Measurement and modelling of growing-season canopy water fluxes in a mature mixed deciduous forest stand, southern Ontario, Canada

During the growing-season of 1995 throughfall, stemflow, and canopy interception loss fluxes from a mature temperate deciduous stand in southern Ontario, Canada were observed to be 77.5±1.0, 3.7±1.3, and 18.8±3.8% of the 259.3 mm incident precipitation input, respectively. Simple empirical equations were derived for estimating the magnitude of each canopy water flux, while equations were also derived for estimating the spatial variability associated with throughfall and stemflow within this stand. The number of throughfall collectors required to sample this flux to within 5 and 10% of the mean at the 95% confidence interval was found to average 37 (range=17–81) and 10 (range=5–21), respectively for incident precipitation events >4 mm. For events <2 mm it was found that an unmanageable number of collectors would be required, on average, to adequately sample this flux. The Helvey and Patric [Water Resources Res. 1 (1965) 193] and the revised Gash [J. Hydrol. 190 (1997) 141] models were evaluated for further application within this forest-type. The Helvey and Patric model was found to systematically underestimate canopy interception loss for events >7 mm, while the revised Gash model was found to systematically overestimate this flux for events with corresponding rainfall intensities≥7 mm h−1. When optimised canopy storage capacities were applied to the revised Gash model the overestimate was reduced from 6.1 to 1.1 mm, or from 12.5 to 2.3% of the observed flux. The finding that the optimised canopy storage capacity value for storms with rainfall intensities≥7 mm h−1 is smaller than that derived for events with rainfall intensities <7 mm h−1 supports the Calder [J. Hydrol. 185 (1996) 363] hypothesis that the storage capacity of a stand is reduced when rain-drops and thus rainfall intensities are large. The need for increased research on the influence stand and meteorological factors have on this and other Gash model parameters is discussed.