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INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 1, No 7, 2011

© Copyright 2010 All rights reserved Integrated Publishing Association

Research article ISSN 0976 – 4402

Received on April 2011 Published on June 2011 1734

PedoGeomorphic analysis of soil loss in the lateritic region of Rampurhat I block of Birbhum district, West Bengal and Shikaripara block of Dumka

district, Jharkhand Sandipan Ghosh 1 , Tithi Maji 2

1 M.Phil. Scholar, Department of Geography, The University of Burdwan, Burdwan713104, StateWest Bengal, India (Email: [email protected])

2 Parttime Lecturer, Department of Geography, Maharajadhiraj Uday Chand Women’s College, Burdwan713104, StateWest Bengal, India

ABSTRACT

Declining productivity of lateritic soil of India indicates land degradation, which occurs mostly through soil erosion and physical or chemical deterioration of soil. In the tropical wet dry type of morphoclimatic region (western Birbhum and Eastern Jharkhand districts of India), the primary process of soil erosion usually takes place when falling raindrops beat the bare soil surface in heavy storm. Rainsplash erosion, sheet and interrill erosion (overland flow), rill and gully erosion are considered as major forms of water erosion. Due to high erodibility of lateritic soil, bare soil cover (deforestation), high erosivity of monsoonal rainfall, low clay, moisture and organic matter content of soil, the study area (border area between Rampurhat I block of Birbhum district, West Bengal and Shikaripara block of Dumka district, Jharkhand) is very much susceptible to rill and gully erosion. So the present article is an attempt to quantify the soil loss and sediment yield of catchments of sample gullies under different forms of water erosion.

Keywords: Laterites, erosivity, erodibility, rainsplash erosion, overland flow, gully erosion and multivariate analysis

1 Introduction

In the broadest sense generally acceptable to the geomorphologists, erosion is the progressive removal of soil or rock particles from the parentmass by a fluid agent (Strahler, 1964). In this discussion water erosion of lateritic uplands will be recognized as taking two basically different forms: (i) slope erosion and (ii) channel erosion. The first is the relatively uniform lowering of soil surface under the eroding force of overland flow or sheet flow which is more or less continuously spread over the ground and is not engaged in carving distinct channels into the surface. The second form of erosion consists of the cuttingaway of bed and banks of a clearly marked. Channel erosion takes place in and produces both the gullies and deep shoestring rills incised into previously smooth slopes. Total erosion of soil surface includes rainsplash erosion, sheet, rill and gully erosion. But these types of erosion are varied in spatial and temporal scales due to the uneven impact of many pedogeomorphic and morpho climatic factors. Therefore, quantitative estimation of soil loss will be more precise if we take into account those dominant or principal factors of fluvial erosion. This fact is emphasised here to get a concise view of soil erosion of lateritic uplands.

2. Objectives

Every field study of physical geography has one or numerous objectives to reach the pre defined goals. Here the major goal is to assess the pedogeomorphic significance and a

PedoGeomorphic analysis of soil loss in the lateritic region of Rampurhat I block of Birbhum district, West Bengal and Shikaripara block of Dumka district, Jharkhand

Sandipan Ghosh, Tithi Maji International Journal of Environmental Sciences Volume 1 No.7, 2011

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complete quantitative assessment of soil erosion on the lateritic uplands of western Rampurhat I block of Birbhum district. Therefore the objectives of this study are as follows:

1. In depth analysis of the development of lateritic soils in every corner of the study area,

2. Finding out the significances of climatic geomorphology in this particular area,

3. Multivariate analysis of geomorphic and hydrologic variables to find out a predictive equation of sediment yield,

4. Finding out the dominant factors of soil erosion and Estimating several forms of water erosion.

3. Materials and Method

It is worthwhile to consider the various methods and approaches to the soil erosion study of the study area. Pedogeomorphology is essentially a field study. The emphasis has, therefore, been laid, wherever possible, on field work. This study involves three principal processes observation, recording and interpretation. This study involves three principal processes observation, recording and interpretation. Drainage basins (nine 3rd order and eight 2nd order sample basins) and slope facets are taken as major geomorphic unit. In the prefield session topographical map (72 P/12/NE, 1979), geological map of Geological Survey of India, Climatic data of Indian Meteorological Department, district planning map of Birbhum (NATMO), satellite images (LANDSAT and IRS), SRTM data (2006) and numerous literatures are collected and a base map is prepared using G.I.S. The geomorphic data are composed from toposheet and SRTM data. Then climatic data, hydrological data and soil data are gathered from the soil laboratory analysis, websites (Indian Meteorological Department, 2010), NBSS (National Bureau of Soil Survey, India, 2005), books and research papers.

Gathering that field information several erosion and geomorphic maps are prepared. In the postfield session morphogenetic, hydrologic, climatic analysis and different statistical analysis are worked out to understand and to represent the accurate ground reality. The uses of computer technology have amazingly reduced the time, which is necessary for large calculations and data analysis in SPSS 14.0, Microsoft Excel 2003 and 2007. Then numerous empirical equations, estimation of soil loss and modeling of soil erosion are formulated. All the cartographic works, ranging from delineation of basin area to thematic mapping are done in MapInfo 9.0 and ArcGis 9.2. Processing of SRTM data, LANDSAT image, IRS image, digital elevation model (DEM) and subsetting of study area are finished in ERDAS 9.1 imagine software (.img file format).

4. Location of the study area

The selected region of erosional study (geographical area of 65.84 km 2 ) is situated in the marginal area of western Rampurhat I block of Birbhum district, West Bengal and western Shikaripara block of Dumka district, Jharkhand. This geomorphic region is associated with the eastern plateau fringe of Rajmahal Basalt trap. It is the lateritic interfluve portion of Brahmani (north) and Dwarka (south) rivers. The study area is located at 5 km west of Rampurhat railway station, near Baramasia busstop. The study area is located between



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24 0 10’ and 24 0 13’N and 87 0 39’ and 87 0 45’E (figure 1 and 4). The maximum and minimum altitudes are 86 metre and 36 metre from mean sea level respectively.

Figure 1: Location map of the eLateritic region of Rampurhat block of Birbhum district, West Bengal and Shikaripara block of Dumka district, Jharkand

5. Results

5.1 Major Physical Conditions

The region has some exclusive physical features and climatic conditions with respect to the eastern fringe of Chotanagpur Plateau of India. Among these the important physical conditions are as follows:

1. The study area is covered with Rajmahal basalt, china clay and mostly laterite. Hard massive basalt is of Jurassic to Cretaceous age, soft and medium hard laterite is of Cainozoic age and china clay is of Late Pleistocene to Early Eocene age (Geological Survey of India, 2001).

2. According to Pascoe (1973) laterites of Birbhum is marked as low level laterite which was broken off from the high level laterites of Rajmahal hills (Eastern parts) and then



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carried to the eastern lower level by the action of streams, rain wash, surface runoff and then redeposited in this area. In this wet and dry type of monsoonal climate and due to ground water fluctuations those materials become cemented again through segregative action of hydrated oxides (figure 2).

3. Local name of the soil is ‘kankara’ which is literally uricru and it is a reddish, loose and friable lateritic soil (low fertility) containing ferruginous concretion (Duricrust morum bed). Presence of laterites indicates the former existence of tropical wetdry periods (Tertiary) in this area.

4. The lowlevel laterites is further eroded or lowering of the surrounding unlateritized areas (or areas where the laterite is unindurated) be left standing above the adjacent country. In effect, the relief becomes inverted here (soupplate laterites, remained as domal uplands, figure 3).

5. The climate of the study area has wetdry, subhumid and subtropical climate with mean annual rainfall 1420 mm (average of 19301960). Mean summer air temperature (April, May and June) is 37.3°c and mean winter air temperature (December, January and February) is 26.4°c. The annual potential evapotranspiration (PET) vary from 1400 to 1600 mm.

6. The study area is a small portion of interfluvial lateritic tracts between Brahmani and Dwarka rivers. As it is a rolling highland, many small streams are bifurcated from this region, such as Chila Nadi, Kandor Nala etc. Except some gullies all the streams are east flowing.

7. We have found that in premonsoon the average water level is above 10 metre depth (April, 2006) and in postmonsoon the average water level is near about 3 metre depth (November, 2005).

8. The vegetations of the study area belong to the tropical dry type deciduous with few evergreens occurring here. Though once upon a time the whole study area was covered with forest of Sal but due to encroachment of Stone crushers and Agriculture there is only few glimpse of forest. In the drier parts of highland the characteristics shrubs and grass field are found.

9. Morphogenetic region ModerateSelva (Peltier, 1950) and Tropical WetDry Savanna (Chorley, Schumm and Sugden, 1984).

10. Major Pedogeomorphic processes: Modmax chemical weathering, moderate physical weathering, modmax mass wasting, modmax fluvial processes (sheet wash, rainsplash, rill and gully erosion), laterisation.

11. Morphological features: Rolling lateritic uplands, wide planation surface (< 3.5 0 ), deep red weathering zone, uricrust of Feoxides, badlands.



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Figure 2: Profile of lateritic soil in the study area

Figure 3: Locus of indurated lateritic crust (soupplate laterites) in surrounding region of Aerodrome (based on field visits, SRTM data, and Global Mapper 11.1)



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Figure 4: IRS 1D LISS III FCC image (2001) of the study area (greenishblue patches are

gully prone lateritic land).

Figure 5: DEM showing the ruggedness of topography and steam network of the study area

5.2 Erosion is a Function of the Erosivity and the Erodibility

The fundamental cause of soil erosion is that rain acts upon the soil, and the study of erosion can be divided into how it will vary for different conditions of soil. The amount of erosion is therefore going to depend upon a combination of the power of the rain to cause erosion and the ability of the soil to withstand the rain (Hudson, 1984). In mathematical terms Erosion is a function of the Erosivity (of the rain) and the Erodibility (of the soil), or Erosion=f (Erosivity) × (Erodibility) (figure 6).



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Figure 6: The factors which affect rainfall erosion (after Hudson, 1984)

5.3 Rainfall Erosivity

Erosivity can be defined as the potential ability of the rain to cause erosion and for given soil conditions one storm can be compared quantitatively with another and a numerical scale of values of erosivity can be created. Soil loss is closely related to rainfall partly through the detaching power of rain drops striking the soil surface and partly through the contribution of rain to surface runoff. The erosivity of a rainstorm is a function of its intensity and duration and of the mass, diameter and velocity of the raindrops.

Two important things are found here

1. It has been reported that typical heavy monsoon rainfall intensity is around 23.51 25.51 mm/hour in this area (Water Resource and its Quality in West Bengal, A State of Environmental Report, WBPCB, 2009) and

2. Median drop diameter of monsoon rainfall is almost 2.2 mm with terminal velocity of almost 6 metre/second (N. Hudson, 1965).

According to Morgan, Morgan and Finney (1984), rainfall energy (J m 2 ) of this area is estimated using the annual rainfall (1437 mm) and typical rainfall intensity (25.51 mm). The equation is as follows (figure 7):

E (J m 2 ) = R (11.9+8.7 log10I)

where, R =annual rainfall (mm) and I =intensity of erosive rain (mm h 1 ).



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Table 1.Rainfall erosivity measurement at Rampurhat (23 o 15’N, 88 o 35’E) based on records of heaviest annual rainfall (Irrigation and Waterways Department, Govt. of West Bengal,

2010)

Year Annual Rainfall (mm)

E (Joule m 2 )

p 2 /P*

1965 1420.4 34286 65.57 1975 1251 30191 77.31 2000 2015 48639 430.15 2001 1338 32297 82.37 2002 1522 36739 66.02 2003 1183 28656 57.14 2004 1472 35532 62.37 2005 1264 30511 163.06 2006 1607 38790 200.05 2007 1712.4 41335 150.58 2008 1399.3 33777 112.09 2009 1240.2 29936 74.52 2010 1437.2 34692 69.48

*p 2 /P= Fournier (1960) index of concentration of rainfall (p 2 /P) is a measure of intensity of rainfall, where p= rainfall in the month with greatest precipitation (mm) and P= annual rainfall (mm).

y = 42.883x + 29709 R 2 = 0.6386

0

10000

20000

30000

40000

50000

60000

0 50 100 150 200 250 300 350 400 450 500

p 2 /P

Rainfall E

nergy (J m 2 )

Figure 7:With increasing concentration ratio of rainfall the rainfall energy is also increased

5.4 Soil Erodibility

The erodibility of a soil is its vulnerability or susceptibility to erosion that is the reciprocal of its resistance to erosion. A soil with a high erodibility will suffer more erosion than a soil with low erodibility if both are exposed to the same rainfall. Erodibility varies with these variables soil texture, aggregate stability, shear strength, infiltration capacity, organic carbon, chemical content etc. It was shown the large particles (coarse sand and gravel) are resistant to transport because of the greater force required to entrain them and that fine particles (clay)



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are resistant to detachment because of their cohesiveness. The least resistant particles are silts and fine sands. Thus soils with high silt content are erodible (Morgan, 1986).

NBSS (2005) measures erodibility factor (K) for West Bengal by the following equation: K=1.2917[2.1x10 4 M 1.14 (12a)+3.25(b2)+2.5(c3)]/100

Where M= %silt (100 %clay); a= %organic matter; b= the soil structure code and c= the profile permeability code. Low value indicates high erodibility of soil.

Table 2: Soil structure code and permeability code (after Wischmeier, Johnson and Cross, 1971)

Soil structure (b) Soil Permeability (c) Very fine granular 1 very slow 6 Fine granular 2 slow 5 Coarse granular 3 slow to moderate 4 Blocky, platy or massive 4 moderate 3

moderate to rapid 2 rapid 1

Table 3.Calculation of erodibility factor of collected field soil samples

Soil Sample

% Sand

% Silt

% Clay

% Organic matter b c K

1 49.8 27.6 22.6 0.25 4 3 0.28 2 65.3 24.6 10.1 0.60 4 2 0.31 3 64.0 22.4 13.6 0.68 4 2 0.22 4 50.1 27.3 22.6 0.25 3 3 0.22 5 52.6 28.3 19.1 0.21 4 2 0.26 6 70.2 19.1 10.7 0.57 3 3 0.19 7 48.3 22.6 29.1 1.60 3 4 0.20 8 49.1 28.3 22.3 1.30 4 3 0.27

Source: soil laboratory analysis by author.

5.5 Rainsplash Erosion

Farmer (1973) show that it is the medium and coarse particles that are most easily detached from the soil mass and that clay particle resist detachment. This may be because the raindrop energy has to overcome the adhesive or chemical bonding forces by which the minerals comprising clay particles are linked.

Experimental and theoretical studies show that the rate of detachment of soil particles by rain splash varies with the 1.0 power of the instantaneous kinetic energy of the rain (Free, 1960; Quansah, 198) or with the square of the instantaneous rainfall intensity (Carson and Krikby, 1972; Meyer, 1981).

Morgan, Morgan and Finney (1984) developed an equation to measure rate of rain splash detachment (kg m 2 ):

F (kg m 2 ) = K (E e aP ) b .10 3 Where E (J m 2 ) = R (11.9+8.7log10I)

R=Annual rainfall (mm),



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I= intensity of erosive rain (mm/hr), K= index of soil detachability, P= percentage of rainfall contributing to permanent interception and stem flow, a= 0.05 and b= 1.0

Table 4: Prediction of soil loss (kg m 2 ) from sample sites due to splash erosion

Soil sample

Soil texture

Surface cover K P F (kg m 2 )

1 clay loam bare soil 0.4 0 13.87

2 sandy loam

savanna grass 0.3 25 1.41

3 sandy clay loam


4 sandy clay loam


5 sandy loam


6 sandy loam


7 clay loam

bare soil and grass 0.4 15 3.09

8 clay loam bare soil 0.4 0 13.87

Note: E=34691.7 J m 2 for the year 2010; the values of K and P are used by Morgan, Morgan

and Finney (1982) and soil loss estimated by author.

5.6 Overland Flow

According to the source from which the flow is derived, runoff may consist of surface runoff, subsurface runoff and ground water runoff. The ‘surface runoff’ is that part of the runoff which travels over the ground surface and through channels to reach the basin outlets. The part of the surface runoff that flows over the land surface toward stream channels is called ‘overland flow’. Horton (1945) describes overland flow as covering twothird or more of the hillsides in a drainage basin during the peak period of a storm. Morgan (1986) suggests that except vegetative area, in bare soil overland flow occurs and is of Hortonian type. Overland flow particularly that of the Hortonian type, acts with the detaching power of raindrops to erode soil particles and transfer them downslope.

The annual volume of overland flow is predicted from the annual rainfall using an equation presented by Carson and Krikby (1972). This assumes that runoff occurs when the daily rainfall total exceeds a critical value which represents the soil moisture storage capacity (Rc) of the soilland use combination.

Q (mm) = R exp (Rc /Ro) Where Rc =1000 MS.BD.RD (Et/Eo) 0.5 MS=soil moisture content at field capacity (%),



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BD=bulk density of the top of soil layer (Mg m 3 ), RD= top soil rooting depth (m) as the depth of soil from the surface to an impermeable or stony layer (usually 0.05 m for grass), Ro =R/Rn Rn= number of rain days in the year and R= Annual Rainfall in mm.

The transport capacity of overland flow is determined from an equation developed by Krikby (1976) and depends on the volume of overland flow, the slope steepness and the effect of crop cover. The equation is:

G= C Q d sin S.10 3 Where C= crop cover management factor,

Q= volume of overland flow (mm), and S= steepness of the ground slope (degree).

Table 5: Estimated values of volume and transport capacity of overland flow of the sample sites

Sample site Rc Ro E (J m 2 ) S C Q

(mm) G (kg/ m 2 )

1 5.81377 2 o 40' 1 913.41 38.81 2 16.37461 4°30' 0.1 401.04 1.26 3 16.37461 4°00' 0.01 401.04 1.12 4 24.25118 2°10' 0.1 217.06 17.34 5 16.37461 3°10' 0.1 401.04 0.89 6 16.37461 2°17' 0.1 401.04 0.64 7 24.25118 2°30' 0.01 217.06 0.02 8 5.81377

12.83035 34691.7

3°21' 1 913.41 48.75 Note: Rn= 112 days and R=1437 mm for year 2010 Source: Irrigation and Waterways Department, Govt. of West Bengal, 2010 and computed by author.

Surface runoff follows a system of downslope flow paths from the drainage divide (basin perimeter) to the nearest channel. This flow net, comprising a family of orthogonal curves with respect to the topographic contours, locally converges or diverges from parallelism, depending upon position in the basin. Horton defined ‘length of overland flow’ Lg as the length of flow path, projected to the horizontal of nonchannel flow from a point on the drainage divide to a point on the adjacent stream channel. During evolution of the drainage system, Lg is adjusted to a magnitude appropriate to the scale of the firstorder drainage basins and is approximately equal to onehalf the reciprocal of the drainage density (Strahler, 1964):

Lg =1/2Dd The spatial analysis of the distribution of Lg reveals the fact that the basins of all orders are more youth in the cyclic stage because these basins are characterized by lower values of Lg (0.050.27 km) in respect of drainage densities (table 10). These show that the orthogonal distances between drainage divides and the tip of gullies are very small. Again small values denote the low magnitude of sheet erosion in this area.

5.7 Gully Erosion



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Rills are preceded by small undulations formed on the surface of the ground by the impact of raindrops during hard rains. As the water continues to concentrate and acquires additional energy for scouring, these grooves become deeper and broader and eventually some of them develop into gullies. It is the severe form of soil erosion in the present decade. Actually, the splashing raindrops may keep the shallow layer of surface water even more highly changed then the gully flow, if the soil is bare and highly detachable (Stallings, 1976). R. P. C. Morgan said that the main cause of gully formation is too much water, a condition which may be brought about by either climatic change or alterations in land use. If the velocity or tractive force of the runoff exceeds a critical or threshold value, gullying will occur (Schumm, 1979).

Figure 8: Views of Gullies at the west of Bhatina village, Rampurhat I block

R. J. Blong’s (1982) method has been employed here to estimate the contribution of sidewall erosion and channel erosion to the total gully erosion. The area defined by the active width of the channel has been assumed to have been eroded by channel i.e. linear erosion and it is presumed that the remaining area represents the areas removed by sidewall erosion. In order to calculate the volume of eroded area, the two cross sections of a single gully are taken with the help of G.P.S. (measuring elevation and location) in upstream and downstream section respectively (figure 8 and 9). The contribution of left and right bank and channel erosion as well as the total volume of the sediment yield between two cross sections was calculated by using the following equation:

Tv = Au+Ad . Lab 2

where Tv= volume in cubic metre Au= crosssectional area of the upstream section in m 2 , Ad= Crosssectional area of the downstream section in m 2 and Lab= distance between Au and Ad in metre.

Table 6: Estimating sediment yield of sample gullies from field database



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Sample Gullies

cross section

width (m)

depth (m)

area (m 2 )

Lab (m)

Volume of

sediment yield (m 3 )

upstream 120 9 1080 Gully 1 downstream 126 7 882 287 1083.073

upstream 18.17 4.65 84.5 Gully 2 downstream 16.5 4 66 178 84.870

upstream 62.5 5.5 312.5 Gully 3 downstream 101.5 10 1015 337 315.512

Figure 9: Upstream and downstream crossprofiles of selected gullies

5.8 Multivariate analysis

Application of the principles of statistics in quantitative geomorphology is essential if meaningful significant conclusions are to be achieved. Statistical method is concerned with the making of inferences from a small sample about the characteristics of a vast population whose absolute parameters can never be known. Here an attempt is made to analyse and establish the relationships between the dependents and the independent variables and to indentify the major morphometric parameters which have a significant role in the erosional landforms and sediment yields of the drainage basins. Since the morphometric and hydrologic variables do not work in isolation but as closely interlinked phenomena, a multivariate analysis seems to be quite necessary to find out the relative importance of each variable. The ‘Principal Component Analysis’ provides the basis of sorting out a number of few components which account for the major amount of explained variation of the variables. Rests of the components are of negligible importance. Again the importance of the variables



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in order of their ranking can be worked out statistically through PCA. Here we have selected nine morphometric variables which have good contribution in erosional intensity and landform development. Based on data of 17 subbasins of study area we can infer the significant factors of the development of erosional landforms among the others. It is cleared from the correlation matrix how far the types of relations are established in this case. It has been found that with 40.72% of explanation (PC 1) most of the variables are positive and they have significant influence in the topographic appearance of the basins. But constant of channel maintenance is the chief dominant variables operating here. Also basin relief, average length of overland flow, ruggedness number and lemniscate ratio are other next dominant variables (77.52%).

Table 7: Extraction of Principal Components with Cumulative percentages of Variance

Variables Dd Lg C Rn Sg LR Rb H QR PC 1

(40.72%)

0.92041 0.722108 0.940898 0.59062 0.60843 0.536588 0.23142 0.072827 0.587926

PC 2 (65.66%) 0.14746 0.15841 0.11661 0.701424 0.00895 0.701567

0.12867 0.905318 0.614745

PC 3 (77.52%)

0.05272 0.485395 0.174613 0.035082 0.351342 0.04475

0.76144 0.182124 0.24131

*Eigen values 3.66 (PC 1), 2.24 (PC 2) and 1.06 (PC 3) **Dd =Drainage Density, Rn =Ruggedness Number, H= Basin Relief, C =Constant of Channel Maintenance, Lg =Avg. Length of overland flow, Sg =Mean ground slope, LR= Lemniscate Ratio and QR= Potential volume of runoff (Q=C.I.A)

Here we have considered the geological erosion which is the rate at which the land would normally be eroded without disturbance by human activity. Any single measurement of erosion rates is affected by numerous variables, of which rock type, climate, vegetation and drainagebasin characteristics (such as area, steepness of slope, drainage density, relief and length of slope) have been found to be the most important (Chow, 1964).

Table 8: Correlation matrix of interrelated variables of soil erosion

Pearson Correlation Matrix

Sediment Yield Dd Lg

Runoff volume in

m 3 /s R/L

Sediment Yield 1 0.14745 0.428562097 0.090814277 0.385085

Dd 0.14745 1 0.678495576

0.427745104 0.552851

Lg 0.428562 0.6785 1 0.206728416 0.22342 Runoff

volume in m 3 /s

0.090814 0.42775 0.206728416 1 0.47623

R/L 0.385085 0.552851 0.223422305

0.476225593 1

To estimate the sediment yield of gully basins we have employed the Fournier index. Fournier (1960) used the following empirical equation to predict sediment yield of sample basins from the knowledge of relief and climate:



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Log E= 2.65 log (p 2 /P) +0.46 log H.tan Ø 1.56 where, E= suspended sediment yield (tons/sq. km/ per annum); p= rainfall in the month with greatest precipitation (mm); P= mean annual precipitation (mm); H=mean height of basin (metre)*; Ø= mean slope (degree) in a basin**; and p 2 /P is used as a To take into account of basin shape, length of overland flow, runoff volume and relief/length ratio (R/L) we have formulated a multiple regressional equation in the sediment yield estimation of small 2 nd and 3 rd order basins of the study area (analysis done for the year of 2010). Table 9: Predictive equation of sediment yield and important factors for soil erosion in the

study area

standard coefficient Equation a b c d

SS= 2.87 0.08 Dd +0.46 Lg +0.27 RV +0.66 R/L

2.87 0.08 0.46 0.66

where SS= suspended sediment yield (tons/km 2 /annum), Dd=drainage density (km/km 2 ), Lg=average length of overland flow (km), RV =runoff volume of sample basin (m 3 /s) and R/L=relief/length ratio of basin (metre/km)

Table 10. Estimation of potential sediment yield of basins (gullies) of the study area

Sample basins

Sediment Yield (tons/km 2 /annum) Dd Lg RV R/L

3a 3.09 4.76 0.11 24.67 13.0625 3b 3.08 3.42 0.14 25.11 12.94117647 3c 3.08 2.59 0.19 86.15 12.65384615 3d 3.08 2.09 0.24 23.05 13.63636364 3e 3.07 3.69 0.13 20.35 10 3f 3.11 6.17 0.08 6.94 32.22222222 3g 2.92 4.36 0.11 11.05 10 3h 3.08 3.32 0.15 14.48 13.75 3i 3.09 4.65 0.11 48.42 12.96296296 2a 3.08 1.8 0.27 26.64 8.076923077 2b 3.07 2.42 0.21 16.57 15 2c 3.10 3.67 0.22 3.84 26 2d 3.10 7.17 0.13 4.55 26.25 2e 2.92 2.67 0.07 22.12 12.66666667 2f 3.10 5.31 0.17 6.04 21 2g 3.12 5.27 0.09 7.63 24.16666667 2h 2.92 8.86 0.05 7.27 16.92307692

5.9 Conclusion



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Lowlevel laterites are notified as one of very much erosion prone soil in India because this soil has high erodible kaolinitic clay (B horizon), surface crusting of ironoxides, light textured, low moisture retention capacity and less vegetative growth. The major findings of research are as follows

1. These laterites are of multicyclic in nature because the plant fossils are found at different depth at the base of gully floors around the aerodrome.

2. Short period of heavy rainfall finds their ways on bare surfaces which are weakened due to lose cohesiveness. Once the top crust or morum beds are removed, a network of rills and gullies are generated in the resultant slope directions.

3. On bare lateritic soil and thin grass cover Horton overland flow, saturated overland flow, tunneling, roof collapsing, slumping, head cut, coalescence of rills, micro piracy, cross grading and meandering are the chief processes of rill and gully development.

4. In this region V shape (youth) and U shape (mature) gullies are observed. Large mature gullies are formed in the western parts of aerodrome. The average depth of large gullies ranges between 38 metre and width ranges from 565 metre.

5. Maximum basins exhibit low values of constant of channel maintenance (<0.4) which means these basins are most erodible, because to maintain one km of channel length there is a low requirement of drainage area.

6. Low value of average Lg (0.050.17 km) signifies that there is a high occurrence of channel erosion (high Dd) than sheet erosion.

7. Using the Morgan, Morgan and Finney Method, it is estimated that mean annual soil loss ranges from 0.0213.87 kg/m 2 /year in the eight sample sites and

8. In addition less vegetative cover, less conservation practice, grazing and illegal morum quarrying enhances the gully extension and more soil loss.

Acknowledgement

The authors expresse his great profound gratitude to the Head of Department of Geography (20092011), Dr. Sanat Kumar Guchhait, Reader of Postgraduate Dept. of Geography, The University of Burdwan, for his encouragement, cooperation and valuable advices, commitment of time throughout the course of this work from checking, examining and compilation of information to the summing up of this article in spite of his demanding academic and administrative schedule.

6. References

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