Spatial variability of near-saturated soil hydraulic properties in Moghan plain, North-Western Iran

The spatial variability of the hydraulic properties of near-saturated soil was investigated in Moghan plain, northwestern Iran. To include all types of cultivated crops and examine the effects of the distance, through nested grid design, 212 sites were selected, with a distance interval of 200, 400, and 800 m. Soil samples were collected from 0- to 20-cm depth for determination of selected soil chemical and physical properties in the laboratory. A tension infiltrometer was employed to consecutively measure the unsaturated infiltration at matric suctions (h (m)) of 2, 5, 10, and 15 cm. The infiltration data was modeled using Wooding's analytical method, and best-fit values of Gardner's parameters of saturated hydraulic conductivity (K (s)) and macroscopic capillary length (lambda (c)) were derived. The data was also modeled using numerical method in DISC software, and the van Genuchten parameters (theta (s), alpha, n, and K (s(DISC))) were optimized. The results of the study revealed that K (s(DISC)) had the highest coefficient of variation (CV), i.e., 212%, among the hydraulic parameters of the soil; shape parameter n, conversely, had the lowest CV. Once the means of the hydraulic parameters were compared, no significant differences were found in the hydraulic parameters among the cultivated crop types. To map the spatial variability of soil parameters by means of ordinary kriging, a spherical model, chosen based on mean error (ME) values and root-mean-square error (RMSE), was used. Based on the semi-variogram parameters, i.e., range, slope, and nugget to sill ratio, the spatial distribution of soil properties was not consistent in the studied area. The lowest and largest ranges of spatial dependency were 1021 and 4500 m for unsaturated hydraulic conductivities at matric suctions of 15 and 2 cm (K (15) and K (2)), respectively. The spatial dependencies of most variables under investigation were moderate to strong. Overall, the findings of this study put forward the view that the variability of soil hydraulic parameters might be controlled conjointly by variability in intrinsic soil properties, namely, particle size distribution, and bulk density, and several management practices in the plain have paramount importance. Policy makers and farm managers can effectively make use of the maps made in this study to manage their in site-specific irrigation practices.