3.1 Soil erosion related properties:
The soil properties of Moridhal awatershed pertaining to the estimation of soil loss and erodibility indices are presented in Table 1 through statistical parameters viz. minimum, maximum, mean, median, standard deviation, skewness and kurtosis. Various box plots as shown in Fig. 4 display the dataset based on a five-number summary of minimum, maximum, median, first and third quartiles of the physiographic units. The textural properties of soils are described by parameters like total sand, very fine sand, silt and clay content. The total sand and very fine sand content of the studied soils ranged from 5.1 to 86.0 per cent and 2.2–39.2, respectively. The silt content of the soils of watershed varied from 8.0 to 61.0 per cent, while the clay content was in the range of 6.0-46.8 per cent. Among the different physiographic units, the alluvial plain soils showed the highest value of total sand (Mean 48.2 per cent) and very fine sand (Mean 19.6 per cent) content in the upper piedmont plain. The values of total sand were found to be lowest (Mean 41.5 per cent) in the flood plain, while the very fine sand was lowest (Mean 14.8 per cent) in the lower piedmont plain. However, the value of coefficient of variation with respect to total sand (47.14 per cent) and very fine sand (37.68 per cent) were highest in the flood plain. The lowest values of coefficient of variation of total sand (27.42 per cent) and very fine sand (20.32 per cent) were observed in upper piedmont plain and alluvial plain soils, respectively. The highest value of silt content (Mean 31.6 per cent) was found in flood plain and lowest in upper piedmont plain (Mean 27.9 per cent). The clay content was found to be highest (Mean 27.0 percent) in flood plain and the lowest value of clay content (Mean 21.3 per cent) was found in the alluvial plain. The coefficient of variation for silt (46.10 per cent) and clay (46.96 per cent) content was highest in the lower piedmont plain. The increasing trend of finer materials from upper piedmont plain towards flood plain could be attributed to the fact that these materials were washed down from the upper piedmont plain and, thereby, got deposited in the flood plain. The textural properties of studied soils varied from loamy sand to clay (Table 1). The coarser soil texture ranging from loamy sand to loam was observed in the upper piedmont plain and lower piedmont plain soils. The finer soil texture ranging from clay to clay loam was found in the alluvial plain and flood plain soils. Deka et al. [37] also reported finer texture in lower elevation areas and coarser texture in higher elevation areas in the soils of north bank plains of Assam. The structural properties of studied soils varied from subangular blocky to massive structure (Table 1). The flood plain and alluvial plain soils were mostly massive in structure which might be due to occasional water stagnation that inhibits aggregation. Being high in silt and clay content, the soils of the upper piedmont plain and lower piedmont plain had mostly single grained structure which may be ascribed to high coarse sand content in these soils. The soils of alluvial plain were dominated by subangular blocky structure. This is in accordance with the findings of Deka et al. [38] who also reported similar structures in the rice-growing alluvial soils of the Brahmaputra valley of Assam.
Table 1
Physiographic distribution of soil erosional properties in Moridhal watershed
|
Minimum
|
Maximum
|
Mean
|
Median
|
Std. Deviation
|
Skewness
|
Kurtosis
|
Physiographic unit: Upper piedmont plain
|
Sand (%)
|
24.20
|
82.20
|
47.37
|
47.15
|
12.99
|
0.86
|
1.84
|
Very fine sand (%)
|
12.31
|
39.21
|
25.21
|
23.61
|
7.23
|
0.48
|
-0.17
|
Silt (%)
|
9.60
|
42.65
|
27.87
|
26.45
|
8.55
|
-0.38
|
-0.07
|
Clay (%)
|
8.20
|
36.45
|
24.77
|
27.24
|
9.05
|
-0.42
|
-0.95
|
Organic matter (%)
|
0.55
|
2.10
|
1.31
|
1.27
|
0.41
|
0.05
|
-0.52
|
Hydraulic conductivity (cm hr− 1)
|
0.34
|
5.66
|
1.79
|
1.36
|
1.48
|
1.73
|
2.72
|
Water holding capacity (%)
|
21.04
|
47.00
|
29.14
|
26.58
|
6.91
|
1.03
|
0.75
|
Available water content (%)
|
4.00
|
22.45
|
8.94
|
7.40
|
5.42
|
1.18
|
0.59
|
Macroaggregate (%)
|
10.25
|
40.10
|
24.86
|
25.91
|
7.76
|
-0.24
|
-0.28
|
Microaggregate (%)
|
59.90
|
89.75
|
75.14
|
74.09
|
7.76
|
0.24
|
-0.28
|
Mean weight diameter (mm)
|
1.01
|
2.40
|
1.41
|
1.29
|
0.37
|
1.34
|
1.78
|
Physiographic unit: Lower piedmont plain
|
Sand (%)
|
5.20
|
82.11
|
47.25
|
51.13
|
21.60
|
-0.03
|
-0.93
|
Very fine sand (%)
|
2.24
|
26.60
|
14.76
|
15.36
|
5.02
|
-0.07
|
0.17
|
Silt (%)
|
9.50
|
60.00
|
29.21
|
28.07
|
13.47
|
0.31
|
-0.54
|
Clay (%)
|
7.70
|
46.80
|
23.53
|
20.17
|
11.05
|
0.33
|
-1.24
|
Organic matter (%)
|
0.56
|
2.82
|
1.72
|
1.72
|
0.54
|
0.05
|
-0.60
|
Hydraulic conductivity (cm hr− 1)
|
0.11
|
6.32
|
2.09
|
2.12
|
1.75
|
0.84
|
-0.06
|
Water holding capacity (%)
|
19.88
|
57.91
|
32.95
|
31.56
|
8.79
|
0.83
|
0.41
|
Available water content (%)
|
3.43
|
20.71
|
9.13
|
5.96
|
5.42
|
0.76
|
-0.91
|
Macroaggregate (%)
|
15.50
|
82.25
|
31.63
|
25.03
|
16.44
|
1.59
|
1.68
|
Microaggregate (%)
|
17.75
|
84.50
|
68.37
|
74.97
|
16.44
|
-1.59
|
1.68
|
Mean weight diameter (mm)
|
1.00
|
2.45
|
1.66
|
1.67
|
0.34
|
-0.02
|
-0.54
|
Physiographic unit: Alluvial plain
|
Sand (%)
|
27.40
|
76.30
|
48.16
|
50.11
|
13.29
|
0.00
|
-0.97
|
Very fine sand (%)
|
12.31
|
30.12
|
19.64
|
19.68
|
3.99
|
0.48
|
0.43
|
Silt (%)
|
14.30
|
43.45
|
30.52
|
30.20
|
7.04
|
-0.12
|
-0.26
|
Clay (%)
|
9.40
|
38.40
|
21.32
|
18.02
|
7.80
|
0.63
|
-0.76
|
Organic matter (%)
|
1.10
|
2.48
|
1.54
|
1.51
|
0.37
|
0.86
|
0.22
|
Hydraulic conductivity (cm hr− 1)
|
0.14
|
4.65
|
1.87
|
2.13
|
1.17
|
0.00
|
-0.17
|
Water holding capacity (%)
|
24.12
|
48.15
|
34.69
|
32.18
|
6.87
|
0.52
|
-0.72
|
Available water content (%)
|
1.01
|
17.55
|
8.17
|
7.13
|
3.91
|
0.57
|
-0.04
|
Macroaggregate (%)
|
21.50
|
73.28
|
46.41
|
35.99
|
18.55
|
0.17
|
-1.80
|
Microaggregate (%)
|
26.72
|
78.50
|
53.59
|
64.02
|
18.55
|
-0.17
|
-1.80
|
Mean weight diameter (mm)
|
1.10
|
2.68
|
1.81
|
1.73
|
0.37
|
0.46
|
0.16
|
Physiographic unit: Flood plain
|
Sand (%)
|
5.10
|
86.01
|
41.46
|
39.85
|
19.55
|
0.26
|
-0.56
|
Very fine sand (%)
|
2.33
|
35.22
|
14.93
|
14.54
|
5.62
|
0.62
|
1.09
|
Silt (%)
|
8.01
|
61.00
|
31.58
|
30.00
|
11.94
|
0.27
|
0.00
|
Clay (%)
|
5.98
|
44.20
|
26.96
|
27.00
|
10.90
|
-0.12
|
-1.18
|
Organic matter (%)
|
0.89
|
2.96
|
1.91
|
1.93
|
0.44
|
0.01
|
0.05
|
Hydraulic conductivity (cm hr− 1)
|
0.12
|
6.54
|
1.80
|
1.76
|
1.69
|
0.93
|
0.14
|
Water holding capacity (%)
|
23.12
|
63.12
|
40.40
|
41.12
|
9.92
|
0.28
|
-0.63
|
Available water content (%)
|
2.61
|
18.70
|
9.77
|
9.10
|
4.75
|
0.36
|
-1.19
|
Macroaggregate (%)
|
14.50
|
81.85
|
47.39
|
40.12
|
23.84
|
0.05
|
-1.68
|
Microaggregate (%)
|
18.15
|
85.50
|
52.61
|
59.88
|
23.84
|
-0.05
|
-1.68
|
Mean weight diameter (mm)
|
1.02
|
2.74
|
1.86
|
1.82
|
0.40
|
0.00
|
-0.41
|
Whole watershed
|
Sand (%)
|
5.10
|
86.01
|
44.85
|
46.73
|
18.86
|
0.09
|
-0.55
|
Very fine sand (%)
|
2.24
|
39.21
|
16.75
|
16.29
|
6.41
|
0.68
|
1.22
|
Silt (%)
|
8.01
|
61.00
|
30.31
|
29.26
|
11.49
|
0.26
|
0.04
|
Clay (%)
|
5.98
|
46.80
|
24.84
|
24.90
|
10.47
|
0.14
|
-1.18
|
Organic matter (%)
|
0.55
|
2.96
|
1.73
|
1.79
|
0.50
|
0.08
|
-0.39
|
Hydraulic conductivity (cm hr− 1)
|
0.11
|
6.54
|
1.89
|
2.01
|
1.61
|
0.92
|
0.29
|
Water holding capacity (%)
|
19.88
|
63.12
|
36.08
|
33.81
|
9.72
|
0.59
|
-0.28
|
Available water content (%)
|
1.01
|
22.45
|
9.25
|
7.18
|
4.91
|
0.64
|
-0.78
|
Macroaggregate (%)
|
10.25
|
82.25
|
40.09
|
32.00
|
21.49
|
0.63
|
-1.15
|
Microaggregate (%)
|
17.75
|
89.75
|
59.91
|
68.00
|
21.49
|
-0.63
|
-1.15
|
Mean weight diameter (mm)
|
1.00
|
2.74
|
1.74
|
1.71
|
0.40
|
0.18
|
-0.47
|
The organic matter content of studied soils varied from medium to high (5.50 to 29.60 g kg-1)
(Table 1) with a standard deviation of 4.96. Among the physiographic units, the highest (Mean 19.10 g kg-1) and lowest (Mean 13.08 g kg-1) amount of organic matter was found in flood plain and upper piedmont plain, respectively. The organic matter status of the soils in the flood plain was higher because of intensive cropping associated with the application of organic manures by the farmers during the cultivation of different crops in their field as well as transportation and leaching of bases and clay particles from upper elevation to lower elevation. Debnath et al. [39] also reported a higher amount of organic matter in rice-growing soils of the Terai zone of West Bengal where the farmers usually apply more organic manure.
The hydraulic conductivity of the studied soils varied from 0.11 to 6.54 hr -1 (Table 1). Among the physiographic units, the highest value of hydraulic conductivity was found in the lower piedmont plain (Mean 2.09 cm hr-1 1), which might be due to a higher amount of coarser material in the area. A significant negative correlation of hydraulic conductivity with clay content (r = -0.742**), mean weight diameter (r = -0.486**) might be due to dispersion and migration of finer particles into conducting pore. Dutta and Barkakoty [40] also obtained similar relationships in some soils of Assam.
The water holding capacity of the studied soils varied from 19.88 to 63.12 per cent (Table 1). The highest value of water holding capacity (Mean 40.40 per cent) was found in the flood plain. The upper piedmont plain soils exhibited lowest value of water holding capacity (Mean 29.14 per cent) which might be due to higher sand content. There was an increasing trend of water holding capacity from upper piedmont plain towards flood plain due to the dominance of finer material that provided sufficient capillary pores and maximum surface area to hold water. A significant positive correlation of water holding capacity was observed with silt (r = 0.441**), clay content (r = 0.491**), porosity (r = 0.368**) and organic matter (r = 0.381**). Similar relationships were also reported by Deka et al. [38] in the soils of the North Bank Plains of Assam, India.
The available water capacity of studied soils varied from 1.0 to 22.5 per cent (Table 1). Among the physiographic units, the highest value of available water capacity (Mean 9.8 per cent) was found in the flood plain which might be due to the dominance of finer material. In contrary, the lowest value of available water capacity (Mean 8.2 per cent) was found in the alluvial plain. The upper piedmont plain soils exhibited highest value of the coefficient of variation (60.62 per cent). A significant negative correlation of available water capacity was observed with total sand content (r = -0.707**), while a positive significant correlation of available water capacity was observed with clay content (r = 0.677**). Borgohain et al. [14] also reported a similar relationship of pabho watershed.
3.2 Estimation of soil loss factors: For estimating soil loss of the Moridhal watershed, the factors associated with the USLE model were first computed. The details about the estimation of the factors are described below:
3.2.1Rainfall erosivity factor (R): Rainfall data for the period from 1989–2019 was obtained from the Regional Meteorological Centre, North East India. The annual precipitation, maximum 24 hrs precipitation of two year and one hour maximum precipitation interval of two year in the studied area were found to be 306.41 cm, 10.40 cm, and 2.58 cm, respectively. The rainfall erosivity factor (R) for the entire watershed area was determined to be 922.67 mm (Table 2) and it was found to be a driving factor of soil erosion processes in the study area.
3.2.2 Soil erodibility factor (K): The range and mean K value for different physiographic units are given in (Table 2). The soil erodibility factor (K) of the studied soils varied from 0.01 to 0.12. The K value was found to be highest (Mean 0.081 t ha MJ− 1 mm− 1) in upper piedmont plain. The lowest K value (Mean 0.067 t ha MJ− 1 mm− 1) was found in Alluvial plain. This might be due to coarser nature of piedmont plain soils which contributed higher permeability of water and, thereby, lesser runoff. Similar findings were also reported by Deka et al. [8].
3.2.3 Topography factor (LS): The LS factor of the physiographic units (Table 2) varied based on the degree and length of slope. The lowest LS factor values of 0.58 was observed in the very gently to gently sloping soils of the flood plain areas. In contrary, the high sloppy lands of upper piedmont plain which was at higher elevation showed the highest LS value (3.34).
3.2.4 Cover and management factor (C): The C factor helps in determining the relative effectiveness of soil and crop management systems in reducing or preventing soil loss. The crop cover and management factor (C) values used for different types of land units were: paddy cultivation: 0.29 (for lower relief) and 0.40 (for higher relief), tea cultivation (0.01), waste land (0.50) and fallow land (0.69). These values were determined using information on land use/land cover and following the tables given by Obi Reddy et al. [9]. The C factor indicated the effect of conservation plans on soil loss and the possible distribution of soil loss potential with subsequent adoption of crop rotations or other management practices [41].
3.2.5 Conservation practice factor (P): The P factor reflects the effects of practices that would reduce the quantity and rate of runoff water and, thereby, could reduce the intensity of erosion. The P values were chosen based on field survey information. It was observed that the studied area had no any soil or water conservation measures, and hence the P values were assigned based on land utilization types. Under agricultural land the single crop land had poor bunding as compared to double crop and, hence, the ‘P’ value of 0.54 was assigned to single crop land and 0.30 to double crop land. For tea plantation ‘P’ value was assigned to be 0.70 as growing of plantation crop is a type of conservation practice. Fallow land and wasteland were assigned ‘P’ value as 1.00. Amongst the different physiographic units lower piedmont plain, alluvial plain and flood plain showed the highest P value (Mean 0.54), while the upper piedmont plain exhibited the lowest ‘P’ values (Mean 0.48).
Table 2
Range and mean of USLE factors and soil loss in different physiographic units of Moridhal watershed
Parameters
|
R
|
K
|
LS
|
C
|
P
|
Soil loss
(t ha− 1 yr− 1)
|
Physiographic unit: Upper piedmont plain
|
Range
|
-
|
0.03–0.12
|
2.83–4.06
|
0.38–0.50
|
0.30–0.54
|
19.94–67.77
|
Mean
|
922.67
|
0.081
|
3.34
|
0.40
|
0.48
|
46.78
|
SD
|
0.00
|
0.02
|
0.47
|
0.04
|
0.11
|
14.64
|
SE
|
0.00
|
0.01
|
0.11
|
0.01
|
0.02
|
3.36
|
CV (%)
|
0.00
|
28.19
|
14.03
|
8.66
|
22.77
|
31.29
|
Physiographic unit: Lower piedmont plain
|
Range
|
-
|
0.01–0.12
|
1.49–2.37
|
0.29–0.38
|
-
|
2.74–52.50
|
Mean
|
922.67
|
0.067
|
2.30
|
0.30
|
0.54
|
23.06
|
SD
|
-
|
0.04
|
0.23
|
0.03
|
0.00
|
13.06
|
SE
|
-
|
0.01
|
0.03
|
0.00
|
0.00
|
1.85
|
CV (%)
|
-
|
53.31
|
9.81
|
10.42
|
0.00
|
56.64
|
Physiographic unit: Alluvial plain
|
Range
|
-
|
0.01–0.12
|
0.75–1.10
|
0.29–0.38
|
-
|
0.78–18.60
|
Mean
|
922.67
|
0.064
|
0.97
|
0.29
|
0.54
|
9.01
|
SD
|
-
|
0.03
|
0.17
|
0.02
|
0.00
|
4.96
|
SE
|
-
|
0.01
|
0.03
|
0.00
|
0.00
|
0.97
|
CV (%)
|
-
|
53.17
|
17.99
|
6.01
|
0.00
|
55.09
|
Physiographic unit: Flood plain
|
Range
|
-
|
0.03–0.12
|
0.20–0.85
|
0.29–0.38
|
-
|
0.95–4.18
|
Mean
|
922.67
|
0.07
|
0.58
|
0.29
|
0.54
|
6.14
|
SD
|
-
|
0.04
|
0.26
|
0.01
|
0.00
|
4.34
|
SE
|
-
|
0.00
|
0.03
|
0.00
|
0.00
|
0.50
|
CV (%)
|
-
|
51.72
|
45.20
|
4.99
|
0.00
|
70.59
|
Whole watershed
|
Range
|
-
|
0.01–0.12
|
0.20–4.06
|
0.29–0.50
|
0.30–0.54
|
0.87–67.77
|
Mean
|
922.67
|
0.07
|
1.45
|
0.31
|
0.53
|
99.39
|
SD
|
-
|
0.04
|
1.03
|
0.04
|
0.04
|
16.00
|
SE
|
-
|
0.00
|
0.08
|
0.00
|
0.00
|
1.23
|
CV (%)
|
-
|
50.02
|
71.00
|
13.57
|
7.63
|
99.39
|
3.2.6 Estimation of soil loss
After estimating the different USLE factors (R, K, LS, C, and P), the total soil loss (A) was computed by multiplying all the factors. The soil loss in the studied area varied from 0.87 to 67.77 t ha-1 yr-1 with a mean value of 16.19 t ha-1 yr-1 (Table 2). Based on estimated soil loss values, the watershed area was grouped into different erosion classes [32] viz., very slight (< 5 t ha-1 yr-1), slight (5–10 t ha-1 yr-1), moderate (10–15 t ha-1 yr-1), moderately severe (15–20 t ha-1 yr-1), severe (20-40t ha-1 yr-1) and very severe (40–80 t ha-1 yr-1). Among the physiographic units, soil loss in the upper piedmont plain area was moderately severe to very severe with a value varying from 19.9–67.8 t ha-1 yr-1 (Mean 46.78 t ha-1 yr-1). The amount of soil eroded in lower piedmont plain areas varied from 2.7–52.5 t ha-1 yr-1 (Mean 23.06 t ha-1 yr-1) which is very slight to very severe erosion. Very slight to moderately severe soil erosion (Mean 9.03 t ha-1 yr-1) was found in the alluvial plain areas. Due to gentle slope flood plain areas exhibited very slight to moderate erosion and the soil loss varied from 0.87–12.79 t ha-1 yr-1 (Mean 6.14 t ha-1 yr-1). The soil loss map (Fig. 5) revealed that nearly 8.35 per cent (2,565 ha) area of the watershed is under very slight erosion hazard having soil loss less than 5 t ha-1 yr-1. About 30.15 per cent (9,265 ha) area of the watershed is under slight erosion 5–10 t ha-1 yr-1. Moderate erosion (15–20 t ha-1 yr-1) and moderately severe erosion (20–40 t ha-1 yr-1) covers 21.73 per cent (6,678 ha) and 7.54 per cent (2,318 ha) areas, respectively. Nearly 6.32 per cent (1,943 ha) area of the watershed suffers from very high erosion having soil loss between 40–80 t ha-1 yr-1. The studied soils exhibited a decreasing trend of soil loss from upper piedmont plain towards flood plain which might be due to washing away of finer material from upslope towards plain. A strong positive correlation of soil loss with relief as evident from the trend analysis curve (Fig. 6) also corroborates the findings. The positive correlation of soil loss with relief indicated that the increase in degree and length of slope resulted more soil loss [8].
3.3 Erodibility indices
Soil erodibility depends primarily on the physical characteristics of the soils viz., soil aggregates, nature and amount of organic matter content, and particle size distribution. These physical characteristics of soils are much affected by the land use system. To determine the erosional behavior of soils under different land use systems, various erodibility indices like water-stable aggregates, dispersion ratio, erosion ratio and erosion index were estimated by incorporating the data on basic soil properties in different empirical formulas. The results are presented in Table 3.
Table 3
Range and mean value of aggregate and erodibility indices in different physiographic units of Moridhal watershed
Physiographic unit: Upper piedmont plain
|
Parameters
|
MAG
|
MIG
|
MWD
|
CR
|
SCR
|
MCR
|
DR
|
ER
|
EI
|
(%)
|
(%)
|
(mm)
|
Range
|
10.3–40.1
|
59.9–89.8
|
1.0-2.4
|
1.74–11.20
|
0.60–3.39
|
1.67–9.52
|
0.13–0.57
|
0.02–0.34
|
0.07–0.57
|
Mean
|
24.9
|
75.1
|
1.41
|
3.88
|
1.32
|
3.61
|
0.31
|
0.13
|
0.21
|
SD
|
7.76
|
7.76
|
0.37
|
2.65
|
0.76
|
2.34
|
0.11
|
0.09
|
0.14
|
SE
|
1.78
|
1.78
|
0.08
|
0.61
|
0.18
|
0.54
|
0.03
|
0.02
|
0.03
|
CV (%)
|
31.22
|
10.33
|
26.04
|
68.31
|
57.98
|
64.67
|
35.79
|
73.43
|
64.04
|
Physiographic unit: Lower piedmont plain
|
Range
|
15.5–82.3
|
17.8–84.5
|
1.0-2.5
|
1.14–11.99
|
0.28–2.85
|
1.09–10.24
|
0.08–0.49
|
0.01–0.34
|
0.03–0.33
|
Mean
|
31.6
|
68.4
|
1.66
|
4.44
|
1.34
|
3.97
|
0.22
|
0.09
|
0.17
|
SD
|
16.44
|
16.44
|
0.34
|
2.85
|
0.52
|
2.41
|
0.10
|
0.07
|
0.07
|
SE
|
2.32
|
2.32
|
0.05
|
0.40
|
0.07
|
0.34
|
0.01
|
0.01
|
0.01
|
CV (%)
|
51.97
|
24.05
|
20.44
|
64.30
|
38.72
|
60.62
|
45.35
|
71.41
|
40.85
|
Physiographic unit: Alluvial plain
|
Range
|
21.4–73.3
|
26.7–78.5
|
1.1–2.7
|
1.26–15.76
|
0.27–2.76
|
1.19–12.44
|
0.06–0.26
|
0.01–0.14
|
0.05–0.27
|
Mean
|
46.4
|
53.6
|
1.81
|
3.98
|
1.53
|
3.58
|
0.15
|
0.06
|
0.12
|
SD
|
18.55
|
18.55
|
0.37
|
1.88
|
0.38
|
1.65
|
0.04
|
0.02
|
0.03
|
SE
|
3.64
|
3.64
|
0.07
|
0.37
|
0.08
|
0.32
|
0.01
|
0.00
|
0.01
|
CV (%)
|
39.97
|
34.61
|
20.56
|
43.64
|
25.07
|
41.60
|
32.94
|
45.91
|
25.52
|
Physiographic unit: Flood plain
|
Range
|
14.50-81.85
|
18.15–85.50
|
1.02–2.74
|
1.31–11.41
|
0.57–2.54
|
1.25–9.47
|
0.08–0.23
|
0.02–0.55
|
0.06–0.71
|
Mean
|
47.4
|
52.6
|
1.86
|
3.44
|
1.20
|
3.13
|
0.13
|
0.07
|
0.13
|
SD
|
23.84
|
23.84
|
0.40
|
2.79
|
0.53
|
2.27
|
0.05
|
0.06
|
0.08
|
SE
|
2.75
|
2.75
|
0.05
|
0.32
|
0.06
|
0.26
|
0.01
|
0.01
|
0.01
|
CV (%)
|
50.31
|
45.32
|
21.29
|
75.73
|
41.24
|
68.79
|
31.10
|
101.16
|
63.93
|
Whole watershed
|
Range
|
10.3–82.3
|
17.8–89.8
|
1.00-2.74
|
1.14–15.72
|
0.27–3.39
|
1.09–12.44
|
0.06–0.57
|
0.01–0.55
|
0.03–0.71
|
Mean
|
40.1
|
59.9
|
1.74
|
4.02
|
1.35
|
3.63
|
0.18
|
0.08
|
0.15
|
SD
|
21.49
|
21.49
|
0.40
|
2.67
|
0.54
|
2.24
|
0.09
|
0.07
|
0.09
|
SE
|
1.65
|
1.65
|
0.03
|
0.21
|
0.04
|
0.17
|
0.01
|
0.01
|
0.01
|
CV (%)
|
53.59
|
35.86
|
22.79
|
66.49
|
40.21
|
61.66
|
50.75
|
88.25
|
58.57
|
3.3.1 Aggregate status
3.3.1.1 Macroaggregate:
The water-stable aggregates (> 0.25 mm in diameter) of the studied soils varied from 10.3 to 82.3 per cent (Table 3). There was an increasing trend of macroaggregate from upper piedmont plain (Mean 24.9 per cent) to flood plain (Mean 47.4 per cent). The higher macroaggregate was found in the alluvial plain and flood plain soils which might be due to a higher amount of organic matter and clay content, which acted as binding agents for the formation of macroaggregate. Water-stable macroaggregation (which prevents crusting) and carbon content are important determining factors of soil erodibility [42]. This is supported by the existence of a significant positive correlation of macroaggregate with organic matter (r = 0.234**) and clay (r = 0.588**). The decreasing trend of macroaggregate towards upslope might be due to increase in total sand content as evident from the significant negative correlation of macroaggregate with total sand (r = -0.595**). Similar results were reported by Deka and Dutta [43] while assessing erodibility indices in the Northern Brahmaputra plains Assam, India.
3.3.1.2 Microaggregate
The microaggregate in the studied soils varied from 17.8 to 89.8 per cent (Table 3). Among the physiographic units, the highest value of microaggregate was recorded under upper piedmont plain (Mean 75.1 per cent) and lowest in flood plain (Mean 52.6 per cent) soils, while the highest value of coefficient of variation was observed in the flood plain (45.32 per cent). There was a decreasing trend of microaggregate from upper piedmont plain to flood plain. It might be due to a decrease in clay and organic matter content towards down slope. This is substantiated by the existence of a significant negative correlation with clay content (r = -0.588**) and organic matter (r = -0.234**). Borgohain et al. [14] also reported similar results while assessing the soils of the Pabho Watershed of Lakhimpur district of Assam, India.
3.3.1.3 Mean weight diameter
The mean weight diameter in the studied soils varied from 1.00 to 2.74 mm (Table 3). Among the physiographic units, the highest value of mean weight diameter was recorded under flood plain (Mean 1.86 mm) and the lowest in upper piedmont plain (Mean 1.41 mm). The increasing trend of mean weight diameter from upper piedmont plain to flood plain indicated better aggregation which might be brought about by clay and organic matter [43]. This is supported by the existence of a significant positive correlation of mean weight diameter with clay (r = 0.534**) and organic matter (r = 0.308**). Borgohain et al. [14] also reported similar results while assessing the soils of the Pabho Watershed of Lakhimpur district of Assam.
3.3.2 Dispersion ratio (DR)
As per the data presented in Table 3, the dispersion ratio of the studied soils varied from 0.06 to 0.57. According to the criterion of Middleton [20], soils having dispersion ratio > 0.15 are erodible in nature. Among the physiographic units, the highest value of dispersion ratio was recorded in upper piedmont plain (Mean 0.31) and the lowest value of dispersion ratio was recorded in alluvial plain (Mean 0.13) soils. Higher dispersion ratio value indicates more proneness to soil erosion. A significant negative correlation of dispersion ratio with total sand (r = -0.331**), organic matter (r = -0.337**), hydraulic conductivity (r = -0.383**) and a positive significant correlation with silt (r = 0.288**) and clay (r = 0.280**) content indicated that soils which were dominated by the finer particles exhibited more dispersion ratio [38]. The organic matter content was found to be correlated significantly and negatively (r = -0.337**) with the dispersion ratio which indicated that organic matter helped in binding of soil particles and, thereby, prevent dispersion [44].
3.3.3 Erosion ratio (ER)
The erosion ratio of the studied soils varied from 0.01 to 0.55 (Table 3). All the soils under the present study which had values of erosion ratio of above 0.10 could be considered as erodible as per the criteria of Middleton [20]. Among the physiographic units, the highest value of erosion ratio was recorded under upper piedmont plain (Mean 0.13). In contrary, the lowest value of erosion ratio (Mean 0.05) was recorded under alluvial plain. The erosion ratio exhibited a significant negative correlation with sand (r = -0.186*), organic matter (r = -0.234**) and hydraulic conductivity (r = -0.295**). Higher the hydraulic conductivity more will be the infiltration of water and, thereby, the erosion through runoff is reduced [44].
3.3.4 Erosion index (EI)
The erosion index of studied soils varied from 0.03 to 0.71 (Table 3). Among the physiographic units, the highest value of erosion index (Mean 0.21) was found in the soils of upper piedmont plain. The lowest value of erosion index (Mean 0.11) was found in the soils of alluvial plain. The erosion index showed significant positive correlation with total sand (r = 0.183*), fine sand (r = 0.249**), and relief (r = 0.426*). However, it exhibited significant negative correlation with clay (r = -0.288**) and organic matter (r = -0.168*). Similar results were also reported by Bhatia and Sharma [45] for some soils of Assam, India. It was observed that almost all the studied physico-chemical properties influenced the erodibility indices to a great extent. A strong positive correlation of the erodibility indices with relief supported the effect of elevation on soil properties as well as soil erosion. This is similar to the findings of Deka & Dutta [43] those who also obtained similar results in the soils of the north bank plain of Assam, India. Based on the calculated values of different erodibility indices the upper piedmont plain soils could be regarded as the most erodible soils of the studied watershed area.
dispersion ratio, erosion ratio, erosion index and soil loss exhibited positive loading in principal component 2 (P2). In contrary organic matter showed a negative loading in this component. All these erodiblity parameters were significantly correlated with each other and represented a set of related parameters with respect to soil erosion. Thus this second factor was termed as ‘Erosion Factor'. The factor loadings of soil parameters in P1 and P2 are depicted in Fig. 8 which revealed that the studied parameters were distributed in all the 4 quadrants. The first quadrant had parameters with positive loadings of both PCs; while the second quadrant had negative loadings on P1 and positive loadings on P2. The third quadrant had negative loadings of both PCs; and the fourth quadrant had positive loadings on P1 and negative loadings on P2.