Geostatistical analysis of sorptivity for a soil under tilled and no-tilled conditions

The properties of soils, even in short distances differ from point to point. These spatial variabilities may be described by geostatistics. This method determines the spatial structure of a specified variable. In this study, we measured the sorptivity of soil in field under tilled and no-tilled conditions with different sampling distances by using a simple method and performed the geostatistical analysis on the obtained data. The measurements were done on the corners of a regular network having dimensions of 5 m × 5 m in a tilled and no-tilled soil, and 0.5 m × 0.5 m in the same tilled soil. The results revealed that sorptivity had spatial structure in a tilled soil with 0.5 m × 0.5 m grid distances and no spatial structure occured in 5 m × 5 m distances, so it was believed that sorptivity is a structural variable only for short distance. The range of variogram was between 3.09 and 3.42 m for the sorptivity with 0.5 m × 0.5 m grids. The spherical model was the best-fitted theoretical variogram. In addition to the simple method for sorptivity measurement, we measured the sorptivity using double rings, which is known as Talsma method. Statistical tests showed a significant and non-significant difference between the two methods in 5 m × 5 m and 0.5 m × 0.5 m grids, respectively. In the simple method for sorptivity measurement, a ring with 10 cm height, 10 cm diameter, and about 2 cm depth of water was applied, but the rings used in the Talsma method were about three to four times larger. Therefore, it is concluded that for short distances, the sorptivity of simple method is in accordance with that obtained in Talsma method. The kriging method was used to estimate the presumed unknown values of sorptivity in some locations in the 0.5 m × 0.5 m grid distances. Results revealed that kriging method is capable enough to estimate the sorptivity if the variable has a spatial structure.