Journal of the Indian Society of Soil Science

Soil samples (104) representing two regions in New Zealand (Canterbury in the South Island and Waikato in the North Island) differing in parent materials, physiography and climate were used. If a ‘few parameter’ approach is used to model water retention [e.g. Brooks-Corey or power-function model, Inψ = a + b In (θ/θ_s)], the correlations of model parameters with composition data are adequate for parameter estimation only. The reasons of this important general principle are discussed. Clay content of the soil is the most important parameter and is highly correlated with the exponent b (the ‘pore-size distribution index’) of the power function model. This variable alone accounts for 66% of the variation in the b parameter. Inclusion of bulk density of soil further improves the prediction. Correlation between the a-parameter (the intercept parameter in the power-function model) and composition parameters is weak, presumably reflecting the relatively narrow range of the values of the a parameter. The best practicable estimation for a is to assume that it is a constant, given by its mean value for a soil set (a = -1.0 in this study). However, this approximation results in large (> ± 3%) differences between observed and estimated θ_ν values for more than half of the soils.