There are some papers in literature which discuss stream-aquifer interaction. They describe that the stream aquifer interaction ranges from connection to disconnection. When there is disconnection, then there is an $\textbf{inverted water table} $ below the stream and above saturated zone. I have not understood what is meant by this kind of water table?
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3 Answers
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An Inverted or Perched water table is a water table that is above the main or regional water table in an unconfined aquifer. The perched water table is generally above a layer of low permeability material such as clay. In the image below, notice that there is an "inverted" water table along bottom of the perched water table.
answered Jun 6, 2017 at 17:22
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$\begingroup$ I am sorry but I am still confused with the word "inverted". Normally inverted means upside down. What is upside down regarding water table? Also can you highlight the 'inverted water table' in the image you added. $\endgroup$ Commented Jun 9, 2017 at 9:44
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$\begingroup$ On the perched water table, there is a water boundary along the top of the impremeable layer which is the upside down water table where the water is above that plane. $\endgroup$ Commented Jun 9, 2017 at 14:38
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To amplify on Gary's answer for the situation under a stream: Surface water will be connected to an unconfined groundwater aquifer if the sediments are saturated between the aquifer and the stream. If the head (the elevation of the water i.e. its pressure) is lower at the stream than the adjacent sediments, then it will be a "gaining stream" the water in the aquifer will flow into the stream. If the head is higher at the stream it will be a "losing stream" and water will flow into the aquifer.
However sometimes, the water level in the aquifer drops enough that it is lower than the sediments right under the stream and the stream becomes a "disconnected stream". This can occur when the sediments in the stream bed have lower permeability than the rest of the aquifer. That's a common occurrence because fine sediment transported by the stream can build up in the stream bed - called a skin effect. If a stream has intermittent flow then the stream can be disconnected when it starts to flow but the water seeping into the bed hasn't had time to fully saturate the sediments underneath.
Note that the gaining or losing flow can be very low. The difference in head tells you the potential flow direction but the permeability of the sediments determines how much flow you will get for a given potential. It is analogous to a battery voltage (potential) and a resistor (permeability) governing currrent (flow).
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When water is infiltrating an aquifer system which contains an unsaturated zone (vadose zone), whether from an infiltration bassin or a river, The first centimeters usually are saturated, the bottom of this saturated layer is called the inverted water table because under the layer there is a soil which is not completely saturated.
