To improve water budgeting of forested catchments and inform relevant hydrologic theory regarding forest water cycling, the scientific community has been seeking simple, inexpensive, direct methods for determining rainwater storage on in-situ tree canopies. This paper evaluates an installation arrangement/routine for one such method: mechanical displacement sensors placed on the trunk to directly monitor compression under canopy water loading from rainfall. The evaluated installation routine aligns mechanical displacement sensors along orthogonal axes passing through the trunk's mechanical center to reduce wind-induced noise. Experimental attainment of neutral bending axes for a subject hard- and softwood tree suggest the routine is precise and approximates the trunk's mechanical center well regardless of differences in cellular axial stiffness between heart and sapwood. When installed in this precise sensor arrangement, bending tests of different direction produced consistent signal ratios between sensor pairs about -1 (1 unit compression/1 unit elongation), allowing the identification and removal of bending strains from raw strain signals to isolate the compression component attributable to canopy water storage. The same experiments performed on sensors 5cm off the computed mechanical center were unable to produce neutral bending axes or consistent signal ratios during directional bending. Results from the method evaluation were translated into a data processing technique that is applied to strain data from 2 sample storms (1 each for the hard- and softwood trees). Processed strain data showed clear synchronicities between rainfall and canopy loading, and periods of maximized canopy water loading (capacity). Our results indicate the evaluated arrangement/installation procedure for mechanical displacement sensors may provide scientists with simple, direct canopy water storage estimates at high temporal resolution and sensitivity.
Supported by US-NSF EAR-0724971, CBET-0931780.
Van Stan J.T., K.A. Martin, J. Friesen, M.T. Jarvis, J.D. Lundquist, & D.F. Levia (2013): Evaluation of an instrumental method to reduce error in canopy water storage estimates via mechanical displacement. Water Resources Research. DOI: 10.1029/2012WR012666
This Paper/Book acknowledges NSF CZO grant support.