Field Performance of the Walker Branch TDE 1993 through 2005

We are conducting a large-scale manipulative field experiments in an upland oak forest on the Walker Branch Watershed in eastern Tennessee USA to identify important ecosystem responses that might result from future precipitation changes. The manipulation of soil moisture is being implemented by a gravity-driven transfer of throughfall precipitation from one treatment plot to another. Throughfall is intercepted in ‰2000 subcanopy troughs (0.3 x 5 m) suspended above the forest floor of the "dry" plots (‰33% of the ground area is covered) and transferred by gravity flow across an ambient plot for subsequent distribution onto the "wet" treatment plot. Percent soil water is being monitored with time domain reflectometers at 310 sampling locations across the site. The experimental system is able to produce statistically significant differences in soil water content in years having both extremely dry and extremely wet conditions. Furthermore, comparisons of pre- and post-installation soil temperature measurements have documented the ability of the experimental design to produce these changes without changing the microclimate of the forest understory.


Hanson, P.J., D.E. Todd, N.T. Edwards, and M.A. Huston (1995)
Field performance of the Walker Branch Throughfall Displacement Experiment,
In: A. Jenkins, R.C. Ferrier, and C. Kirby, eds.,
Ecosystem Manipulation Experiments: scientific approaches, experimental design and relevant results
Ecosystem Research Report #20, Commission of the European Communities, pp. 307-313

Hanson, P.J., D.E. Todd, M.A. Huston, J.D. Joslin, Jennifer Croker, and R. M. Auge (1998)
Description and field performance of the Walker Branch Throughfall Displacement Experiment: 1993-1996,
ORNL/TM-13586, Oak Ridge National Laboratory, Oak Ridge, Tennessee.
36 pages and appendicies

Hanson P.J. (2000)
Large-scale Water Manipulations -- chapter 23
In: O.E. Sala, R.B. Jackson, H.A. Mooney, and R.W. Howarth, eds.
Methods in Ecosystem Science
Springer-Verlag, New York.  pp. 341-352

Hanson, P.J., D.E. Todd and J. S. Amthor (2001) 
A six year study of sapling and large-tree growth and mortality responses to natural and induced variability in precipitation and throughfall. 
Tree Physiology 21:345-358
Hanson PJ, Todd DE, Huston MA (2003)
Walker Branch Throughfall Displacement Experiment (TDE)
In: Hanson PJ, Wullschleger SD, Eds,
North American Temperate Deciduous Forest Responses to Changing Precipitation Regimes.
Springer, New York, pp. 8-31.

Experimental Design and Site Description Figures:

TDE Soil Water and Related Data:

Summary graphs:

Annual Pattern of Soil Water Content 1993-2005

Soil Moisture Release Curves for the TDE Soils and the Annual Pattern of Soil Water Potential 1993-2005

Pre-treatment Soil Water Contour Plots by Date Through 2000: Post-treatment Soil Water Contour Plots by Date:           1994          1995           1996           1997           1998 1999 2000 Starting in 2001, only one upper, middle and lower slope transects (i.e., transectes 3, 6, and 9) were sampled for the periodic evaluation of soil water content.  These data were insufficient for the production of contour plots.