This equation represents flow to a pumping well (a sink of strength G), located at the origin. Both this equation and the Cartesian version above are the fundamental equation in groundwater flow, but to arrive at this point requires considerable simplification. Some of the main assumptions which went into both these equations are:
* the aquifer material is incompressible (no change in matrix due to changes in pressure &mdash; aka subsidence),
* the water is of constant density (incompressible),
* any external loads on the aquifer (e.g., overburden, atmospheric pressure) are constant, 
* for the 1D radial problem the pumping well is fully penetrating a non-leaky aquifer,
* the groundwater is flowing slowly (Reynolds number less than unity), and
* the hydraulic conductivity (K) is an isotropic scalar.
Despite these large assumptions, the groundwater flow equation does a good job of representing the distribution of heads in aquifers due to a transient distribution of sources and sinks.