# HydroGeoSphere/Units and Physical Constants

The units used in the program are not preset, although a default of kilogram-metre-second units is assumed and used to define the values of certain physical constants as discussed below. The user should decide which units will be used for mass (M), length (L) and time (T) for the various input variables, issue the appropriate units instruction (or assign appropriate values for the physical constants) and then consistently use those chosen units for all other input data. For example, if you want to specify the dimensions of your domain in metres and the time at which you want a solution is in seconds, then all measures of time and length will have to be in seconds and metres, respectively. The hydraulic conductivity should therefore be specified in m s$^{−1}$, a pumping rate in m$^3$ s$^{−1}$ etc. The program does not perform any checks to ensure unit consistency.

## Default values

Default values are assigned for the gravitational acceleration and fluid properties which correspond to standard values in the kilogram metre-second system. These parameters are used when defining the properties of fractures, open wells and tile drains.
The following default values will be used for the physical constants and correspond to typical values in the kilogram-metre-second system:
* Gravitational acceleration $g$ = 9.80665 m s$^{−2}$, Equation 2.3.
* Fluid density ${\rho }$ = 1000.0 kg m$^{−3}$, Equation 2.13.
* Fluid viscosity ${\mu }$ = 1.124 × 10$^{−3}$ kg m$^{−1}$ s$^{−1}$, Equation 2.13.
* Fluid compressibility ${\alpha }_{w}$ = 4.4 × 10$^{−10}$ (m s$^2$) kg$^{−1}$, Equation 2.15.
* Fluid surface tension ${\chi }$ = 0.07183 kg s$^{−2}$, Equation 4.5.
If you are using different units or you want to change the default values you can do so using the following instructions:

## Units: kilogram-metre-minute

Converts the default values given above into the kilogram-metre-minute system. This instruction also converts the porous media, dual continuum, fractured media and surface flow default properties which are defined in the code. NOTE: It does not convert properties specified in any prefix.grok, .mprops, etc. file. Similar instructions exist for converting to the following systems:
* Kilogram-metre-hour.
* Kilogram-metre-day.
* Kilogram-metre-year.
* Kilogram-centimetre-second.
* Kilogram-centimetre-minute.
* Kilogram-centimetre-hour.
* Kilogram-centimetre-day.
* Kilogram-centimetre-year.
You can change the default values of the physical constants using the following instructions. If you change the default units from the kilogram-metre-second system make sure the values given here are in the new system.

## Gravitational acceleration

* grav Gravitational acceleration constant [L T$^{−2}$], $g$ in Equation 2.3.

## Fluid density

* rho Fluid density [M L$^{−3}$], ${\rho }$ in Equation 2.13.

## Fluid viscosity

* visc Fluid viscosity [M L$^{−1}$ T$^{−1}$)], ${\mu }$ in Equation 2.13.

## Fluid compressibility

* wcomp Fluid compressibility [(LT$^2$) M$^{−1}$], ${\alpha }_{w}$ in Equation 2.15.

## Zero fluid compressibility

Assigns a value of zero for fluid compressibility (i.e. incompressible).

## Fluid surface tension

* tensn Fluid surface tension [M T$^{−2}$], ${\chi }$ in Equation 4.5.