Water use by plants

Vegetation transpires at least 100 times more water per annum than is present in the plant as biological water. This figure varies greatly with different plant types, according to their specific adaptations for preserving water. For instance, drought-adapted plants have a much greater capacity to close down stomata (pores on the leaf surface) to limit transpiration. Plants are therefore a type of water pump. They move large volumes of water because they use it as a medium to extract dissolved nutrients, and as an evaporative cooling mechanism “ a room with plants will always feel cooler. A plant with a large leaf area, such as a mature apple tree, is therefore capable of removing many tonnes of soil water during the growing season.

Plants have many different adaptations to help them obtain, and conserve moisture. A grape vine, for instance, may put down water-seeking roots to a depth of 40metres. In Princess Margaret Rose Caves, on the Glenelg River, there are roots 50mm across nearly 50metres below the surface. Presumably they are seeking groundwater at the level of the river or below, still nearly 10 metres lower, but the trees on the surface are less than 15metres tall.

While plants may have deep roots for survival, they will generally prefer to seek water at or near the soil surface, where air and nutrients are readily available. If there is not a continual input of water into soil, plant roots will dry out the soil around their main feeding roots. Some of this water can be replaced by capillary action (water seeping upwards through the soil pores), but there is always a tension between the two forces of gravity (pulling water down) and capillarity (pulling water up).