Vertical variability of soil properties in a small watershed

Abstract

Coefficient of variation, spatial trends, and correlation structure of successive depth increments were determined for several soil properties along a 200 m line transect in a sloping field. Undisturbed soil cores were taken at 21 sites in nine depth increments (15 cm) to a 135 cm depth, and used for measurements of soil bulk density, texture, water retention at several different suctions, macroporosity, organic matter content, pH, and available phosphorus (P). For each depth interval, a trend was found in all of the soil properties horizontally along the line transect, which was described adequately by a second-degree polynomial. There was also a strong vertical trend in each soil property. This vertical trend was removed from the data by subtracting from them the values given by the polynomial fitted to the data for each depth interval along the line transect. The detrended data were then analyzed for correlation and semivariance between 21 values for one depth and corresponding values at depths separated by one, two, three, or four lags. These sets of correlations and semivariances provided a means to test the stationarity in their values for a given lag, at different depths in the soil. The results indicated a high degree of nonstationarity in the correlation coefficients and semivariances for all properties. However, for most properties several of the correlation coefficients were significant at small lags. The mean of the correlation coefficients decreased only slowly with increase in the number of lags, and the mean semivariance showed the opposite trend. The mean semivariograms were essentially linear within the distance of four lags (60 cm). These results enhance our understanding of the vertical spatial variability structure for applications in hydrologic modeling.