ON THE SPATIAL VARIABILITY OF PARAMETERS OF THE UNSATURATED HYDRAULICCONDUCTIVITY

In situ measurements of discharge-head (steady state rate of water dis charge and water head (Q, H)) pairs were used to estimate the saturate d conductivity K-s and the parameter alpha of the Gardner [1958] model of local unsaturated hydraulic conductivity at N = 152 observation po ints in a field plot (40 m long, 6 m wide, and 2 m deep) by means of t he Geulph permeameter method. Twenty-two of these (Q, H) pairs yielded negative K-s and were excluded from the subsequent analyses. Estimate s of In K-s and In alpha from N = 130 observation points were used to estimate parameters of the spatial covariance and the drift functions of the two soil properties, using the restricted maximum likelihood es timation procedure and the weighted least squares procedure, respectiv ely. For both soil properties a deterministic drift was identified, su ggesting that in the site under investigation, approximately 59% and 3 7% of the total variability off = In K-s and a = In alpha, respectivel y, may stem from large-scale variations. Results of the analyses of th e uncorrelated residuals of In K-s and In alpha suggest that both foll ow an approximately normal distribution and are moderately cross-corre lated (rho(fa) = 0.68). The fitted covariance models of both soil prop erties exhibited slight statistical anisotropy in the horizontal plane and a significant anisotropy in the vertical planes. Components of th e range of In K-s were 2.5 m and 0.6 m in the horizontal and the verti cal directions, respectively, while those of In alpha were 1.4 m and 0 .4 m, respectively. Estimates of the correlation scales of both proper ties in the horizontal plane were associated with relatively large unc ertainty, which probably stemmed from the less than optimum selection of the number and locations of the observation points in this plane. T he results of the model validation test, however, indicated that the f itted covariance and drift models were theoretically consistent with t he observations of both soil properties.