Ground bottom flow formula (Water Overlay)

Ground water flowing in from deeper ground layers through the bottom boundary, due to head pressure from water bodies situated outside the project area is calculated per grid cell. The formula is based on the principles used in the formula of Pankow for estimating this ground bottom flow^[1].

The water level change due to ground bottom flow is calculated using the following formula:

\Delta wl_{x,y}={\frac {h_{d}(t)-h_{x,y}}{c_{x,y,tf}}}\cdot {\frac {\Delta t}{24\cdot 60\cdot 60}}

The external bottom head pressure is variable over time by the introduction of $h_{v}(t)$ :

h_{d}(t)=p_{x,y,tf}+h_{v}(t)

Finally, the actual amount of water flowing into the ground from the deeper ground layers through the bottom boundary is calculated as followed:

\Delta s_{x,y}={\Delta wl_{x,y}}\cdot {ws}

where:

\Delta wl_{x,y}

= The water level change due to the ground bottom flow, in meters.

\Delta t

= Computational timestep in seconds.

h_{x,y}

= Ground water head in the grid cell.

h_{d}(t)

= Additional global pressure at time t, in meters

p_{x,y,tf}

= Ground water level (or head pressure) in meters at timeframe

tf

.

h_{v}(t)

= Additional variation of bottom head pressure over time

c_{x,y,tf}

= Resistance of the bottom boundary in days at timeframe

tf

ws

= Water storage fraction of the soil

\Delta s_{x,y}

= Water flowed in through the bottom boundary from deeper ground layers, in m

Notes

The calculated $\Delta wl_{x,y}$ is essentially the change in groundwater level, inherently taking into account the porosity of the soil. The final $\Delta s_{x,y}$ is representative of the actual amount of water, i.e. the height of the water, if the relevant quantity of water wasn't in the ground but sat on the surface instead.
$p_{x,y,tf}$ and $c_{x,y,tf}$ can both be provided as spatially variable values using prequel overlays. These prequels can have multiple timeframes, as indicated by the formula, allowing changing head and resistances over time.
$h_{v}(t)$ can be provided as a set of values, variable over time.
A head pressure lower than the water level in the project is also allowed, resulting in ground water lowering and flowing into the deeper ground layers and out of the project area.