Interception is a key hydrological process whereby the plant canopy captures and holds precipitation. Typical methods of estimating interception are laborious and difficult to apply over extended time-series, may rely on calculating interception as the residual of an uncertain water balance, and/or produce qualitative metrics of leaf wetness. An in-situ technique was reinvestigated for directly measuring intercepted water mass by applying to a tree branch a parametrized and simplified form of the Euler-Bernoulli beam equation. A mature Acer macrophyllum (Bigleaf maple) branch was instrumented with two strain gauges in a half Wheatstone bridge configuration. The strain gauges took measurements at 1 minute intervals for 3 months (Jul-Sep 2016) while a nearby micrometeorological station simultaneously collected weather data (wind speed and direction, solar radiation, precipitation, air temperature, humidity, and pressure). In addition, the branch was instrumented with two common, commercially available leaf wetness sensors, one at the canopy edge near the branch tip and one inside the canopy on the branch approximately above the strain gauges. After field monitoring, the monitored branch was harvested at the point of the strain gauge sensors and its diameter, mass, location of center of mass, and leaf area were measured in the laboratory.
Results/Conclusions
Interception events and the diurnal signal of night tree water uptake and daytime branch mass loss from transpiration were both evident in the time series of strain gauge data. Empirical and modeling approaches were compared to remove the diurnal signal of internal branch mass variation (capacitance), while accounting for differences in day lengths and meteorological conditions. The residual signal showed the effects of branch loading by intercepted water. Eight rain events were recorded during the experiment, ranging from 0.6 to 17.6 mm of rainfall over a 24-hour period. Each was recorded as a rapid increase in branch mass followed by a gradual drying period. Small pre-dawn “events” interpreted as loading by dew were also recorded on many days. Comparing the records of the strain gauges and the leaf wetness sensors suggested that this model of leaf wetness sensor is hysteretic, producing two very different readings given the same mass of canopy water storage depending on whether during wetting or drying. With methodological improvement, such as begun by this study, the strain gauge-approach to measuring branch interception is promising and may further allow extracting important metrics and relationships, such as between maximum interception and storm size or intensity.