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

© 2011 Sandipan Ghosh et al., licensee IPA- Open access - Distributed under Creative Commons Attribution License 2.0

Research article ISSN 0976 – 4402

Received on April 2011 Published on June 2011 1743

Pedo-Geomorphic 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, Burdwan, West

Bengal, India (

2- Part-time Lecturer, Department of Geography, Maharajadhiraj Uday Chand Women‟s

College, Burdwan, West Bengal, India

[email protected]

doi:10.6088/ijessi.00107020032

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 morpho-climatic 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 inter-rill 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 parent-mass 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 cutting-away 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 pedo-geomorphic 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.

Pedo-Geomorphic 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 1744

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 pedo-geomorphic significance and a

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. Pedo-geomorphology 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 pre-field 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

post-field 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 sub-setting 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 km2) 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 bus-stop. The study area is located between

24010‟ and 24

013‟N and 87

039‟ and 87

045‟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

carried to the eastern lower level by the action of streams, rain wash, surface runoff

and then re-deposited 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 wet-dry

periods (Tertiary) in this area.

4. The low-level 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 (soup-plate laterites, remained as

domal uplands, figure 3).

5. The climate of the study area has wet-dry, sub-humid and subtropical climate with

mean annual rainfall 1420 mm (average of 1930-1960). 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 inter-fluvial 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 pre-monsoon the average water level is above 10 metre depth

(April, 2006) and in post-monsoon 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- Moderate-Selva (Peltier, 1950) and Tropical Wet-Dry

Savanna (Chorley, Schumm and Sugden, 1984).

10. Major Pedo-geomorphic processes: Mod-max chemical weathering, moderate

physical weathering, mod-max mass wasting, mod-max fluvial processes (sheet wash,

rain-splash, rill and gully erosion), laterisation.





11. Morphological features: Rolling lateritic uplands, wide planation surface (< 3.50),

deep red weathering zone, uricrust of Fe-oxides, badlands.

Figure 2: Profile of lateritic soil in the study area

Figure 3: Locus of indurated lateritic crust (soup-plate laterites) in surrounding

region of Aerodrome (based on field visits, SRTM data, and Global Mapper 11.1)





Figure 4: IRS 1D LISS III FCC image (2001) of the study area (greenish-blue 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).





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

).





Table 1.Rainfall erosivity measurement at Rampurhat (23o15‟N, 88

o35‟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

)

p2/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

*p2/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

R2 = 0.6386

0

10000

20000

30000

40000

50000

60000

0 50 100 150 200 250 300 350 400 450 500

p2/P

Ra

infa

ll E

ne

rgy

(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)





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

M1.14

(12-a)+3.25(b-2)+2.5(c-3)]/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),





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

savanna

grass 0.1 25 2.32

4

sandy

clay

loam

savanna

grass 0.1 25 0.48

5 sandy

loam

savanna

grass 0.3 25 0.48

6 sandy

loam

savanna

grass 0.3 25 1.41

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 two-third 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 soil-land 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 (%),





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 Qd 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/

m2)

1 5.81377

12.83035 34691.7

2o40' 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 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 first-order drainage

basins and is approximately equal to one-half 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

(0.05-0.27 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





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= cross-sectional area of the upstream section in m2,

Ad= Cross-sectional area of the downstream section in m2 and

Lab= distance between Au and Ad in metre.

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





Sample

Gullies

cross-

section

width

(m)

depth

(m)

area

(m2)

Lab

(m)

Volume

of

sediment

yield

(m3)

Gully 1 upstream 120 9 1080

287 1083.073 downstream 126 7 882

Gully 2 upstream 18.17 4.65 84.5

178 84.870 downstream 16.5 4 66

Gully 3 upstream 62.5 5.5 312.5

337 315.512 downstream 101.5 10 1015

Figure 9: Upstream and downstream cross-profiles 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





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 sub-basins 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

drainage-basin 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

m3/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

m3/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:





Log E= 2.65 log (p2/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 p2/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 2nd

and 3rd

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

Equation standard coefficient

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/km2/annum), Dd=drainage density (km/km

2),

Lg=average length of overland flow (km), RV =runoff volume of sample basin (m3/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/km2/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

Low-level 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 iron-oxides, light-

textured, low moisture retention capacity and less vegetative growth. The major findings of

research are as follows

1. These laterites are of multi-cyclic 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 3-8 metre and width ranges from 5-65 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 (0.05-0.17 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 0.02-13.87 kg/m2/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

(2009-2011), Dr. Sanat Kumar Guchhait, Reader of Post-graduate Dept. of Geography, The

University of Burdwan, for his encouragement, co-operation 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.

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pedo-geomorphic analysis of soil loss in the lateritic ...sandipan ghosh, tithi maji international...

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