The Necessity of Sensor Calibration for the Precise Measurement of Water Fluxes in Forest Ecosystems

Abstract

In forested watersheds, interception loss (EI) and transpiration (ET) constitute the majority of evapotranspiration. Accordingly, their precise evaluations are necessary to understand and quantify fluxes within the hydrologic cycle. EI is commonly measured by tipping-bucket rain gauges and flow meters, while ET is often estimated by sap flow techniques. To obtain reliable estimations of EI and ET, we describe detailed procedures to calibrate tipping-bucket rain gauges and flow meters as well as sap flow techniques. For tipping-bucket rain gauges and flow meters, we measure the one tip static volume, and then changes in the one tip amount with different inflow rates for dynamic calibration. Without proper calibration, the significant evaluation error in EI can range from 40% underestimation to 20% overestimation. We calibrate three sap flow techniques—thermal dissipation (TD), heat field deformation (HFD), and heat ratio (HR) methods—for Japanese cedar (Cryptomeria japonica) from two sites. The clear radial and azimuthal trends in sap flux density (FD) are confirmed for the artificial sap flow generated by a vacuum pump. Among segments sampled at a site, TD and HFD methods do not have any tendencies to overestimate and underestimate FD. While at the other site, TD and HFD methods underestimate FD, and therefore ET, by at least 30%, the HR method shows a 30% overestimation. Thus, we highly recommend the calibration of tipping-bucket rain gauges, flow meters, and sap flow techniques to obtain valid estimates of EI and ET.