Spate Irrigation

Spate irrigation is a water management system that is unique to semi-arid regions. It is found in the Middle East, North Africa, West Asia, East Africa and parts of Latin America. Surface runoff from mountain catchments is distributed to riverbeds (so-called wadis, the Arabic term for valley, referring to a dry riverbed that contains water only during times of heavy rain) and spread over large areas. Some of the floodwater in sandy riverbeds sinks into the voids between the sand particles where it replaces air, which escapes to the surface of the flood water in thousands of bobbles making the floodwater look as if it was boiling.

Spate irrigation systems contain a big risk and uncertainty. The uncertainty comes both from the unpredictable nature of the floods and the frequent changes of the riverbeds from which the water is diverted. Moreover, it is important that water rights and requirements of down-stream farmers are considered to avoid conflicts. Spate irrigation is often used by the poorest segments of the rural population. They have developed substantial local wisdom in organising spate systems and managing both the flood water and the heavy sediment loads that go along with it (SPATE IRRIGATION NETWORK n.y.). However, spate irrigation is a largely neglected and forgotten form of resource management, in spite of its potential to contribute to poverty alleviation, adaptation to climate change and local food security.

Wadi hydrology is characterised by a great variation in the size and frequency of floods, which directly influence the availability of stormwater for irrigation and agriculture. Wadis have very high sediment loads (therefore sedimentation beds are necessary) and important groundwater recharge through seepage in the wadi bed. For the design of spate irrigation a good estimate of these main hydrological characteristics is required (FAO 2010). This estimate can be done based on technical measurements and estimation, but also the knowledge of the local farmers should be considered.

Generally, a spate irrigation system consists of intake areas or canals within the wadi where the stormwater flow is diverted, sedimentation basins, the channels bringing the water to the field, and overflow to flush the sedimentation basins. When designing the intake, it is important that water requirements of down-stream farmers are considered.

On the Field

(adapted from FAO 2010)

After the water is diverted from the wadis it has to be distributed on the fields. Even though the water flows over the field surface, it is a big difference in contrast to the common surface irrigation methods. Basically, there are four methods, which are commonly known for distributing water at field level in spate irrigation. These methods are grouped into two practices: (1) field-to-field distribution or individual field distribution; and (2) extensive distribution or intensive distribution.

In field-to-field irrigation, all the flow in a canal is diverted to a group of bunded fields by an earthen bund that blocks the canal. When the upstream field of the group commanded by the canal bund is irrigated, water is released by making a cut in the downstream field bund to release water to the next field. This process is repeated until all the fields in commode have been irrigated. The alternative to field-to-field distribution is to supply fields from individual inlets on secondary or tertiary canals. They offer a higher control of water distribution. They help to reduce scours and allow irrigating downstream fields without damaging upstream fields. On the other hand they need more land area for the secondary or tertiary canal construction.

Another difference in the method of water distribution at field level is whether irrigation is spread widely or concentrated in a small area. Whereas in extensive systems a single irrigation is common over the whole surface (extensive distribution), fields may be irrigated twice or three times before cultivation when floods are concentrated on a small area (intensive distribution). Both types can exist in the same system and are dependent on the moisture holding capacity of the soil. If the intensive method is used, crops varieties need to be well adjusted to soil moisture stress.The main challenges for the optimisation of spate irrigation system design are (IFAD 2011):

• Improve local diversion structures, ensuring that improvements do not interfere with established water distribution rules
• Avoid damages of the command area by high floods or heavy sedimentation.
• Improve water productivity and soil moisture management.
• Improve field preparations, seed treatment, use of improved seed, early planting and targeted use of agrochemicals.
• Introduce new crops (see also crop selection)
• Improve land and water tenure, issuing individual titles where they do not exist and codifying or reviewing water rights so as to minimise conflicts and accommodate new realities.
• Work on the bigger picture – improve access roads to spate-irrigated areas, general amenities and market facilities (see also integrated water resource management). For example, there is often little equipment and few market facilities that could upgrade production systems. In several spate-irrigated areas in Ethiopia, there is no access to the earth-moving equipment that would make a big difference in developing diversion structures and rehabilitating land and channel systems.

Costs have to be adjusted to the local conditions, project, machinery and material. Furthermore, operation and maintenance have to be considered. In Pakistan for example, costs for material (improved traditional structures) are US$ 10-300 / ha and US$ 10-40 / ha year (NWP 2007). Usually farmers have to be supported by the government and/or an NGO.

A low-cost spate irrigation system has several advantages; it is a simple technology that can be easily maintained, it is less dependent on heavy machinery and important materials and construction / reparation work can be done by farmers themselves (less costly, and it can be done fast and with locally available materials) (FAO 2010).

The main problem for the operation of spate irrigation systems is that stormwater transports a lot of suspended solids and debris. Therefore the climatic conditions and the hydrological characteristics of the wadi need to be known and channels and sedimentation basins have to be flushed and/or cleaned out during the dry season (EMBAYE 2009).

Also the fields and the field bunds have to be maintained regularly (see also the factsheet about field bunds and field trenches). The fields have to be levelled to guarantee an equally water distribution. Moisture conservation (tillage techniques, soil cover, soil amendment or mulching) is as important as water supply and also the soil fertility management has to be considered (read more about this topics in FAO 2010).

The upstream/downstream conflict also affects the standard of living. While a few favoured landowners located at the head of some schemes generate high incomes from commercial-scale farming, most spate irrigators further downstream are poor subsistence farmers, who lack potable water and sanitation, electricity and health care leading to high infant mortality due to malnutrition among children and pregnant women, as well as anaemia, malaria and other health problems (FAO 2010).

If old spate irrigation systems are modernised, its owners/users are often able to increase the water storage volume. In many cases that leads to an upstream/downstream conflict. Therefore, not just the design should be considered, but also social settings and tenure issues; including international water rights (see also the right to water and sanitation) and regional regulations (see also IWRM).

This problem is illustrated by the example of wadi Mawr in Yemen: Here, farmers on the south bank and lower parts of the system have lost access to the water that they could have formerly diverted and have had to rely on the reduced water volumes available in the infrequent, very large floods that pass over the diversion weir. This case shows how an improved diversion system that should have benefited all farmers in a scheme was diverted from its intended role for the benefit of a few (read more in FAO 2010).