Developing Drainage Design Criteria

The design requirement expresses the agricultural function of the drainage system in terms that can be used as input information for any of the drain spacing equations or graphs. Drainage criteria can be evaluated for a steady-state condition, falling or fluctuating water table, salinity control, or trafficability of the soil, depending on the main function of the drainage system. These objectives could be used in the evaluation of the drainage requirement, but the drainage requirement itself could be expressed in terms of an equivalent steady-state criterion. If the steady-state criterion is known for a soil of porosity, it can be evaluated for a soil of different porosity by calculating the frequency of high water-table positions for soils using analog or digital models. The natural drainage intensity should be deducted from the drainage criterion so that, when artificial drainage is needed, the system can be designed on the basis of the drainage deficiency of the field.