**4. Temporal variation of salinity in Sri Lanka**

Soil salinity varies seasonally whereby both coastal and inland salinity is high during the dry season. During the wet season with high rainfall salts are washed off from the soils, but in dry periods, salts accumulate causing salinity. Furthermore, the impact of coastal salinity is reduced during the wet months due to high rainfall causing a pushing effect and runoff inflow toward the sea reducing sea water backflow and saline water intrusion into the land [69]. With the heavy rain, salts are washed off in the main cultivation season, but with less water availability and with high water evaporation, the minor cultivation season is more salinity prone especially toward the end of the season. The phenomena create a seasonal cycle in salinity in the irrigated and rain-fed systems whereby salinity deposited from primary and secondary sources (seawater and irrigation water) during the dry season is washed off during the wet periods (**Figures 4** and **5**) [48, 50, 69–71]. The amount of rainfall and water availability, therefore, are primary factors that determine soil salt concentration and the variability in rainfall causes seasonal and annual fluctuations in salinity levels.

The west coast where the rainy (March to August: first inter monsoon followed by southwest monsoon; September to February: second inter-monsoon followed by northeast monsoon) and dry periods observe bimodal patterns. Two clear salinity peaks were recognized. In time series salinity estimates of soil and water in selected sites in the rain-fed and irrigated rice lands in the west coast the highest salinity levels are recorded from August to September at the end of the minor cultivation season (EC > 24 dSm<sup>1</sup> ). A second minor salinity peak was observed at the end of the major

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

#### **Figure 4.**

*Temporal variation in salinity and rainfall in Northwestern [A], Southwestern [B], and eastern [C] coastal line of Sri Lanka. The vertical axis on the left indicates the monthly average rainfall (mm) and on the right indicates soil electrical conductivity (dSm<sup>1</sup> ). Gray boxes indicate monthly rainfall (mm), whereas the line electrical conductivity (dSm<sup>1</sup> ). The green boxes from left to right present crop stages of 2.5, 3-, and 3.5-months rice varieties, including seedling (susceptible), tillering/vegetative, reproductive/PI, and mature/ripening crop. The crop-salinity susceptibility window is indicated in red boxes (based on [45, 48, 50, 63]).*

#### **Figure 5.**

*Temporal variation of salinity and rainfall in irrigated "mega production areas" of Sri Lanka. The vertical axis on the left indicates monthly average rainfall (mm) and on the right indicates the electrical conductivity (dSm<sup>1</sup> ) in irrigation water. Gray boxes indicate the rainfall, whereas the line indicates electrical conductivity (dSm<sup>1</sup> ) in irrigated water. The green boxes from left to right present growth stages of crops of 2.5, 3-, and 3.5-months, including seedling (susceptible), tillering/vegetative, reproductive and PI (susceptible), and mature/ripening stages. The crop/salinity susceptibility window is indicated in red boxes (based on [69–71]).*

cultivation season during February to March, where the EC values in soil >20 dSm<sup>1</sup> (**Figure 4**) [45, 48, 50, 63]. A similar pattern is reported in the Jaffna peninsula [72]. When salts were not washed off due to insufficient rain, the seedling crop of the both minor and major cultivation season is affected due to early season salinity. Unlike in the west coast, in the north-central and east parts of the country EC values gradually built over a period of six months during the dry season starting from the later part of

the major cultivation season throughout the minor cultivation season [72] (**Figure 5**). Accordingly, the major irrigated rice farming systems are affected due to high salinity over the dry months from April to October, where EC value can be as high as 7 to 9 dSm<sup>1</sup> . Therefore, farmers experience transitional saline conditions with significant yearly (depending on the annual rainfall) and seasonal fluctuations in salinity level in both irrigated and rain-fed local farming systems.
