1
The Hydrological Cycle
1.1 Overview
The hydrological cycle describes the continuous movement of water above, on and below the surface of the Earth. It is a conceptual model that describes the storage and movement of water between the biosphere (the global sum of all ecosystems, sometimes called the zone of life on Earth), the atmosphere (the air surrounding the Earth, which is a mixture of gases, mainly nitrogen (about 80%) and oxygen (about 20%) with other minor gases), the cryosphere (the areas of snow and ice), the lithosphere (the rigid outermost shell of the Earth, comprising the crust and a portion of the upper mantle), the anthroposphere (the effect of human beings on the Earth system) and the hydrosphere (see Table 1.1).
Table 1.1 Water in the hydrosphere and the distribution of fresh water on the Earth (from Martinec, 1985)
| (a) Distribution of water in the hydrosphere |
| Forms of water present | Water volume
(106 km3) | As % |
| Polar ice, sea ice, glaciers | 28 | 2.0 |
| Surface water, ground water, atmospheric water | 8 | 0.6 |
| (b) Distribution of fresh water on Earth |
| Forms of water present | Water volume (106km3) | As % |
| Polar ice, glaciers | 24.8 | 27.9 | 76.93 | 77.24 |
| Soil moisture | 0.09 | 0.06 | 0.28 | 0.17 |
| Ground water within reach | 3.6 | 3.56 | 11.17 | 9.85 |
| Deep ground water | 3.6 | 4.46 | 11.17 | 12.35 |
| Lakes and rivers | 0.132 | 0.127 | 0.41 | 0.35 |
| Atmosphere | 0.014 | 0.014 | 0.04 | 0.04 |
| Total | 32.236 | 36.121 | 100.0 | 100.0 |
*Based on Volker (1970).
†Based on Dracos (1980), referred to in Baumgartner and Reichel (1975).
Models of the biosphere are often referred to as land surface parameterization schemes (LSPs) or soil–vegetation–atmosphere transfer schemes (SVATs). An example of an SVAT is described by Sellers et al. (1986). The water on the Earth’s surface occurs as streams, lakes and wetlands in addition to the sea. Surface water also includes the solid forms of precipitation, namely snow and ice. The water below the surface of the Earth is ground water.
Most of the energy leaves the ocean surface in the form of latent heat in water vapour, but this is not necessarily the case for land surfaces. Hence maritime air masses are different to continental air masses. The atmosphere and oceans are strongly coupled by the exchange of energy, water vapour, momentum at their interface, and precipitation. The oceans represent an enormous reservoir for stored energy and are denser than the atmosphere, having a larger mechanical inertia. Therefore ocean currents are much slower than atmospheric flows. The atmosphere is heated from below by the Sun’s energy intercepted by the underlying surface, whereas the oceans are heated from above. Lakes, rivers and underground water can have significant hydrometeorological and hydroclimatological significance in continental regions. The hydrological cycle is represented by the simplified diagram in Figure 1.1.
Figure 1.1 Simplified representation of the hydrological cycle
(NWS Jetstream NOAA, USA, www.srh.noaa.gov/jetstream/atmos/hydro.htm)
1.2 Processes comprising the hydrological cycle
There are many processes involved in the hydrological cycle, the most important of which are as follows:
- Evaporation is the change of state from liquid water to vapour. The energy to achieve this may come from the Sun, the atmosphere itself, the Earth or human activity.
- Transpiration is the evaporation of water from plants through the small openings found on the underside of leaves (known as stomata). In most plants, transpiration is a passive process largely controlled by the humidity of the atmosphere and the moisture content of the soil. Only 1% of the transpired water passing through a plant is used by the plant to grow, with the rest of the water being passed into the atmosphere. Evaporation and transpiration return water to the atmosphere at rates which vary according to the climatic conditions.
- Condensation is the process whereby water vapour in the atmosphere is changed into liquid water as clouds and dew. This depends upon the air temperature and the dew point temperature. The dew point temperature is the temperature at which the air, as it is cooled, becomes saturated and dew can form. Any additional cooling causes water vapour to condense. When the air temperature and the dew point temperature are equal, mist and fog occur. Since water vapour has a higher energy level than liquid water, when condensation occurs the excess energy is released in the form of heat. When tiny condensation particles, through collision or coalescence with each other, grow too large for the ascending air to support them, they fall to the surface of the Earth as precipitation (Chapter 2). Precipitation is the primary way fresh water reaches the Earth’s surface, and on average the Earth receives about 980 mm each year over both the oceans and the land.
- Infiltration of water into the land surface occurs if the ground is not saturated, or contains cracks or fissures. The flow of water into the ground may lead to the recharge of aquifers, or may move through unsaturated zones to discharge into rivers, lakes or the seas. Between storm or snowmelt periods, stream flow is sustained by discharge from the ground water systems. If storms are intense, most water reaches streams rapidly. Indeed, if the water table – the boundary between the saturated and unsaturated zones – rises to the land surface, overland flow may occur.
- The residence time of water in parts of the hydrological cycle is the average time a water molecule will spend in a particular area. These times are given in Table 1.2. Note that ground water can spend over 10,000 years beneath the surface of the Earth before leaving, whereas water stored in the soil remains there very briefly. After water evaporates, its residence time in the atmosphere is about nine days before it condenses and falls to the surface of the Earth as precipitation. Residence times can be estimated in two ways. The first and more common method is to use the principle of conservation of mass, assuming the amount of water in a given store is roughly constant. The residence time is derived by dividing the volume of water in the store by the rate by which water either enters or leaves the store. The second method, for ground water, is via isotropic techniques. These techniques use either in-stream tracer injection combined with modelling, or measurements of naturally occurring tracers such as radon-222 (see for example Lamontagne and Cook, 2007).
- Human activities release tiny particles (aerosols) into the atmosphere, which may enhance scattering and absorption of solar radiation. They also produce brighter clouds that are less efficient at releasing precipitation. These aerosol effects can lead to a weaker hydrological cycle, which connects directly to the availability and quality of fresh water (see Ramanathan et al., 2001).
Table 1.2 Average residence times for specific stores (see for example www.physicalgeography.net/fundamentals/8b.html)
| Reservoir | Average residence time |
| Seasonal snow cover | 2 to 6 | months |
| Soil moisture | 1 to 2 | months |
| Ground water shallow | 100 to 200 | years |
| Ground water deep | 10,000 | years |
1.3 Global influences on the hydrological cycle
Differential heating by the Sun is the primary cause of the general circulation of the atmosphere. There are a number of regional differences which influence the hydrological cycle in addition to the relative...
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