between 5 m and 15 m high (lower limit for large dams established by the International Commission on Large Dams). They are constructed of rubble in small rural catchments in areas of moderate relief. They have rustically designed lateral spillways with a discharge capacity of tens of cubic meters per second or, in some cases, just over 100 m3 per second. Some, but not all, have a sluice gate. The dams cost around 500,000 euros, sometimes far less. Their reservoirs are relatively small, a few hectares and have a holding capacity from a few tens of thousands to 106 m3 (Albergel and Rejeb, 1997). Their main objectives are to reduce losses in agricultural land (estimated at 10,000 ha per year) and dam siltation, to increase water table recharge and to provide water for irrigated cropping (Albergel and Nasri, 2001).

Small dams have been known in West Asia since ancient times. The dam on the Nahr El Asi, near Homs, in Syria, was built in the reign of Seti 1 (1319-1304 BCE). Many were built at the start of the Christian era, one of which was Badieh Dam on the road to Palmyra. Numerous ruins testify to their presence in the dry steppes. Some still exist, but are completely filled with sediment. The first small dams built using modern techniques were those constructed during the 1960s in the province of Swaida to supply drinking water to villages on a basalt plateau that has no underground water. These reservoirs are usually stocked with fish. The lakes are used for fishing and for leisure activities, as with the small dam at Al Corane, not far from Damascus, in a small high valley. In the Middle East, the idea of a hill reservoir is not as well defined as in the Maghreb, but numerous small reservoirs have been constructed to create water reserves for the cattle of the nomadic Bedouin tribes.

In North Africa, the drought in the early 1980s, considered to be the longest ever experienced, marked the start of a policy of labor-intensive, small dam and hill reservoir construction. Small dams were primarily designed for irrigation, livestock watering, flood protection or supplying drinking water to rural areas that had no readily exploitable underground water. North African countries built a large hydraulic infrastructure in the 1970s and 1980s. Almost all the large dams are affected by significant sediment. Numerous small dams have been built to slow down siltation. For example, the largest dam in Morocco, the Al Wahda Dam on the Ouergha River in the province of Sidi Kacem (88 m high, with a capacity of 3.4 109 m3), is protected by many small dams in the upstream catchment designed to hold the erosion coming from the steep marly slopes of the Rif. Some have already been constructed, while others are planned. Erosion from the Ouergha catchment, estimated at 98 tonnes ha-1 annually over 6150 km2, causes the dam to lose 60 106 m3 volume each year (Anonymous, 2001).

2.1.4.3 Infrastructures for water harvesting

Water harvesting is a way to increase water available for crops. Section 2.5.2 on traditional knowledge presents the history of water harvesting, describing modern or rehabilitated infrastructures for water harvesting. Water harvesting is a proven technology to increase food security in droughtprone areas. Erosion control and recharging groundwater are additional advantages. Water harvesting can also be considered for rudimentary irrigation or storage of  

drinking water for animals. The farmers and pastoralists have no control over timing. Runoff can be harvested only when it rains. Although it exists in extremely old cultures (Nasri et al., 2004) and has been observed in most countries, extension and irrigation staff often have quite limited knowledge about various water-harvesting techniques and the associated socioeconomic implications. A growing awareness of the potential of water harvesting for improved crop production arose in the 1970s and 1980s. The stimulus was the well-documented work on water harvesting in the Negev Desert of Israel (Evanari et al., 1971). ICARDA is helping disseminate information on water technologies, promoting adoption and providing training for field staff in CWANA (Oweis et al., 1998, 1999).

Three main kinds of infrastructure for water harvesting are distinguished (Prinz, 1996; Prinz and Prinz et al., 1998).

• Rainwater harvesting: These infrastructures induce, collect, store and conserve surface runoff in arid and semiarid regions (Boers and Ben-Asher, 1982). This type of harvesting covers water collected from rooftops, courtyards and treated surfaces. The water is used for domestic purposes or garden crops. Microcatchments collect runoff from a small catchment, to feed the root zone of a tree, bush or annual crops. Macrocatchments convey runoff from hill-slope catchments to a cropping area at the hill foot or to a tank or artificial pond for watering cattle.
• Floodwater harvesting, also called "large catchment water harvesting" or "Spate Irrigation," is collecting and storing of flow from floods in intermittent wadis for irrigation. Floodwater harvesting has two forms: (1) Floodwater harvesting within the stream bed. The water flow is dammed and stored in reservoirs or is allowed to inundate the flood plain; the wetted area can be used for agriculture or pasture improvement. (2) Floodwater diversion. The wadi water is forced to leave its natural course to nearby cropping fields. Various technologies and different names exist for these two floodwater harvesting types (see section 2.5.2 on traditional knowledge). Pakistan has more than 1.5 106 ha under floodwater harvesting. The irrigated area under floodwater harvesting in North Africa and the Middle East is increasing.
• Groundwater harvesting covers different infrastructures that concentrate and extract groundwater using little energy or only gravity. Qanat systems, underground dams and special wells are a few examples of groundwater-harvesting infrastructures. Qanats, widely used in Iran, Pakistan, North Africa and Spain, consist of a horizontal tunnel that taps underground water in an alluvial fan and brings it to the surface due to gravitational effect. Qanat tunnels have an inclination of 1 to 2% and a length of up to 30 kilometers. Many are still maintained and steadily deliver water to fields for agriculture and villages for drinking water. Groundwater dams are subsurface dams built in the wadi beds. They obstruct the underground flow of ephemeral streams. The water is stored in the sediment below ground and can be used after floods. A more sophisticated infra-