Linear equation

In the simplest physically based approach (“linear model”), the horizontal flow rate, q_wp, is assumed to be proportional to the hydraulic gradient and to the thickness and saturated hydraulic conductivity of each soil layer:

                                                (2.40)

where d_u  is the unit length of the horizontal element i.e. 1m, z_p is the lower depth of the drainage pipe i.e. the drainage level, z_sat is the simulated depth of the ground water table and d_p is a characteristic distance between drainage pipes. Note that this is a simplification where the actual flow paths and the actual gradients are not represented. Only flows above the drain level z_p are considered. See viewing function Physically based drainage equation.

 

Figure 2.6. The geometrical assumptions behind the groundwater flow towards a sink point in the saturated zone of the soil.