morphometric aspects of a small tropical mountain river system

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Morphometric aspects of a small tropical mountain river system, thesouthern Western Ghats, IndiaJobin Thomas a; Sabu Joseph a;K. P. Thrivikramaji b

a Department of Environmental Sciences, University of Kerala, Thiruvananthapuram, Kerala, India b

Department of Geology, University of Kerala, Thiruvananthapuram, Kerala, India

First published on: 08 January 2010

To cite this Article Thomas, Jobin , Joseph, Sabu andThrivikramaji, K. P.(2010) 'Morphometric aspects of a small tropicalmountain river system, the southern Western Ghats, India', International Journal of Digital Earth, 3: 2, 135 — 156, Firstpublished on: 08 January 2010 (iFirst)To link to this Article: DOI: 10.1080/17538940903464370URL: http://dx.doi.org/10.1080/17538940903464370

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Morphometric aspects of a small tropical mountain river system,the southern Western Ghats, India

Jobin Thomasa*, Sabu Josepha and K.P. Thrivikramajib

aDepartment of Environmental Sciences, University of Kerala, Thiruvananthapuram 695 581,Kerala, India; bDepartment of Geology, University of Kerala, Thiruvananthapuram 695 581,

Kerala, India

(Received 27 April 2009; final version received 4 November 2009)

The Muthirapuzha watershed (MW) is one among the major tributaries ofPeriyar � the longest west flowing river in Kerala, India. A morphometric analysiswas carried out to determine the spatial variations in the drainage characteristicsof MW and its 14 fourth order sub-watersheds (SW1�SW14) using Survey ofIndia topographic maps and Landsat ETM� imagery. The study revealed that thewatershed includes a sixth order stream and lower order streams dominate thebasin. Results did indicate that rainfall has a significant role in the drainagedevelopment whereas structure and relief of rocks dictate the drainage pattern.The asymmetry in the drainage distribution is correlated with the tectonic historyof the Munnar plateau in the late Paleocene age. The watershed is moderate towell-drained and exhibited a geomorphic maturity in its physiographic develop-ment. The shape parameters revealed the elongated nature of MW and drainagenetwork development in the watershed. Further, the analysis provided significantinsight into the terrain characteristics. This study strongly brings to light, (a) thetendency of the watershed to soil loss and (b) the hydrological makeup of the sub-watersheds, which combined helped to formulate a comprehensive watershedmanagement plan.

Keywords: morphometry; digital earth; Muthirapuzha watershed; Western Ghats;India

Introduction

Since the early research investigations of Horton (1945), Thornbury (1954), and

Strahler (1964) emphasized the advantages of drainage pattern analysis in

characterizing geomorphic features and inferring the degree of structural and

lithological controls in the evolution of fluvial landforms. Geology, relief, and

climate are the key determinants of running water ecosystems functioning at the

basin scale (Lotspeich and Platts 1982, Frissel et al. 1986). Morphometric

descriptors represent relatively simple approaches to describe basin processes and

to compare basin characteristics (Mesa 2006) and enable an enhanced understanding

of the geological and geomorphic history of a drainage basin (Strahler 1964). The

morphometric assessment helps to elaborate a primary hydrological diagnosis in

order to predict approximate behavior of a watershed if correctly coupled with

geomorphology and geology (Esper 2008). The hydrological response of a river basin

*Corresponding author. Email: [email protected]

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can be interrelated with the physiographic characteristics of the drainage basin, such

as size, shape, slope, drainage density and size, and length of the streams, etc.

(Chorley 1969, Gregory and Walling 1973). Hence, morphometric analysis of a

watershed is an essential first step, toward basic understanding of watershed

dynamics.

In terrain characterization studies, and especially on spatial variabilities ofmorphometric parameters, the contributions of Mather and Doornkamp (1970),

Gardiner (1978), and Gregory (1978) are considered immensely important. In the

Indian regional context, morphometric analysis was employed for characterizing

watersheds (Nag 1998, Vittala et al. 2004), for the prioritization of micro watersheds

(Ratnam et al. 2005) and for the development of groundwater resources (Sreedevi

et al. 2004, 2009). Locally in Kerala, examples of similar approaches have been

applied in the Kuttiyadi (James and Padmini 1983), Chalakkudy (Maya 1997), and

in Pamba (Rajendran 1982) watersheds. Recently, Vijith and Satheesh (2006) as well

as Manu and Anirudhan (2008) analyzed the drainage characteristics of Meenachil

and Achankovil Rivers using remote sensing and Geographic Information System

(GIS) as tools.

The emerging trends in the applications of computers especially in mapping,

development of information systems and virtual world enabled to integrate a wide

range of information about the physical system and to use these digital data for

research or to solve practical problems. The advantages of this digital earth concept

over the conventional methods are its ability to create, manipulate, store, and use

spatial data much faster and at a rapid rate. In addition, the development of thedigital earth concept supplied the mechanism and the data to allow a coupling

between the form and process. Moreover, it made the quantitative approach for

surface characterization and the mechanism for the interpretation and manipulation

of the quantitative datasets easy. The present study employed the same concept on

a watershed level and this paper primarily focuses on the description and nature

of spatial variations of physical characteristics of the drainage system of the

Muthirapuzha watershed (MW), in order to describe and evaluate the linear, areal,

and relief characteristics, using data aggregated from Survey of India (SOI)

toposheets (scale, 1:50,000) and corresponding Landsat 7 ETM� imagery (spatial

resolution: 30.0 m; 14 January 2000; WRS-2, Path 144, Row 053).

Study area

The MW (n�6th, L�37.81 km, Area�275.71 km2; N Lat. 108 01? 55ƒ to 108 11?31ƒ and E Long. 768 59? 45ƒ to 778 14? 52ƒ), a major sub-watershed (at elevations

spanning between 740 and 2690 m) of the west flowing Periyar in Kerala, India

(Figures 1 and 2), is etched in the Precambrian rocks of the southern Western Ghats

and specifically those of the Munnar Plateau, which is home to an important peak,

viz., Anai Mudi (2690 m) which is the tallest peak south of the Himalayas.

The sixth order Muthirapuzha main stem trends approximately in a NE�SW

direction, while its two fifth order tributaries, viz., Kannimala creek follows an

approximately southerly trend while Gudrale creek has established an essentially

westerly trend. Stream network of the MW dissects nearly 75% of the Munnar

plateau � a cardinal motif of the southern Western Ghats � with a roughly E�W

trending long axis and bounded to the north by the Kannan Devan hills and

136 J. Thomas et al.


Figure 1. Muthirapuzha watershed (MW) and fourth order sub-watersheds.

Figure 2. Landsat ETM� imagery (2000) representing MW.

International Journal of Digital Earth 137


Cardamom hills in the south. According to Soman (2002), the Munnar plateau is

portion of an extensive plantation surface with a southwesterly slope tending to

descend in a stepped manner. While U shaped valleys and broad ridges characterize

the plateau, the MW has two other local plantation surfaces.

Geological setting and soils

The MW is dominated by migmatites of the Precambrian age (Soman 2002). Thechief lithologies, in the order of decreasing areal spread, are granitic gneiss,

migmatite (hornblende-biotite gneiss), intrusive granite bodies, calc-granulite, and

minor quartzite patches (Figure 3). Pegmatites, quartz veins, and basic intrusives

characterize the older host rocks. Macroscopically, migmatite is a composite rock,

with alternating (folded and non-uniformly pinching or swelling) bands, enriched in

quartzo-feldspathic and mafic-minerals (Thampi 1987). Locally, the rock grades into

typical biotite or hornblende-biotite gneiss. The granitic gneiss is medium grained,

pinkish in color, and foliated. Foliation is expressed by parallel planar arrangementof flakes of biotite, prisms of hornblende, and lenticular flattened quartz grains. In

addition, both concordant and discordant patches and veins of non-foliated granite

and aplite are present in the granitic gneiss. Granite is seen exposed as WNW�ESE

trending linear body with irregular outline surrounded by granitic gneiss, migmatite,

and calc-granulite. In the area, south of Devikulam, the granite is exposed in the core

of a major fold. The calc-granulite is a medium grained rock and the weathered

surfaces are puckered, particularly near contact with migmatite, due to resistant

veins of quartzo-feldspathic material (Thampi 1987). A thin layer of laterite of about

Figure 3. Geological map of MW.



15.0 cm has developed in the high plateau around Eravikulam, 12.0 km to the NNE

of Munnar.

The foliation trends indicate the presence of a major synclinal-axial trace,

contained mostly in the hornblende-biotite gneiss, displaying a high amplitude planview which is a characteristic of Precambrian terrains. A minor anticlinal-axial trace

of NW�SE orientation appears within an enclave of granitic gneiss to the immediate

south of the center in Figure 3. Two major lineaments, crossing roughly at right

angle, have been discerned in area-one trends NE�SW, while the other (though only a

portion appears in the map) has a NW�SE alignment. A third minor lineament of

nearly E�W trend is noticed toward the southwestern border of the MW.

The soils of MW are categorized into major soil taxonomic units as fine loamy,

mixed, thermic family of Mollic Paleudalfs, clayey mixed, thermic family of TypicPalehumults, and clayey skeletal, mixed, isohyperthermic family of Ustic Palehu-

mults. These are very deep, well-drained hill soils developed on gneissic parent

material where in the former, gneissic material occupy 30�40% of the volume below

100�125 cm (Anon 2006). The O horizon, highly enriched in organic matter, is dark

and reddish brown to black in color. The soil under forest cover is quite fertile and

supports prolific undergrowth.

Climate and vegetation

As part of this study, weather data relating to 1989�2004 gathered by the Center for

Water Resources Development and Management (CWRDM), Kerala, India was

examined. The annual mean temperature is 178C with a mean minimum of 78C in

January, and a maximum of 268C in March. The mean annual rainfall in the region

is 3400 mm, whereas mean-monthly maximum stands at 873 mm (in July) and mean-

monthly minimum at 14 mm (in January). According to Trevartha’s (1954) climateclassification scheme, the MW classifies under ‘humid’ climate.

In addition, the MW is a part of the ‘India aquosa’ or ‘Malabar’ phytogeographical

province (Hooker 1907), which is a high species diversity ecoregion and is typical of the

tropical mountain realm, spanning the full extent of the Western Ghats (WWF 1997).

Again, the MW, per se is covered by several vegetation belts including montane

grasslands and southern montane wet temperate forests (Shola forests), occurring at

elevations upward of 1900 m above msl. Tea (Thea sinensis) plantations cover nearly

41% of the watershed, while another 19% is shared by eucalyptus (Eucalyptus globulus

and Eucalyptus grandis) plantations. Intensive and ubiquitous vegetable farming is

practiced in the sediment fills of the interfluves.

Materials and methods

The SOI topographic maps (1:50,000) and Landsat 7 ETM� imagery (spatial

resolution: 30.0 m; 14 January 2000; WRS-2, Path 144, Row 053) were used as a base

for delineation of MW including its fourth order sub-watersheds. The stream

network and elevation contours were on-screen digitized and prepared a geodatabaseusing GIS platform. Based on the drainage network and contour data, MW was

divided into 14 fourth order sub-watersheds. The drainage channels were character-

ized according to their corresponding drainage order. Drainage network of the

watershed was analyzed following Horton’s (1945) scheme and stream ordering after



Strahler (1964). The morphometric parameters were divided into three categories:

linear, areal, and relief aspects and these parameters. The basic parameters such as

basin area, perimeter, length, and stream length were extracted from the geodatabase

and other parameters were derived from these basic parameters by means of variousmathematical equations (Table 1).

Results

Figure 1 is a sketch of MW boundary including the stream network and the 14 fourth

order sub-watersheds. The linear, areal, and relief parameters have been examined and

detailed in the following along with the highlights of the results.

Drainage pattern

The MW in general, exhibits a dominantly parallel pattern; while semi-centripetal,

trellis, and rectangular patterns co-exist. The parallel pattern with low order

sub-parallel streams forms slopes joining higher order streams at nearly uniform

intervals are a characteristic of areas with steep slopes where channels are controlled

by structure, though departure from the former also appears. Semi-centripetal pattern

is inferred toward the head of Devikulam valley, where lower order streams head to a

central depression before draining out. The drainage system upstream of theMaduppatty reservoir (SW1 and SW2), is rectangular and the mainstream makes

several sharp and nearly right-angled bends.

Barbed drainage pattern is noticed in Kannimala creek watershed and at the

confluence of the mainstream with Gudrale creek. Thampi (1987) reported that to the

west of Munnar (Figure 1), i.e. in SW12, third order streams frequently bend at near-

right angles to cut through ridges resulting in a trellis pattern of drainage. Further,

straight channel segments and preferred direction of alignment of streams reflect

fracture/lineament control on drainage. Diverse stream orientations like N�S, NW�SE, NE�SW, etc. observed in the MW. The ‘boat hook bend’ shape of channel at the

confluence of Kannimala creek with Muthirapuzha at Munnar is a relict of paleo

drainage (Thampi 1987). The asymmetry of the MW, with eight left bank fourth order

sub-watersheds and six right bank fourth order sub-watersheds, is an attribution to the

tectonic history of the Munnar plateau, and Soman (2002) assigns a late Paleocene age

to the latter.

Linear aspects

Perimeter (P)

The data on perimeter of MW (109.93 km) and that of 14 fourth order

sub-watersheds are given in Table 2. Among the sub-watersheds, SW10 has the

largest P (24.45 km), registering a larger basin area (23.07), while the perimeter of

SW8 (7.16 km) is the smallest of all.

Basin length (Lb)

The Lb of MW is 37.81 km and that of 14 sub-watersheds are given in Table 2. SW5,

SW10, SW12, and SW13 are relatively longer ones (Lb�8 km), while SW7 and SW8



Table 1. Morphometric parameters used for the morphometric analysis.

Sl. No. Parameters Definition Units References

Linear aspects

1. Perimeter (P) Length of the

watershed boundary

km

2. Basin length (Lb) Maximum length of the

watershed measured

parallel to the main

drainage line

km

3. Stream order

(Nu)

Hierarchical ordering Dimensionless Strahler (1957)

4. Stream length

(Lu)

Length of the major

stream

km Horton (1945)

5. Bifurcation ratio

(Rb)

Rb�Nu/N(u�1),

where Nu is number of

streams of any given

order and N(u�1) is

number in the next

higher order

Dimensionless Horton (1945)

6. Stream length

ratio (Rl)

Rl�Lu/L(u�1), where

Lu is stream length

order u and L(u�1) is

stream segment length

of the next lower order

Dimensionless Horton (1945)

7. Rho coefficient

(r)

r�Rl/Rb Dimensionless Horton (1945)

Areal aspects

8. Area (A) Area of watershed km2

9. Drainage density

(Dd)

/Dd�aLt

A; where aLt

is the total length of all

the ordered streams

km km�2 Horton (1945)

10. Stream frequency

(Fs)

/Fs�aNt

A; where Nt is

total number of stream

segments of all orders

km�2 Horton (1945)

11. Drainage texture

(T)

T�Dd�Fs km km�4 Smith (1950)

12. Length of

overland flow

(Lg)

Lg�1/2Dd km Horton (1945)

13. Constant of

channel

maintenance (C)

C�1/Dd km Schumm

(1956)

14. Form factor (Ff) Ff�A/Lb2 Dimensionless Horton (1945)

15. Circularity ratio

(Rc)

Rc�4pA/P2 Dimensionless Miller (1953)



are of short values (Lb53 km). Further, the sub-watersheds are relatively elongate,

consequently covering larger basin areas (r�0.96), and hence affirming the role of

head-ward erosion in making lengthy channels.

Stream order (Nu)

The classification of streams based on the number and type of tributary junctions,

has proven to be a useful indicator of stream size, discharge, and drainage area

(Strahler 1957). Tabulation of the order (u) specific number of streams (N) is in Table

2. MW is designated as a sixth order watershed with three fifth order tributaries and

whole sub-watersheds considered for the present study are of fourth order.

Stream length (Lu)

The mean and total stream length of each stream order is tabulated in Table 2. The

mean length of channel segments of a given order is more than that of the next lower

order, but less than the next higher order, indicating that the watershed evolution

follows erosion laws acting on geologic material with homogeneous weathering-

erosion characteristics (Nag and Chakraborty 2003). Some variations from this

general behavior observed in SW3, SW5, SW6, SW7, SW8, and SW9 may indicate

Table 1 (Continued)

Sl. No. Parameters Definition Units References

16. Elongation ratio

(Re)

/Re�1:128

ffiffiffiffi

Ap

LbDimensionless Schumm

(1956)17. Shape index (Sw) Sw�1/Ff Dimensionless Horton (1932)

Relief aspects

18. Bain relief (R) R�H�h, where H is

maximum elevation and

h is minimum

elevation within the

basin

km Schumm

(1956)

19. Relief ratio (Rr) Rr�R/Lb Dimensionless Schumm

(1956)

20. Ruggedness

number (Rn)

Rn�R�Dd Dimensionless Strahler (1958)

21. Dissection index

(DI)

DI�R/Ra, where Ra

is absolute relief

Dimensionless Singh and

Dubey (1994)

22. Gradient ratio

(Rg)

Rg�Es�Em/Lb, where

Es is the elevation at the

source, Em is the

elevation at the mouth

Dimensionless Sreedevi et al.

(2004)

23. Melton

ruggedness ratio

(MRn)

MRn�H�h/A0.5 Dimensionless Melton (1965)



Table 2. Linear parameters of MW and sub-watersheds.

Parameters SW1 SW2 SW3 SW4 SW5 SW6 SW7 SW8 SW9 SW10 SW11 SW12 SW13 SW14 MW

P 15.66 15.29 11.21 18.32 19.41 17.52 9.02 7.16 14.65 24.45 9.36 19.49 21.16 10.69 109.93

Lb 5.29 5.35 4.43 6.12 8.23 6.45 2.68 2.34 4.01 8.18 4.09 8.39 8.92 3.69 37.81

Number of streams N1 62 61 26 41 86 62 21 15 27 113 33 94 94 27 1243

N2 15 16 7 9 16 17 5 4 6 26 8 23 29 6 320

N3 3 6 2 3 3 4 2 2 2 4 2 3 5 2 81

N4 1 1 1 1 1 1 1 1 1 1 1 1 1 1 14

N5 � � � � � � � � � � � � � � 3

N6 � � � � � � � � � � � � � � 1

Nt 81 84 36 54 106 84 29 22 36 144 44 121 129 36 1662

Mean stream length L1 0.51 0.47 0.58 0.54 0.50 0.49 0.51 0.45 0.56 0.52 0.40 0.49 0.53 0.64 0.51

L2 0.57 0.48 0.71 0.81 0.56 0.47 0.74 0.44 0.59 0.63 0.48 0.49 0.52 0.51 0.55

L3 1.69 0.61 1.83 1.12 3.75 1.63 0.67 0.62 1.11 1.59 1.06 1.42 0.85 1.40 1.11

L4 1.60 2.92 0.96 2.21 1.07 2.82 0.15 0.22 0.79 5.00 1.01 7.31 6.90 0.43 2.39

L5 � � � � � � � � � � � � � � 6.40

L6 � � � � � � � � � � � � � � 17.37

Total stream length LT1 31.85 28.43 15.09 22.05 42.76 30.64 10.61 6.76 14.99 58.43 13.25 45.91 49.97 17.16 635.17

LT2 8.52 7.71 4.96 7.32 8.92 8.06 3.68 1.76 3.52 16.33 3.80 11.20 15.07 3.05 175.79

LT3 5.07 3.63 3.66 3.35 11.26 6.51 1.34 1.24 2.21 6.37 2.12 4.27 4.23 2.80 90.21

LT4 1.60 2.92 0.96 2.21 1.07 2.82 0.15 0.22 0.79 5.00 1.01 7.31 6.90 0.43 33.39

LT5 � � � � � � � � � � � � � � 19.20

LT6 � � � � � � � � � � � � � � 17.37

LT 47.04 42.69 24.67 34.93 64.01 48.03 15.78 9.98 21.51 86.13 20.18 68.69 76.17 23.44 971.13

Rb1�2 4.13 3.81 3.71 4.56 5.38 3.65 4.20 3.75 4.50 4.35 4.13 4.09 3.24 4.50 3.88

Rb2�3 5.00 2.67 3.50 3.00 5.33 4.25 2.50 2.00 3.00 6.50 4.00 7.67 5.80 3.00 3.95

Rb3�4 3.00 6.00 2.00 3.00 3.00 4.00 2.00 2.00 2.00 4.00 2.00 3.00 5.00 2.00 5.79

Rb4�5 � � � � � � � � � � � � � � 4.67

Rb5�6 � � � � � � � � � � � � � � 3.00

Rb 4.04 4.16 3.07 3.52 4.57 3.97 2.90 2.58 3.17 4.95 3.38 4.92 4.68 3.17 4.26

Rl 2�1 1.12 1.02 1.22 1.50 1.12 0.96 1.45 0.98 1.05 1.21 1.20 1.00 1.02 0.80 1.08

Intern

atio

na

lJo

urn

al

of

Dig

ital

Ea

rth1

43


Table 2 (Continued)


Rl 3�2 2.96 1.27 2.58 1.38 6.70 3.47 0.91 1.41 1.88 2.52 2.21 2.90 1.63 2.75 2.02

Rl 4�3 0.95 4.79 0.52 1.97 0.29 1.73 0.22 0.35 0.71 3.15 0.95 5.15 8.12 0.31 2.15

Rl 5�4 � � � � � � � � � � � � � � 2.68

Rl 6�5 � � � � � � � � � � � � � � 2.71

Rl 1.68 2.36 1.44 1.62 2.70 2.05 0.86 0.91 1.21 2.29 1.45 3.02 3.59 1.29 2.13

Rho 0.42 0.57 0.47 0.46 0.59 0.52 0.30 0.35 0.38 0.46 0.43 0.61 0.77 0.41 0.50

Note: P, perimeter; Lb, basin length; Rb, bifurcation ratio; Rl, stream length ratio; and Rho, Rho coefficient.

14

4J.

Th

om

as

eta

l.


anomalous development of catchments. The mean length in SW10, SW12, and SW13

increased abruptly from the general trend, which is indicative of consequences of

exertion of pressure by structural elements. Hack (1957) empirically defined the

relationship between watershed area and stream length as L�1.4�A0.6, while in this

study, though a similar relation (L�1.238�A0.63) manifests, but with different

intercept and exponent, which also indicates head-ward erosion as the driver of

channel network growth and extension. This confirms the significant role of rainfall

in the drainage network development.

Bifurcation ratio (Rb)

Bifurcation ratio, a measure of the degree of ramification of drainage network (Mesa

2006), exercises a significant control over the ‘peakedness’ of runoff (Chorley 1969).

The Rb values usually fall in the range of 3.0 and 5.0 for networks formed on

homogeneous rocks (with least/minimum structural disturbances), on the one hand

and hits values higher than 10.0, where structural controls play dominant roles on the

other (Mekel 1970, Chow et al. 1988). The shape of watersheds also exerts a significant

control on Rb (Verstappen 1983). The variations in Rb values are a reflection of the

differences in the shape of stream network (Ghosh and Chhibber 1984). In respect of

MW, Rb attains a value of 4.26, while the values for the 14 sub-watersheds vary from

2.58 to 4.95 (Table 2), which is comparable to that of mountainous or highly dissected

areas (Horton 1945). The closer range, in the variations of mean bifurcation ratio of

the sub-watersheds (SW1, SW2 and SW6; SW3, SW4, SW9, SW 11, and SW14; SW10

and SW12; SW7 and SW8; SW5 and SW13) is ascribed to the geometrical similarities

among the watersheds. High Rb values in SW5, SW10, SW12, and SW13 may indicate

high overland flow and discharge due to hilly nature of terrain plus steeper disposition

of slopes, while low Rb values in SW3, SW7, and SW8 can be a reflection high

infiltration rate and lesser number of channels.The Rb of the successive stream orders (in SW2, SW5, SW10, SW12, and SW13),

with much larger spread is interpreted as a predominant outcome from geological

attributes (Strahler 1952). The hypothesis proposed by Giusti and Schneider (1965)

suggests that the general trend of the bifurcation ratios confirms that the Rb values

within a region decreases with increase in order. The deviation from the above

hypothesis in the sub-watersheds (SW1, SW2, SW6, SW10, SW12, and SW13)

indicates that geology and relief have affected the branching of streams. The second

postulation implies that the basins of equal order, but variable areas tend to have the

smallest Rbs in the smallest areas and a high positive correlation between basin area

and Rb (r�0.96) confirms the same. The morphometric analysis of Achankovil

River, flowing through the Achankovil Shear Zone in the Southern Kerala reported

Rb values in the range of 3.46 and 5.50 (Manu and Anirudhan 2008). Further, the

poor correlation reflected by low correlation coefficients for Rb with Dd (r�0.09),

with Fs (r��0.10), and with Lb (r�0.38) clearly demonstrate the axiom

that stream organization depends on variables like overall geological structure,

lithological characteristics, climate, and vegetation.



Stream length ratio (Rl)

The mean Rl of MW is 2.13 and varies from 0.86 to 3.59 in the 14 sub-watersheds

(Table 2). The variability in Rl, among successive stream orders, is a reflection of

differences between slope and topography and hence it has an important control on

discharge and erosional stage of the watershed (Sreedevi et al. 2004). Though, the Rl

between successive orders of streams in the sub-watersheds does not obey any

empirical rule or follow any systematic variations, some anomalous values were

observed in a few sub-watersheds. The anomaly may be interpreted as a sign of

disequilibrium in the drainage system. It must also be associated with either as

downstream extension of the higher order segment or an upward extension of

tributaries or inception.The high positive correlation of mean Rl with A (r�0.89) indicates the higher

erosional activity and consequent tendency for a more rapid bifurcation of streams

and development of higher order streams. Wide variability among the Rl values of

MW suggests the domination of local geology over length of channel segments. The

increase of Rl from lower to higher orders is exemplified by MW may be indicative of

attainment of geomorphic maturity.

Rho coefficient (r)

The Rho coefficient is an important parameter relating drainage density to

physiographic development of a watershed which facilitate evaluation of storage

capacity of drainage network and hence, a determinant of ultimate degree of

drainage development in a given watershed (Horton 1945). The climatic, geologic,

biologic, geomorphologic, and anthropogenic factors determine the changes in this

parameter. Rho values in the MW and sub-watersheds span from 0.30 to 0.77

(Table 2). SW10 reports the largest value (r�0.77) while, SW2, SW5, SW6, and

SW12 also show second higher values (r�0.50), suggesting higher hydrologic

storage during floods and attenuation of effects of erosion during elevated discharge.

Areal aspects

Area (A)

The MW drains an area of 271.75 km2 and the area of each fourth order

sub-watersheds are specified in Table 3. Among the 14 sub-watersheds, SW8 is the

smallest of all (A�2.89 km2), whereas SW10 is the largest (A�23.07 km2). Six

sub-watersheds have areas less than 10 km2 while SW10 and SW13 have areas in

excess of 20 km2. The mean area of fourth order watershed stands at 11.43 km2.

Drainage density (Dd)

Drainage density is a parameter sensitive to the erosional development and provides

a link between form attributes of a watershed and processes operating along the

stream course (Strahler 1954, Gregory and Walling 1973). According to Verstappen

(1983), Dd measures the degree of fluvial dissection and is under the influence of

numerous factors, but the resistance to erosion of rocks, infiltration capacity of land

and climatic conditions rank high. The Dd of MW is 3.57, while Table 3 reports



Table 3. Areal parameters of MW and sub-watersheds.


A 12.28 11.68 6.29 10.50 17.72 12.04 4.41 2.89 7.87 23.07 4.63 18.62 22.09 5.98 271.75

Dd 3.83 3.65 3.92 3.33 3.61 3.99 3.58 3.45 2.73 3.73 4.36 3.69 3.45 3.92 3.57

Fs 6.60 7.19 5.72 5.14 5.98 6.98 6.58 7.61 4.57 6.24 9.50 6.50 5.84 6.02 6.12

T 25.28 26.24 22.42 17.12 21.59 27.85 23.56 26.25 12.48 23.28 41.42 23.99 20.15 23.60 21.85

Lg 0.13 0.14 0.13 0.15 0.14 0.13 0.14 0.14 0.18 0.13 0.11 0.14 0.14 0.13 0.14

C 0.26 0.27 0.26 0.30 0.28 0.25 0.28 0.29 0.37 0.27 0.23 0.27 0.29 0.26 0.28

Ff 0.44 0.41 0.32 0.28 0.26 0.29 0.61 0.53 0.49 0.34 0.28 0.26 0.28 0.44 0.19

Rc 0.63 0.63 0.63 0.39 0.59 0.49 0.68 0.71 0.46 0.48 0.66 0.62 0.62 0.66 0.28

Re 0.75 0.72 0.64 0.60 0.58 0.61 0.88 0.82 0.79 0.66 0.59 0.58 0.59 0.75 0.49

Sw 2.28 2.45 3.12 3.57 3.82 3.46 1.63 1.89 2.04 2.90 3.61 3.78 3.60 2.28 5.26

Note: A, area; Dd, drainage density; Fs, stream frequency; T, drainage texture; Lg, length of overland flow; C, constant of channel maintenance; Ff, form factor; Rc,circularity ratio; Re, elongation ratio; and Sw, shape index.

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results of all sub-watersheds, suggesting that terrain is steep and impervious and

highly dissected as well as the region receives high precipitation (Horton 1932,

Langbein 1947). SW11 has the highest (4.36) value and SW9 possesses the lowest

(2.73). The MW and its fourth order sub-watersheds are moderate to well-drained,

wherein geological factors, particularly lithology, resistance of rocks to erosion, and

infiltration capacity determine the drainage density variations.

Stream frequency (Fs)

The Fs for MW is 6.12 km�2, while Fs of the sub-watersheds are presented in Table

3. In addition, Fs possess a strong positive correlation with Dd values (r�0.71,

significant at 0.01 level).

Drainage texture (T)

Smith (1950) suggested that drainage texture is a measure of relative channel spacing

in a fluvial-dissected terrain, which is greatly influenced by climate, vegetation,

lithology, soil type, relief, and stage of development of a watershed. The T values of

MW and 14 sub-watersheds appear in Table 3. Smith (1950) identified five different

texture classes based on Dd values viz., very coarse (B2), coarse (2�4), moderate

(4�6), fine (6�8), and very fine (�8). In the report here, MW and the sub-watersheds

like SW1, SW2, SW5, SW10, SW12, and SW13 group under moderate texture, while

rest of the sub-watersheds are texturally coarse.

Length of overland flow (Lg)

Length of overland flow is the length of water over the ground before it gets

concentrated into definite stream channels which affect both hydrologic and

physiographic development of drainage basins (Horton 1945). The MW reports an

Lg value of 0.14, where as all the sub-watersheds, show values varying between 0.11

and 0.18 (Table 3). The MW and the sub-watersheds is in a mature geomorphic stage,

characterized by a relatively higher Lg value; while SW11, characterized by lower Lg

value is in late youth or early mature stages of development.

Constant of channel maintenance (C)

The C values of 14 sub-watersheds ranges between 0.23 and 0.37 and that of MW is

0.28 (Table 3). Most of the sub-watersheds with low values indicate the region with

close dissection and these are moderately influenced by structural parameters (Vijith

and Satheesh 2006). SW9, characterized by large C value implies significantly higher

infiltration rates than the rest.

Form factor (Ff)

Ff is a parameter used to predict the flow intensity of a watershed of a defined area

and this has a direct linkage to peak discharge (Horton 1945, Gregory and Walling

1973). The Ff of MW is 0.19 and that of 14 sub-watersheds (Table 3) range between

0.26 and 0.61. But Ff values for SW7 and SW8 are relatively larger values (Ff�0.50)



indicating higher flow peaks but of shorter duration, while SW4, SW5, SW6, SW11,

SW12, and SW13 have low Ff (50.30) implying a more elongate plan view of

watersheds and suggesting consequent flatter peak flows of longer duration.

Circularity ratio (Rc)

The circularity ratio is expressed as the ratio of the basin area (A) and the area of a

circle with the same perimeter as that of the basin. The Rc values can attain a

maximum of 1.0 where the outline of the watershed is approaching near circularity

(Miller 1953). The MW has an Rc of 0.28, whereas in all sub-watersheds, the range is

between 0.39 and 0.71 (Table 3). A numerically low Rc indicates an elongated shape,

while higher values are expression of approach to near circularity. SW4, SW5, and

SW6 have low Rc values. High Rc values (Rc�0.60) of SW1, SW2, SW3, SW7,

SW8, SW11, SW12, SW13, and SW14 indicate the predisposition to flood hazard

during peak periods of concentrated flood flow. The value of Rc is a reflection of the

stage of evolution of the watersheds, wherein low (e.g. SW4, SW6, and SW9) and

high (the remaining sub-watersheds) Rc values correspond, respectively, to youth

and mature stages of watershed development.

Elongation ratio (Re)

The Re of MW is 0.49, while it is between 0.58 and 0.88 in the 14 sub-watersheds

(Table 3). The higher Re for SW7 and SW8 indicates that these watersheds are far

less elongated than others causing a higher discharge during a short period

(Verstappen 1983). Based on the classification by Strahler (1964), SW7 and SW8

has outlines of oval shape (0.90�Re�0.80) and SW1, SW2, SW9, and SW14 are

less elongated (0.80�Re�0.70) and all other sub-watersheds are of elongated

(ReB0.70) outline. The elongated shape of the watersheds with high relief and steep

slope with a smooth hydrograph which is explained by greater time lag for water

from upper regions of the catchment to reach outlet.

Shape index (Sw)

The Sw of MW is 5.26, while the values of the sub-watersheds range between 1.63

and 3.78 (Table 3). The drainage-network development of MW is in a length to width

ratio of 1:5 and so drainage channels tend to develop more along the width than east

west directions.

Relief aspects

Basin relief (R)

R is a parameter that determines the stream gradient and influences flood pattern

and volume of sediment that can be transported (Hadley and Schumm 1961). It may

be unduly influenced by one isolated peak within the watershed. Basin relief is an

important factor in understanding denudational characteristics of the basin

(Sreedevi et al. 2004). The MW is endowed with an R of 1950 m, while that of



14 sub-watersheds are given in Table 4. The larger R values are a result of the paleo

and neo tectonic regimen of the southern Western Ghats.

Relief ratio (Rr)

Rr is a dimensionless height to length ratio, i.e. basin relief and basin length and widely

accepted as an effective measure of gradient aspects of the watershed (Schumm 1956).

The Rr of MW is 0.05, while that of all sub-watersheds given in Table 4. SW2, SW12,

and SW13 have relatively low Rr values (RrB0.10) indicating the exposure of

basement rocks as small ridges and mounds with lower slope values, while SW7, SW8,

and SW14 show the higher Rr values (Rr�0.20), indicating presence of areas of

steeper slope and higher relief underlain by resistant rocks (Vittala et al. 2004).

Ruggedness number (Rn)

The ruggedness number is expressed as the product of basin relief and drainage

density (Strahler 1958). The Rn for MW is 6.96 and that of 14 sub-watersheds are

tabulated in Table 4. The Rn values for the sub-watersheds range between 1.46

(SW2) and 4.44 (SW10). The high ruggedness value of MW and sub-watersheds

implies that these tracts are more prone to soil erosion and have intrinsic structural

complexity in association with relief and drainage density (Vijith and Satheesh 2006).

Dissection index (DI)

DI is a parameter implying the degree of dissection or vertical erosion and expounds

the stages of terrain or landscape development in any given physiographic region or

watershed (Singh and Dubey 1994). On average, the values of DI vary between ‘0’

(complete absence of vertical dissection/erosion and hence dominance of flat surface)

and ‘1’ (in exceptional cases, vertical cliffs, it may be at vertical escarpment of hill slope

or at sea shore). DI value of MW and the sub-watersheds (Table 4), imply that the

watershed is a highly dissected one. Most of the sub-watersheds are highly dissected

while SW2, SW7, SW8, and SW12 group under moderately dissected type. Further,

SW2, SW7, SW8, and SW12 are in an equilibrium condition, while the other sub-

watersheds and MW are at an in-equilibrium stage (Singh and Dubey 1994).

Gradient ratio (Rg)

Gradient ratio is an indicator of channel slope which enables assessment of the

runoff volume (Sreedevi et al. 2004). MW has an Rg of 0.04 and that of all

sub-watersheds (Table 4) vary from 0.04 (SW13) to 0.25 (SW14). The large Rg values

reflect the mountainous nature of the terrain. Approximately 75% of the main

stream flows through the plateau and the relatively low values of Rg confirm the

same.

Melton ruggedness number (MRn)

The MRn is a slope index that provides spatialized representation of relief

ruggedness within the watershed (Melton 1965). MW has an MRn of 0.12, while



Table 4. Relief parameters of MW and sub-watersheds.


R 760 400 880 890 1070 950 560 600 725 1190 742 490 743 1096 1950

Rr 0.14 0.07 0.20 0.15 0.13 0.15 0.21 0.26 0.18 0.15 0.18 0.06 0.08 0.30 0.05

Rn 2.91 1.46 3.45 2.96 3.86 3.79 2.00 2.07 1.98 4.44 3.24 1.81 2.56 4.30 6.96

DI 0.36 0.20 0.44 0.42 0.50 0.43 0.30 0.29 0.37 0.59 0.40 0.30 0.44 0.73 0.87

Es (m) 2122 2203 2468 2640 2600 2593 1834 2058 2249 2556 2053 1848 1822 1880 2122

Em (m) 1760 1760 1620 1760 1580 1700 1560 1700 1580 1500 1500 1440 1440 960 740

Rg 0.07 0.08 0.19 0.14 0.12 0.14 0.10 0.15 0.17 0.13 0.14 0.05 0.04 0.25 0.04

MRn 0.22 0.12 0.35 0.27 0.25 0.27 0.27 0.35 0.26 0.25 0.34 0.11 0.16 0.45 0.12

Note: R, basin relief; Rr, relief ratio; Rn, ruggedness number; DI, dissection index; Es, elevation at source; Em, elevation at mouth; Rg, gradient ratio; and MRn, Meltonruggedness number.

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in the sub-watersheds, MRn ranges from 0.11 to 0.45 (Table 4). These values are

comparable to the results of Marchi et al. (1993) in the Eastern Italian Alps.

According to the classification of Wilford et al. (2004), SW3, SW8, SW11, and SW14

are debris flood watersheds where bed load component dominates sediment undertransport, while the remainder of sub-watersheds and MW are water flood

watersheds. But incidence of debris flows or fluvial sediment transport obviously

depends on availability of debris and yet low ruggedness of landscape indicates the

availability of loci suitable to trap debris from upstream areas and out of tributaries

dominated by debris flow, causing a transition from debris flows to bed load

transport (Marchi and Fontana 2005).

Hydrological implications

The stream network development in MW is asymmetric in that eight tributaries

originate from the left bank while only six rise from the right bank of the watershed,

creating a hydrologic disparity within the watershed. So, it can be assumed that

watershed geometry and drainage properties influenced the hydrologic regime in thesame magnitude within and among the sub-watersheds. Though many approaches

have been adopted for watershed development and management by providing

attention toward physical hydrology; parametric methods, including deciphering of

relationships among morphometric parameters and their control on hydrologic

variables earned very little consideration. The hydrological system is very complex

and morphometric analysis provides adequate information on the hydrological

behavior of the drainage watershed. An understanding of how the watershed

responds to different natural processes is one component of the essential knowledgebase for applying principles of watershed management. But this knowledge needs to

be applied in the context of how morphometrical parameters will also affect the

stream flow, sediment transport, and debris flows.

Summary

The evaluation of drainage characteristics of MW and its fourth order

sub-watersheds unveiled the importance of morphometric studies in terrain

characterization and basin evolution studies, which led to the following points.The drainage network of the MW is well-developed and systematically organized

to provide sufficient draining, with a large number of first and second order streams.

The results emphasize the fact that the terrain, underlying the Muthirapuzha, the

chief drainage system of Munnar Plateau, is a tectonically active and uplifted

landmass exercising structural control on the drainage pattern. The results of

Horton analysis ratify Horton’s laws. The MW confirms the drainage network

development through homogeneous weathering and head-ward erosion. The Rb

values of MW characterize highly dissected mountainous watersheds with maturetopography and higher drainage integration. A high proportion of first order streams

(�70%) indicates structural breaks, chiefly as, lineaments, fractures, and antiforms

and synforms, of rocky basement of the watershed. The Dd values provide sufficient

insight into surface geology (i.e. impervious basement and steeper slopes) causing



higher surface run off, and humid climate resulting in a moderate to well-drained

basin and a higher level or degree of dissection. The relief parameters indicate that

MW is structurally complex with mountain landscape and how the latter influences

development of stream segments. Again, it is inferred that the drainage pattern in thewatershed is controlled by relief and structure.

MW and its sub-watersheds are elongate in shape and hence the sub-units will

tend to have lower flood peaks but longer duration flood flows � hence affording

flood management. These shape parameters may aid in the flow forecasting of

streams in the basin, where data are lacking or the watersheds are inaccessible. The

spatial variations in the distribution of tributary channels of Muthirapuzha, is

indicative of the role of the drainage network in determining the hydrological regime.

The higher bifurcation ratios, along with higher drainage density and low elongationratios and form factors suggest the geological control of later (neo) tectonic activities

on drainage organization. The watershed ‘enjoys’ sheet, rill and gully erosion, and as

a consequence a large volume of sediment is under transport. The extensive

monoculture plantations of tea and secondary eucalyptus, maintained in the

watershed may severely augment the impact. Though the hydrological system is

highly complex, the analysis of the morphometric parameters provides adequate

information about both terrain characteristics and hydrological behavior of the

watersheds. An understanding of the watershed response to different processes is onecomponent of the knowledge base required in applying principles of watershed

management. Hence, it would be concluded from the above study that the

integration of morphometrical analysis along with conventional watershed assess-

ment methods would have a beneficial effect on judicious watershed management.

Acknowledgements

I (JT) am indebted to late Dr. R. Satheesh (Reader, School of Environmental Sciences,Mahatma Gandhi University, Kerala) for strongly motivating me in high altitude mountainresearch, Dr. Rajesh Raghunath (Department of Geology, University of Kerala), Mr. GeorgeAbe (CWRDM, Kottayam) for help rendered at various stages of this study, and Dr. A.P.Thomas (Director) and Mr. H. Vijith (Centralized Remote Sensing and GIS facility, School ofEnvironmental Sciences, Mahatma Gandhi University, Kerala), for rendering immenseassistance during data analysis. Finally, we are grateful to Kerala Forest Department (forpermission and logistic support in the field studies) and Kerala State Council for Science,Technology, and Environment (KSCSTE), Thiruvananthapuram (financial support).

Notes on contributors

Jobin Thomas is currently doing his Ph.D. as a Junior Research Fellow in the Department ofEnvironmental Sciences, University of Kerala. He was awarded the M.Sc. in EnvironmentalSciences from Mahatma Gandhi University, Kerala. The broad goal of his contemporaryresearch is to understand the influence of various environmental variables on river channelmorphology. His broad research interests range from river channel morphology to applica-tions of remote sensing and GIS.

Dr. Sabu Joseph is currently working as Senior Lecturer in the Department of EnvironmentalSciences, University of Kerala. He acquired his M.Sc. in Geology and Ph.D. in Sedimentologyfrom University of Kerala. His current research addresses the process geomorphology of



fluvial systems and his lab projects range from hydrogeochemical analysis of river systems toisotopic applications in environmental studies.

Prof. Thrivikramji is presently working as the Project Director, Climate Change & Energy,Centre for Environment and Development, Thiruvananthapuram, Kerala, India. He took hisM.S. in Geology (Computer applications in stratigraphic analysis) and Ph.D. in Sedimento-logy (Paleo-hydraulics) from Syracuse University, New York. His research programs center onthe study of river metamorphosis due to human interventions and paleoclimatic significance ofsediments.

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morphometric aspects of a small tropical mountain river system

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