surface and groundwater from neighbouring ecosystems, rainforests of the lowland equatorial monsoon (the LEM) depend upon rainfall and are more subject to rainfall fluctuations.
However the net amount of rainfall actually available may be reduced by evaporation before it enters the biological regime.
Water In Plants: There are three main reasons plants need water:
- for maintaining shape through turgidity, as when a balloon holds its shape when filled with air. (When there is insufficient water in non-woody plant tissues, they lose shape and wilt);
- as an essential ingredient in photosynthesis
- as a transportation system within the plant body. Jobs include carrying products of photosynthesis away to other parts of the plant (for example, taking sugars from leaves to roots of the cassava plant, to be stored as starch), bringing soil nutrients in solution up from the roots, and circulating the hormones that control plant growth.
However, too much water can be damaging; for example if the soil surrounding the roots of a plant is waterlogged, oxygen uptake is restricted.
Evaporation: the conversion of liquid water into vapour by the heat of the sun.
Transpiration: specifically the loss of water as vapour from a plant, mainly through the stomata of the leaves. Plants can control their rate of transpiration to some degree; the negative pressure it causes in the transporting system serves to draw water and any dissolved nutrients in at their roots to compensate.
Evapotranspiration: the combined evaporation and transpiration from an area of land, a body of vegetation or an ecosystem.
Condensation: the conversion of gaseous water vapour back into liquid water when it cools.
Moreover, rainfall not lost by evaporation may be lost by leaching through the soil and by run-off. Nonetheless a healthy rainforest ecosystem acts like a sponge: the soil system (including litter, detritus and humus) soaks up the water, subsequently releasing it slowly to plants and to rivers by percolation.
Percolation: the movement of water down through the soil due to gravity, especially where the soil is saturated.
Capillary Moisture: is water left in the soil after percolation, being held to soil particles by surface tension (in the same way as water sticks to the side of a plastic, metal or glazed surface).
Evapotranspiration is greatest in a clearing in the tropical rainforest, where a lot of vegetation are disturbed and exposed to the heat of the sun; it is least (as one would expect) in the shaded undergrowth. Also, as one would expect, for any given season, daily evapotranspiration potential is at its maximum at noon.
Temperature varies during the day but also within the vertical space of the forest. These inequalities create localised rainfall, when transpired vapour from one area condenses back to water in a cooler area or on a cooler surface. On a still day
