A continuous contour trench is dug at a right angle to the slope and are planned along contour lines. Doing so stops the water flowing downhill in its tracks by the trenches, and water percolation into the soil below is facilitated.
Trenches are dug along the contour lines (perpendicular to the slope of the farmland) in the site for a depth of around 1.5 to 2 feet.
The distance between each trench depends on the slope of the terrain. The more the slope, the closer the trenches are made.
There are a lot of advantages to making these trenches.
These trenches help control the water from flooding downstream farm areas, which helps in saving water and channeling it in the right direction.
The water that percolates into these trenches after a rainfall, keeps the soil moisture intact for a long time that may even extend up the following dry season. The same water can be directly pumped out for irrigation or extracted from shallow wells in the area.
Without trenches, a lot of soil erosion happens which increases the salt build-up in the water downstream. This becomes unfavourable for groundwater quality as well as for crops. Also, the roots and foliage of the vegetation trap sediment that would otherwise overflow from the trench during heavy rainfall.
Any pollutants other than salts that may have mixed during the runoff are also contained within these trenches without getting concentrated and accumulating downstream. Runoff happens when rainfall intensity exceeds the ability of the soil to absorb and transmit rain-water.
These trenches prevent soil erosion from happening as the water flow is arrested at each contour. As can be seen below, silt has accumulated in trenches after the rains recently. The silt that has accumulated in the trenches will be collected and reused in the farmland.
Over time, these trenches create a micro-climate locally and improve bio-diversity due to the availability of moisture and topsoil runoff.