Irrigation - Spate
The management of sediment loads is as important as the management of flood water. Soil moisture conservation (recharge of shallow aquifers) is the key to high productivity. Spate irrigation is an ancient form of water management in arid and semi-arid environments, practised most widely in Pakistan, but also in Asia, Yemen, the Horn of Africa and North Africa. It is typically applied where highland plains meet alluvial flat slopes and where annual rainfall is erratic, often below 200 mm. In Pakistan, sporadic floods from temporary rivers are diverted and spread over a large area of land by earthen bunds, about 1 km long, several metres high and up to 20 m wide at the base. Near the mountains, the bunds divert part of the fast flowing flood; lower down they divert the entire flow. Water is guided through a system of flood channels to the bunded fields, often as large as 15 hectares, sub-divided into sections. The collected water is used for irrigation, the filling of water ponds and the recharge of groundwater. As such, spate irrigation provides considerable opportunities for reviving and improving the agricultural productivity and livestock production.
Spate irrigation occurs in areas as varied as South Asia, the Middle East, West Africa, North Africa, the Horn of Africa, Central Asia and Latin America. Spate irrigation is typically found in arid and semi-arid regins, where highlands border plains. It uses seasonal floods for irrigation – but as the floods differ from year to year the area served by it fluctuates widely.
Wadi catchments generally have sparse vegetation cover, and thin rocky soils. Soils are exposed to raindrop impact and soil crusting, which results in a low infiltration capacity. Storm rainfall generates local overland flow, which converges into wadi channel networks, generating spate flood run off events. Runoff generation is usually localised, reflecting the size of convective rainfall cells that generate run off events.
| - Large systems can be constructed manually with local materials and small civil works.
- Control over floodwater and sedimentation reduces flooding and gullying downstream.
| - Spate irrigation and pond farming systems are risk-prone, due to the unpredictable floods and frequent changes in the riverbeds from where water is diverted. |
- Spate irrigation is simply a method of using water optimally. It does not control the supply of water or prevent shortage of water, which can cause big income fluctuations
Construction, operations and maintenance
Construction and maintenance requires considerable human and animal labour or the use of tractors and bulldozer and consequently a strong local organization.
- Material (improved traditional structures): US$ 10-300/ha.
- Operation and maintenance: US$ 10-40/ha per year.
In the last few decades extensive civil engineering investments have been made, mostly in large spate irrigation systems in Yemen, and to a lesser degree in Pakistan, Eritrea and Tunisia. The track record of all these large civil engineering investments is at best patchy. Investments in flow division and regulation in Pakistan (for instance on the Gaj Nai in Sindh) have performed reasonably well, but the same cannot be said for modern flow diversion structures. An evaluation of 47 relatively minor spate systems built with national funding in Balochistan between 1960 and 1990 established that only 16 were still operational.
In the Tihama plains of Yemen the designs of the modernised systems became more sophisticated over time. The first scheme to be modernised, Wadi Zabid, suffered from serious water diversion and sedimentation problems, and is now operated rather like a traditional system with diversion essentially controlled by bunds constructed by bulldozers. Later schemes included effective diversion and sediment handling arrangements, albeit at a high investment cost. A profound change in some large modernised systems is that they ceased to be managed by farmers. The most extreme manifestation has been in Yemen, where the Tihama Development Authority (TDA) assumed full responsibility for operation and maintenance after the civil works on the various systems were completed. TDA has struggled to find the funds needed to carry out the maintenance, particularly removing sediments from canals, required to keep the schemes functioning. In other cases where new structures have been provided responsibility for maintenance is ambiguous, particularly when local capacity to manage complex civil engineering projects is limited.
The main lessons learned from these experiences are:
- The planning and design of the rehabilitation and improvement works have mostly been carried out without effective partnership with farmers and land users. Farmers’ valuable knowledge of spate irrigation, and their preferences regarding the scope and type of works and changes in the layout of their irrigation system were often not properly considered during the design process.
- The investment costs have been very high and it is doubtful if they can be justified in purely economic terms.
- The operation and maintenance of the larger diversion structures and canal systems, is difficult and expensive. In particular sedimentation at intakes and in canals is often not properly controlled in ‘modernised systems’.
- New structures have promoted larger inequity in the distribution of irrigation water due to the collapse of traditional evolving water rights – “modernised” diversion structures give much larger control over spate flows to favoured groups of the upstream farmers.
- The benevolence of a flood Floods are generally considered damaging but this story is one where the farmers of Bihar willingly invited a flood to bless their fields!
- The Spate Irrigation Network The Spate Irrigation Network aims to improve the livelihoods of those living in the spate irrigated areas. It exchanges experiences and good practices, initiates and supports new programs and policies, and mainstreams education and training. The network consists of professionals, practitioners and farmers. At present the network has more than 400 members.
- Meta Meta. Many resources for spate irrigation.
- Guidelines for spate irrigation: Irrigation and drainage - paper 65. Food and Agriculture Organization (FAO), 2010. Agriculturesnetwork.org.