Determining the Effective Factors on Soil Susceptibility to Surface Sealing Using Multivariate Statistical Analysis

Soil surface sealing is mostly influenced by the specific prevailing conditions during its formation, and the variables affecting soil surface sealing are the physical characteristics of the soil, the chemical conditions of the soil-water system, the characteristics of rain, and the initial and boundary conditions of the flow (Mualem et al, 1990). It is a common phenomenon in most soils of Iran due to arid and semi-arid climate, weak vegetation and loose soil aggregates. In Kowar plain, as an important Agricultural pole of Fars province, it is very important to determine the factors affecting the formation of surface sealing, to identify the most important characteristics of the soil in controlling the surface sealing formation, and to provide a suitable index to determine the sensitivity of the soil to this formation based on the characteristics of soil erosion, which are very important issues and have been addressed in this research study.

To carry out this research, 80 samples (0-20 cm) were selected from Kowar plain with a variety of physical and chemical characteristics, and after sampling, the samples were air dried and passed through a 2 mm sieve. Then, the parameters of particle size distribution, humidity, organic matter, electrical conductivity and pH of saturated soil extract were measured. In addition, the average weight and geometric mean diameter of soil aggregates, calcium carbonate equivalent, fractal dimensions of one-centimeter soil particles (Bartoli, et al, 1991), percentage of moisture saturation, sodium, calcium and magnesium were determined. The saturated hydraulic conductivity of the samples were measured and the evaluation of soil sensitivity to the surface sealing with seven different indices of SI, RSI, CI, WAS, PR (Permeability Resistance), CSI,   C5-C10 had been performed. Also multivariate statistical methods (cluster analysis, discriminant analysis and canonical correlation) were used for conducting the required analyses.

 After conducting the multivariate statistical method investigations, the results of cluster analysis led to the creation of a discriminant function with a chi-score of 12.73 reaching significance at the probability level of 0.001. This function completely showed the power (ability) to create separation between the two cluster analysis result groups. The organic matter was the most important primary variable that separated these two groups from each other and was significant at the probability level of 0.001. Based on the structure matrix, it was determined that CI, SI and CSI indexes with coefficients of 0.85, 0.58 and 0.30, respectively, are among the most important indices in the separation of these two groups. The performed canonical correlation provided a canonical function with a correlation of 0.99 and a chi score of 626.2. The highest canonical correlation belongs to primarily organic matter and then to clay. Two indices of Soil Stability Index (SI) and Crusting index (CI) were the most important indices having the highest correlation level among the indices. The most important variables influencing the evaluation indices of the sealing surface in the two group’s separation were organic matter and clay, which provided a function with high capability and the highest canonical correlation, respectively.

  The highest coefficient of variation is related to the mass of soil particles with a length of one centimeter (Bartoli model) and the highest canonical correlation in the first degree belongs to the organic matter and clay, it also shows the most important variables, with the highest correlation between these two variables with the mentioned indicators (CI,SI). In addition, two indices of SI and CI were the most important indices having the highest correlation among the indices. The results of cluster analysis, the discriminant function analysis and canonical correlation showed that the most important variables affecting the sealing surface evaluation indices in separating two groups and providing a function with high capability and the highest canonical correlation are organic matter and clay, respectively.  Also based on the structure matrix, the crusting index and aggregate stability are the most important indices that separate the two groups. In fact, the main samples differentiating factors in terms of the sealing surface are the above named indices. Organic matter and soil stability indexes have become significant with a very high correlation and showing a better justification of this indicator (SI) with the characteristics of the soil in determining the sensitivity of the soil to the seal structural formation layer.