Precision weighing lysimetry for trees, using a simplified tared-balance design.

Four weighing lysimeters (1.8 m diameter, 1 m depth, weight 3.5-4.5 Mg) were used to estimate transpiration from single, isolated trees. The weighing mechanism incorporated a simple, new system of wire ropes around torsion tubes, producing a lever action through which most of the weight was tared off with a counter weight. Weight differences measured by load cell under the main lever arm were unaffected by hysteresis or temperature change. Sensitivity better than 100 g was achieved in windless conditions. Daily measurements over yearly periods (trees could be regularly replaced using extra pots) revealed both sigmoid seasonal patterns, and sigmoid diurnal changes closely related to micrometeorological variables, for a range of tree species. Measurements taken during rainfall demonstrated that an isolated tree intercepts considerably more precipitation than does a uniform crop covering the same ground area, and that substantial evaporation from the canopy takes place while it is raining.