I don't know what to make of this calculation which I picked up in a New Scientist story. A few weeks ago there was a study using NASA's Grace satellite measurements that showed an increase in groundwater extraction from North Indian aquifers.
A second study on these satellite measurements asserts that the groundwater loss amounts to about 54 cubic km per year over a time period of 2002 and 2008. A lot of this extracted groundwater ends up in the sea and could be contributing to raising sea-levels by 0.16 millimeters every year, about 5% of the total sea level rise. That is about the same as contributed by runoff from melting Alaskan glaciers the authors conclude.
I don't have access to the full paper so I don't know the details of the calculation but here is what the scientists have to take into account:
Part of the groundwater extracted will be taken up by plants and remain there over the life of the plant and make its way into the food chain.
Part of it will be lost through the plants through evapo-transpiration.
Part of the water will remain in soil adhering to clay and sand particles.
Part of it will make its way back to the aquifer.
Part of it will make its way through the soil to local streams and eventually to the sea.
Part of it will be lost by direct evaporation. That evaporated water (and the water lost by evapo-transpiration) will fall as rain and part of it will infiltrate as groundwater and part of it will be surficial runoff into streams and eventually into the sea.
Just giving you something to think about what happens to extracted groundwater.