# HydroGeoSphere/Dual Continuum (Saturated)
## Default Dual-Continuum Saturated Flow Properties

Unless you modify the default values, all dual-continuum zones (and elements) in the domain will be assigned the default properties which are listed in Table 5.7. These values are representative of a sand:
Table 5.7: Default Values for Dual-continuum Saturated Flow Properties :
| Parameter | Value | Unit |
| Name | Default Sand | - |
| Hydraulic conductivity terms: | - | - |
| $K_{xx}$ | 7.438 × 10$^{−5}$ | m s$^{−1}$ |
| $K_{yy}$ | 7.438 × 10$^{−5}$ | m s$^{−1}$ |
| $K_{zz}$ | 7.438 × 10$^{−5}$ | m s$^{−1}$ |
| Specific storage $S_{sd}$ | 1.0 × 10$^{−4}$ | m$^{−1}$ |
| Porosity | 0.375 | - |
| Volume fraction of porous medium $w_{d}$ | 0.01 | - |
| Unsaturated flow relation type | Pseudo-soil | - |
Note that the default state of the hydraulic conductivity tensor ($K_{d}$ in Equation 2.17) is that it is isotropic. It should also be noted that for dual continua, off-diagonal terms are not considered.
You can use the general methods and instructions outlined in Section 5.8.1 to modify the default distribution of saturated dual-continuum properties.
As was the case for the instructions which modify porous medium properties, the following instructions also have a scope of operation, the only difference being that they would appear in the .dprops file instead of the .mprops file.

## K isotropic

Scope: .grok .dprops
* kval Hydraulic conductivity [L T$^{−1}$].
Assign an isotropic hydraulic conductivity (i.e. $K_{xxd}$ = $K_{yyd}$ = $K_{zzd}$).

## K anisotropic

Scope: .grok .dprops
* kvalx, kvaly, kvalz Hydraulic conductivities [L T$^{−1}$] in the x-, y- and z-directions respectively.
Assigns anisotropic hydraulic conductivities.

## Specific storage

Scope: .grok .dprops
* val Specific storage [L$^{−1}$], $S_{sd}$, but defined in a similar way to $S-s$ in Equation 2.10.

## Porosity

Scope: .grok .dprops
* val Porosity [L$^3$ L$^{−3}$], $theta_{sd}$ in Equation 2.16.

## Volume fraction dual medium

Scope: .dprops
* val Volume fraction [L$^3$ L$^{−3}$], $w_{d}$ in Equation 2.16.
The volume fractions of the dual medium and porous medium always add up to 1.0.

## First-order fluid exchange coefficient

Scope: .dprops
* val First-order fluid exchange rate, ${\alpha }_{wd}$ in Equation 2.69.

## Interface k

Scope: .dprops
* val Interface hydraulic conductivity [L T$^{−1}$], $K_{a}$ in Equation 2.69.

## Convert pm k to macropore k

Scope: .dprops
* val Porous medium background hydraulic conductivity $K_{bkgrd}[L/T]$.
We can express the bulk hydraulic conductivity of a dual-continuum $K_{bulk}$ as the sum of the porous media $K_{bkgrd}$ and fracture $K_{d}$ components:
    $K_{bulk}=K_{bkgrd}(1-w_{d})+K_{d}w_{d}$ (Equation 5.11)
where $w_{d}$ is the volume fraction [L$^3$ L$^{−3}$] in Equation 2.16.
If we assume that the observed (porous medium) hydraulic conductivity is equal to $K_{bulk}$, and supply an educated guess for $K_{bkgrd}$, we can rearrange the equation and calculate $K_{d}$ as:
    $K_{d}=[K_{bulk}-K_{bkgrd}(1-w_{d})]/w_{d}$ (Equation 5.12)
For all elements in the currently chosen dual zones, the porous medium hydraulic conductivity is replaced by $K_{bkgrd}$ and the fracture hydraulic conductivity $K_{d}$ is set equal to the calculated value.