Kayla Frew, of Ardgowan School, asks :-

Does much water from a lake soak into the ground?

Bob Spigel, a scientist at the National Institute of Water and Atmospheric Research (NIWA), responded.

It all depends on the kinds of soils that make up the lake basin and on how much water there already is in the soils that surround and lie under the lake. We call this water "groundwater" when it fills all the spaces between soil grains.

In a study of 63 lakes in the United States, scientists found considerable exchange between lake water and groundwater in 45 of the lakes. About 40 per cent had water seeping into the lake from the ground, 40 percent had groundwater flowing both into and out of the lake and 20 per cent had water soaking into the ground.

Lake Rotoma, Lake Rotoehu and Lake Okataina all have basins in the very porous soils of the North Island's central volcanic plateau near Rotorua. None of these lakes has a surface outlet; that is, there are no streams draining them, although they do have small inflowing streams. Rainfall on these lakes generally exceeds 2000 mm every year, while they lose less than 900 mm of water per year by evaporation. Thus, to keep from overflowing, these lakes must lose 1100 mm of water through the ground every year.

Perhaps the best known example in New Zealand of a lake that loses considerable volumes of water through the ground is Lake Waikaremoana. It was formed around 2000 years ago by landslides that dammed a gorge. Much to the annoyance of engineers (who have been generating hydroelectric power there since 1929), the natural earth and rock dam formed by the landslide is far from watertight and efforts to plug the leaks have not been completely successful.