Impact of Climate Change on Agriculture in Sri Lanka

By C. Shanthi de Silva

Human activities have been recognized as the primary cause for global warming. The build-up of greenhouse gases (GHGs) threatens to set the earth on the path to an unpredictably different climate. A key challenge, in that instance would be food security, crucial to sustain the growing world population. An enormous amount of water is required to produce food. Irrigation needs vary according to the balance between rainfall and evapotranspiration, and the resultant fluctuations in soil moisture status. Anthropogenically induced climate change is expected to influence rainfall and temperature patterns.  Since climate change will influence temperature and rainfall patterns, there are likely to be direct impacts on soil moisture.  Changes in soil moisture due to global warming will have other hydrological effects which will, in turn, affect agriculture around the world.   

Sri Lanka is no exception to this, as it is also prone to natural disasters. During the last few decades, for instance, Sri Lanka has witnessed a number of extreme rainfall events in south western regions during south west monsoon season (May to September) and in contrast northeast and north central regions experienced severe drought during the paddy and other field crop growing seasons. Farmers used to start paddy cultivation during maha season, expecting the north east monsoonal rains. Sri Lanka is experiencing uncertainty and inadequacy of north east and inter-monsoonal rains, during these years which caused damage to the paddy and other field crop cultivations. Each year, the Government of Sri Lanka spends huge amounts of funds on drought, floods and other, relief services.

Studies on past climate data will not predict the future, as drastic changes are taking place in climatic factors due to climate change. To predict the future, there are general circulation and weather forecasting models which are capable of predicting the future climate. This article is based on the findings of a study undertaken to focus on the impacts of climate change on water resources in Sri Lanka, with the collaboration of Cranfield University, UK. The HadCM3, which is a coupled atmosphere-ocean general circulation model (AOGCM) developed at the Hadley Centre for Climate Prediction and Research in the UK, has been used for this study. This model is considered as significantly more sophisticated than the earlier versions, and the HadCM3 model provides information about climate change occurring across the globe during the 21st century. In order to provide information on possible changes in the world climate, the climate change models are forced to consider future scenarios. 

According to IPCC-SRES, those future scenarios are A1, economic-global; B1, environmental-global; A2, economic-regional; and B2, environmental–regional.  In this study, scenarios A2 and B2 for 2050 were selected to compare with the baseline data (1961-1990). A2 scenario is considered as representing the highest impact of climate change and the B2 is the second highest impact. Further the results of the HadCM3 predictions of monthly rainfall for 2050s were also compared with that of the baseline (1961-1990) and data for the period of the last ten year (1999-2008) of Meteorological Department, Colombo.

Under these two scenarios tested, the average annual rainfall is predicted to increase by 14% (A2) and by 5 % (B2) across the country. Among the wet zone areas, the average annual rainfall in Colombo, Galle, Kandy, Nuwara Eliya and Ratnapura is predicted to increase. The highest increase in Colombo by 32% (A2) and 24% (B2). However the rainfall is predicted to decrease in other dry zones areas such as Anuradhapura, Batticaloa and Trincomale. The highest decrease is predicted in Batticaloa 14% (A2) and 12% (B2). 

During the southwest monsoon (May to September), rainfall across the country is predicted to increase considerably, by over 30%, compared to the baseline. Therefore, south western regions (wet zone areas) of the country such Colombo, Ratnapura, Galle and Nuwara Eliya will be experiencing a significant increase in rainfall.  In addition, predicted higher rainfall in Nuwara Eliya and Ratnapura areas will cause floods and land slides. This increase in rainfall will have serious impacts on the country’s infrastructure to cope with floods in the south western region. Therefore, it will be advisable to store the excess runoff water safely within these regions. 

According to the results, the north east monsoon (December to February) is predicted to decrease on average by 34% (A2) and 26%( B2).  The north east monsoon rainfall in dry zone areas such as Jaffna, Mannar, Vavuniya, Anuradhapura, Batticaloa, Tricomalee and Hambantota are predicted to decrease (Table 1). The highest decrease is predicted in Tricomalee and Batticaloa 27% (A2) 29%( B2) followed by Anuradhapura, Vavuniya, Mannar and Puttlam. It will create severe drought related problems in these areas. 

Table 1: Percentage decrease in Northeast Monsoon rainfall according to HadCM3 prediction for A2 scenario in 2050s compared to the baseline (1961-1990) in the dry zone areas in Sri Lanka.  

For example, according to HadCM3 model prediction for Anuradhapura there will be a 27% decrease in rainfall during northeast monsoon in 2050, and the gradual decrease will be 9%, 13.5%, 18%, 22.5% and 27% in 2010, 2020, 2030, 2040 and 2050, respectively. According to the Meteorological department data, the northeast monsoon rainfall in Anuradhapura has decreased by 6% during the last ten year period (1999-2008).  Further, there was a significant decrease in rainfall in the months of November and December (16%) over the same period compared to the base line (1961-1990) in Anuradhapura, whereas the HadCM3 predicted decrease in rainfall for 2050s for the month of December is only 6%, which is considerably less than the observed decrease over the past ten years (1999-2008)(Figure 1). It shows that climate change effects on northeast monsoonal rains in north, central and eastern regions is taking place at an accelerated pace. It will have serious impacts on paddy cultivation as it depends on the northeast monsoon rainfall.  Further, the rainfall during yala season (May- August) also has been decreased during the past ten years (1999-2008) when compared with the base line (1961-1990). 

Figure 1: Monthly average rainfall of Anuradhapura   by Meteorological Department for (1961-1990), (1999-2008) and HadCM3 model prediction for 2050s.

The temperature increase predicted by HadCM3 is mainly in the north, north eastern and north central regions of the country.  The average annual temperature is predicted to increase by 1.6° C (A2) and 1.2° C (B2).  Highest increase in temperature is predicted in Anuradhapura by 2.1° C (A2) and 1.6° C (B2). The lowest annual average temperature increase is predicted in Nuwara Eliya by 1.1° C (A2) and 1° C (B2). Due to the predicted increases in temperature and the decrease in rainfall, the evapotranspiration is predicted to increase in the north, north central and eastern regions of Sri Lanka. Further, the soil moisture deficit is also predicted to be high in the north, north central and eastern areas (Jaffna, Mannar, Vavuniya, Trincamalee, Anuradhapura and Batticaloa) where agricultural activities are intensive. These predicted changes in rainfall, temperature, evapotranspiration and the soil moisture deficit, will demand additional irrigation water to compensate for the crop water requirement now and in the coming years. Therefore the climate change effects on maha and yala seasonal rains will cause serious problem for agricultural activities, such as paddy and other field crop cultivations in the north, north central and eastern regions, unless suitable adaptation measures are implemented to provide additional irrigation water as the rainfall will not be sufficient for food production in these areas. 

Adaptation measures must be clearly planned to complement climate change impacts in the various regions. For sustainable agricultural production of paddy and other field crops, additional irrigation water is a must by any means as the rainfall alone will be sufficient to fulfill the crop water requirements. Therefore policy makers should implement methods to store any additional water by renovating the existing surface tanks in the north, north central and eastern regions. Further, providing grants for farmers to build their own agro wells will be another option, but it should be planned carefully to maintain at least 200m spacing between the two nearest wells. Further, agro wells are more suitable for other field crop cultivation as the water in an agro well will not be sufficient to irrigate paddy cultivation. In an Agro well, the major portion of water that can be abstracted during pumping is from the well storage and because of the over burden of hard rock aquifer underneath in many of these regions it results in very low permeability. Further, farmers must learn to use the well water efficiently by using drip and sprinkler irrigation. In addition, researchers must focus on adaptation measures for crops to cope with high temperature and water stress. It may lead to changing crops, varieties and timing of cultivation.

The author, C. Shanthi de Silva is Professor in Agricultural Engineering at Faculty of Engineering Technology, The Open University of Sri Lanka, Nawala, Nugegoda