Infiltration is ceased once the flow reaches steady-state and allowed drainage under gravitational gradient. The temporal variation of suction and moisture content at different depths in the soil during drainage can be plotted for obtaining hydraulic gradients.

Fig. 4.31 Conceptual illustrations of (a) Transient water content profiles and b) Suction profiles measured during field instantaneous profile test (after Lu and Likos, 2004)

At the moment drainage begins in soil, the moisture content is θ_s and the corresponding suction is zero. As the drainage continues the water content decreases with time and vice-versa for matric suction. Further, the soil layers at greater depth have smaller rates of moisture loss. Thus, the soil layers at greater depths maintain relatively high water content for longer period of time. Similar to earlier analysis, the water flux through any layer at depth z_j and time t_j is

.........................................................................(4.11)

This slope can be obtained from the illustration in Fig 4.31. The cumulative flux is obtained by summing the water fluxes through all the overlying layers. The hydraulic gradient is then calculated from Darcy's law from the above computed flux and the hydraulic gradient,