8. Ecology of spate irrigation

Ephemeral rivers are often unexpectedly rich depositories of vegetation. Spates collect seeds from a large part of the catchment and deposit them in the river bed and flood irrigated fields. The moist and often organic-rich layers of silt provide a favourable environment for  wild trees, plants and mushrooms to germinate  and develop.  Logs and branches, often carried over considerable distance, may add to this. As they are lodged against trees growing in or along the river channel, they create small blockages, trapping organic material, further supporting vegetative growth (Jacobson et al. 1995).

8.1       Wild vegetation

A sample of native species occuring in the spate irrigated area of DG Khan in Pakistan is given in box 8.1. The wild species often represent aconsiderable value. The grasses and shrubs sustain livestock population. Trees such as tamarisk are used for fuel, utensils and tanning; acacia is used as timber and fuel wood. Harvesting is often done in bad years, helping  families to survive adverse periods. The spate s also carry wild vegetables and cucurbits to the fields.  Often special interest are mushrooms - in the spate irrigated areas of Pakistan the harvesting of various types of mushrooms is a lucrative side activity, with truffels fetching particularly good prices. To locate these 'underground mushrooms' hoever is a skill that is not everyone's forte - it requires a special 'eye'.

Box 8.1:           Native species in Suleiman Spate Area (Pakistan)

8.2       Vegetation and river bed stabilization

The vegetation that develops in the river beds often plays an important role in stabilizing the river bed. This is particularly true in spate rivers in soft alluvial plains, that do not have the armouring of the gravel and stone river beds and are therefore far more prone to rapid sedimentation or scour.

An example is the  Yandefero RiverKonso, Ethiopia. When the vegetation of the riveraine forest downstream of the system of flood channels was disturbed in Yandefero, the river changed its course and dropped at a lower level at the main Sehan river. This oontributed to accelerated scour of the river in the upper section, rendering the flood channels located there unserviceable. Vegetation also sometimes helps in raising the river beds. When trees such as tamarisk colonizes the bed of spate rivers flows are slowed down and bed levels increase. In the Korakan River in Balochistan, ariver particurly prone to degradation, a ban on cutting vegetation in the river bed was put in place by the spate irrigation farmers.

Damage to the riverain forest at the end of the Yandafero system in Konso, Ethiopia contributed to the deepening of the river bed upstream

There is usual a gradual transition in the vegetation of spate rivers - as one travels downstreams. Whereas the upper reaches of spate rivers tends to get more frequent floods, the physical disturbance that comes with has the effect of removing the vegetation will. In the lower reaches of ephemeral rivers discharges decreases as a result of upstream abstraction and infiltration. Infrequent floods result in harsh environments, where only hardy drought-resistant plants survive (Jacobson et al. 1995).  Vegetation can also be used as an indicator to assess the pattern and reliability of flooding.

Rouchiche, quoted in Camacho (2000) has also suggested that vegetation can be used to protect the outer curves of spate streams. As the speed of water in these stretches is higher little natural vegetation take root. Rouchiche proposed to:

- armor the most exposed parts of the rviers with a tamarix cover under the protection of a retaining wall. Camacho (2000) has suggested that in the absence of tamarix lowland grawa could be used too

- undertake wadi training by establishing dense vegetative patterns in spur patternd

- protect the banks of the wadi by planting for instance tamarix and saccharum aegyptica

Proposed bank protection using retaining wall with shrubs (Rouchiche, quoted in Camacho (2000))

Eritrea - repair of flood diversion spurs with shrubs - denuding the surrounding hills

8.3       Spate irrigation and natural degration

The construction of brushwood spurs and weirs also requires a substantial amount of trees and branches. These are usually collected in the surroundings. In Eritrea the numerous bunds (aqm's) that have developed in some parts of the country have been associated with  the denudation of the hills. There are several other factors that cause  upland deforestation obviously. A link exists with the development of spate irrigation and the changing hydrographs that result from deteriorating vegetation cover in the uplands - floods become more peaked and more silt laden.  The phenomena is not new.  Research into pollen near the pre-Indus site of Nauwshero on the Kacchi Plains, Pakistan suggested that in 2600 BC already dramatic shift in vegetation took place. One theory is that this was caused by the development of spate irrigation and the larger population pressure on local natural resources.

Print this document

Back to the top of this document