Indeed, around 8% of India’s waterways (measured by length) are found in Kerala. The most important of Kerala’s forty-four rivers include the Periyar (244 km in length), the Bharathapuzha (209 km), the Pamba (176 km), the Chaliyar (169 km), the Kadalundipuzha (130 km), and the Achankovil (128 km). Most of the remainder are small and entirely fed by monsoon rains. These conditions result in the nearly year-round waterlogging of such western regions as Kuttanad, 500 km² of which lie below sea level.

Kerala’s climate is mainly wet and maritime tropical,[10] heavily influenced by the seasonal heavy rains brought by the Southwest Summer Monsoon. In eastern Kerala, a drier tropical wet and dry climate prevails. Kerala receives an average annual rainfall of 3,107 mm - some 70.3 km3 of water. This compares to the all-India average of 1,197 mm. Parts of Kerala’s lowlands may average only 1,250 mm annually while the cool mountainous eastern highlands of Idukki district - comprising Kerala’s wettest region - receive in excess of 5,000 mm of orographic precipitation (4,200 mm of which are available for human use) annually.

Kerala’s rains are mostly the result of seasonal monsoons. As a result, Kerala averages some 120-140 rainy days per year. In summers, most of Kerala is prone to gale-force winds, storm surges, and torrential downpours accompanying dangerous cyclones coming in off the Indian Ocean. It is also prone to occassional droughts,[11] as well as rises in sea level and cyclonic activity resulting from global warming.[12][13] Kerala’s average maximum daily temperature is around 36.7 °C; the minumum is 19.8 °C.[7] Mean annual temperatures range from between 25.0-27.5 °C in the coastal lowlands to between 20.0-22.5 °C in the highlands.

Climate

Kerala, which lies in the tropics, is mostly subject to the type of humid tropical wet climate experienced by most of Earth’s rainforests. Meanwhile, its extreme eastern fringes experience a drier tropical wet and dry climate. Kerala receives an average annual rainfall of 3107 mm - some 7,030 crore m3 of water. This compares to the all-India average is 1,197 mm. Parts of Kerala’s lowlands may average only 1250 mm annually while the cool mountainous eastern highlands of Idukki district - comprising Kerala’s wettest region - receive in excess of 5,000 mm of orographic precipitation (4,200 crore of which are available for human use) annually.

Kerala’s rains are mostly the result of seasonal monsoons. As a result, Kerala averages some 120-140 rainy days per year. In summers, most of Kerala is prone to gale-force winds, storm surges, and torrential downpours accompanying dangerous cyclones coming in off the Indian Ocean. Kerala’s average maximum daily temperature is around 36.7 °C; the minumum is 19.8 °C.

Physiogeography
Eastern Kerala consists of land encroached upon by the Western Ghats; the region thus includes high mountains, gorges, and deep-cut valleys. The wildest lands are covered with dense forests, while other regions lie under tea and coffee plantations (established mainly in the 19th and 20th centuries) or other forms of cultivation.

Forty-one of Kerala’s forty-four rivers originate in this region, and the Cauvery River descends from there and drains eastwards into neighboring states. Here, the Western Ghats form a wall of mountains penetrated near Palakkad; here, a natural mountain pass known as the Palakkad Gap breaks through to access inner India. The Western Ghats rises on average to 1500 m elevation above sea level.