FEATURE-Climate change: A looming threat to irrigated agriculture 

By Eng. Thushara Dissanayake

Sri Lanka has a legacy of massive ancient irrigation systems developed in the dry zone during the ancient kings’ era. The utterance of Great King Parakramabhu: “Let not even a drop of rain water go to the sea without benefiting man", no doubt, may have inspired the professionals in the water sector to further develop the irrigated agriculture of the country harnessing the high rainfall received. Total annual renewable surface water resources of Sri Lanka is about 52 billion cubic meters (BCM), while total average annual rainfall is about 112 BCM.  

Since the construction of the Senanayake Samudra in 1950s as the largest reservoir in Sri Lanka, organisations in the water sector have completed many new reservoir projects of large capacities. Lunugamwehera, Weheragala, Deduru Oya, Kalugal Oya and Yan Oya are some of the recent achievements of a long list which targeted mainly the water supply to irrigated agriculture. The total storage capacity of major and medium irrigation reservoirs in Sri Lanka is about 3.4 BCM. In addition, major reservoirs constructed under Accelerated Mahaweli Development Project are considered to be multipurpose reservoirs as the main objective is the generation of hydro-electricity. Nevertheless, the contribution of Mahaweli reservoirs to irrigated agriculture of the country is indispensable. With the newly constructed Moragahakanda-Kaluganga reservoirs total Mahaweli reservoir capacity stands at around 3.2 BCM. In addition, the total number of minor tanks in the island are estimated to be over 10,000, though the total water storage capacity is not accurately known. 

Reliability of these reservoirs depend on rainfall

Irrigated agriculture is the largest user of raw water in Sri Lanka. The country receives almost a constant volume of rainfall annually. But, this does not mean that we have no water issues whatsoever. Generally, reservoir designs are based on past rainfall characteristics of the areas they are located. Hence, they depend not only on annually expected rainfall volume but also the temporal pattern of rainfall. They will have the desired storage levels to supply the demand only if they receive expected rainfall at the right time in right quantities. In addition, it is important to ensure that the water withdrawal rates remain within the envisaged limits during the reservoir design. When rainfall becomes more variable within season and over time, reliability of reservoirs will decrease.

Climate change impacts reservoir operations
As explained above if a reservoir does not receive expected rainfall in right quantities in the right time it will more or less fail to serve the water demand sites up to the expectation. On the contrary, if it gets rainfall more than expected during a operation cycle the additional amount of water may spill out of the reservoir and be a waste flowing to the sea unless there are any other storage reservoirs located in the downstream which are capable of arresting that spilled volume of water. 

Climate change has caused rainfall to behave according to an aforesaid unpredictable manner. Technically speaking what we experience today is a temporal variation accompanied with fluctuation in intensities. Climate change experts predict that Sri Lanka will experience longer dry spells frequently in future. During such dry spells, often unforeseen, our reservoirs would be incapable of supplying the irrigation water demands continuously throughout the cultivation season.

The global warming or the increase in atmospheric temperature is the root cause of climate change. Hence, the other major impact of climate change is the increase in evapotranspiration due to increased atmospheric temperature. As a consequence, water requirement of the agricultural crops go up demanding an excess water requirement from reservoirs. In the meantime water losses from the reservoirs themselves by way of evaporation will also go up. These scenarios will create more water stresses on irrigated agriculture in future. 

When the conditions are not favorable for agriculture there will be a sharp decrease in people engaged in agriculture abandoning their lands. Hence, this crisis should not be understood merely as a water crisis as it has the potential of ramifying further into other social and economic spheres. So much so that the decrease in agricultural production will lead to higher selling prices, which neither the government nor the consumer is happy about. 

Possible interventions to resolve the issue
We are not in a position to control a natural phenomenon like rainfall nor to predict the pattern of climate change and its impacts in future accurately. Hence, the only option available with us is improving our water management strategies and practices. Construction of storage reservoirs where possible will only be a part of the solution. However, we have utilised or identified almost all possible locations for reservoir construction by now and most of the remaining places have got high social and environmental implications. Under these circumstances some alternatives that require stakeholder attention are discussed below. It should be acknowledged that some of these concepts are already in practice but in an ad hoc manner. What is needed is to implement them with clear goals and to monitor and assess the outcomes after implementation. 

Irrigation system modernisation
We are not in a position to control a natural phenomenon like rainfall. Besides, we are not able to predict the pattern of climate change and its impacts in future accurately. Hence, the only option available with us is improving our water management strategies and practices.

At present the actual water requirement for producing 1 kg of paddy is around 2500 - 3500 litres while the crop requirement is about 1400 litres. This higher requirement is mainly due to the water losses during conveyance and application. Our overall irrigation water use efficiencies are assumed to be just 40 per cent, though no adequate research has been done on this at field level to accurately calculate it system-wise.

Presently many countries are working on irrigation modernization (IM) for the sustenance of irrigated agriculture. The Food and Agriculture Organization (FAO) defines IM as "a process of technical and managerial upgrading (as opposed to mere rehabilitation) of irrigation schemes combined with institutional reforms, with the objective to improve resource utilization (labor, water, economic, environmental) and water delivery service to farms". Simply speaking, IM consists of a set of interventions to improve water management and level for farm services to farmers, which eventually leads to improved crop production and resilience to climate change under the present context. It includes both engineering and management interventions.

These interventions for better water management are mainly;

1.    Upgrading of water conveyance systems so that evaporation and infiltration losses can be minimized. Deficient canals are replaced with concrete lined canals and buried pipes
2.    Process improvement in water allocation to farmers with modern techniques like supervisory control and data acquisition (SCADA), that intervenes in the process without changing the rules of the water management
3.    Pragmatic desilting of reservoirs to regain their original capacities where applicable
4.    Improvement of agricultural roads and access roads in farming areas to facilitate easy operations and management
5.    Capacity building of farmers 

Watershed conservation
All the irrigation systems are not reservoir-based. There is a large number of anicut-based irrigation systems as minor irrigation systems. They depend on water diverted from rivers or streams. The success of these systems depends on the water availability of such rivers and streams.

During rain, a considerable portion of water is infiltrated into the ground and rivers and streams are fed gradually by this groundwater making them perennial. For this mechanism forests in the river basins play a major role by delaying rain water runoff, which in turn helps ground water recharge. Yet the issue is diminishing forest cover in our watersheds. This rate of forest cover reduction will adversely affect ground water infiltration and eventually result in dried up rivers and streams.  Further, agriculture depending on ground water will face water stresses. Hence, the importance of watershed conservation goes without saying.

Farmers’ responsibilities and the role of the extension services

Sri Lankan farmers are supplied with irrigation water free. This does not mean that water is freely available. Even though the relevant authorities could intervene to minimise conveyance water losses of the systems, the control of water application is mainly in farmers’ hands. Therefore, they also have a big role to play in minimising water application losses. 

Traditional water application methods should be replaced with best water saving methods. The method known as alternate wetting and drying (AWD), introduced far back, is a water management technique practiced in paddy cultivation that need much less water than the usual practice of keeping ‘standing water’ in the paddy field and proven to give higher yields. In areas where water stress is frequent crops that require less water should be grown with appropriate cultivation methods. If there is an evident shift in rainy season, crop cultivation periods can be adjusted to allow earlier or later planting so that both coincide, in order to reduce irrigation water use. 

Where most of the farmers are concerned, dominant factor in the crop selection process seems to be market price while not giving enough attention to crop water requirements. In some cases, it is the farmers’ status quo that matters. Hence, relevant authorities should carefully focus on these aspects when they render their extension services to the farmer community. In this respect, better inter-agency cooperation and communication and active participation are essential. No need to mention how the modern technology can be used by the authorities for creating an effective work environment. 

Final remarks

The objective of this article is to acknowledge the looming threats to our irrigated agriculture and discuss few proactive measures briefly. It is noteworthy that escalating food demands due to increasing population have not been taken into account here. Implementing aforesaid proactive measures need the dedicated action of many sectors that include politicians, public sector organisations and the farmer community. Failure to take prompt action will result in additional stress in food production in future that in turn would affect the economy of the country as more and more food items will have to be imported. 

(The writer is a chartered Civil Engineer. This article is based on his personal views and does not reflect the positions he holds in any organisation).