Published online by Cambridge University Press: 24 May 2017
The equilibration time   $\unicode[STIX]{x1D70F}$  in response to a change in flux from
 $\unicode[STIX]{x1D70F}$  in response to a change in flux from   $Q$  to
 $Q$  to   $\unicode[STIX]{x1D6EC}Q$  after an injection period
 $\unicode[STIX]{x1D6EC}Q$  after an injection period   $T$  applied to either a low-Reynolds-number gravity current or one propagating through a porous medium, in both axisymmetric and one-dimensional geometries, is shown to be of the form
 $T$  applied to either a low-Reynolds-number gravity current or one propagating through a porous medium, in both axisymmetric and one-dimensional geometries, is shown to be of the form   $\unicode[STIX]{x1D70F}=Tf(\unicode[STIX]{x1D6EC})$ , independent of all the remaining physical parameters. Numerical solutions are used to investigate
 $\unicode[STIX]{x1D70F}=Tf(\unicode[STIX]{x1D6EC})$ , independent of all the remaining physical parameters. Numerical solutions are used to investigate   $f(\unicode[STIX]{x1D6EC})$  for each of these situations and compare very well with experimental results in the case of an axisymmetric current propagating over a rigid horizontal boundary. Analysis of the relaxation towards self-similarity provides an illuminating connection between the excess (deficit) volume from early times and an asymptotically equivalent shift in time origin, and hence a good quantitative estimate of
 $f(\unicode[STIX]{x1D6EC})$  for each of these situations and compare very well with experimental results in the case of an axisymmetric current propagating over a rigid horizontal boundary. Analysis of the relaxation towards self-similarity provides an illuminating connection between the excess (deficit) volume from early times and an asymptotically equivalent shift in time origin, and hence a good quantitative estimate of   $\unicode[STIX]{x1D70F}$ . The case
 $\unicode[STIX]{x1D70F}$ . The case   $\unicode[STIX]{x1D6EC}=0$  of equilibration after ceasing injection at time
 $\unicode[STIX]{x1D6EC}=0$  of equilibration after ceasing injection at time   $T$  is a singular limit. Extensions to high-Reynolds-number currents and to the case of a constant-volume release followed by constant-flux injection are discussed briefly.
 $T$  is a singular limit. Extensions to high-Reynolds-number currents and to the case of a constant-volume release followed by constant-flux injection are discussed briefly.