Traditional Water Harvesting Techniques:
Traditional water harvesting systems in North African and Southwest Asia include some of the oldest sustainable methods of water harvesting in arid lands.
An understanding of these systems which have lasted for millennia is crucial for current efforts to combat desertification and rehabilitate degraded desert habitats at a time when climate change is compounding the threat to many areas in North Africa and Southwest Asia which are extremely vulnerable to shifts in climatic conditions.
At present there are two main classifications of traditional water harvestng systems one by Oweis et al. (2004) and the other by Prinz (1996. 2000).
The classification by Oweis et al. (2004)(see below) starts by subdividing techniques on the basis of the size of the catchment area, which is the area from which water is collected. Small catchment areas (micro-catchment) are then further classifed as roof-top and on-farm technqiues. By contrast, techniques that depend on large catchment areas (macro-catchment) are further subdivided into wadi bed and off-wadi systems. Prinz bases his classification initially on the basis of the source of water; namely, rainwater, floodwater, and groundwater.
The current proposed classification follows Prinz in using the source of water as its starting point, however, in the current system a recognition is made of harvesting moisture (dew and air moisture), snow and snow meltwater. In addition, a differentiation is made between ephemeral wadi water and floodwater from perennial water, like the Nile.
Water from surface runoff is also recognized as a category on its own.
In many cases, harvested water is not used immediately and is stored for use later in cisterns, reservoirs, tanks, or wells. These storage facilities are often lined to prevent water loss, and may be covered to minimize evaporation and prevent introudction of unwanted sand or litter. Similar facilities, however, are used to harvest groundwater. Since they depend on seepage from groundwater, they are mostly unlined cavities. In many cases water harvesting, as in the case of surface runoff, is based on either channeling runoff to storage areas or enhancing infiltration to maximize amount of moisture in the ground.
Water harvesting depends not only on the sources of water, but also on bedrock, surface relief, vegetation, evaporation rates, and more importantly know-how. This or any other classification is only a means for exchanging information within a conceptual framework and a standardized terminology. We hope that this contribution will bring to the attention of policy makers, professionals, and the General Public the innovative approaches by which people in Aridlands all around the Mediterranean have been able to overcome the adversity of water scarcities in a fragile environment. These methods may be used as they have been traiditonally used, they may be coupled with new technologies and new materials for greater efficiency and better performance, or they may just be a source of inspiration for equally innovative techniques or even just as an illustration of the endemic problems of water scarcity in desert regions and the ingenuity required to survive where water, the source of life, is a rare gift.
Classification Of Water Harvesting Systems Oweis, Hachum And Bruggeman
In 2004, ICARDA issued a monograph (Oweis, Hachum, and Bruggeman, 2004) with case studies from Tunisia, Jordan, Morocco, Syria, Libya, Iraq, Egypt, and Yemen. In the introductory chapter to that volume Oweis et al. (2004) present a classification system after Oweis et al. (2001) and Prinz (1996, 2001).
Fig. 1.- Classification of water-harvesting systems (after: Oweis et al. 2001, 2004)
Oweis, Hachum, and Bruggeman (2004) provide the following description of traditional water harvesting methods:
The microcatchment systems are those in which surface runoff is collected from a catchment area where surface runoff prevails over a short distance. The system, though simple, requires continuous maintenance with a relatively high labour content.
Contour ridges are "bunds", usually constructed along the contour line at intervals between 5 and 20 meters.
Semi-circular and trapezoidal bunds consist of bunds assuming geometrical winged shapes facing the direction of maximum slope. They are constructed so that they can catch water from a large area to supply water for plants behind the bund.
Small pits consist of holes 0.3-2 m in diameter, and 5-15 cm deep. Manure and grass are mixed with soil and placed in the pits. These pits are excellent for rehabilitation of degraded agricultural land.
Small runoff basins, which are sometimes called "negarim", are small shallow basins 5-10 m in width and 10-25 m in length. They can be constructed on almost any slope and are suitable for growing trees.
Runoff strips are narrow cropped strips alternating with vegetated [does this mean cultivated?] strips. The strips (1-3 m) facilitate the movement of runoff water to irrigate the cultivated crops.
Contour bench terraces are terraces supported by stone walls and constructed on very steep slopes.
Courtyard and Rooftop systems include systems constructed to harvest water from roofs and courtyards of houses, large buildings, and greenhouses. Water collected is used mostly for domestic purposes.
Macrocatchment and floodwater systems harvest water from very large areas. The catchment area is often outside the boundaries of the farm. Accordingly, they involve problems of water rights and water allocation. They include wadi-bed systems (a wadi is an ephemeral stream) in which water is used from wadi surface water or water infiltrating the wadi bed. Small dams are often cosntructed to store water in reservoirs. Jessourare dams or embankments constructed across steep slopes. Sediments in water stopped by the dams settles to form new farm land. Off-wadi systems make use of wadi water to irrigate areas outside the wadi-bed. The most important off-wadi systems include water-spreading systems which are based on diverting water from its original course to adjacent areas. Large bunds (also called tabia in Tunisia) consist of large V-shaped earthen bunds spaced at a distance of 10-100 m. They can store large amounts of water.
Tanks and reservoirs are often excavated in the ground. They are called hafa'ir in the Sudan, Jordan and Syria.
Cisterns are subsurface reservoirs with a capacity ranging from 10 to 500 cubic meters. They are often used for human and animal water consumption.
Hillside runoff systems also called sylaba or sailaba are cosntructed to direct runoff downhill to flat fields (often before it joins wadis).
Prinz Classification of Traditional Water Harvesting Systems
Another classification has been provided by Prinz (1996, 2000). His classification is as follows:
- Rainwater Harvesting
- Floodwater Harvesting
- Groundwater Harvesting
Conventional irrigation methods use the rainfall after it has infiltrated into the ground, using underground water or the water of permanent streams and rivers.
Rainwater harvesting techniques collect the rainfall before it enters the soil, i.e. as surface runoff.. The collection and concentration of rainfall and its use for the irrigation of crops, pastures and trees, for livestock consump-tion and household purposes is called rainwater harvesting.
Each rainwater harvesting system requires:
- A "runoff area" (catchment) with a sufficiently high run-off yield, and
- "run-on" area for utilisation and/or storage of the accumu-lated water. These methods can be subdivided according to their way of water collection and the kind of storage. The greater the aridity of an area, the larger is the required catchment area in relation to the cropping area for the same water yield.
Three types of water harvesting for agricultural purposes are covered by "Rainwater Harvesting":
- Water collected from roofs and courtyards and similar paved, bituminized, or otherwise treated surfaces. The col-lected water is used for domestic purposes, for animal water consumption or for garden crops.
- Micro-catchment water harvesting is a method of collecting surface runoff (sheet or rill flow) from a small catchment area and storing it in the root zone of an adjacent infiltration basin. The basin is planted with a single tree or bush or with annual crops.
- Macro-catchment water harvesting is also called "water har-vesting from long slopes" or "harvesting from external catchment systems". In this case, the runoff from hill-slope catchments is conveyed to the crop-ping area located below the hill foot on flat terrain.
Floodwater harvesting is also called Large catchment water harvesting or "Spate irrigation", and comprises two forms:
- In case of "floodwater harvesting within the stream bed" the water flow is dammed and, as a result, inundates the valley bottom of the flood plain. The water is forced to infiltrate and the wetted area can be used for agriculture or pasture improvement.
- In case of "floodwater diversion", the wadi water is forced to leave its natural course and conveyed to nearby cropping areas. These two systems - the catchments being many square kilometers in size - require more complex structures of dams and distribution networks and a higher technical in-put than the other two water harvesting methods. It is difficult to give exact figures on the present total area under the various forms of "Rain and Floodwater Harvesting".
Groundwater damsobstruct the flow of groundwater or the subterranean flow of ephemeral streams and rivers in a river bed. The water is stored in the sediment below the ground sur-face. The stored water can be used to recharge an aquifer or to raise the level of an aquifer thus making it more accessible for lifting.
There are basically two types of groundwater dams: sub-surface dams and sand storage dams. A sub-surface dam is constructed below ground level and arrests the flow in a natural aquifer. A trench is dug across the valley, reaching down to the bedrock, and a dam is constructed in the trench. Water is extracted at the upstream side of the dam by shallow wells.
This contribution has been conceived and actualized within the SHADUF Project. The use of iconic representation has been initiated by P. Laureano (IPOGEA, Italy), but his descriptions and classification of water harvesting system are not followed here. The graphics are the work of Adham Bakry based on schematic sketches by Fekri Hassan.