Sri Lanka has a wide variation of climates and seasons and receives abundant rainfall. The soil resource varies by region and thus the crops, forest types and natural vegetation. These variations lead to a rich natural endowment of biodiversity. These are presented as the bio resource base of Sri Lanka. These bioresources determine the food security and influences the way people live. Thus as in every country that is predominantly dependent on forest and agriculture for livelihood and food, Sri Lanka illustrates eloquently the lifescape and landscape relations.
2.1 Climate, seasons and rainfall
The climate in Sri Lanka is tropical monsoon with five seasons. These seasons have a bearing on the production cycles in their own land and availability of seasonal crops and vegetables that determine household's availability and access to food.
Rainfall: The five seasons in Sri Lanka are primarily marked by rainfall periods (Baldwin, 1991) that are described below.
The convection - convergence period (March to mid April). The rainy days follow the pattern of clear sky in the morning, formation of rain clouds in the early afternoon followed by a thunderstorm in late afternoon. This pattern of rains covers most of the island area.
The pre-monsoon period (mid April to late May). This represents the surge of advancing Southwest monsoon, which results in rainfall in south and southwestern portion of the country. It marks the end of convectional weather.
The Southwest monsoon period (late May to late September). This brings largest quantity of water in the southwestern region of the island. The central highland massif intercepts the monsoon clouds, where the moisture carrying currents, rise high with fall of temperature and cause heavy precipitation. However, the wind currents glide past, massive rampart in northwesterly direction, thereby depriving rest of the country with the benefit of rain. The Southwest monsoon brings rainfall varying between 200 mm – 2 000 mm. It is restricted to southwestern region of the island. Dry strong winds blow in the north, north central region, and southeastern region. The agricultural season associated with this spell is called Yala.
The cyclonic period (late September to late November). This is the revival of earlier pre-south west monsoon phase of rains. The cyclonic systems developing in Bay of Bengal and Indian Ocean can cause heavy down pour accompanied by strong winds.
The northeastern monsoon period (November to January). This winter monsoon carrying moisture from Bay of Bengal is comparatively weak, but it covers almost the entire country. It marks the beginning of Maha agriculture season. The next significant spell occurs during the months of October and November. Convection currents and the formation of deep depressions, which bring about widespread precipitation over the island. Adequate rainfalls varying between 400–750 mm, which helps in pre-winter monsoon agricultural activities of the following Maha season.
Temperature: Two main factors influence the temperature regime in Sri Lanka, its location in the tropical zone and land being surrounded by sea. The prevailing mean annual temperature ranges around 27 Celsius with a mean daily range of about 6 Celsius. This value held good for 75–80 percent of the land area, specifically for low-country, dry zone. However, in the central highlands, due to altitudinal effect, the mean annual temperature is around 15 Celsius. In Nuwara Eliya region ground frost conditions developed occasionally (Panabokke, 1996).
2.2 Soil resources
Major groupings of world soils are present in Sri Lanka. The 14 major soil groups are distributed in 4 topographic classes: (i) Flat to undulating; (ii) rolling to hilly; (iii) hilly and mountainous; and (iv) extremely steep. Due to high intensity rainfall Sri Lanka's soils are vulnerable to erosion, soils like immature brown loams, non-classic brown and reddish brown earth's are highly erodible. Even the comparatively stable soils like red yellow padsolic and reddish brown latosolic soils, which occur on steep slopes, are susceptible to erosion. Soil conservation measures are most essential input for sustaining high levels of agricultural productivity, (Panabokke, 1996).
2.3 Water resources
Sri Lanka receives annually about 12 million ha. /m of water through atmospheric precipitation, chiefly through two monsoon seasons. Nearly 50 percent of the water is lost, 20 percent percolates into the soil to re-charge the ground water and remaining, 30 percent (3.5 million ha/m) is available as surface water for agricultural, industrial and civic uses (Baldwin, 1991). Historically and to date the waters sources influenced human settlement choices and today water resource management interventions affect the human resettlement policies.
2.3.i River systems
The central highland massif, which intercepts the two major spells of monsoon rains and other precipitation, is focal point, where from 103 river systems radiate out. The collective length of rivers is about 4 560 km and basins drain an area of 59 217 sq. kms. Thus covering 90 percent of the island. The rivers falling in the dry zone are small and seasonal (Baldwin, 1991).
The Mahaweli Ganga is the longest river (325 km) and has the largest basin of 10 327 sq. km., covering one sixth of the country. It rises in the Southwest central highland wetzone and carries water to the dry zone in Northeast.
The Mahaweli Ganga has been harnessed by a series of dams to produce hydro-electricity with an aggregate installed capacity of 938 megawatts, which meet 90 percent of the electricity requirements of the country. Its reservoirs irrigate over 500 000 ha. of land (Baldwin, 1991).
The Walwe Ganga, which also rises from the central highland wet zone, drains southwards and merges into sea at Ambalamtota. With the construction of a large Uda Walwe reservoir, the irrigation facilities have vastly improved to develop land for an integrated system of multiple cropping. New settlements are being developed under the Mahaweli Economic Agency Development programme. This programme has given great impetus to socio-economic development in the Humbantota district, which falls in the dry-arid zones.
2.3.ii Water reservoirs
The rainwater flow in rivers and streams has been harnessed into reservoirs and village tanks scattered in the extensive dry zone area. A large number of man made and naturally occurring water bodies is a prominent feature of lowlands. These water bodies are called village tanks or wewa. The area of a water body varies from the smallest of 1 ha to the largest 6 500 ha of most of others range from 100–300 ha. There are about 10 000 reservoirs, which irrigate about 500 000 ha. of lands. The total spread of water bodies is about 170 000 ha. About 12 000 small village tanks irrigate 269 000 ha. of land in the dry zone.