Infiltration is the movement of water into an unsaturated soil profile. When the precipitation is intense enough to saturate the soil surface, part of it remains ponded or runs off, and part of it infiltrates at the maximal rate; this is the infiltration capacity. Initially the infiltration capacity is mainly driven by the pressure gradient due to capillarity and the effect of gravity is relatively small; thus for small times the problem can be treated as one of horizontal flow or simple sorption. For larger times the effect of gravity gradually takes over, and the infiltration rate approaches the saturated hydraulic conductivity. When the precipitation intensity is low, initially all the water seeps into the pores; this is rainfall infiltration. As precipitation continues, the increasing surface soil-water content is unknown, but the flow at the surface is the known precipitation rate. In time, the soil at the surface becomes saturated, after which water ponds the surface. Important parts of the solution are the time to ponding and the ensuing infiltration rate. Many problems of evaporation from soils can be analyzed on the basis of the isothermal flow equation, viz. Darcy’s law.