# HydroGeoSphere/Adaptive Timesteps

If required, **HydroGeoSphere** can modify timestep values as the solution proceeds, based on the transient behaviour of the system (see Equation 3.102). The following instructions can be used to activate this feature, and set targets for specific variables (e.g. pressure head, saturation etc.). These targers are used to modify timestep size as the solution proceeds.

## Head controlEdit

**dhead_allowed**Maximum allowed absolute change in nodal head during any time step [L].

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## Water depth controlEdit

**ddepth_allowed**Maximum allowed absolute change in nodal surface water depth during any time step [L].

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## Saturation controlEdit

**dsat_allowed**Maximum allowed absolute change in nodal saturation during any time step [L].

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## Newton iteration controlEdit

**nnri_allowed**Maximum allowed number of Newton-Raphson iterations during any time step [L].

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## Concentration controlEdit

**dconc_allowed**Maximum allowed absolute change in nodal concentration during any time step [L].

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## Mass change controlEdit

**dmass_change_allowed**Maximum allowed absolute change in mass during any time step [L].

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## Mass error controlEdit

**dmass_error_allowed**Maximum allowed absolute mass error during any time step [L].

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The following controls are used to generate a timestep multiplier according to Equation 3.102. There are limits to how large or small the multiplier can be, and these are set by default to be in the range of 0.5 to 2 respectively. If you want to modify these limits you can do so using the following instructions:

## Maximum timestep multiplierEdit

**val**Maximum timestep multiplier value.

Assigns a new value for the maximum timestep multiplier, which defaults to 2.

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## Minimum timestep multiplierEdit

**val**Minimum timestep multiplier value.

Assigns a new value for the minimum timestep multiplier, which defaults to 0.5. Currently, **HydroGeoSphere** ignores this parameter and allows *any non-zero minimum, no matter how small*.

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