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Large Scale , Long Term , Physically Based Prediction of Water Yield in Forested Catchments

A water balance model was used to simulate the long term increases in water yield with forest age observed in Victoria’s Ash forests and characterised by the Kuczera curve. Specifically, the hypothesis was tested that water yield changes could be explained by changes in evapotranspiration (ET) resulting from changes in leaf area index (LAI). A curve predicting changes in the total LAI of Mountain Ash forest was constructed based on ground based LiCor plant canopy analyser (PCA) observations and their correlation with Landsat Thematic Mapper (TM) measurements of the transformed normalised difference vegetation index (TNDVI). A further curve for Mountain Ash canopy LAI was constructed, based on destructive LAI measurements and stem diameter measurements. The large scale, physically based water balance model Macaque was used to evaluate the effect of changes in LAI on predicted streamflow over an 82 year period spanning the extensive 1939 wildfires. The use of the LAI curves induced a positive change in the predicted hydrographs relative to the case for constant LAI, but the change was not large enough. Predicted water yield from older forests was not as high as that observed. Of a number of possibilities, concomitant changes in leaf conductance were suggested as an additional control on streamflow. These were estimated using data on stand sapwood area per unit leaf area and coded into the model. 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. Effectively, the Kuczera curve was able to be reproduced. 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, most likely associated with declining leaf conductance with forest age.