Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 24

The primary source of pre-precipitation moisture is the atmosphere. Precipitation requires cooling of moist air by upward convection or mixing between two air masses of different temperature. The moist air becomes saturated by cooling and condensation. With further cooling, the moisture falls as rain or snow, depending on the ambient temperature.

Most of the atmospheric water is stored in the troposphere, especially in the warm air of tropical latitudes (Hayden 1988). Evaporation from the seas happens efficiently in the warm climate in the tropics, adding moisture to the atmosphere. Evaporation of more than 60% of water takes places between 30° north and south latitudes. In contrast, only about 5% of total evaporation takes place beyond the 50th parallels. More than 60% of evaporation is from the oceans (Lamb 1972; Hayden 1988). This implies a higher presence of moisture in the tropical air and on the windward side of continents. The stored atmospheric water is condensed before precipitation in two broad ways: barotropic and baroclinic.

Barotropic conditions prevail in the low latitudes. In a barotropic atmosphere, the horizontal thermal gradients are small. The condensation is carried out by vertical lifting of the heated air, and where vertical wind shear is low, may give rise to huge convective clouds. Lifting of humid air is accompanied by a continuous production of latent heat by condensation which uplifts air in the barotropic atmosphere in several ways. The uplift commonly happens by:

Figure 3.1 Average discharges of (a) suspended sediment and (b) water in the Orinoco River and its tributaries.

Source: Meade 2007 and references therein.

 Convergence of northeast and southeast trade winds along the Intertropical Convergence Zone (ITCZ).

 Circulation of air giving rise to tropical storms (which may reach even the rotating velocity of tropical cyclones) and easterly waves.

 Orographic uplift of air when moist airstreams reach the windward slopes of mountain regions.

A high amount of rain may fall where such conditions are fulfilled, increasing river discharge. Where such conditions are weak or absent in low latitudes, arid conditions prevail, as in North Africa or Central Australia, and large rivers are either absent or survive only by importing a high discharge from the upstream basin area. The ITCZ and its associated belt of rainfall moves north and south annually, giving rise to a pronounced seasonality in rainfall. A pattern of rainy summer and dry winter is known as the monsoon system which brings copious rainfall to many parts of the tropical world, especially where the incoming moist summer air is lifted against an orographic zone. The southern slopes of the Himalaya and the eastern slopes of the Andes are excellent examples. Both regions nurture a set of major rivers.

Episodic rainfall occurs from large-scale cyclonic circulations in the lower latitudes, some of which may develop into tropical cyclones producing destructive and heavy rainfall (for details, see Gupta 2011). Tropical cyclones generally do not form near the Equator or over the South Atlantic but are found in other parts of the tropics. These storms tend to give rise to immense volumes of rainfall while moving west within the belt of trade winds. Significant rainfall in the tropics also occurs from the converging meteorological phenomenon known as the easterly waves. A number of large rivers thus exist in the tropics.

A baroclinic atmosphere is typical of extratropical latitudes with sharp horizontal thermal contrasts. The contact between two converging air masses with different level of properties, such as pressure, temperature, and moisture, is known as a front. For example, in the northern hemisphere, a front could be a meeting of dry cold polar air coming from the north and wet and warmer air coming from the south. The horizontal contrast in pressure and temperature is followed by a vertical movement of air. The warmer air rises above the colder one which leads to cooling, condensation, and precipitation. A jet stream, if present at a level high above the front, increases its intensity.

The frontal storms of the baroclinic atmosphere are large but variable in size. Diameters range from several hundred to a thousand kilometres. Hayden (1988) has described the areas of precipitation from such storms as matching the size of large river basins of the middle latitudes. Usually, along a frontal area, multiple storms occur, following one another, filling the channels and flooding the rivers. Although compared with the deep convection pattern of the barotropical atmosphere, rainfall rates are much less, the compensating longevity of baroclinic systems leads to a substantial amount of rainfall.

Flooding may also occur from melting of snow and ice, accumulated earlier, from a number of storms in the winter season. Flooding in the middle latitudes therefore often happens in spring or early summer. A second source of river discharge therefore is the accumulated snow and ice on the land surface of river basins which melts into annual floods as the climate turns warmer.

We can therefore have two classes: a low-latitude barotropic and a higher-latitude baroclinic section. This pattern controls the rise and fall of the river hydrographs and floods. Large floods in big rivers occur under specific circumstances. For example, in the tropics, cyclonic circulations give rise to large rain-bearing storms which may develop up to the strength of tropical cyclones. Heavy, intensive, and episodic rainfall from such storms commonly arrives in the middle of the wet season when the river is high and the ground is wet, giving rise to flood discharges, extensive erosion, and sediment transfer (Gabet et al. 2004). In higher latitudes, floods arrive from a series of large-scale frontal storms, often as rain on snow. Rain-bearing tropical cyclones moving towards higher latitudes, may also contribute to floods in major rivers.

Precipitation may vary considerably from year to year (Amarasekera et al. 1997). The basin of a large river may go through a spell of wet or dry years due to various types of climatic shifts such as the short-term El Niño Southern Oscillation (ENSO) or other climate drivers which operate over longer periods. A full ENSO cycle usually runs for five to eight years. It includes particularly dry years (known as El Niño), and wet years (La Niña). The resulting wet or dry climate is found over various parts of the world at the same time explaining variable flows in large river basins. For example, precipitation and river runoff rise during La Niňa for the Magdalena, Orinoco, and the northern tributaries of the Amazon across northern South America.

The Pacific Decadal Oscillation (PDO) is similar but lasts for 20–40 years. Rivers of the tropics and subtropics are commonly affected by the short-term ENSO and PDO climate drivers. The North Atlantic Oscillation (NAO) affects rivers flowing to the Atlantic and Arctic Oceans. Its influence on precipitation and runoff can be recognised in the runoff pattern of the northeastern and mid-Atlantic rivers of the United States but rivers in Europe are difficult to interpret. Other long-term climatic oscillations are the Atlantic Multidecadal Oscillation (AMO) and the Southern Annular Mode (SAM). Given that a long series of discharge data is available only for a few rivers, the effects of long-term climate drivers on rivers are difficult to investigate.

Runoff of a large river thus reflects various climatic criteria: annual rainfall, seasonality in rainfall, and episodic rain from synoptic disturbances. Not only are the large rivers thus maintained, but their behaviour is also characterised by the run of changing wet and dry years determined by climate drivers, and episodic storm rainfalls. A high average rainfall or floods of limited recurrence interval is required to maintain the channel of a large river.