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