Ground infiltration formula (Water Overlay)

Water can infiltrate from the unsaturated zone to the saturated zone.

The infiltration from the unsaturated zone to the saturated zone is calculated per cell. When the unsaturated zone fraction drops below the terrain's MIN_UNSATURATED_FRACTION this process is stopped. This 'min fraction' can also be dynamically adjusted, based on other factors, during the simulation using iterations.

First the height of the unsaturated zone is calculated.

$d_u=B_c-w_g$

Then the ratio of water amount to unsaturated height is calculated.

$r=\mathrm{\Delta }{w}_u/d_u$

Then we calculate the distance of the unsaturated zone which can infiltrate.

$d_i=min(d_u,\mathrm{\Delta }t\cdot f_{i,t}\cdot I_g)$

Finally, we calculate the actual amount of water that infiltrates from the unsaturated zone to the saturated zone.

$\mathrm{\Delta }{w}_i=d_i\cdot r$

Where:

$w_g$ = The groundwater level, relative to datum.

$B_c$ = the datum height of the surface of cell c, set by the elevation or a Terrain elevation prequel.

$d_u$ = The height of the unsaturated zone.

$\mathrm{\Delta }{w}_u$ = The amount of water in the unsaturated zone. The height of the water column if the equivalent amount of water was placed on the surface.

$r$ = Ratio of water to height in the unsaturated zone.

$\mathrm{\Delta }t$ = Computational timestep in seconds.

$f_{i,t}$ = The interpolated infiltration factor at time $t$. Configured using INFILTRATION_FACTOR_S.

$I_g$ = The GROUND_INFILTRATION_MD of the underground terrain type.

$d_i$ = The height distance in the unsaturated zone which can be subject to infiltration to the saturated zone.

$\mathrm{\Delta }{w}_i$ = The infiltration which takes place from the unsaturated zone to the saturated zone.

Uptake (BETA)

When the unsaturated zone fraction is below the terrain's MIN_UNSATURATED_FRACTION ground water is transported from the saturated zone to the unsaturated zone to compensate.

Until the ground water (saturated zone) drops below the GROUND_WATER_MAX_UPTAKE_DEPTH_M. Below this depth no uptake (capillary or root) is active, the only flow is downwards.

This functionality is disabled by default and can be activated by setting GROUND_WATER_MAX_UPTAKE_DEPTH_M attribute to e.g. 5.

Related

The following topics are related to this formula.

Formulas

Groundwater level formula (Water Overlay)

Surface infiltration formula (Water Overlay)

Ground bottom flow formula (Water Overlay)

Models

Ground model (Water Overlay)

Infiltration model (Water Overlay)

Tracer flow model (Water Overlay)

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