Environment of Sri Lanka

Sri Lanka is a small island economy with a land area of 65,525 sq km, and located between 5. 54'and 9.52' north latitude of the equator and 79 39' and 81 53' east longitude. The climatic pattern of Sri Lanka is determined by the generation of monsoonal wind patterns in the surrounding oceans. Four basic seasons based on rainfall exist. These are, the south - west monsoonal period during May to September; an inter-monsoonal period during October–November; the north-east monsoonal period from December to February; and another inter-monsoonal period lasting from March to April.

Rainfall is monsoonal, convectional and digressional. 55% of the island's rainfall comes from the monsoons. The mean annual rainfall ranges between 900mm to 6000mm, with an island wide average of about 1,900 mm, which is about two and a half times more than the world annual mean of 750mm. The average rainfall varies from below 1000 mm in the arid regions of the dry zone (North West area and the southeast corner of the island), to over 5000 mm in the wet season in south west of the country. Thus on the basis of the rainfall regimes, the country is divided into three broad climatic zones

These are designated as the Wet Zone, Dry Zone and the Intermediate Zone. Sri Lanka is further divided into 24 agro-ecological regions based on rainfall expectancy, altitude, soil class, and landform.

The physiography of Sri Lanka comprise of three pen plains or erosion plains made up of a central highland massif, rising in tiers from a low gently undulating plain surrounding it and extending to the sea, (the lowest peneplain). Rising from the inner edge of the lowest peneplain, in a steep step of about 300 meters is the middle peneplain with a maximum elevation of about 800 m above sea level. Within it and rising from it in another step of 1000 to 1300 m is the highest peneplain at a general level of about 2000 m above mean sea level, but rising in places to 2300 to 2700 m (P.G. Cooray, 1984, Introduction to the Geology of Sri Lanka). 

A major part of the country is made up of Precambrian crystalline rocks, which constitute 90% of the land area (i.e., the Highland Complex, Vijayan Complex and Wanni Complex), with Miocene limestone sedimentary deposits extending from Puttalam to the Jaffna Peninsula. There are Jurassic deposits in Tabbowa and Andigama of the North-western province, and quaternary deposits in the coastal area (i.e., sand, sandstone, clay and gravels, and coral reef). Inland deposits of coral debris and gem-bearing gravels are mainly found in Sabaragamuva Province

There are 103 natural river basins with catchments ranging from 9 to 10,448 sq. km. Seventeen river basins have catchment areas of over than 1000 sq. km. The Mahaweli river, which is the longest river (3352 km), has the largest catchment area (10448 km ). Out of the 103 river basins, some are perennial flowing continuously throughout the year. The ephemeral rivers flow only during or immediately after periods of precipitation, and their flow is less than 30 days per year. The flows of the intermittent streams occur during certain times of the year and their seasonal flow usually lasts longer than 30 days per year

The mean annual temperature is approximately 27.5 C in most parts of the island's low-lying areas, while it 0 is around 18 C in the higher altitudes of the central part of the country. The average relative humidity varies from 70% during day to 95% in the night. Sri Lanka's varied climatic conditions and topography have given rise to a unique biological diversity. Biodiversity, which means diversity of life forms and the variety of systems in which life exists, is defined at three levels, namely, species diversity, genetic diversity and ecosystem diversity. It is significant that more than 35% of the flora are endemic to the country, while more than 65% of those found in the wet zone are endemic. Standing out among the fauna are 59 endemic species of land/fresh water crabs, while more than 50% of land snails, amphibians, and reptiles are endemic. The majority of the endemic species are found in the wet zone.