What is Spate Irrigation?

Spate irrigation is a type of water management, that is unique to semi-arid environments. It is found in the Middle East, North Africa, West Asia, East Africa and parts of Latin America. Flood water from mountain catchments is diverted from river beds (wadi’s) and spread over large areas. Spate systems are very risk-prone. The uncertainty comes both from the unpredictable nature of the floods and the frequent changes to the river beds from which the water is diverted. It is often the poorest segments of the rural population whose livelihood and food security depends on the spate flows. Substantial local wisdom has developed in organizing spate systems and managing both the flood water and the heavy sediment loads that go along with it.

Spate irrigation provides a livelihood for farmers in often marginal environments

Where does one find spate irrigation systems?
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. Estimates for the area under flood irrigation are not easy to make, as the area under spate irrigation changes from year to year and as spate irrigation has never had the amount of attention from development agencies or tax authorities, that perennial irrigation has had. 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.

The most comprehensive information on how much spate irrigation there is comes from FAO

Source: FAO Aquastat

The figures for these selected countries indicates an order of magnitude. Some figures are debatable. The area under spate irrigation in Eritrea is also quoted as 90,000 ha. For Pakistan – where spate irrigation is found in all four provinces – an estimate of 3.250,000 has been mentioned by other sources. Further spate irrigation is reported from North Chile and Bolivia, Iran, Afghanistan, Mauretania and Senegal, Ethiopia, Kenya and the Northwest coast of Egypt.

Spate irrigation in Konso, Ethiopia

Is spate irrigation similar to other flood irrigation and water harvesting systems?
As spate irrigation uses seasonal floods for irrigation, it is akin but different from two other categories of flood-based irrigation systems, i.e inundation canals (that start to flow as soon as the flood in a perennial river reaches a certain level) or flood rise or recession irrigation, where a rising perenial river overtops its banks and inundates the plains alongside the river. In flood rise or recession irrigation crops are grown on the rising or receding flow or on the residual moisture. In spate irrigation instead water is diverted from normally dry river beds (wadi’s) when the river is in spate. The flood water is then diverted to the fields. This may be done by free intakes, by diversion spurs or by bunds, that are build across the river bed. The flood water – typically lasting a few hours or a few days – is channeled through a network of primary, secondary and sometimes tertiary flood channels. Command areas may range from anything between a few hectares to over 25,000 hectares.

Wadi Zabid in spate (Yemen)

How are spate irrigation systems managed?
Some of the larger spate irrigation rank among the largest farmer managed irrigation systems in the world. The structures are sometimes spectacular: earthen bunds, spanning the width of a river, or extensive spurs made of brushwood and stones. Spate systems are made in such a way that ideally the largest floods are kept away from the command area. Very large floods would create considerable damage to the command area. They would destroy flood diversion channels and cause rivers to shift. This is where the ingenuity of many of the traditional systems comes in. Spurs and bunds are generally made in such a way that the main diversion structures in the river break when floods are too big. Breaking of diversion structures also serves to maintain the flood water entitlements of downstream land owners.

Repairing a brushwood spur (Eritrea)

What is the importance of sedimentation?

Sedimentation is another important feature of spate irrigation and spate irrigation is as much about managing water as it is about managing sedimentatin. The spate waters are usually laden with sediment. Scour and siltation are part and parcel of spate irrigation. Rivers in spate lift and deposit huge quantities of sediment. As a result there is constant change in bed levels, resulting in changes in bed levels and water distribution. The impact of these processes differs between the various systems. It depends on the amount and composition of the sediment load that a river carries, which depends on the rainfall pattern and the characteristics of the catchment area; its geology, morphology and vegetation cover. Farmers are usually able to identify the origin of a flood by the type of sediment that is transported by it. The degree of siltation and scour also depends on the local topography and the type of material. In spate irrigated areas with low gradients, as are found on the plains, a river is always in danger of choking itself with its own silt deposits and finding another way. Moreover, in the fine sandy deposits of the plains, the scouring of the riverbed is a larger danger than it is in the armuoured river beds of the highlands. As a result, the lowland flood irrigation systems are particularly dynamic.

Farmers, however, are not passive actors in these scour and siltation processes. They actively manipulate the scour and sedimetnation processes. They may deepen the headreach of a flood channel, in order to attract a larger flood that will further scour out the channel. If a flood river breaks its banks, farmers may close the breaches, if it deflects water away from their land or on other occasions, they will leave the breaches intact, so that these will act as escapes, creaming off the peaks of the very high floods and maintaining the flow at their own system at a manageable level. In other cases farmers will manipulate the siltation process to force the river bed to purposely silt up. The latter is in practice where the river has become uncontrollable, because its bed may has become to deep or to steep. The remedy is to built a strong permanent bund across the river and force the river to deposit its sediment load upstream of the bund.

Scour and siltation (Pakistan)

What is the history of spate irrigation?
Spate irrigation has a long history. Several sources assume that in Yemen spate irrigation started when the wet climate of the Neolithic gave way to more arid circumstances and that spate irrigation thus has been in use for five thousand years. Similarly, archeologists have discovered the remains of checkdams for spate rivers in Tauran, Iran and Balochistan, Pakistan. In Yemen spate irrigation witnessed its zenith during the Shebean period in the first millenium BC. The great Mar’ib Dam, constructed on Wadi Dhana, irrigated two oases on either banks, estimated to cover 9600 ha.

One can only speculate how the technique spread across the world. The intense development of trade after the Islamic period may have helped spread innovations from the Yemen area. The recent development of spate irrigation in Eritrea is for instance traced back to the arrival of Yemeni migrants 80-100 years ago. Yet it is likely that spate irrigation techmology has sprung up independently in several areas – particularly as it is found in areas as diverse and remote as West Africa, Arabia, Central Asia and Latin America.

What is the future of spate irrigation?

As a testimony of the diversity in development in the world, spate irrigation on the decline in rich areas such as Saudi Arabia, but is on the increase in low income countries such as Ethiopia and Eritrea. Generally spate irrigation is associated with low returns per labour, great variability in income between good and bad years and a high degree of social organisation to maintain the systems. Where more rewarding sources of income come up, where a period of long droughts force people to abandon their area or where the local organisation is undermined spate irrigation systems may disappear.

Another important change in several areas, that are traditionally spate irrigated is the introduction of groundwater irrigation. In many spate irrigated areas groundwater resources are relatively rich due to long periods of recharge. With the availability of relatively inexpensive pumpsets groundwater has become an important source of irrigation, for instance in spate areas in Dera Ghazi Khan (Pakistan), Tunesia or Yemen. This has resulted in a neglect of the spate infrastructure and a change towards perennial cropping.

The number of public programs to support spate irrigation have been relatively limited. One reason has been the difficulty to justify investments in civil engineering works on systems, dominated with low value farming. The second reason has been that it has been hard to identify successful interventions in spate systems, because spate systems are often hydraulically and socially generally very complex.

An alternate approach to support spate systems has been the subsidization of mechanical traction. This approach has been followed with a relative high degree of success in Pakistan and Tunesia. Bulldozer programs have put a very useful resource at the hand of local spate farmers – who have remained in charge of the design and implementation. The cost effectiveness of bulldozer has been relatively high, moreover.

Bulldozer program in Balochistan (Pakistan)

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