Revised Gash Model Research Articles

The influence of seasonal changes in canopy structure on interception loss: Application of the revised Gash model

Interception loss from a multi-species, deciduous, broad-leaved secondary forest in Japan was derived from gross rainfall, throughfall, and stemflow. The seasonal changes in the canopy structure were expressed using the leaf area index, which ranged from 1.65 to 4.31. In addition, the relationship between leaf area index (LAI) and relative light index (RLI) was examined using the Beer-Lambert equation. We established that RLI was associated with the canopy cover fraction. The relationships between throughfall, stemflow, and interception loss and gross rainfall were approximated by a linear regression equation for the growing and dormant seasons. Spatial variability in throughfall could be estimated from the relationship between the ratios of throughfall and RLI or LAI measured at many points, and had a high coefficient of variation. For stemflow, the regression coefficient and constant were strongly related to the diameter at breast height and were approximated by exponential equations. The differences in interception loss between seasons were small despite the distinct differences in LAI (the percentages of the total gross rainfall were 17.6 and 14.3% in the growing and dormant seasons, respectively). The revised Gash model was used to estimate interception loss more precisely by dividing the calculation period into (i) the entire study period, (ii) the growing and dormant seasons, and (iii) 3-month periods. The estimated interception loss was over- and underestimated by 7.8% for (i), 1.4% for (ii), and −2.2% for (iii). Regarding the parameters in revised Gash model, the canopy storage capacity varied with changes in LAI, and the evaporation rate was related not only to meteorological conditions, but also to the canopy structure in terms of aerodynamic resistance.

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.

An evaluation of the Gash interception model in a northern hardwood stand

During the growing season of 1995, canopy water fluxes were measured within a northern hardwood stand in southern Ontario, Canada. Observed canopy interception loss, throughfall, and stemflow fluxes from the stand were 19.3±3.5%, 76.4±2.9%, and 4.3±2.0% of incident precipitation, respectively. Both the original and revised Gash analytical rainfall interception loss models simulated these fluxes within the standard error of the observed estimates, suggesting that the analytical model may be appropriate for further applications within this forest type. The revised Gash model is recommended for further applications as it is better physically based. Both the original and revised models suggest that ∼60% of interception loss during the study period was evaporation from the canopy once rainfall has ceased while evaporation from the saturated canopy during rainfall accounted for ∼27%–33% of interception loss. Additional components of interception (e.g., evaporation from trunks) were computed to be minor contributors to total canopy interception loss.