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Agro Ecological Zones


Climate of rice growing regions in Sri Lanka

Rice is grown under more diverse environmental conditions than any other major food crop in the world and situation remains as the same in Sri Lanka as well. In spite of its relatively small aerial extent, Sri Lanka exemplifies a variety of climatic conditions depending on geographical settings of respective locations. The average annual rainfall varies from about 900 mm (Maha Lewaya, Hambantota) to over 5,500 mm (Kenilworth Estate, Ginigathhena). Being located in the low latitudes between 6° and 10° N and surrounded by the Indian Ocean, Sri Lanka shows very typical maritime-tropical temperature conditions. These conditions are characterized by greater daily than annual temperature ranges and moderate average temperatures in comparison with the more continental tropics.  Temperature conditions in Sri Lanka are also characterized by a significant temperature decrease in the central highlands according to the vertical atmospheric lapse rate of the atmosphere.

Sri Lanka has traditionally been generalized in to three climatic zones in terms of “Wet Zone” in the southwestern region including central hill country, and “Dry Zone” covering predominantly, northern and eastern part of the country, being separated by an “Intermediate zone,” skirting the central hills except in the south and the west (Figure 1).  In differentiating aforesaid three climatic zones, rainfall, soils, land use and vegetation have been widely used.  The Wet zone receives relatively high mean annual rainfall over 2,500 mm without pronounced dry periods.  The Dry zone receives a mean annual rainfall of less than 1,750 mm with a distinct dry season from May to September.  The Intermediate zone receives a mean annual rainfall between 1,750 to 2,500 mm with a short and less prominent dry season.  As low temperature is an important climatic factor affecting plant growth in the Wet and Intermediate zones of Sri Lanka, a sub-division based on the altitude takes into account the temperature limitations in these two climatic regions. In this delineation, the Low-country is demarcated as the land below 300 m in elevation and the Mid-country with elevation between 300 - 900 m while the Up-country is the land above 900 m elevation (Figure 2).  Both Wet and Intermediate zones spread across all three categories of elevation while the Dry zone is confined to the Low-country resulting seven agro-climatic zones covering the entire island (Figure 3). These seven agro-climatic zones have further sub-divided into Agro-Ecological Regions (AER) with a total of 46 AERs covering the entire island (Figure 4). The delineation of AER boundaries of Sri Lanka has been based on the rainfall regime, terrain characteristics, predominant soil type, land use and vegetation so that each AER represents an uniform agro-climate, soils and terrain conditions and as such would support a particular farming system where certain range of crops and farming practices find their best expression.

Detailed studies on climatology of Sri Lanka has identified that "climatic year" or "hydrological year" of the island begins in March and not in January so that seasonal weather rhythm or more specifically the rainfall seasons ranges from March to February. It is generally accepted that there are four rainfall seasons in Sri Lanka:

March - April   ® First Inter Monsoon (FIM) rains
May - September ® South West Monsoon (SWM) rains
October - November ® Second Inter Monsoon (SIM) rains
November - February ® North East Monsoon (NEM) rains

These rainfall seasons do not bring homogeneous rainfall regimes over the whole island and it is the main cause to exhibit such a high agro-ecological diversity of the country despite its relatively small aerial extent.  Out of these four rainfall seasons, two consecutive rainy seasons make up the major growing seasons of Sri Lanka, namely Yala and Maha seasons. Generally Yala season is the combination of FIM and SWM rains. However, since SWM rains are not effective over the Dry zone it is only the FIM rains that fall during the Yala season in the Dry zone from mid March to early May. Being effective only for two months, the Yala season is considered as the minor growing season of the Dry zone. The major growing season of the whole country, Maha begins with arrival of SIM rains in Mid September/October and continues up to late January/February with the NEM rains.

Except in almost all AERs in the Up country Wet and Intermediate zones where minimum temperature at night is limiting, paddy is the most common land use in valley bottoms in the all other AERs of the country. Solar radiation is not a limiting factor for rice growth in almost all rice growing regions of Sri Lanka. However, when all other conditions such as water, nutrients and temperature are non-limiting the intensity of sunlight may determine the yield level depending on the location and season. For example, in the Wet zone, solar radiation may limit the rice yield during Yala season due to high cloud cover arising from the south west monsoonal circulation whereas a similar situation could expect in the Dry zone during Maha season due to overcast conditions that may result due to “weather systems” formed in the Bay of Bengal and north east monsoonal circulation.

Climate of the Low Country Wet Zone

This agro-climatic region has been sub-divided in to five AERs (Figure 4) where rice is the sole land use in inland valleys and flood plains. The expected annual rainfall at the 75% probability level in this region ranges from 1,700 to 3,200 mm depending on the agro-ecological region. Its average maximum temperature ranges from 32 to 35 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 22 to 24 0C where the lowest values are generally observed during the period of December to February, the winter months of the island. The day time relative humidity is generally ranged from 60 to 75 percent where as night time values may reach even up to 90 per cent at anytime of the year.

Climate of the Mid Country Wet Zone

Even though this agro-climatic region has been sub-divided in to six AERs (Figure 4) rice is predominantly found only in four AERs.  The two AERs located in higher elevations, namely WM1a and WM1b are not suitable for rice as cool injuries are likely to occur. The expected annual rainfall at the 75% probability level in this region ranges from 1,400 to 3,300 mm depending on the agro-ecological region. Its average maximum temperature ranges from 27 to 33 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 18 to 22 0C where the lowest values are generally observed during the period of December to February, the winter months of the island. The further low values of night temperature are likely to experience in higher elevations of the region. The day time relative humidity is generally ranged from 55 to 80 percent where as night time values may range from 75 to 85 per cent.

Climate of the Up Country Wet Zone

This agro-climatic region has been sub-divided in to four AERs (Figure 4) and rice is hardly found in this region. As the elevation of this agro-climatic region is well above the 900 m, temperature has become a limiting factor for growth of rice plant.

Climate of the Low Country Intermediate Zone

This agro-climatic region has been sub-divided in to five AERs (Figure 4) where rice is the predominant land use in valley bottoms and terraced upland slopes in some areas. The expected annual rainfall at the 75% probability level in this region ranges from 1,100 to 1,600 mm depending on the agro-ecological region. Its average maximum temperature ranges from 29 to 35 0C. The highest values are being recorded during the period of late February to early May. The average minimum temperature is ranged from 20 to 26 0C where the lowest values are generally observed during the period of December to February, a common phenomenon for the entire island. The day time relative humidity is generally ranged from 55 to 75 percent where as night time values may reach even up to 90 per cent especially during winter months of the year.

Climate of the Mid Country Intermediate Zone

Although this agro-climatic region has been sub-divided in to eight AERs (Figure 4), rice is being cultivated only in five AERs.  In these AERs rice is the major land use in valley bottoms and terraced slopes at least in one season out of two seasons in a year. In the other season, farmers may switch in to vegetable cultivation depending on the suitability of their lands for the same. The expected annual rainfall at the 75% probability level in this region ranges from 1,100 to 2,000 mm depending on the agro-ecological region. Its average maximum temperature ranges from 28 to 330C. The highest values are being recorded during the period of late March to early May. The average minimum temperature is ranged from 18 to 23 0C where the lowest values are generally observed during the period of December to February. The day time relative humidity is generally ranged from 55 to 75 percent where as night time values are generally around 75 to 85 per cent.

Climate of the Up Country Intermediate Zone

Although this agro-climatic region has been sub-divided in to seven AERs, rice is being cultivated only in two AERs due to limitation of the temperature regime in the rest of  AERs owing to their relatively higher elevations.  In those two AERs rice may be the major land use in valley bottoms during minor rainy season (Yala). In the other season (Maha), farmers may switch in to high value temperate vegetable crops, especially for potato to harness the potential of low temperature regime prevailing in these regions. The expected annual rainfall at the 75% probability level in those two regions ranges from 1,400 to 1,600. Its average maximum temperature ranges from 22 to 290C. The highest values are being recorded during the period of late March to September. During the said period, high wind speeds that blow from the southwest direction is a common weather phenomenon to experience in this region. The average minimum temperature is ranged from 13 to 18 0C where the lowest values are generally observed during the period of December to March. Hence, low temperature injuries in rice plants could be a recurrent problem if rice is grown in those regions during the major rainy season, Maha season.  Relative humidity during day time in this agro-climatic region is generally ranged from 60 to 82 percent where as night time values may reach even up to 90 per cent especially during winter months of the year.

Climate of the Low Country Dry Zone

This agro-climatic region is the country’s driest part and it has been sub-divided in to 11 AERs. Even though water is a limiting factor in this part of the country for year round crop production, trans-basin diversion of some rivers of Wet and Intermediate zones and large number of tanks that were built during ancient times have made it possible to cultivate lowlands in to rice or rice based cropping systems.  Out of 11 AERs rice is the predominant man made land use in 10 AERs except in DL3 AER, the Oxisol belt which spreads from northwestern coastal region to northern peninsular. The expected annual rainfall at the 75% probability level in this region ranges from 650 to 1,100 mm depending on the agro-ecological region. In some AERs monthly rainfall distribution depicts a bi-modal pattern where as AERs found in the northeastern and part of the Dry zone shows a uni-modal monthly rainfall distribution and hence, cultivation of rice in lowland in those regions is possible only during the major rainy season (Maha season) unless irrigation water is provided. When the Wet zone of Sri Lanka experiences South West Monsoon rains, the same monsoonal wind blows over the Dry zone as a warm and dry wind, a Föhn like wind locally known as Yal Hulang or Kachchan. Hence, crop water requirement during this period, May to September (Yala season) is very much higher than that of the other times of the year (Maha season). The general wind speed of the Dry zone is 3 – 5 km/hr. However, during said period it may reach even 12 – 15 km/hr speed. The average maximum temperature in the Dry zone ranges from 29 to 38 0C depending on the AER. The highest values are being recorded during the period of late February to late September irrespective of the location. Thus, high temperature injuries are being experienced in rice grown during Yala season in the Dry zone.  Continuous weather observations have shown that it is becoming more and more common feature in rice cultivation during recent times and it could be a repercussion of global warming. The average minimum temperature is ranged from 20 to 26 0C where the lowest values are generally observed during the period of December to February, a common phenomenon for the entire island. However, further low nighttime temperatures are experienced during winter months in the northern peninsular of the island due to the influence of the huge land mass of the Indian sub-continent making it possible to grow potato. However, rice is not grown in this region due to some other edaphic limitations. The day time relative humidity in the Dry zone is generally ranged from 50 to 75 percent where as night time values may reach even up to 90 per cent, especially during winter months of the year.



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Last updated: October 30, 2007