Water Portal / Rainwater Harvesting / Groundwater recharge / Contour trenches

Contour trenches are not irrigation channels, rather they are trenches dug to slow down and attract runoff water which then infiltrates into the soil. Small scale contour trenches can also be used within field level.

Suitable conditions

• Locate trenches in natural runoff areas, but not on slopes over 10%.
• Soil in vicinity needs to have sufficient infiltration capacity and potential sub-surface storage capacity.

Construction, operations and maintenance

In order to efficiently capture runoff in a catchment through appropriately sized trenches, the following information and analysis is needed:

• Catchment area, gathered from topographical maps
• Detailed rainfall data which is used to create a rainfall frequency analysis where precipitation level and probable recurrence interval.

 * Using this data, the predicted runoff for a particular intensity of rainfall can be shown using the Soil Conservation Service (SCS) method, which is a simple way to calculate runoff in ungauged catchments. To calculate runoff, the following is needed: precipitation for a particular rainfall recurrence interval (e.g. 5 years), catchment area, soil characteristics, and land use in catchment. For a given recurrence interval, the total amount of runoff for the catchment can then be calculated. 

• Soil infiltration rates based on physical soil investigations on site.
• Total trench capacity can then be determined that can store the runoff volume minus what would infiltrate in the trenches during the rainfall event.
• Trench dimension and spacing in the catchment can then be calculated. This then has to be checked with local preferences and

adjusted accordingly. Trenches in Vietnam were originally designed as rectangular trenches 4m wide by 1m deep, but modified according to requests from local people to trapezoidal trenches 2.5m wide at top and 1m wide at base and 0.75m deep.118 Trenches should also then be over-dimensioned to allow long-term runoff volume to be stored despite a lack of maintenance – things like siltation and erosion of the banks will reduce infiltration capacity and volume over time.

• It seems wise to pilot contour trenches in an area before scaling up.
• Trenches are dug in line with topographical contours.
• Constructing trenches primarily to favour plant growth and increase agricultural productivity (as was the case in Vietnam) rather than just

as a means to increase groundwater levels, seems to be a good approach because there can be increased interest to ensure operation & maintenance of the trenches (in Vietnam, people wanted to establish a group of landowners to take care of the structures in order to ensure continued crop yields. The knock-on effects on shallow groundwater levels are thereby made sustainable since the primary concern for people was an economic one.

• Excavated soil can be used to fill up existing gullies.
• Involvement of local people in the design of the project ensures that their participation continues.
• In Vietnam, after trenches were completed farmers were discussing ways to access to low-cost loans with long-time repayment

conditions so that they could replicate the technology. Access to finance therefore seems to be important in scaling up this technology.