The hydrological cycle is the sum of all the motions of water around the planet. As such, it provides the link between the oceans and the atmosphere, and between these two systems and the cryosphere (or the world's ice). The motion of water is another consequence of the cascade of energy fuelled by sunlight, and serves to transport energy, as well as mass, around the planet. The hydrological cycle is also a major geomorphological agent, transporting sediment from the land masses to the oceans, and is crucial to Life (the biosphere).
Water in the hydrological cycle passes between reservoirs, or major systems. By far the largest of these is the oceans, which at any one time contain over 97% of the world's water.The Table below gives the size of the main water reservoirs. Note how little is contained in the atmosphere and in rivers.
Table 1: Inventory of the World's water reservoirs
| RESERVOIR | VOLUME (cubic kilometres) | PERCENTAGE OF TOTAL |
| Oceans | 1,370,000,000 | 97.25 |
| Glaciers and Ice Sheets | 29,000,000 | 2.05 |
| Underground aquifers | 9,565,000 | 0.685 |
| Lakes | 125,000 | 0.01 |
| Rivers | 1,700 | 0.0001 |
| Atmosphere | 13,000 | 0.001 |
| Biosphere | 600 | 0.00001 |
| TOTAL | 1,408,705,300 | 100 |
Water constantly passes between these reservoirs in fluxes, as shown in Table 2.
Table 2: Global values for the major fluxes between reservoirs.
| RESERVOIRS | PROCESS | FLUX (cubic kilometres per year) |
| OCEANS-ATMOSPHERE | Evaporation | 400,000 |
| Precipitation | 370,000 | |
| LAND MASSES - ATMOSPHERE | Evaporation | 60,000 |
| Precipitation | 90,000 | |
| LAND MASSES - OCEANS | Runoff | 30,000 |
The following important points emerge from this Table:
(1) More water is evaporated from the oceans than falls on to them as precipitation, and more water falls as precipitation on to the land masses than is evaporated. The balance is made up by river runoff. If the precipitation and evaporation budget did not work in this way, the land masses would progressively dry up, and oceans would progressively gain all of the world's water.
(2) The annual flux of water through the atmosphere is about 460,000 cubic kilometres per year, about 35 times larger than the amount held in the atmosphere at any one time. This means that the average residence time of water in the atmosphere is very short. In contrast, the size of the ocean reservoir is over 3,000 times larger than the annual flux to the atmosphere or from the atmosphere and land masses, so the average residence time of water in the oceans is very long.
These global rates mask important regional differences. Most of the evaporation is from the Tropical oceans, particularly below the Sub-Tropical Highs, where there are clear skies and large receipts of solar radiation. This moisture is carried along in the lower layers of the atmosphere by the Trade Winds, until it reaches the Inter-Tropical Convergence Zone. Here it is uplifted in rising convective systems, and condenses into clouds, and then falls as rain, commonly in intense downpours. Thus, intense convection over Tropical land masses accounts for much of the world's rainfall, and much of the flux of water from the oceans to the land. This is strikingly shown in this image, which shows the amount of water vapour in the atmosphere for 7th December 2000. The brightest areas contain the most water vapour, and clearly form a speckled band around the Tropics. This is especially prominent around the East Indies and Northern Australia. Note the bright circular patch off north-western Australia: a Tropical Cyclone!
This next image shows the annual total precipitation. Again, note the importance of the Tropics, especially the East Indies and Amazonia.
The rainfall and water vapour images both highlight the overwhelming importance of the Tropics in terms of water fluxes through the atmospheric reservoir. Note also the low vapour content and low rainfall in the Sub Tropics: the arid, Sub-Tropical Highs. Additionally, in the water vapour image we can see the swirling patterns of water vapour transport in Mid-Latitude cyclones, and the high precipitation amounts in these Latitudes. Hence, the patterns of vapour transport and precipitation are closely controlled by the large-scale patterns of atmospheric circulation identified in Lecture 2.
The influence of precipitation patterns on river runoff are clearly evident in the next Table:
Table 3: Areas, Discharges, and Sediment discharges of the world's largest rivers.
| River | Drainage Area | Length | Water discharge | Sediment discharge | |
| (thousands of square km) | (km) | (cubic m / sec | (cubic km / yr) | (thousands of tonnes / yr) | |
| Amazon | 6,150 | 6,275 | 200,000 | 6,300 | 900,000 |
| Zaire | 3,820 | 4,670 | 40,000 | 1,250 | 43,000 |
| Orinoco | 990 | 2,570 | 34,880 | 1,100 | 210,000 |
| Ganges-Brahmaputra | 1,480 | 2,700 | 30,790 | 971 | 1,670,000 |
| Yangtze | 1,940 | 4,990 | 28,540 | 900 | 478,000 |
| Mississipi-Missouri | 3,270 | 6,260 | 18,390 | 580 | 210,000 |
| Yenisei | 2,580 | 5,710 | 17,760 | 560 | 13,000 |
| Lena | 2,500 | 4,600 | 16,300 | 514 | 12,000 |
| Mekong | 790 | 4,180 | 14,900 | 470 | 160,000 |
| Parana-La Plata | 2,830 | 3,940 | 14,900 | 470 | 92,000 |
The four largest rivers in terms of discharge are in the Tropics (Amazon and Orinoco in South America; Zaire in Africa, and the Banges-Brahmaputra in monsoonal Asia). The Amazon annual discharge (6,300 cubic kilometres per year) is equal to about 21% of the total discharge of all the world's rivers. Note that the area of the Amazon basin is not quite double that of the Mississippi-Misouri, but has over ten times the discharge: a stunning illustration of the vast amount of rainfall over the Amazon basin. Note also the implications that the huge discharge of Tropical rivers has for sediment transport and erosion.
The flows of water vapour through the atmosphere have been termed Tropospheric rivers (the Troposphere is the lowermost 10 - 15 km of the atmosphere, where most of the weather happens). The water vapour image really helps us to appreciate the nature of such "rivers": persistent flows of water joining the regions of evaporation in the oceansto the return flows of rivers back to the oceans.
Fluxes of moisture through the atmosphere vary on different time-scales. Some of these changes are regular and fairly predictable, like the seasonal variations in rainfall and river discharge associated with the Indian monsoon. Such seasonal changes determine the regime of rivers, which exerts a strong control on patterns of vegetation, agriculture, and earth-surface processes.
However, within such annual cycles, there is much year-by-year variation, which in India may mean the difference between a good year and a drought, or widespread flooding. The weather in the UK in the autumn of 2000 is a good example of such variability: rainfall in Britain usually increases in the autumn, but 2000 was particularly wet, with the result that river discharges have been exceptionally high causing widespread damage.
The figures in Table 2 show a balance in the fluxes between the oceanic and continental reservoirs. This would indicate that all of the reservoirs maintain a constant mass of water. This, however, is not the case today, and was certainly not the case in the past. Over the past century, the storage in lakes has increased as large dams have been constructed to retain river water. Additionally, the world's mountain glaciers and ice caps have melted back, decreasing the storage in glaciers and returning water to the oceans. The net result of these and other changes has been to increase the mass of the oceanic reservoir, with the result that global sea level has increased. The observed rise in sea-level is 10 - 25 cm over the past century. Not all of this is due to water leaving other reservoirs such as the glaciers and ice sheets.About half is attributable to thermal expansion of the oceans (recall that water expands when it is heated above 4°C).
Sea-level change due to exchanges between reservoirs has been dramatic in the past. At the peak of the last major glaciation (about 25,000 years ago), global sea-level was about 120 metres lower that today, because so much water was locked up in the great ice sheets. It is useful to think of the growth and decay of the ice sheets, and the associated fall and rise of global sea level, in terms of imbalances in the hydrological cycle. During ice sheet growth, low global temperatures meant that water precipitated on land as snow was unlikely to melt and be returned to the oceans: hence during this period, evaporation from the oceans exceeded river runoff, and the ice sheets expanded. Conversely, during ice sheet retreat, rising global temperatures melted more ice than accumulated each year as snow: hence runoff (and melting of icebergs) returned more water to the oceans each year than was precipitated on to the land. Sea-level rise flooded huge areas of the continental shelves at the end of the last glaciation, in a dramatic demonstration of the consequences of the effects of global climate change on the hydrological cycle.
Other reservoirs can undergo changes due to global change. For example, the amount of groundwater in accessible aquifers may decrease if inputs decrease (i.e. rainfall in the catchment) or outputs increase (i.e. withdrawals by farmers). Another example is the amount of water retained in wetlands (surface water), which may be decreased by climate change or artificial drainage.