Irrigation - Spate irrigation
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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.
| Advantages | Disadvantages/limitations |
|---|---|
| - Large systems can be constructed manually with local materials and small civil works. - Control over floodwater and sedimentation reduces flooding and gullying downstream. |
Considerations
- Lowland systems depend on the water supply from their upper watershed.
- 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.
- 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.
- Construction and maintenance requires considerable human and animal labour or the use of tractors and bulldozer and consequently a strong local organization.
- Spate irrigation is associated with big income fluctuations between good and bad years.
- There is scope to improve the productivity of spate irrigation through improved traditional engineering, field-level management, and new or improved crops.
Costs
- Material (improved traditional structures): US$ 10-300/ha.
- Operation and maintenance: US$ 10-40/ha per year.
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