Soil water collection: Underground dam

Solution type: Soil water collection

• Objectives: Increase the capacity for accumulation and storage of rainwater, under conditions of low rainfall, increasing the availability of water during the crop cycle.

• Description: This water storage technique, through the use of underground dams, is used in agriculture in situations of limited availability of surface water that prevents the construction of surface reservoirs. 

The groundwater storage and conservation system consists of accumulating rainwater, from surface runoff, in two ways: 

1) Surface accumulation

2) Underground accumulation

Location and selection of soil

The water retention capacity of the soil is a determining factor for the implementation of this technique. The depth between 0.4 to 0.5 m is essential to maintain the water for longer. Most underground dams are built on river or stream beds, using stone walls jointed with cement and sand mortar.

Construction

The dams have a height of no more than 5 m. from the bed to the surface level. Dimensions vary between 10 to 15 m long and 5 m wide. For the waterproof wall or partition within the pre-existing alluvium, the following materials are used, among others:

• Stone wall repointed with cement and sand mortar;
• Brick wall with sand and lime mortar;
• Compacted clay wall;
• Plastic sheet wall or membrane layer up to the surface.

Components

1. Catchment Area (CA). The land area represented by a small hydrographic basin. It is formed by topographic and phreatic divisions; Its objective is to capture rainwater from surface runoff and direct it through natural or artificial drainage to the storage or planting area.
2. Exploration zone with dry season or plantation agriculture. It is the area represented by the hydraulic basin of the dam. At the time of formation of the system, this area settles, covering the surrounding areas, simultaneously allowing a greater volume of water to be stored in the alluvial macropores, achieving greater coverage of a greater production area.
3. Dam Wall (DW). The purpose is to intercept the flow of surface and groundwater, leading to the formation and/or elevation of the ground water table.

Downstream of each of these dams, a cistern is built to store excess water from the dam. In this cistern, the storage tank is located underground, regularly covered by a waterproofing material. This cistern has two filtration systems, one internal to the storage tank and another upstream, in the underground dam, as well as aerators for internal air circulation.

Cost and resources

• • • Approximately USD 3,000 (Cost of an underground barragem varies according to local conditions)

Advantages

• • • Some results demonstrate an average groundwater storage per dam of 500 l/m3, using a dam with the capacity to store 1,317.9 m3, favoring the irrigation of crops between 0.6 and 0.90 ha.
    • A dam requires low-cost but continuous maintenance to prevent erosion, sedimentation, infiltration, tree growth and damage caused by insects or animals.
    • It allows having a volume of water greater than the demand for agricultural crops and/or animal husbandry, ensuring the supply of water, especially during the dry period, improving productivity and the income of producers.

Complexity

The construction of small rainwater accumulation dams does not entail complexities. Local materials and natural resources are used, construction procedures and technical requirements for topography, hydraulic system and soil, for foundations for a small-scale project are generally simple. The broad base of an earthen dam distributes the load over the foundation.

Comparatively, it is a technique that requires little investment and tangible fixed assets for small producers.

Sources:

• Porto, E.R.; Vivallo Pinare, Ag.; Wllllams Fuentes, C.O.; Silva, A De S.; Lopes, L. H. De O. Small farmers V: methods for implementing integrated agricultural production systems (SIP). Petrolina, PE; EMBRAPA-CPATSA, 1990.
• Stephens, T. Handbook on small earthen dams. Guide to location, design and construction. Pomegranate; FAO, 2011.