Hydrological cycle

The hydrological cycle is simple to conceive (although its elements are by no means always easy to measure) (Figure 18.19). The principal source of water is the oceans; radiant energy makes water evaporate into the atmosphere, winds distribute it over the surface of the globe, and precipitation brings it down to earth (with a net movement of atmospheric water from oceans to continents), where it may be stored temporarily in soils, lakes and icefields. Loss occurs from the land through evaporation and transpiration or as liquid flow through stream channels and groundwater aquifers, eventually to return to the sea. The major pools of water occur in the oceans (97.3% of the total for the biosphere; Berner & Berner, 1987), the ice of polar ice caps and glaciers (2.06%), deep in the groundwater (0.67%) and in rivers and lakes (0.01%). The proportion that is in transit at any time is very small - water draining through the soil, flowing along rivers and present as clouds and vapor in the atmosphere constitutes only about 0.08% of the total. However, this small percentage plays a crucial role, both by supplying the requirements for survival of living organisms and for community productivity, and because so many chemical nutrients are transported with the water as it moves.

Atmosphere (0.013) Vapor transport 0.037

Evaporation Precipitation 0.423 0.3BB

Precipitation 0.110

Precipitation 0.110

Evaporation 0.073

Evaporation Precipitation 0.423 0.3BB

Evaporation 0.073

Figure 18.19 The hydrological cycle showing fluxes and sizes of reservoirs (X 106 km3). Values in parentheses represent the size of the various reservoirs. (After Berner & Berner, 1987.)

Rivers and lakes

(0.13) Groundwater

Rivers and lakes

(0.13) Groundwater

Figure 18.19 The hydrological cycle showing fluxes and sizes of reservoirs (X 106 km3). Values in parentheses represent the size of the various reservoirs. (After Berner & Berner, 1987.)

The hydrological cycle would proceed whether or not a biota was present. However, terrestrial vegetation can modify to a significant extent the fluxes that occur. Plants live between two counterflowing movements of water (McCune & Boyce, 1992). One moves within the plant, proceeding from the soil into the roots, up through the stem and out from the leaves as transpiration. The other is deposited on the canopy as precipitation from where it may evaporate or drip from the leaves or flow down the stem to the soil. In the absence of vegetation, some of the incoming water would evaporate from the ground surface but the rest would enter the stream flow (via surface runoff and groundwater discharge). Vegetation can intercept water at two points on this journey, preventing some from reaching the stream and causing it to move back into the atmosphere by: (i) catching some in foliage from where it may evaporate; and (ii) preventing some from draining from the soil water by taking it up in the transpiration stream.

We have seen on a small scale how cutting down the forest in a catchment in Hubbard Brook can increase the throughput to streams of water together with its load of dissolved and par-ticulate matter. It is small wonder that large-scale deforestation around the globe, usually to create new agricultural land, can lead to the loss of topsoil, nutrient impoverishment and increased severity of flooding.

Another major perturbation to the hydrological cycle will be global climate change resulting from human activities (see Section 18.4.6). The predicted temperature increase, with its concomitant changes to wind and weather patterns, can be expected to affect the hydrological cycle by causing some melting of polar caps and glaciers, by changing patterns of precipitation and by influencing the details of evaporation, transpiration and stream flow.

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