* Once area, rainfall data, and soil data are collected, then total trench capacity can be determined. The runoff volume minus what would infiltrate in the trenches during the rainfall event would be the trench capacity.
* Trench dimension and spacing in the catchment can now be calculated. This 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. Trenches should also be over-dimensioned (exaggerated) 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. ====Digging the trenches==== * Trenches are dug in line with topographical contours. Ploughing is not recommended as it induces evaporation.
* 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.
* Trenches can be connected to the wall of a sand dam, increasing filtration.
* Involvement of local people in the design of the project ensures that their participation continues.