In order to derive the Green-Ampt equation, we begin with drawing of the 3 energy lines for the case of a wetting front between the water table and the soil surface. This is the same diagram drawn in your homework problem a few weeks ago. Potential energy, z, at the datum is zero, and increases linearly with elevation. Thus, the orange line is drawn as shown. Pressure, P, at the water table is zero and decreases in the dry zone below the wetting front.
H is also zero throughout the soil profile below the wetting front.
The pressure at the ponded water surface is zero and increases with depth. Thus, the green line, P, in the ponded water can be drawn. Likewise, infiltration is rapid immediately after ponding and decreases over time as the wetting front advances down the soil profile. The difference in energy, H, between the soil surface and the wetting front remains constant, but the length between the soil surface and the wetting front, L f , increases. Energy lines with wetting front near the soil surface during early part of infiltration event.