How much money do you have in your bank account? ‘Not enough’ is the usual answer! Of course, the amount varies over time, depending on how much we pay in or draw out. In simple terms, we are interested in the difference between income and expenditure and remain happy as long as the balance stays positive.
It is much the same with any physical system — there are inputs, outputs and storages. Such systems are often called ‘cascading systems’ because the output from one subsystem forms the input to the next. Thus, in the hydrological cycle, rainfall is the major input, with evaporation and discharge as the main outputs. In between, there are many storages, like soil moisture and groundwater, and many f low processes linking the stores together, such as infiltration, percolation and overland f low.
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