Agro-ecology of Sri Lanka

An agro-ecological region represents a particular combination of the natural characteristics of climate, soil and relief. 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. Rainfall distribution in Sri Lanka has traditionally been generalized in to three climatic zones; "Wet Zone" in the southwestern region including central hill country, "Dry Zone" covering predominantly northern and eastern parts of the country and "Intermediate Zone" running between the other two zones. In differentiating aforesaid three climatic zones land use, forestry, rainfall and soils have been widely used. The Wet Zone covers the area, which receives relatively high mean annual rainfall over 2,500 mm without pronounced dry periods. The Dry Zone is the area, which receives a mean annual rainfall of less than 1,750 mm with a distinct dry season from May to September. The Intermediate Zone demarcates the area, which 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. 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.

Based on many decades of work, these seven agro climatic zones have further been divided in to 24 agro-ecological regions in 1979 (Fig. 1). The differentiation of the Wet Zone into its distinctive agro-ecological regions was governed primarily by differences in rainfall distribution. In the Dry Zone, on the other hand, it was the nature of the soil that primarily determines the identity of individual agro-ecological region. In the Intermediate Zone, it was observed that rainfall distribution and soil play an equally important role. According to this map, there are 10 agro-ecological regions in the Wet Zone, 9 in the Intermediate Zone and 5 in the Dry Zone.

Availability of more spatial and temporal data, and advancement of GIS technology have led to the sub-division of 24 agro-ecological regions of Sri Lanka into a map with 46 agro-ecological sub-regions on an enhanced scale in 2002. The demarcation of the island into 46 agro-ecological sub-regions is shown in Fig. 2. The main distinguishing characteristics of each agro-ecological region are denoted by a 4-character code consisting of letters and a number. Three major climatic zones are indicated by the first upper case letter of the code (W, I and D). The second upper case letter of the code (L, M and U) denotes three categories of elevation. The numerical character in the third place of the code represents a more detailed moisture regime (rainfall and evaporation combined) with a degree of wetness on the scale of 1 to 5 where 1 being the most favorable. The lower case letter in the fourth place indicates a sub-region as determined by rainfall distribution and other physical environmental factors where degree of wetness decreases a > f.

Soils of Sri Lanka

The soils of Sri Lanka have been classified at Great Group level for the whole country and Series level for some parts. However, mapping has been done mainly at Great Group level and the generalized soil map of Sri Lanka is given in Fig. 3. Accordingly there are fourteen Great Soil Groups and their extents in the country are given in Table 1.

Table 1. Extents of Great Soil Groups in Sri Lanka

Great Group	Extent ('000 ha)
Reddish Brown Earths	1610
Low Humic Gley	950
Non Calcic Brown	163
Red Yellow Latasols	280
Calcic Red Yellow Latosols	40
Immature Brown Loams	205
Solodized Solonetz	210
Grumusols	15
Red Yellow Podsolic	1490
Reddish Brown Latosolic	60
Alluvials	450
Regosols	190
Bog and Half Bog	60
Lithosols	210

The chemical and physical properties of these soils have been discussed by De Alwis and Panabokke (1972). The physical properties (except for water holding properties) of major Soil Groups such as Reddish Brown Earths, Red Yellow Podzolic, Red Yellow Latosols and Reddish Brown Latosolic are favorable for plant growth. The chemical fertility of Wet Zone soils is poor because these soils have been extensively leached due to high rainfall. The CEC values of most of the soils are low. This requires the use organic manure and special fertilizer management practices on these soils. The base saturation of the Dry Zone soils remains at a higher range. Solodized Solonetz, Bog and Half Bog soils are the major problem soils found in the country. The agricultural potential of soils of Sri Lanka has been discussed by De Alwis and Panabokke (1972). In general, soils of Sri Lanka do not pose major problem for crop production.

The placement of major Soil Groups of Sri Lanka within the Orders of soil taxonomy is shown in Table 2.

Table 2. Placement of Great Soil Groups within seven of the ten Orders of soil taxonomy.

Order	Sub Order	Great Group	Equivalent Great Soil Group (Sri Lanka)
Entisols (Recently formed soils)	Aquents Fluvents Psamments	Tropaquents Tropofluvents Quartxipsammenta	Alluvials Sandy Regosols Soils on old alluvium
Vertisols (Shriniking & swelling (dark clay soils)	Usterts	Pellusterta	Grumusols
Inceptisols (Embryonic soils with a few diagnostic features)	Ustrepts Tropepts	Ustropepts Eutropepts	Immature Brown Loams Immature Brown Loams in Wet Zone
Alfisols (High Base status forest carrying soils)	Aqualfs	Tropaqualfs Natraqualfs	Low Humic Gley Solodized Solonetz
	Ustalfs	Rhodustalfs Haplustalfs	Reddish Brown Earths Non-calcic brown soils
	Udalfs	Rhodudalfs	Reddish Brown Earths of Intermediate Zone
Ultisols (Low base status forest soils)	Udults	Tropudults Rhodudults Tropudults Plinthudults	Reddish Brown Latosolic Red Yellow Podzolic Red Yellow Podzolic Red Yellow Podzolic with soft laterite
	Humults	Tropohumults	Red Yellow Podzolic soils with prominent A-1 horizon
	Ustults	Rhodustults	Red Yellow Podzolic of the semi-dry Intermediate Zone
Oxisols (Sequioxide rich highly weathered soils of the tropical regions)	Ustox	Haplustox Eutrustox	Red Yellow Latosols Calcic Red Yellow Latosols
Histosols (Organic soils)	Hemmists	Tropohemmists	Bog and Half Bog
References: De Alwis KA and CR Panabokke (1972) Handbook of the soils of Sri Lanka. J. Soil Sci. Soc. Ceylon 2, 1-98

Water Resource in Sri Lanka

Surface water

The amount of water remains from rainfall after evaporation and infiltration losses are considered as available surface water. Sri Lanka’s total annual runoff appears to be roughly 5.0 million ha m. It is estimated that nearly 65% of annual rainfall escapes to the sea as runoff by rivers of 103 river basins and by 94 coastal basins, thus there exists a great potential for irrigation development.

Schemes for the purpose of storage and / or regulation of runoff water for mainly irrigation have been in existence mainly in the Dry Zone of Sri Lanka from the ancient time. These schemes have been constructed as a series of small reservoirs or a chain of tanks at successive locations down one single watercourse forming a cascade of tank system. These systems are interconnected storage and regulating reservoirs, which serve multiple functions of resource management including irrigation, domestic supply, water for livestock, and subsurface water for perennial cropping. Some of these tanks have very long histories dating back to over thousand years and were once the backbone of an ancient hydraulic civilization, which flourished in the north central part of the country. However there have been large tanks, which were meant for meeting water deficiencies with trans-basin diversions. The ancient irrigation schemes such as Kalawewa, Nuwarawewa and Kantalewewa are examples for these.

Other than ancient irrigation schemes there are recent irrigation as well as other schemes existing in Sri Lanka. Altogether there are 521 schemes of which 307 are storage irrigation schemes, 104 are diversion irrigation schemes while 110 are drainage, flood control or salt-water exclusion schemes. Moreover, there are 12 schemes in operation with lift irrigation facilities. All schemes, which provide irrigation water for over 80 ha, are considered as major schemes and such schemes are maintained by the Department of Irrigation. Total irrigable extent under these schemes is nearly 340,000 ha. In addition there are about 25,000 minor schemes, which are maintained by the Department of Agrarian Development each serving less than 80 ha. Minor irrigation schemes provide irrigation facilities for 162,000 ha approximately. These irrigation schemes provide water mainly for the cultivation of paddy while cash earning short age crops are also grown quite frequently.

Ground Water

The ground water availability varies within the country depending on the geology and the rainfall. Accordingly, following hydrological regions have been identified.

The western and northern coastal belts consisting of Miocene limestone, sand and wind blown sand
The eastern and south eastern coastal belts consisting of gravel, sand and wind blown sand
The areas occupied by laterites
The narrow faulted basins
Crystalline terrain in the Wet and Dry Zones

Groundwater constitutes a significant contribution for irrigation water in Sri Lanka. Though shallow wells have been used over a long period in the Jaffna peninsula they have become very popular in the Dry Zone only in recent times as a mean of extracting shallow ground water for irrigation. The water is used to irrigate cash earning short age crops grown around the wells. Since shallow wells are being constructed at an alarming rate in the Dry Zone it is anticipated that it could lead to serious environmental problems in near future. Many deep wells are also located in the crystalline terrain in the Dry Zone to provide water for domestic purposes. In the Wet Zone, shallow wells are used to extract ground water mainly for domestic purposes because irrigation is not needed in this area, as there is plenty of rain throughout the year for crop production.

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