Natural ground catchment and Open water reservoir

This category covers a range of large open water ponds that store rainwater. Natural depressions (pans) also hold rainwater in a similar way but are not modified or designed. Ponds described in this section include those that are either excavated and/or which might make use of the natural topography, and which in most cases involve an embankment around part of the pond to retain the water (the material for which may have come from excavation works). They come by different names in different countries, but names include johads and “hafirs”. These reservoirs can also be formed in existing seasonal water courses or valleys, in which case they may also be called valley dams, which are essentially the same as gully plugs (see relevant section). They can have limited to high aquifer recharge capacity – for ponds purposely built to increase groundwater recharge, see section on infiltration ponds under Managed Aquifer Recharge (MAR). This section discusses ponds constructed with the primary goal of storing surface water for various water uses (e.g. irrigation, livestock), although they may also recharge groundwater. Ponds can be lined as well as unlined.

Suitable conditions

• The base of the pond needs to be impermeable, e.g. unfissured rock or clay, in order to save costs and prevent having to find a form of lining.
• Try to site ponds to minimize excavation – use natural or man-made topographical features, e.g. borrow

pits from road construction, or sloping ground.

• Site in areas of high intensity rainfall, as this leads to high runoff, causing ponds to fill with water rather than water infiltrating into the soil.
• Build small reservoirs (5-10 ha) in large watersheds – when built with a good spillway, there is no problem and reservoirs fill up quickly. Siting in this case is best determined by proximity to a village, topographical geometry or presence of roads/access. Hydrology comes into play in the design for larger reservoirs (>15 ha). However, when constructing valley dams specifically (those in a seasonal watercourse), the rule of thumb is not to build small reservoirs (below 10,000 m3) in catchments larger than 400 ha (1,000 acres), otherwise the amount of overflow is excessive to the point of creating washed-out spillways.
• In pastoralist areas, it might be good to site ponds in areas where traditionally pasture is used first after the rains. In this way, as much water as possible can be used to cover water demand before it is taken by seepage and evaporation, leaving other sources with less seepage and evaporation, e.g. sand dams, to be used later on in pasture accessed during the dry season.