Spatial and temporal variability of soil water content

Soil water content was monitored at 12 cm incremental depths from the surface to a depth of 60 cm on a 0.6 ha field plot (Alfisol) with a portable hand held time domain reflectometer. Variability in soil water content was generally lowest in the surface 0-12 cm which corresponds to the plough layer, apparently as a result of homogenizing effect of ploughing. The nugget variance of soil water content generally increased with depth of sampling with lowest (2.55%, vol.) obtained at the surface 0-12 cm and highest (36%, vol.) at 48-60 cm soil depth. The topsoil water content fitted to the linear model while other depths at all sampling dates was fated to the spherical model. The range of influence of spatial dependence of soil water content varied between 18 m at 24-36 cm depth and 27 m at 48-60 cm depth. The contour maps obtained with kriged estimate of soil water content showed more homogeneous soil water content on the surface compared with the subsurface layers. The temporal variability in soil water content with dates of sampling was found to be mainly conditioned by rainfall while the spatial variability was influenced by clay content and, subsequently, water holding characteristics of soil layers.