Document Type : Complete scientific research article
Authors
1
Dept. of Soil Science and Engineering, Faculty of Water and Soil, University of Zabol, Zabol, Iran
2
Dept. of Soil Science and Engineering, University of Zabol, Zabol, Iran
3
Soil and Water Research Department, Golestan Agricultural and Natural Resources Research and Education Center, AREEO, Gorgan, Iran
4
Devision of Soil Formation, Classification and Survey Researches, Soil and Water Research Institute, Karaj, Iran
Abstract
Soil maps are one of the important needs for different land users and decision-makers. Saturation percentage is one of the easily-available physical parameters of soil moisture, Therefore, the present study was conducted with the aim of digital mapping the saturation percentage of soil in three dimensions and using geostatistical methods along with environmental variables in the Sistan Plain, which is located on the Hirmand River delta in a dry climate.
Materials and methods: To carry out this research, the information on 576 soil profiles located in the Sistan Plain was used. The msaturation percentage of soil was measured using the standard method at depths of 0-15, 15-30, 30-60, and 60-100 cm using the weighted average method. Several 35 environmental variables extracted from satellite images as remote sensing variables and 22 environmental variables extracted from the digital elevation model (DEM) were created as land variables, and the environmental variables that showed a significant correlation with the saturation percentage at each depth were included in the modeling process and geostatistical analyses. The inverse distance weighting method (in three states of first, second, and third powers), simple and ordinary kriging, and simple and ordinary cokriging were used as geostatistical analysis methods.
The results showed that the average value of soil saturation percentage at a depth of 60-100 cm had the highest amount (39.30%) and at a depth of 0-15 cm had the lowest average value (33.92%). The best variogram model for the saturation percentage at depths of 0-15, 15-30, and 30-60 cm was the spherical model and 60-100 cm was the exponential model, and the spatial fit for all the studied depths was in the medium spatial fit class. The results of the correlation between environmental variables and the saturation percentage of soil showed that the variables derived from remote sensing had a significant correlation only in the first and second depth, which were close to the surface, but the variables derived from DEM had a significant correlation in all studied depths. These variables were mainly related to fluvial and aeolian activities, which had the greatest effect on soil changes in the studied area. The results of the geostatistical estimators showed that for the first depth, the simple cokriging method with drainage covariate, for the second depth, the simple cokriging method with valley depth covariate, and for the third and fourth depths, ordinary cokriging with drainage covariate were the best and most accurate methods. The three-dimensional modeling of soil saturation percentage showed that the value of saturation percentage is the lowest in the south and medium values in the middle of the Sistan plain, and the highest values of saturation percentage are in the north of the plain at the edge of the Hamoun wetlands. From the surface to the depth the saturation percentage is repeated with the same spatial trend, but the average value of saturation percentage increases from the surface to the depth.
The saturation percentage of soil is an easily-available soil characteristic that should be given more attention for land management, especially in dry areas. Also, a three-dimensional view of soil and its mapping can give a more complete understanding to land users in the direction of development and planning. Geostatistical (cokriging) methods using auxiliary variables can be effective in preparing digital and three-dimensional maps of soil characteristics and can help different land users for better management. This issue depends on the finding that environmental variables are used that reflect the conditions of soil formation and factors affecting it in the study areas.
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