* 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 and soil properties based on physical soil investigations on site (e.g. how does sandy, loamy, or clay soils absorb water?)
* Total Once area, rainfall data, and soil data are collected, then total trench capacity can then be determined that can store the . The runoff volume minus what would infiltrate in the trenches during the rainfall eventwould be the trench capacity.
* 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. 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.