Executive Summary
Field trenches increase precipitation harvesting by breaking the slope of the ground and therefore reducing the velocity of water runoff. By decreasing runoff, they enhance water infiltration and prevent soil erosion. Trenches can be seen as an extended practice of ploughing fields. They may be applied to all soil types and are not dependent on slope or rainfall conditions (SUSSMAN 2007).
| In | Out |
|---|---|
Freshwater, Food Products |
Introduction
Adapted from RAINWATERHARVESTING (2011)
Basically, trenches can be seen as extensive ploughing to the right angle of a field’s slope. By breaking the slope and therefore reducing the velocity of water runoff, field trenches filter runoff water from rainfall and hence reduce soil degradation, erosion and enhance infiltration of surface run-off and soil moisture.
A great advantage of field trenches in comparison to other types of water harvesting measures (such as bunds, planting pits, micro basins, retention basins, and controlled drainage) is their applicability to all soil and rainfall conditions.
Trenches may be added to steep slopes as well as even land and can be used for all types of soil depth.
Basic design principles
Adapted from SUSSMAN (2007) and RAINWATERHARVESTING (2011)
For optimal performance, trenches are constructed along contour lines, similar to contour bunds. Therefore, the lines need to be marked before starting shovelling (for a simple technique to define contour lines, see SUSSMAN (2007)). When digging the trench, the excavated soil is placed downslope along the edge of the trench. Crops are then planted on the elevated land between the trenches.
The design of contour trenches can be continuous or intermittent. While the continuous ones are best suited for moisture conservation in regions with low rainfall, interrupted trenches are best suited to high rainfall areas.
The optimal distance between two trenches depends upon the slope of the field, where steeper grounds require less distance.
Entirely preventing water runoff may be dangerous if extremely heavy rainfall occurs. To control excess water, it can be channelled away by building drains or waterways.
Cost considerations
The costs for field trenches depend on the cost of the filling material, and the labour needed (which can differ according to soil conditions).
Due to their simple construction, only basic material is needed for building trenches, such as stakes, shovels, picks, crops, and possibly a tractor (depending on the slope of the field).
Operation and maintenance
For reaching best outputs, sediment should be removed from the bunds and be reapplied to the field uphill the trench from time to time (SUSSMAN 2007). Berms may need more frequent repairing as long as vegetation has not been established to stabilise soil.
At a glance
| Working principle | By making field trenches, water runoff is hindered, leading to increased infiltration. |
| Capacity/adequacy | Construction is simple, requiring only basic tools and skills. But, depending on soil conditions, labour can be intensive. Trenches may be adapted to most areas and soil types. |
| Performance | Field trenches increase soil moisture and prevent soil erosion. |
| Costs | Costs depend on the filling material as well as the labour needed. As basic material can be used, there are hardly any expenses for special tools. |
| Self-help compatibility | As only little material is needed and the construction is comparably simple, trenches may be implemented and maintained without outside help. |
| O&M | Sediment should be removed from the trenches and reapplied to the field uphill the trench. Vegetation may stabilise the trench and therefore decreases the need for repairing. |
| Reliability | If construction and maintenance is done well, reliability of trenches is high. |
| Main strength | Broad suitability: field trenches are useful for most soil conditions. |
| Main weakness | On-going maintenance is inevitable and can be time consuming. |
Field trenches suit most soil and rainfall conditions. Their design may be adapted to different rainfall conditions: While continuous trenches are good for dry regions, interrupted bunds can be helpful for water harvesting in regions with higher rainfall.
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