Charco Dams

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Description

Charco dams are a traditional means of water harvesting in arid and semi-arid landscapes of Eastern Africa, especially in Tanzania. This type of dam has a deep reservoir and limited surface area which helps to reduce water-loss from evaporation. Charco dams are built to collect rainwater and runoff water from flat surfaces. As a result, they are suitable for livestock watering and crop irrigation, but less suitable for domestic use (unless the water is treated). Typically, trees and shrubs are planted around the edges of the dam to act as windbreaks to further reduce evaporation. Small earth dams may vary significantly in size based on the water storage capacity needed. Storage of up to 1000m3 is possible, beyond which a pond is considered large and will require different building protocols. Dams need to be thoroughly planned in order to ensure their feasibility, longevity, and safety. Please make sure to consult the additional supporting resources provided for more specific details on how to plan and build a Charco dam, or talk to a local extension officer for help on planning and implementing a Charco dam in your landscape.

  • Erosion prevention
  • Water harvesting
  • Runoff reduction
  • Increase crop yields

Materials required:

  • A tape measure
  • A line level
  • Long strings
  • Wooden pegs
  • Shovel(s) and/or wheelbarrows
  • Draught animals with ploughs

Steps of Implementation:

  1. Dam Mapping.
    Using a topographic map of the land or a simple sketch, locate the position of the reservoir, inflow channels, overflow channels, the silt traps, and the embankment.

Two factors can help you to determine the best location for your dam:

A. Soil type: Coarse grained, clayey soils are best suited for Charco dams as these will retain water better than fine grained sandy soils. 20-30% clay content is the minimum requirement. If the dam reservoir is located in an area with porous soils, consider lining it with a 20-30 cm layer of clayey soil.

B. Topography of your land: Usually, the lowest point of the land is the most suitable place for a dam. If possible, choose a natural depression where rainwater accumulates during the rainy season. Or choose a location close to where rainwater runoff flows naturally.

2. Excavation. 

Clear the vegetation and mark out the area:

Begin by uprooting and removing all vegetation and stones from the construction area. Then, create an outline of the structure by using wooden pegs and a long piece of string tied to the middle of the pond. Wrap the string around the pegs. Follow the proportions indicated in the picture below (in step 3) as much as possible but make adjustments when necessary.

Excavation of a Charco dam can be done:

  • Manually with shovels and wheelbarrows. (3 m3/day per person)
  • By use of draught animals with scoops, ploughs and carts (30m3/day per person)
  • In an automated manner, by hiring a crawler/bulldozer.

3. Construction.

There are a number of different elements that need to be constructed to create a complete Charco dam:

The Pond
Should be at least 3m deep in-order to make the dam worthwhile in the face of evaporation.  Create a slight slope from the bed (deepest part of the pond) towards the inlet. This way, when the water evaporates it may remain in the deeper sections of the dam, thus minimizing the surface area exposed to evaporation.

Inflow
If a natural channel for the water does not already exist, dig an inflow channel that will lead runoff water into the pond. The inflow of the dam should be located where rainwater usually accumulates or passes by in a stream. If needed, the size (length and width) of the inflow area can be extended to increase the water catchment capacity of the pond.

Earth wall
Use the excavated earth from the pond to form an earth wall on the edges of the dam. Heap the excavated earth around the dam immediately after digging. Compaction of the heaped earth is not necessary. Build the earth wall with a moderate slope to avoid erosion: the slopes of the wall should be flatter than 1:1, or 45 degrees. Make sure that enough space is left between the edge of the pond and the heaped soil/earth wall to build the Berm. The earth wall should be highest opposite the inflow section.

Berm
The Berm is an area between the reservoir and the earth wall that is  left untouched. It facilitates transportation in and out of the construction site and stops earth from the wall falling back into the reservoir. If there are plans to enlarge the dam in the future, leave more space for the Berm than the 2m recommended in figure 1.

Spillways
Spillways ensure that water flows safely and in a controlled manner over the dam wall. Two spillways beginning at the inflow and following the exterior of the earth wall should be built: One spillway on either side of the wall. Spillways for Charco dams are built as small gullies paved with stone and long-rooted grass in order to avoid erosion.

A number of additional, but not mandatory, features can be added to enhance the Charco dam:

  • Silt traps: A series of silt traps of 1 to 2 meters deep should be excavated along and beyond the inflow channel. Silt traps should be between 5m and 20m away from the mouth of the reservoir. This helps minimize the silt that enters the reservoir.
  • Fencing: It is best to fence off Charco dams to keep livestock and unwanted people away from the dam. This helps to protect water quality and prevent the spread of diseases.
  • Windbreak: Trees can be planted along the exterior of the dam wall. When planted facing the prevailing wind direction, trees can protect the dam walls from erosion and reduce evaporation. A row of vegetation can also act as a fence (cheaper option).

This intervention contribute to:

Estimated costs of intervention (in Kenyan shillings - Ksh):

Description Costs Cost of a 500m3 dam
Manual excavation Ksh.100 (USD $0.81)/m3

(2006 dollars)

 USD $405
Excavation (using Oxen) Ksh.80 (USD $0.65)/m3

(2006 dollars)

 USD $325
Excavation (using tractor) Ksh.60 (USD $0.49)/m3

(2006 dollars)

 USD $245

Estimated benefits of intervention (in Kenyan shillings - Ksh):

Socio-economic Benefits Value of Benefits
Labor saved fetching water (Ksh 500 x 3 months) Ksh. 1500 (≈USD $20) (2006 Dollars)
Labor saved on watering livestock (Ksh 500 x 3 months) Ksh. 1500 (≈USD $20) (2006 Dollars)
Income from sale of tomatoes and kale (over ¼ irrigated acre) Ksh. 6500(≈USD $90) (2006 Dollars)
Value of household consumption of tomatoes and kale

(Over ¼ irrigated acre)

 Ksh. 500 (≈USD $7) (2006 Dollars)
Total income from a 500m3 water reservoir after a rainy season Ksh. 10 000(≈USD $140)

Based on a case study of the annual value of benefits from a 500m3 water reservoir (Source).

Additional information

Stones

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Rainfall

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Land use

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Slope

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