often invalid in natural biological systems and extremely hard to validate with in situ measurements. Our study aims to present a novel method for accurate in situ determination of probe spacing. The method uses nonlinear curve fitting with two mathematical models applied during low flow conditions. Numerical simulations were conducted with three types of probe misalignment and demonstrated that errors in heat pulse velocity (Vh) (< 2.0%) is less than the error associated with commonly-used probe spacing correction methods. Field experiment on European beech (Fagus sylvatica L.) comparisons with corrected probe spacing applied to the heat ratio (HR) and Sapflow+ method produced improved sap flux densities, with both methods agreeing very well (root mean square deviation < 1 cm3 cm−2 h−1). The proposed method allows real time probe spacing measurements at low flow rates, alleviating some uncertainties associated with zero flow assumption. The new method can hence successfully be used to estimate in situ probe spacing for operational measurements of sap flux density.