Towards a Better Design of Micro-Catchment Water Harvesting Schemes.

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Towards a Better Design of Micro-Catchment Water
Harvesting Schemes.

• By Dave Best

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Direct rainwater harvesting from micro-catchment
schemes (within field catchment systems)

• Overland flow harvested from a short catchment.
  1-30 metres.
• Concentration of surface runoff from a catchment
  into lower lying cropped area.
• Ratio of catchment to cultivated area- from 11
  to 201.
• Offers a passive, low cost means of increasing
  the amount of water available to crops

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Advantages over alternative irrigation schemes.

• Systems relatively easy to maintain and operate.
• Simple and easy to construct
• Once constructed little maintenance is required.
• Runoff water low salt content

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Examples of micro-catchment water harvesting
systems
1) Negarim micro-catchments
2) Contour bunds
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3) Semi circular bunds
4). Contour ridges
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Design Considerations

• To improve catchment efficiency runoff can be
  increased, and infiltration and interception
  reduced.
• Balance between the efficiency of a catchment in
  producing runoff and its susceptibility to
  erosion
• Increasing runoff by clearing slopes of
  vegetation and compaction of surfaces may lead to
  greater erosion with longer slope lengths and
  steeper gradients.

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Determination of Catchment yield.

• Rainfall Characteristics.
• average annual rainfall, peak rainfall intensity,
  and minimum expected annual precipitation.
• Physiographic factors.
• Infiltration Capacity.

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Catchment Cultivated area

• Rainfall
• Runoff
• Crop water requirements.

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Catchment Cultivated area-calculations.

• Water Harvested Catchment area x Design
  rainfall x Runoff coefficient x Efficiency
  factor.

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Example.

• Crop Water Requirement 525 mm
• Design Rainfall 310 mm
• Runoff Coefficient 0.5
• Efficiency Factor 0.75
• C 525 310 1.85
• Ca 310 x 0.75 x 0.5

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Surface treatments

• Mechanical treatments (smoothing and compacting).
• Colloidal dispersion methods (slaking, sodium).
• Hydrophobic applications (water repellents).
• Surface binding materials (cementing and sealing)
• Surface covering (asphalt, rubber and plastic)

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Experiment testing different techniques for
catchment lining on a plantation of Jujube
(Zizyphus mauritaina)

• Shallow conical micro-catchments of 1.0m radius
  were constructed around the plant

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The soil moisture depletion pattern in various
treatments in a 90cm profile
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Jujube
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Using flow models. i.e HILLFLOW