Large-scale modelling of forest hydrological processes and their long-term effect on water yield

A water balance model was used to simulate the long-term increases in water yield with forest age which are observed in the mountain ash (Eucalyptus regnans) forests of Victoria, Australia. Specifically, the hypothesis was tested that water yield changes could be explained by changes in evapotranspiration resulting from changes in leaf area index (LAI). A curve predicting changes in the total LAI of mountain ash forest was constructed from ground-based observations and their correlation with Landsat Thematic Mapper measurements of the transformed normalized difference vegetation index (TNDVI). A further curve for mountain ash canopy LAI was constructed from destructive LAI measurements and stem diameter measurements. The curves were incorporated within Macaque, a large-scale, physically based water balance model which was applied to three forested catchments (total area 145 km(2)). The model was used to evaluate the effect of changes in LAI on predicted stream flow over an 82-year period spanning the 1939 wildfires which burnt most of the area. The use of the LAI curves induced improvement in the predicted hydrographs relative to the case for constant LAI, but the change was not large enough to account for all of the difference in water yield between old-growth and regrowth forests. Of a number of possibilities, concomitant changes in leaf conductance with age were suggested as an additional control on stream flow. These were estimated using data on stand sapwood area per unit leaf area and coded into Macaque. The hydrograph predicted using both the LAI curves and a new leaf conductance versus age curve accurately predicted the observed long-term changes in water yield. We conclude that LAI is a partial control on long-term yield changes, but that another 'water use efficiency per unit LAI' control is also operative. Copyright (C) 1999 John Wiley & Sons, Ltd.