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RICE-the staple food-CLIMATE

Climatic Conditions in Rice Grown Areas

Sri Lanka exemplifies a variety of climatic conditions emphasized by the geographical settings of respective locations.

Temperature:

  • Country’s centralized location in the Indian Ocean (50 – 100 N) facilitates a typical tropical maritime temperature conditions
  • This results greater daily than annual temperature ranges and moderate average temperatures in comparison with other continental tropics
  • Significant temperature drops could be experienced when reaching central highlands based on the vertical atmospheric lapse rate

Annual rainfall:

  • Varies from ~ 900 mm (Maha Lewaya, Hambantota) to over 5,500 mm (Kenilworth Estate, Ginigathhena)

Solar radiation:

  • This is not a limiting factor for rice crop growth.
  • However, when all other factors (water, nutrients, and temperature) are non-limiting, the intensity of sunlight may determine the yield level depending on the location and season.
  • Eg.: in the Wet zone, high cloud cover resulted from the southwest monsoonal circulation may limit the rice yield during Yala season and similarly in the Dry zone during  Maha season due to northeast monsoonal circulation.

3 general climatic zones

3 climatic zones based on

  • precipitation
  • soil type
  • land use patterns
  • vegetation types

Elevation

Vertical atmospheric lapse rate

  • Drop in environmental temperature drops along with the increasing altitude.
  • 07 agro-climatic zones based on the altitude.

Weather rythems

The“hydrological year” (“climatic year”)

  • Begins in March and not in January.
  • Seasonal weather rhythm (rainfall seasons) ranges from March to next February.

Climatic zones

Wet zone

  • southwestern region including western slope of the central hills.
  • relatively high mean annual rainfall (>2,500 mm) without pronounced dry periods.

Dry zone

  • mainly the northern and eastern part of the country.
  • low mean annual rainfall (<1,750 mm) with a distinct dry season from May to September.

Intermediate zone

  • region skirting the central hills (except in the south and the west) that separates Wet and Dry zones.
  • a mean annual rainfall between 1,750 to 2,500 mm with a short and less prominent dry season.

Agro-climatic zones

Low country

  • region below 300 m in elevation
  • 03 agro-climatic zones
  • low country (1) wet zone, (2) dry zone and (3) intermediate zone

Mid country

  • region ranging from 300 – 900 m elevations
  • 02 agro-climatic zones
  • mid country (1) wet zone and (2) intermediate zone

Up country

  • region higher than 900 m in elevation
  • 02 agro-climatic zones
  • up country (1) wet zone and (2) intermediate zone

These 07 agro-climatic zones are further sub-divided into 46 Agro-Ecological Regions (AER) covering the entire island based on:

  • rainfall regime
  • terrain characteristics
  • predominant soil type
  • land use patterns and vegetation
  • expected dryness

Each AER has unique characteristics that provide suitability for unique crops. 

Rice is grown in all AERs other than the up country (low temperature during night is a limiting factor for rice cultivation).

READ MORE –  Map of Agro-ecological regions >>

 

Rainfall seasons

There are 04 rainfall seasons in Sri Lanka:

1March – AprilFirst Inter Monsoon (FIM) rains
2May – SeptemberSouth West Monsoon (SWM) rains
3October – November Second Inter Monsoon (SIM) rains
4November – FebruaryNorth East Monsoon (NEM) rains

Rainfall experienced in these seasons are not homogenous, resulting an immense agro-ecological diversity. In assition, SWM rains are not effective over the Dry zone,

Two major crop seasons are operated in Sri Lanka based on this rainfall seasons. 

Yala season

  • FIM and SWM rains are in effect.
  • As SWM is not effective in Dry zone, Yala season is restricted to 02 months (mid-March to early May).
  • Considered as the minor growing season for the Dry zone.

Maha season

  • SIM and NEM rains are in effect.
  • Major growing season for the entire country.

 

Climatic zones

Sri Lanka has traditionally been generalized in to three climatic zones 

  • “Wet Zone” in the southwestern region including western slope of the central hill country
  • “Dry Zone” represented predominantly by northern and eastern part of the country
  • “Intermediate zone”, skirting the central hills except in the south and the west, separates Wet and Dry Zones

Differentiation is largely based on

  • Precipitation
  • Contribution of south-west monsoon rains
  • Soil type
  • Land use patterns
  • Vegetation types

Annual rainfalls

  • Wet zone – relatively high mean annual rainfall over 2,500 mm without pronounced dry periods
  • Dry zone – a mean annual rainfall less than 1,750 mm with a distinct dry season from May to September
  • Intermediate zone – a mean annual rainfall between 1,750 to 2,500 mm with a short and less prominent dry season.

These climatic zones are further classified to specific agro-ecological regions considering more sub-regionally specific annual rainfall, expected dryness in different months, major soil groups and characteristics of the terrain.

In addition, climatological studies show that the “hydrological year” (“climatic year”) of the island begins in March and not in January. Therefore, seasonal weather rhythm (rainfall seasons) ranges from March to February

Low temperature

Low temperature is an important climatic factor affecting plant growth. Vertical atmospheric lapse  rate could be considered as a factor that causes temperature drop with the increasing altitude; Therefore, a sub-division of climatic zones was recognized based on the altitude, resulting seven agro-climatic zones.

region below 300 m in elevation – “Low-country” (represented by areas belong to all three climatic zones)
region ranging from 300 – 900 m elevations – “Mid-country” (represented only by Wet and Intermediate zones)
region with elevation higher than 900 m – “Up country” (represented only by Wet and Intermediate zones)

These seven agro-climatic zones have further sub-divided into Agro-Ecological Regions (AER) with a total of 46 AERs covering the entire island .

READ MORE –  Map of Agro-ecological regions >>

AER boundaries are delineated based on

  • rainfall regime
  • terrain characteristics
  • predominant soil type
  • land use
  • vegetation

Each AER has unique characteristics which, emphasizes that each AER is specifically provide suitability for a unique crops and crop varieties that could be cultivated there in. Rice is grown under diverse environmental conditions than other major food crops in the world which remains the same for Sri Lanka.

Paddy is the most common land use in valley bottoms. Other than AERs in the Up country Wet and Intermediate zones (as the low temperature during night becomes a limiting factor), rice is grown in all other AERs.

Rainfall seasons

It is generally accepted that there are four rainfall seasons in Sri Lanka:

1March – AprilFirst Inter Monsoon (FIM) rains
2May – SeptemberSouth West Monsoon (SWM) rains
3October – November Second Inter Monsoon (SIM) rains
4November – FebruaryNorth East Monsoon (NEM) rains

These rainfall seasons do not bring homogeneous rainfall regimes to the entire island, causing  immense agro-ecological diversity.

Two consecutive combined rainy seasons make up the major crop growing seasons of Sri Lanka, namely Yala (combination of FIM and SWM rains) and Maha (begins with the SIM rains and continues combined with the NEM rains) seasons.

SWM rains are not effective over the Dry zone, therefore, Yala season in the Dry zone is restricted only from mid-March to early May. Being effective only for two months, the Yala season is considered as the minor growing season for the Dry zone. The major growing season of the whole country-is Maha

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