# Ground flow formula (Water Overlay)

Ground flow is different from surface flow, since it has to account for the slowdown and porousness of the medium. In general, horizontal ground flow is calculated using formulas described in Harbaugh 2005[1][2]. However, when an aquifer is present, the Aquifer formula is applied.

It depends on the configuration of the HYDRAULIC_CONDUCTIVITY_WITH_THICKNESS attribute value in the Water Overlay what Hydraulic Conductivity formula is used:

### Hydraulic Conductivity without Thickness

Two adjacent cells, where ground water level of cell 1 is larger than cell 2.

The flow between the two cells is calculated as:

${\displaystyle \Delta w_{t}=w_{1,t}-w_{2,t}}$

${\displaystyle B_{bb}=max(B_{1}-d_{b,1},B_{2}-d_{b,2})}$

${\displaystyle A_{c,t}=\Delta x\cdot ({\bar {w_{t}}}-B_{bb})}$

${\displaystyle K=min(K_{1},K_{2})}$

${\displaystyle V_{K,t}={\frac {\Delta w\cdot K\cdot A_{c,t}}{\Delta x}}\cdot \Delta t}$

where:

${\displaystyle w_{n,t}}$ = The ground water level of cell ${\displaystyle n}$ at time ${\displaystyle t}$.
${\displaystyle B_{c}}$ = the datum height of the surface of cell c, set by the elevation or a Terrain elevation prequel.
${\displaystyle d_{b,c}}$ = The ground bottom distance of the cell c, defined by a Bottom distance prequel or a general GROUND_BOTTOM_DISTANCE_M of the Water Overlay.
${\displaystyle B_{bb}}$ = the datum height of the bottom boundary at the edge of interaction between the two cells.
${\displaystyle K_{n}}$ = The hydraulic conductivity of the cell, defined in HYDRAULIC_CONDUCTIVITY_MD of the ground terrain.
${\displaystyle A_{c,t}}$ = Area of conductance at time ${\displaystyle t}$.
${\displaystyle \Delta w}$ = Ground water level difference at time ${\displaystyle t}$.
${\displaystyle \Delta t}$ = Computational timestep in seconds.
${\displaystyle \Delta x}$ = Size of grid cell.
${\displaystyle {\bar {w_{t}}}}$ = Averaged ground water level at time ${\displaystyle t}$, based on water levels in ground, WATER_STORAGE_PERCENTAGE and potentially the surface water level, when the ground is filled to the top.
${\displaystyle V_{K,t}}$ = The amount of water to be transported at time ${\displaystyle t}$ between one cell and the other.

### Hydraulic Conductivity with Thickness

The flow between the two cells is calculated as:

${\displaystyle \Delta w_{t}=w_{1,t}-w_{2,t}}$

${\displaystyle KD=min(KD_{1},KD_{2})}$

${\displaystyle V_{KD,t}={\frac {\Delta w\cdot KD\cdot \Delta x}{\Delta x}}\cdot \Delta t}$

where:

${\displaystyle w_{n,t}}$ = The ground water level of cell ${\displaystyle n}$ at time ${\displaystyle t}$.
${\displaystyle KD_{n}}$ = The hydraulic conductivity of the cell, multiplied with the thickness of the layer, defined in HYDRAULIC_CONDUCTIVITY_WITH_THICKNESS_MD of the ground terrain.
${\displaystyle \Delta w}$ = Ground water level difference at time ${\displaystyle t}$.
${\displaystyle \Delta t}$ = Computational timestep in seconds.
${\displaystyle \Delta x}$ = Size of grid cell.
${\displaystyle V_{KD,t}}$ = The amount of water to be transported at time ${\displaystyle t}$ between one cell and the other.

### Aquifer formula

When an aquifer is present, its hydraulic diffusivity is used to calculate the water flow.

Based on conditions being true, the calculated volume of water that is transported through the aquifer is calculated as:

${\displaystyle V_{a,t}={\begin{cases}{\dfrac {\Delta w_{t}\cdot {KD}_{a}\cdot \Delta x}{\Delta x}}\cdot \Delta t&{\text{if }}w_{n,t}>z_{a}{\text{ and }}KD_{a}>0\\V_{K(D),t}&{\text{otherwise}}\end{cases}}}$

Where:

${\displaystyle \Delta w_{t}}$ = Ground water level difference between the two adjacent cells at time ${\displaystyle t}$;
${\displaystyle {KD}_{a}}$ = The AQUIFER_KD attribute of aquifer.
${\displaystyle \Delta x}$ = Size of grid cell.
${\displaystyle \Delta t}$ = Computational timestep in seconds.
${\displaystyle w_{n,t}}$ = Ground water level in cell ${\displaystyle n}$ at time ${\displaystyle t}$;
${\displaystyle z_{a}}$ = the datum height of the aquifer at the cell.
${\displaystyle V_{K(D),t}}$ = The calculated amount of water to be transported at time ${\displaystyle t}$ between one cell and the other.
${\displaystyle V_{a,t}}$ = Volume in ${\displaystyle m^{3}}$ that flows between the two adjacent cells due to the aquifer at time ${\displaystyle t}$.

## Related

The following topics are related to this formula.

Features
Aquifer (Water Overlay)
Formulas
Groundwater level formula (Water Overlay)
Ground infiltration formula (Water Overlay)
Models
Ground model (Water Overlay)
Infiltration model (Water Overlay)
Tracer flow model (Water Overlay)