> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/theoretical-documentation/section-2-hydrology/chapter-2-3-soil-water/2-3.4-perched-water-table.md).

# 2:3.4 Perched Water Table

For an HRU with a seasonal high water table, if the soil profile becomes saturated to the point that percolation for upper soil layers to lower soil layers is inhibited, water will pond in the soil profile and create a perched water table.

SWAT+ allows the user to define the depth to an impervious layer for the HRU. If the depth to the impervious layer is in the soil profile, no water is allowed to percolate out of the soil profile. If the impervious layer is defined below the soil profile, percolation out of the soil profile is adjusted from the value determined with equation 2:3.2.3 using:

$$w\_{perc,btm}=w\_{perc,btm,orig}*\frac{depth\_{diff}}{depth\_{diff}+exp\[8.833-2.598*depth\_{diff}]}$$                                                                       2:3.4.1

where $$w\_{perc,btm}$$ is the amount of water percolating out of the soil profile on a given day (mm H$$*2$$O), $$w*{perc,btm,orig}$$ is the amount of water percolating out of the soil profile on a given day calculated with equation 2:3.2.3 (mm H$$*2$$O), and $$depth*{diff}$$ is the distance from the bottom of the soil profile to the impervious layer (m).

Water builds up in the soil profile from the bottom of the profile. After the bottom layer of the profile reaches saturation, any water exceeding the storage capacity of the bottom layer is allowed to fill the overlying layer. This continues upward until all the excess water has been distributed.

The height of the perched water table is calculated:

$$h\_{wtbl}=\frac{SW-FC}{(POR-FC)\*(1-\phi\_{air})}\*depth\_{imp}$$                                                                                                            2:3.4.2

where $$h\_{wtbl}$$ is the height of the water table (mm), $$SW$$ is the water content of the soil profile (mm H$$*2$$O), $$FC$$ is the water content of the soil profile at field capacity (mm H$$*2$$O), $$POR$$ is the porosity of the soil profile (mm),$$\phi*{air}$$ is the air-filled porosity expressed as a fraction, and $$depth*{imp}$$ is the depth to the impervious layer (mm).

Table 2:3-5: SWAT+ input variables used in perched water table calculations.

| Definition                                       | Source Name | Input Name | Input File |
| ------------------------------------------------ | ----------- | ---------- | ---------- |
| $$depth\_{imp}$$: Depth to impervious layer (mm) | DEP\_IMP    |            | .hru       |
| $$depth\_{imp}$$: Depth to impervious layer (mm) | DEPIMP\_BSN |            | .bsn       |


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