# iMOD User Manual version 5.2 (html)

#### 12.13PWT Perched water table package

A perched water table (or perched aquifer) is a (temporary) water table that occurs above the regional groundwater table in the unsaturated zone. This occurs when there is a (relatively) impermeable layer above the regional groundwater table in the unsaturated zone. With the PWT-package a perched water table can be schematized in iMOD, the perched water table concept is given in figure 12.4.

In the following pages the concept of the perched water table package is described and illustrated by several hydrologic situations. Hereby, the following figure (figure 12.5) is used which that represents the perched water table in terms of model parameters. Important to understand is that there can be only a single perched water table in each vertical column. Once a perched water table exists, both the horizontal and vertical flow component will be reduced up to zero when the pressure head above the perched water table drops below the top of the aquitard that creates the perched water table.

The PWT package is applied using the following assumptions, these are described in the following table in more detail.

The numerical implementation is such that the horizontal conductances and the vertical resistances are calculated on the heads of timestep $t-1$. This is in order to avoid numeric instability.