Long-term growth and water balance predictions for a mountain ash (Eucalyptus regnans) forest catchment subject to clear-felling and regeneration.

We used a physically based ecohydrological model to predict the water balance and growth responses of a mountain ash (Eucalyptus regnans F. Muell.) forest catchment to clear-felling and regeneration. The model, Topog-IRM, was applied to a 0.53 km(2) catchment for a 3-year pretreatment period, and a 20-year period following clear-felling and reseeding of 78% of the catchment area. Simulations were evaluated by comparing observed and predicted streamflows, rainfall interception and soil water values. The model faithfully simulated observed temporal patterns of overstory live stem carbon gain and produced a leaf area trajectory consistent with field observations. Cumulative throughfall was predicted within 1% of observations over an 18-year period. Over a 4-year period, predicted soil water storage in the upper 1.5 m of soil agreed well with field observations. There was fair correspondence between observed and predicted daily streamflows, and the model explained 76% of the variation in monthly flows. Over the 23-year simulation period, the model overpredicted cumulative streamflow by 6%. We argue that there is a useful role for physically based ecohydrological models in the management of mountain ash forest catchments that cannot be satisfied by simple empirical approaches.

Integration of sapflow velocity in elliptical stems.

The estimation of water flux through the stem of a plant by point estimates based on the heat pulse technique requires integration of the velocity profile. The polynomial integration method and the weighted average technique both assume radial symmetry about the bole. A method is introduced that solves the equations for the ellipse in the weighted average technique, and includes an optimizing algorithm for the placement of sensors. The sensitivity of sapflow estimates through stems to increasing eccentricity is examined by means of analytical solutions and simulation. The error in sap flux measurement resulting from the radial approximation of the sapwood conducting area is expressed analytically and shown to be small over the range of eccentricities (e < 0.8) expected in nature. Simulations where radial symmetry is assumed for elliptical stems show substantial error in mean velocity (and thus flux) with modest eccentricities (e > 0.4).