Instrumentation

Access Tube Installation

Drilling rig at oak site.

At least five 45 mm diameter holes were drilled to depths up to 9 metres at each experimental site. An aluminium access tube, sealed at one end, was inserted into each hole. These allow repeated insertion of a neutron probe and the determination of soil water content at pre-determined intervals to the base of each tube.

Access tube in oak woodland.

Taking readings from neutron probe at heath site.

Neutron probe readings occur fortnightly. The probe itself works by emitting fast neutrons, which, upon collision with certain atoms, particularly hydrogen, generate a cloud of slow neutrons. These in turn are recorded by the probe's Geiger counter. By comparing the counts with those generated when the probe is inserted into a tank of water, it is possible to obtain a relative measure of the moisture volume.

Capacitance Probes and Lysimeter Trays

Capacitance Probes measure changes in the apparent dielectric constant of a soil, which is proportional to its moisture content. In the case of the Delta-T (Theta) probes used in this research, readings are taken in voltages that -- given adequate calibration information -- can be subsequently converted into measures of volumetric soil moisture content.

Capacitance (Theta) probe with oak roots.

Reading a Theta probe.

A lysimeter tray is inserted at depth, and captures vertically draining flows of water. These samples are stored until collection and then analysed chemically.

Plastic tubes at ground level enable abstraction from buried lysimeter trays.

Heat Pulse Tracers

Heat Pulse Tracers measure sap flow, and by inference, a plant's transpiration. The probe works by sending a regular pulse of heat directly into the sapwood (xylem). Two sensors detect temperature both just below (usually 5 or 6 mm) and above (10 or 12 mm) the heater probe. The xylem temperature at either sensor is directly related to distance from the heater and sap velocity. Up to six or eight probes would typically be used simultaneously to measure sap flow flux on the same tree.

Heat Pulse sensor in pine tree.

Heat Pulse sensors, datalogger (buried box) and laptop to download data .

Automatic Weather Station

An Automatic Weather Station (AWS) has been placed in open ground near the pine wood to take hourly measurements of several variables, including temperature, relative humidity, rainfall, wind speed and net radiation. These are downloaded at approximately fortnightly intervals. The data will be used to compute potential evapo-transpiration.

Downloading data from the Automatic Weather Station.

Tipping bucket rain gauge with AWS.

Long jpg of deciduous trees

Back to: Trees and Drought front page, Geography Home