Climate in south India is controlled by monsoons periodicity causing an alternation of dry and wet seasons. The dry season extends from mid-December to May and the wet season starts with the arrival of the southwest monsoon from May to September and continues with the northeast monsoon from October to December. The Karnataka State, located in Southern India, presents in its south-eastern part a unique context to carry out comparative studies about the effect of environmental factors. Indeed, the Western Ghâts parallel to the south west coast of the peninsula form an orographic barrier inducing an important climatic gradient. By crossing this barrier from west to east, annual rainfall decreases progressively within a distance of about 80 km from about 5000 mm/yr (Humid area) to 750 mm/yr (Semi arid area) (BARBIERO ET AL. 2007). Both experimental watersheds of Mule Hole and Maddur are located along this transition zone (Figure 4).

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Figure 4. Climatic gradient over the Western Ghâts

The Mule Hole watershed is situated in a protected forested area (the Bandipur National Park) in the climatic transition zone, along the road from Gundlupet (Karnataka State) to Sultan Battery (Kerala State),.
The study site is located in the humid transition area: the average annual rainfall is 1120 mm/yr. The mean temperature is around 27°C per year. The reference evapotranspiration is about 1050 mm/yr. The average (from 2004 to 2006) aridity index (Figure 5), defined as the ratio between rainfall and evapotranspiration (UNESCO, 1979) is 1.2. The climate can be classified as humid.

The Maddur watershed is located further east in more dry conditions: rainfall is about 750 mm/yr. The reference evapotranspiration is about 1250 mm/yr, inducing a lower aridity index of 0.75 (average on 2005 and 2006; Figure 5). The climate tends to shift to sub-humid conditions.

Diagramme de d'indice d'aridité

Figure 5. Rainfall vs. aridity index for the Mule Hole and Maddur watersheds