**3. Spatial variation of soil salinity in Sri Lanka**

Soil salinity varies across the country and the highest salinity levels are reported in the north-central plains and along the coastal belt (**Figure 1**, **Table 1**) [15, 41–53]. Primary salinity due to natural weathering of salt-containing bedrock is restricted to isolated patches in the island. However, secondary salinity is widespread and found in inland and coastal rice farming systems. There are three main processes associated with secondary salinity in local rice farming systems *viz,* irrigation salinity, dryland salinity, and coastal salinity. Irrigation salinity arises in irrigated areas as a result of rising groundwater tables from excessive irrigation or due to the use of poor-quality water. In dryland non-irrigated landscapes, dryland salinity occurs due to the rise of the water table that drives soluble ions deposited by primary salinity to the soil surface. Coastal salinity is a result of the accumulation of salts in soil and water due to seawater intrusion from surface flow or seepage of seawater through sea level rise and tidal activity. The incidence of tides and dikes is the main reason for the intrusion of seawater into inland areas. Primary salinity, found in a few isolated sites such as the Nawagathegama soils series in the northwest part of the country [54], has a minimal economic impact on local agriculture; however, secondary salinity resulting from human activities causes significant economic losses in the local rice farming systems.

The inland salinity is a combination of both irrigation salinity and dryland salinity and has a mosaic distribution island-wide [53]. The main processes that contribute to irrigation salinity are the rise of groundwater tables resulting from poor drainage, excess irrigation, and perennial irrigation systems, which result in shallow water tables that bring salts to the upper layers of the soil profile. The problem is aggravated due to blocked drainage canals that prevent draining of salts [55]. A case study, in the largest irrigation scheme in the country the "Mahaweli," observed that soil salinity is a principal determinant of rice production in affected irrigated areas whereby yield loss of 10–15% is recorded in moderately saline areas; however, the yield was reduced by third in high saline areas [15]. In the segment "Mahaweli H" > 10% of the irrigable area is affected due to high salinity and an additional 40% is affected by moderate salinity providing a relative measure of the scale of the problem [15]. When salinity is >5dSm<sup>1</sup> , rice yield is reduced by more than 20% in affected compared to unaffected cultivations [56]. Inland salinity, therefore, has a significant impact on mega-rice cultivation environments, causing reduced production and direct economic losses.

Multiple studies have shown high concentrations of inorganic ions and EC >2.5 dSm<sup>1</sup> in irrigated water in the dry zone, significantly exceeding the recommended EC range in irrigation water, which is 0.7–0.75 dSm<sup>1</sup> [18]. Despite the high EC values, natural Na<sup>+</sup> concentrations in inland soils, including soils in the megacultivation environments, and except in a few isolated pockets, were low [55]. A significant portion of the northern half of the country recorded soil Na<sup>+</sup> concentration > 200 mg/kg, which is below the toxic limits [57]. However, high CO3 2 , HCO3 , Ca+2, and Mg+2 concentrations in the dry zone soils increase EC [58, 59]. Anuradhapura, Puttalam, Polonnaruwa, and Kurunegala districts record soil Mg2+ and Ca2+ concentrations as high as 730 ppm (optimum level for rice production, 120 ppm, [60] and 509–3452 ppm [61], respectively). Ca2+ saturation of cation exchange capacity (CEC) of soils in the dry zone is Ca 52.5% of CEC (5 cmolc/kg), whereas the recommended Ca CEC is <20% [62]. Further, high HCO3 <sup>2</sup> and SO4 <sup>2</sup> are reported in water in irrigation wells in north-central areas, which can contribute to high soil EC values. In such conditions, although not subjected to Na+ toxicity, the crops are subjected to osmotic stress owing to the high ion concentration in the root zone.



*The Scale and Complexity of Salinity Impacts on Sri Lankan Rice Farming Systems:… DOI: http://dx.doi.org/10.5772/intechopen.112651*

#### **Table 1.**

*Spatial and temporal variation in salinity in Sri Lanka.*

Coastal salinity has a significant impact on the rain-fed farming systems in the southwest coastal belt [48, 63] and in the Jaffna peninsula [41–43]. Due to the low coastal slope rice farming systems in the southwest coast are frequently affected by inundation and irrigation with salty water [48, 52, 64]. An area of 0.112 million ha is affected due to coastal salinity in Sri Lanka [30], where the EC of the soil extraction can exceed 4 dSm<sup>1</sup> [65]. In affected sites, tidal waves can range between 45 cm and 60 cm during spring tide and between 10 cm and 25 cm during neap. In dry conditions, the reduction of water levels in rivers causes backflow of seawater along rivers and intrudes the rice fields with salt water, thereby saltwater intrudes into lands up to 50 cm above mean sea level [32] in dry weather. Further, blockage of drainage canals can also cause salinization in rice fields in the coastal line. The EC range in the southwest coast varied from 0.1 to 0.4 dsm<sup>1</sup> based on soil: water 1:5 extracts. Unlike in inland salinity, soils affected due to coastal salinity contain high concentrations of Na<sup>+</sup> and Cl ions [66, 67]. In the southwest coastal belt (Kaluthara to Matara), Na<sup>+</sup> levels vary from 50 to 100 ppm; however, in Jaffna and on the eastern coastline the Na<sup>+</sup> levels are higher than 200 ppm [53]. Detailed observations were made in the Bentota river basin, where 70% of the land is affected due to coastal salinity, ranging from 8 dSm<sup>1</sup> to 16 dSm<sup>1</sup> along with >10% of land affected by high salinity (>16 dSm<sup>1</sup> ) [45, 46, 53]. Agricultural production loss per year due to salinity in the area is over 3.6 million USD [45]. Rice lands are gradually abandoned in the Jaffna peninsula owing to the high levels of seawater intrusion causing salinization of soils and water in wells used for irrigation. In addition, the major irrigated farming systems in Mannar, Hambantota, and Trincomalee coastal lines are also affected due to coastal salinity, where EC values can vary between 3 to 30 dSm<sup>1</sup> [42, 47, 48, 68]. Coastal salinity has a significant impact on semi-subsistence rice farming systems in the southwest, northwest, and eastern coastal line limiting food availability and livelihoods for the local farmers, and thereby destabilizing local food systems and increase poverty.
