Volume estimation from probability distribution models

Abstract

Two Statistical distribution functions; 3parameter Weibull and 3parameter Dagum were compared to evaluate the most suitable for fitting the  diameter distribution and subsequently estimating the volume of Tectona grandis Plantation at Omo Forest Reserve, Ogun State, Nigeria. Fifteen  temporary sample plots of 25m by 25m were randomly established and complete enumeration of the diameter at breast height over back and total height was carried out. The diameter distributions were then processed into 1cm diameter classes. A 3parameter Weibull function with α, β and γ (representing shape, scale and location respectively) and 3parameter Dagum with k, α (shape) and β (scale) were fitted into the size-class dataset using maximum likelihood method. Weibull recorded smaller mean goodness-of-fit values of 0.9962, 0.0882 and 0.5565 for Kolmogorov-Smirnov, Cramer Von Misser and Anderson Darling statistics respectively hence its preference over Dagum. Stand volume estimation was done using Weibull distribution which estimated 7580.977m3/ha (predicted) as against 8010.146m3/ha (observed). This therefore indicates a shortfall of 429.169m3/ha. Overall, 3Parameter Weibull is suitable volume estimation and fitting diameter distribution of the Tectona grandis plantation at Omo forest Reserve.

Keyword: Weibull, Dagum, Tectona grandis, Volume estimation