USE OF A COVARIANCE VARIOGRAM TO INVESTIGATE INFLUENCE OF SUBSURFACE DRAINAGE ON SPATIAL VARIABILITY OF SOIL-WATER PROPERTIES

Knowledge of the spatial variability of soil-water properties is of pr imary importance for management of agricultural lands. This study was conducted to examine which spatial structure measure, the semi-variogr am or the covariance variogram, is appropriate for inference of the sp atial structure and performing interpolation of soil-water properties from sample data sets. Using the appropriate spatial structure measure , the spatial variability of these properties (saturated hydraulic con ductivity, water table depth, groundwater salinity, and soil salinity and sodicity) as affected by subsurface drainage is also evaluated. Th e soil-water properties were sampled before and after the installation of subsurface drainage on a regular square grid of 500 m at 61 locati ons within 1470 ha in the Nile Delta of Egypt. The results showed that the covariance variogram reveals the character of spatial structure a nd that it is more appropriate for interpolation than the semi-variogr am. Subsurface drainage has highly affected the spatial variability of soil-water properties. On average, the spatial correlation range incr eased by approximately 29%, whereas the ratio of structural heterogene ity to the total variation (relative structured heterogeneity) was dou bled 4 years after drainage installation. Moreover, the nugget effect increased and was present for all soil-water properties with noticeabl y high values. Uneven spatial distributions were also observed. Furthe r study of long-term spatial variation of soil-water properties as aff ected by subsurface drainage is suggested. (C) 1998 Elsevier Science B .V. All rights reserved.