Copeland Creek Watershed Evapotranspiration Project:

Water regulation in response to soil and atmospheric drought at the Fairfield Osborn Preserve

copeland creek photo by Scott Hessbridge over Copeland CreekUSGS technician measuring flowCopeland Creek falls


evapotranspiration diagram

Project Description: Evapotranspiration, the sum of evaporation from the land surface and transpiration from plants, is a significant source and variable source of water loss from drainage basins.Trees can use a highly variable fraction of precipitation, thus leaving a highly variable fraction for runoff into the creek. Older trees typically use more water, but this also varies widely and is sometimes reversed, depending on the species. The effects of soil and atmospheric drought on tree water use varies widely too. Measurements of tree physiology can accurately model these processes on a landscape scale and can be used to compare effects of alternative management scenarios on water yield

To better understand water cycle dynamics in the Copeland Creek watershed, a student research team constructed and installed a sensor network to continuously monitor environmental conditions and sap flow (tree water loss) from trees in the Fairfield Osborn Preserve (headwaters of Copeland Creek).  We couple these measurements to periodic campaigns to measure physiological properties and traits, in order to inform a physiological model of tree water use and test hypotheses about species and seasonal differences in response to water availability. 

The overarching goals of this project are to better understand how water flux from forested catchments east of the Laguna de Santa Rosa is regulated by environmental variation and change, and to assimilate this knowledge in the form of a predictive model that can be used by water resource managers in Sonoma County.

Duration: Spring 2012

Type of Educational Activities: Service-learning, independent research

Project Faculty: Tom Buckley (Biology)

Partners: Sonoma County Water Agency

Participating Courses: BIOL 348 Plant Physiology (Tom Buckley) - 12 students

We constructed and calibrated 26 probe sets from component parts in the laboratory. To measure transpiration in the upper watershed, we chose a S/SW facing toe slope at the Fairfield Osborn Preserve. Approximately 13 trees were instrumented with 26 sap flow probe sets. We inserted one probe set at breast height (1.30 m) under the bark of each sample tree. To minimize bias due to azimuthal variation, probes were installed at the same azimuth in all trees. Each probe set consisted of 2 probes, 0.13 cm in diameter and 3.5 cm in length, spaced 0.5 cm apart axially in the bole. A drill guide (ICT Int’l) was used to minimize errors in spacing and probe alignment. The centre probe contained a heater wire, and the upper and lower probes contained two thermocouples each, located 1.25 and 2.75 cm from the probe hub, which was situated at the outside of the stem after removing a small area of bark. In each tree, the thermocouple pair at 1.25 cm depth comprised the ‘‘outer sensor,’’ and that at 2.75 cm depth comprised the ‘‘inner sensor.’’ Heat pulses (40 or 50 J) were triggered by a 16-bit microprocessor unit attached to the tree adjacent to the probes, approximately 10 cm to the side of the probe insertion point, and temperature ratios were recorded 80 s after each pulse. A Hobo temp/RH/PAR logger was also installed at the site.  

Probes were thermally insulated using reflectix insulation. Probe interfaces are connected to ICT SmartLogger dataloggers, powered by a 12-volt truck battery. Students visit the network twice each week to recharge batteries. Measurements were scheduled to be recorded every 30 min. Data can be used to calculate whole tree sap flux by summing the products of sapwood area and sap flux for outer and inner sapwood regions, all divided by total sapwood area. Sapwood regions can be delineated by a boundary corresponding to the midpoint between the inner and outer probes. Outer sapwood area can be identified as the lesser of total sapwood area (calculated from sapwood length in two cores per tree) and the area of the outer region; inner sapwood area is the remainder of total sapwood area.

Data and Reports: After installation, we encountered problems with the datalogger which required replacement. No reliable data were collected this season.

Lead investigator Tom Buckley accepted a position with a university in Australia and he and his graduate students are no longer able to continue with the project. We are exploring whether the equipment can be used by other researchers in the region.

Scientific Papers: Measurement Techniques

Buckley, T.N., T.L. Turnbull, S. Pfautsch, M. Gharun and M.A. Adams. 2012. Differences in water use between mature and post-fire regrowth stands of subalpine
Eucalyptus delegatensis R. Baker. Forest Ecology and Management 270 (2012) 1–10.

Buckley, T.N., T.L. Turnbull, and M.A. Adams. 2012. Simple models for stomatal conductance derived from a process model: cross-validation against sap flux data. Plant, Cell and Environment (2012) 35, 1647–1662

Scientific Papers: Evapotranspiration in Russian River Watershed

Potter, C. and S. Hiatt. 2009. Modeling river flows and sediment dynamics for the Laguna de Santa Rosa watershed in Northern California. Journal of Soil and Water Conservation. 64 (6): 383-393.