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Forest landscapes of the southern western Ghats, India

B.R. Ramesh
Rajan Gurukkal

Part II. Analysing The Landscape: The Anamalai Region (Nelliyampathi Hills)

2. Physical and Bioclimatic Features of the Selected Landscape Units

B.R. Ramesh, Pierre Couteron, Gérard Bourgeon et G. Muthu Sankar

Texte intégral

1The level of heterogeneity in a landscape is generally determined by its physical, biological and human ecological complexities. Understanding the characteristics, behaviour and interactions of these elements are crucial for evolving a prudent management system. The landscape units in Kerala (see chapter 1) although may have some similarities in landscape elements; the complex environment driven by topographical and bioclimatic variability on the one hand and diverse cultural, social and economic situations driven by various demographic patterns and livelihood issues on the other have made many units unique.

2The Anamalai Hills along the southern rim of the Palghat Gap culminating at Anaimudi (2695 m- the highest point in the Ghats) constitute one of the three endemic centres of the Western Ghats (Nayar, 1996). Based on elevation range, topographic features and soil characteristics three landscape units are identified in this region. Of these, the landscape unit 16 (LU16) that covers the entire area of the Western Anamalai region (Figure 1.7 in chapter 1) has been selected for the study. In addition, the tapered and juxtaposed part of the LU13 that acts as a buffer to LU16 in the west from the highly populated coastal hinterland has been considered for comparative purposes.

3However, due to logistic constraints, only watershed areas of Chalakudi, Karuvannur and Periyar (northern part) rivers within these two landscape units were selected for the study. Thus the study site lies between 76º 21′ 55′′– 76º 56′ 55′′E and 10º 04′ 21′′– 10º 33′ 29′′ N (Figure 2.1) encompassing an area of 1837 km2, in which LU13 and LU16 account to 545 and 1292 km2 respectively. Administratively the selected area covers seven Reserved Forests consisting of three PAs (Parambikulam, Chimmony and Thattekad) and four non-PAs (part of Nemmara, Chalakudy, Vazhachal and Malayattoor). The State of Tamil Nadu borders the northeastern part of the study area.

Figure 2.1 Study area showing LU13 and 16 forest adminstrative units
(Inset: location of the study area based on figure 1.7)

4Abiotic factors like relief, geology, soil, drainage and bioclimate play a significant role in shaping the structure of the landscape. This chapter deals with the physical and spatial outline of such factors that influence the biological elements including human activities.

Relief and drainage

5The DEM (Digital Elevation Model) prepared for the study area (Figure 2.2) is based on SRTM (Shuttle Radar Topographic Mission) data distributed by USGS (United States Geological Survey). The spatial resolution of the data is given by a pixel size of 90 m and the relative vertical accuracy is ±6 m.

6LU16 is generally spread between an average elevation of 400 and 1400 m and represented by partially and strongly dissected surfaces. The partially dissected surface is mostly above 600 m and forms two major plateaus represented by Nelliyampathi and Sholayar. Parambikulam basin in the northeastern corner of LU16 drains off into the Chalakudi River in the gap between these two plateaus. In the southeast, Idamalayar and Pooyamkutti valleys become an enclave with the northern slope of Sholayar plateau and spur hills originating from the High Range. The descending hills and valleys from these two plateaus form the strongly dissected surface. The strongly dissected scarp at the lower reaches interspersed with broad valleys form the LU13.

7The landscape units together encompass one complete (Chalakudi) and two partial (Karuvannar and Periyar) watershed areas (Figure 2.3). The networks of valleys of these watersheds are strongly defined by two faults: one oriented to the north-south and the other from the north-west to the south-east directions. The Chalakudi (fed by Karappara, Parambikulam and Sholayar) and Periyar (fed by Idamala Ar and Pooyamkutty Ar) are two major rivers. The Kurumalipuzha River along with the Chimmony stream is one of the main tributaries from the Karuvannur watershed area. The regular flow of all these rivers is restricted by seven reservoirs covering a total area of about 90 km2. These reservoirs are built to cater to the hydroelectric, irrigation and drinking water demand of Kerala and Tamil Nadu. There is a proposal to have more reservoirs in the Chalakudy River (at Athirapally and Karappara) and Periyar (Pooyamkutty) River.

Figure 2.2 Physiography of the handscape units

Figure 2.3 Drainage network and watershed areas in the landscape units

Geology and soil

8The map showing geology (Figure 2.4) has been clipped from the 1: 1000000 scale Geological and Mineral Map of Kerala (Balasundaram, 1970). According to this map, the main rock formation in the study area is the Precambrian charnockite, about 2600 million years old. Charnockite is a coarse granular rock mainly composed of quartz, feldspar and hypersthene besides other minerals. It is a metamorphic rock, a kind of gneiss formed under great pressure at high temperature in the presence of CO2 and water. Apart from charnockite, there are intrusions of granites, mainly in the Parambikulam basin and Nelliyampathi plateau. Gneisses are also found sporadically and mostly in southern part of LU13.

Figure 2.4 Geology of the landscape units

Figure 2.5 Soil map of the landscape units (adapted from NBSS and LUB)

9The soil map (Figure 2.5) has been adapted from the soil map of Kerala prepared by the NBSS & LUB (Krishnan et al, 1996). Although the map published is at 1: 500000 scale, the information in the map is at a resolution of 1: 250000 scale (Nair, pers. com.).

10The parent material (rock) and climatic factors are important in determining the soil types and their characteristics. Considering the higher level (the Soil Order) of Soil Taxonomy, in the study area, there are two types of soils: Inceptisols and Ultisols. The former represents early stages of soil profile development and the latter represents mature stages.

11As most of the study area receives high rainfall, these soils have udic moisture regime (i.e. the soil control section, from 20 to 60cm depth, does not become dry even for 90 cumulative days in normal years). This is considered as regular for the development of Humults that are therefore classified in the Typic subgroup on the map. Most of the soils present in the study area are rich in organic matter. For the Inceptisols this is indicated by their classification in humic subgroups (presence of an umbric epipedon which is defined as relatively rich in organic matter and having a base saturation <50 percent). For Ultisols, they are classified in the Suborder of Humults because they have more than 12kg C/m2 between the mineral soil surface (litter not included) and a depth of 100 cm.

12For our study area, the soil map (Figure 2.5) shows 4 units, which correspond to broad soil associations (as explained in Table 2.1). Regarding their location:

  • unit 31 is predominantly found on LU13;
  • unit 33 with rock outcrops and potential to severe erosion, is mostly found in Parambikulam basin;
  • unit 36 is generally found on gentle slopes of partially dissected hills of LU16 with dense vegetation;
  • unit 38 with numerous rock outcrops is mainly found on steep slopes of highly dissected hills of LU13 and 16.

Table 2 Legend of the Soil Map of the study area (adapted from Krishnan et al, 1996)

Code Dominant soil type Associated with Description
31 Humic Dystrudepts Typic Palehumults Very deep well drained, gravely loam soils, with moderate erosion
32 Humic Dystrudepts Typic Haplohumults Deep well drained, loamy soils with moderate erosion
33 Oxic Dystrudepts Rock Deep well drained gravely clay soils with severe erosion
36 Typic Haplohumults Oxic Dystrudepts Very deep well drained, clayey soils, with moderate erosion. And gravely loam soils on gentle slopes
37 Typic Palehumults Typic Haplohumults Very deep, well drained clayey soils with moderate erosion
38 Typic Palehumults Rock Very deep, well drained, clayey soils with moderate erosion


13Bioclimate is one of the most important factors that determine the types of vegetation of any region. The Western Ghats with hilly terrain harbour a wide array of bioclimate with corresponding forest types (Ramesh, 2001). Pascal (1982) has published bioclimatic maps (at 1: 500000 scale) that depict the regimes of rainfall, temperature and length of the dry season for the southern and the central part of the Western Ghats.

14In the study area, although there is sufficient data on rainfall, the information on the temperature is not adequate enough to use at a bigger scale. Nevertheless, we have used the above mentioned bioclimatic map to describe the temperature and length of the dry season, in spite of its coarseness (compare to the scale of the study area).


15The rainfall data were collected from 65 rain gauge stations that belong to the Meteorological department, Electricity department and estates in and around the study area. Monthly and annual average rainfall figures have been calculated for all the stations for which more than 5 years of data are available. Spatial interpolations between stations were made to assess rainfall figures at the locations of the field plots used for vegetation description and also with respect to a systematic geographical grid (0.05° in each direction). Interpolation was carried out using a specific computer routine (using Matlab®) that attributes to each interpolated point, the weighted mean of rainfall values found for its five nearest neighbours (weighting is done according to the inverse distance to neighbors). Results of the grid interpolation were used to draw the isohyets map (for every 500 mm; ‘contour’ function of Matlab®) of the landscape units (Figure 2.6)

16On this map, the distribution of isohyets shows two principal gradients due to orographic effect and exposure to the southwest monsoon, linked to the arrival of moisture-laden southwesterly winds from the Arabian Sea. This is a major phenomenon determining the rainfall of the study area between June and September. The western part of the study area (LU13 and part of LU16) receives rainfall between 3000 to 3500 mm, however, towards the southeastern corner, the rainfall gradually increases to more than 5500 mm due to the orographic effect of the high range hills, which culminates at Anaimudi (2695 m). On the other hand, towards the northeastern corner, rainfall gradually decreases to less than 1500 mm due to the sheltering effect of Parambikulam basin by the high surfaces of Nelliyampathi and Sholayar.

17Notwithstanding the altitude, all the stations with westerly exposure receive more than 3000 mm and have a rainfall regime with peak during July (Figure 2.6). Being relatively a rain shadow area, the Parmabikulam basin shows a major peak during July. However, it also shows a secondary peak (Sungam and Parambikulam stations) during October, which is mostly related to cyclones and depressions formed in the Bay of Bengal after the withdrawal of the southwest monsoon.

Figure 2.6 Distribution of animal average rainfall in the landscape units and ombrographs for selected stations


18The temperature gradient is mainly linked to elevation. It has been estimated that between 400 and 1500 m altitude, the decrease in temperature is almost 0.8 to 0.9° C for every increase in 100 m (Pascal, 1982). The bioclimatic map indicates that below 700 m, in LU13 and the highly dissected part of LU16, the mean temperature of the coldest month (t) is more than 23° C, whereas on plateaus in LU16, it varies from 16° to 23° C (Figure 2.7).

Figure 2.7 Mean temperature of the coldest month in the LUs (Pascal, 1982)

Length of the dry season

19The length of the dry season (LDS) is expressed in terms of months. According to Bagnouls and Gaussen (1953) a month is considered dry when the rainfall in mm is less than twice the value of the mean temperature (in °C). The bioclimatic map of the Western Ghats (Pascal, 1982) shows two principal gradients in LDS: (1) south to north increase related to the timing of arrival and withdrawal of the south-west monsoon (2) west to east increase corresponds to a decrease in rainfall. In the study area, the average LDS varies from 2 to 5 months (Figure 2.8). Most of the area has 3 months, however, towards the Parambikulam basin, it increases from 4 to 5 months.

Figure 2.8 Length of dry season (in months) in the LUs (Pascal, 1982)


20The analysis of physical features of the study area highlights the complexities and diversities in the natural and operating environment. Erosion prone young soils (Inceptisols) in association with steep slopes and high rainfall indicate the vulnerability of the landscape. These factors eventually lead to deforestation due to landslides and siltation of streams and rivers. It is imperative for landscape managers to take appropriate steps to strengthen such vulnerable areas from natural calamities as well as human induced disturbances. The existing reservoirs already cover 5 % of the study area and a proposal to build three more dams may further affect the already shrunken natural land cover of the area.

Table des illustrations

Légende Figure 2.1 Study area showing LU13 and 16 forest adminstrative units (Inset: location of the study area based on figure 1.7)
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Légende Figure 2.2 Physiography of the handscape units
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Légende Figure 2.3 Drainage network and watershed areas in the landscape units
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Légende Figure 2.4 Geology of the landscape units
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Légende Figure 2.5 Soil map of the landscape units (adapted from NBSS and LUB)
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Légende Figure 2.6 Distribution of animal average rainfall in the landscape units and ombrographs for selected stations
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Légende Figure 2.7 Mean temperature of the coldest month in the LUs (Pascal, 1982)
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Légende Figure 2.8 Length of dry season (in months) in the LUs (Pascal, 1982)
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© Institut Français de Pondichéry, 2007

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