CORRELATION OF SPATIALLY VARIABLE SOIL WATER RETENTION FOR A SURFACE SOIL

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Abstract
We studied the variability of soil moisture retention in space over a wide range of suctions for an Olivier soil. Soil moisture at field capacity; saturation; and 0.005-, 0.01-, 0.03-, 0.1-, and 1.5-MPa suctions was measured on undisturbed soil cores, which were sampled at 30-cm spacing from the soil surface along an 80-m transect. The soil cores were further used for laboratory measurments of saturated hydraulic conductivity (K), bulk density, and particle-size fractions. As suctions increased, the correlation coefficients (CV) for the corresponding mositure contents increased. Lowest CVs were obtained for bulk density and the silt fractions, whereas the highest CV was for measured K along the transect. Semivariogram analysis indicated extensive spatial structure for soil moisture data sets at most suctions, with 50% of the sample variance attributed to spatial variation. However, lack of spatial structure, i.e., pure nugget effect, was obtained for moisture data sets at low suctions (0.005 and 0.01 MPa). This semivariogram finding was consistent with results based on autocorrelation analysis. Moroever, no clear patterns were observed for the range of spatial influence or length of correlation for soil moisture with increasing suctions. We concluded that the extent of spatial structure for soil moisture was not influenced by the degree of tension in the 0- to 1.5-MPa range. In addition, cross-correlogram results indicated that optimum correlations were obtained for field capacity and .theta. at 0.03 MPa and for field capacity and bulk density observations. Poorly defined ranges with significance but low correlations were obtained for field capacity and K and for field capacity and the silt fraction.

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