TEMPORAL STABILITY OF SPATIAL PATTERNS OF SOIL-WATER STORAGE IN A CULTIVATED VESUVIAN SOIL

Spatial and temporal variability of soil water storage were studied on a sandy soil in Ponticelli (NA), Italy. Aluminium tubes were installed in a plot (0.3 ha and cultivated with barley) at a depth of 150 cm. A neutron probe moisture meter was used to measure the water content at intervals of 15 cm along the vertical soil profiles. The spatial dependence of the soil water storage along a 50 m transect was examined by variograms at different times. These experimental variograms were found to be similar and a combined variogram could be used. On the basis of the spatial correlation a grid of 20 sites were selected with sufficient spatial separation to be uncorrelated. Measurements were performed from November 1986 to April 1987 at six different dates. The temporal stability of spatial patterns of soil water storage was tested in two ways: (1) by ranking the storage values for a given time and comparing the ranks of the sites; (2) by analyzing the rank correlations between storage values at different times. Moreover, contour maps of soil water storage were drawn by punctual kriging. It has been demonstrated that some measuring locations show extreme values of water storage continuously. However, it was not possible to identify a limited number of locations representative of the average field water storage, due to the extreme homogeneity of the soil. Therefore this method offers better possibilities with more heterogeneous soils. Correlation analysis showed only a moderate loss of information at rank-order level during the growing season.