The hydrological system
Click the link below for more about the hydrological cycle from nature.com.
The hydrological cycle as a system (From Waugh – Geography An Integrated Approach)
The Water Balance
Shows the relationship between the inputs (precipitation) and outputs (run-off + evapotranspiration + or – any changes in storage. At some times the inputs may be greater than the outputs, and at others the outputs may be greater than the inputs (see below).
The maximum sustainable yield
This refers to the maximum level of extraction of water that can be maintained indefinitely for a given place or region.