Investigating Spatial Variability of Saturated Hydraulic Conductivity and the Factors Affecting It in Different Layers of Soil Profile of Paddy Fields

Spatial variability of soil hydraulic properties is important for soil-water balance and irrigation management in paddy fields. The objective of this study was to determine the spatial variability of saturated hydraulic conductivity (Ks) and the factors affecting it. Soil samples from 120 profiles were collected using random stratified sampling scheme. Soil samples were taken from top soil, compacted plow pan, and non-saturated subsoil. In these samples, soil texture, organic carbon, bulk density, saturated moisture, and saturated hydraulic conductivity were measured (by the falling head method). The most heterogeneity with a maximum coefficient of variation (CV), positive skewness, and lognormal frequency distribution for Ks indicated preferential flow caused by the presence of biological pores and cracks from previous drought stress. The results of the geostatistical analysis showed that hydraulic conductivity followed spherical and exponent semi-variogram model in puddled top soil and the non-saturated subsoil, respectively. This variable followed linear semi-variogram model with low slope in plow pan. The CV of Ks in plow pan (299%) was 121% and 284% higher than, respectively, the upper and the lower layers. The closeness of the behavior of Ks to random variables in this layer was mainly due to the existence of biological pores and root channels and the movement of animals, which lead to preferential flow. Despite the weak and moderate spatial structure of the variables, the kriging interpolation method was more accurate for Ks compared to inverse distance weighting. The spatial distribution pattern of Ks in puddled top layer was inversely related to clay content. Despite the high clay content in the west half of the study area, the Ks values in this region were high due to the presence of biopores. These results indicate that the compacted plow pan cannot play its key role for reducing hydraulic conductivity and retaining water in the soil. The magnitude and spatial variability of soil hydraulic conductivity and the factors affecting it depend on the site-specific irrigation management.