The hydrological cycle is usually depicted in a form similar to that shown in Figure 2.1, which is taken from a classical reference work by Ackerman et al. (1955). An alternative representation, which is more suitable for the present discussion, is shown in Figure 2.2. In the latter figure the rectangles denote various forms of water storage: in the atmosphere, on the surface of the ground, in the unsaturated soil moisture zone, in the groundwater reservoir below the water table, or in the channel network draining the catchment. The arrows in the diagram denote the various hydrological processes responsible for the transfer of water from one form of storage to another.