Current state and challenges of monitoring tree canopy water storage
2015 Spring Cyberseminar Series
- John Van Stan / Georgia Southern University
Quantifying the amount of water stored in forest canopies during and after precipitation is necessary for predicting interception loss and water receipt to the surface of wooded watersheds. Yet, methods for monitoring in situ forest canopy water storage through storms remains a challenge. This cyberseminar discusses the current state and problems of methods developed to meet this challenge at the individual tree canopy scale. Dr. Van Stan is an Assistant Professor of Geography and Geology at Georgia Southern University.
Forest precipitation interception plays a significant role in the hydrologic cycle for over 25% of the global land area. A critical component of forest interception is the storage of precipitation in the canopy (Sc) and trunk (St) – together these elements represent total tree water storage (S). This presentation discusses the pitfalls of current, common indirect S estimation methods, then describes newly-developed methods for monitoring S throughout storms. These whole tree S monitoring methods include mechanical displacement (compression) and sway interceptometers. As trunks tend to be irregularly shaped, canopies are unevenly distributed, and mechanical displacement can be caused by other factors than S-related compression, compression-based interceptometers require precise installation for the isolation of water loading observations. To identify precise sensor installation locations, high-resolution trunk profiles are generated using the LaserBark automated tree measurement system. These scans are utilized to approximate the location of neutral bending axes. A routine then instructs a mobile rangefinder along the cross-section to optically indicate exact positioning for strain sensors over the bending axes. As imprecise sensor placement linearly increases error and diminishes signal-to-noise ratio, this automated installation routine is designed to remove significant distortions created by wind throw, off-centre loading within unevenly-distributed canopies, and human error that can lead to erroneous measurements of rainfall interception. As for sway interceptometers, changes in an individual tree’s “free” sway frequency can be related to the square root of canopy mass. Calibration is achieved by hoisting a known mass into the canopy at a known height and observing the shift in free sway frequency. Assumptions which limit application of the sway-based interceptometer include: density and modulus of elasticity of the tree are the same throughout the trunk and the effect of weather changes on wood properties is negligible. Relating sway to S is further limited by the unknown placement and distribution of S in the canopy.
CUAHSI's Spring 2015 Cyberseminar Series on Evapotranspiration - Frontiers in measurement, modeling and management from the leaf to the landscape!
Hosted by Steven Brantley, Joseph W. Jones Ecological Research Center