Water from Small Dams

Chapter 4.Charco damsFarmers and cattle owners in semi-arid parts of Tanzania build small earth damsknown as Charco, or Milambo in Kiswahili. These dams are built in a way which triesto reduce evaporation losses by deepening the water reservoirs and minimising theirsurface area. Trees and scrubs are grown on the windy site of the charco dams tofunction as windbreaks that also reduce evaporation.A Tanzanian farmer (with cap) explains the benefits of his charco dam.4.1 Site selectionThe best sites for constructing charco dams are in natural depressions where rainwatereither flows or accumulates during the rainy season. The soil should, preferably, bedeep clay, silt or Black Cotton soil. Coarse textured sandy soils should be avoided asthese are highly permeable and water will drain through them. If seepage is high incharco dams, they can be plastered with clayey soil and compacted using compactorsmade of tree trunks. The most suitable types of soil for dams are shown in Chapter 6.Sites with underlying strata of sand, gravel, limestone or fractured rock at a shallowdepth may also result in high seepage losses, unless they are sealed with clayey soil.Ideally, a charco dam should be located near to a gully or a natural waterway, whichcarries water during and after rainfalls, as this water can easily be diverted into thedam. Avoid building dams near or downstream from livestock enclosures to avoidorganic and/or chemical pollution.Charco dams are usually excavated manually by individuals near their homesteads forwatering livestock. The water may also be used for some domestic purposes, if it isboiled or treated by the sun’s UV rays in transparent bottles (SODIS).Farmers dig their ponds during dry seasons and enlarge them every year, until theowner is satisfied with the capacity of the dam.17