An agro-ecological region represents a particular combination of the natural characteristics of climate, soil and relief (Panabokke, 1996). When an agro-climatic map, which can be considered as areas where the integrated effect of climate is uniform throughout the area for crop production, is superimposed on soil and terrain the resulting map identifies agro-ecological regions. Thus, each agro-ecological region represents a 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.
In the Wet zone, there are 15 agro-ecological sub-regions. Four sub-regions found in the Up-country wet zone show a distinct variation in the distribution of the South West Monsoon (SWM) rains. Being in the most effective area of the SWM rains, WM1a, WL1a and WU1a sub-regions receive the highest amount of rainfall in the country. Apart from the amount and distribution of SWM rains, relative effectiveness of North East Monsoon (NEM) rains has also played a vital role in distinguishing 6 sub-regions in the mid- country wet zone. The four months period from December to March is relatively “dry” in WM3a agro-ecological sub-region while there are two distinct dry periods in the WM3b due to reduced effectiveness of SWM rains over this sub-region. In the Lowcountry Wet zone, amount and distribution of SWM as well as First Inter Monsoon (FIM) rains were important in identifying the 5 agro-ecological sub-regions. Meanwhile, the months July, August and December in WL3 agro-ecological region does not receive adequate amount of rainfall and hence cannot be considered as wet months. As such, 4 months period extending from December to March is relatively “dry” in this region.
The Intermediate zone consists of 20 agro-ecological sub-regions out of which 15 subregions are in the central hills. Varying degree of effectiveness of different rainfall governing mechanisms across the central hills has caused variety of growing environments in this region. There are 7 agro-ecological sub-regions in the Up country Intermediate zone out of which IU1 is reported to receive the highest annual rainfall among all sub-regions of the entire Intermediate zone. Being in the Knuckles range, this region receives ample amount of rains from NEM while the contribution from SWM rains is also substantial. Complex geographical settings of the IU3 agro-ecological region which encompasses almost whole of the so-called “Uva basin” have resulted 5 agroecological sub-regions due to high spatial variability of intermonsoonal and NEM rains in this region. Meanwhile, being located in the rain shadow area of the SWM, this region does not receive adequate rains during June to September resulting in dry and windy environment. The Mid country Intermediate zone has 7 agro-ecological sub-regions. Most of these sub-regions also do not receive adequate rains from SWM and, hence, 4 months period from June to September is relatively dry. Low country Intermediate zone consists of 5 agro-ecological sub-regions. Other than IL2, all other agro-ecological subregions in the Low country Intermediate zone resemble a bi-modal rainfall distribution. Since Second Inter Monsoon (SIM) and NEM rains are the only effective rainy seasons in the region, the IL2 agro-ecological region exhibits a distinctly uni-modal rainfall distribution along with a long and pronounced dry period from April to September.
In the Dry zone, there are 11 agro-ecological sub-regions with different rainfall distribution and edaphic features. The DL3, DL4 and DL5 agro-ecological regions of the Dry zone receive the lowest annual rainfall of the country in combination with some soil limitations that are found in these regions. Out of 11 agro-ecological sub-regions, only DL1a and DL1b are characterized by two discernible peaks in the rainfall distribution and thus, support crops in both Maha and Yala growing seasons. Those agro-ecological subregions found in the eastern sector of the Dry zone, i.e., DL1c, DL1d, DL1e and DL2a and DL2b, exhibit a distinct uni-modal rainfall pattern, and support only the crops in Maha season. The rest of the agro-ecological sub-regions of the Dry zone also support only the Maha crop since Yala rains in those sub-regions are not adequate to meet the evapotranspiration requirements.
Sri Lanka has a heterogeneous agro-ecological environment and many workers have made efforts to classify this situation. A particular agro-ecological region represents fairly even agro-climate, soils and terrain conditions and would support a particular farming system with a certain range of crops and farming practices, including forage cultivation and livestock farming.
On rainfall distribution, Sri Lanka has traditionally been classified into three climatic zones viz; the Wet Zone, Dry Zone and Intermediate Zone. The Wet Zone covers the south-western region including the central hill country and receives relatively high mean annual rainfall over 2,500 mm without pronounced dry periods. The Dry Zone covers predominantly the northern and eastern part of the country, being separated from the Wet Zone by the Intermediate Zone. 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.
In differentiating these three major climatic zones; land use, forestry, rainfall and soils are widely used and as a result, they were divided into 24 agro-ecological regions. Environmental change, availability of more spatial and temporal data and advancement of GIS technology has led to the sub-division of the 24 agro-ecological regions of Sri Lanka into 46 sub-regions.