Читать книгу Introducing Large Rivers - Avijit Gupta - Страница 23

River discharge (Q) is computed as the volume of water passing a given point on the river in unit time. It is measured in m³ s⁻¹, or for annual or long-term periods for large rivers in km³ year⁻¹. Two rivers can be compared by normalising their discharges into runoff (R) which is discharge per unit area of the basin (R = Q/A). The standard expression of runoff is

where

R	=	Runoff
P	=	Precipitation (includes snowmelt for certain basins)
AE	=	Actual evapotranspiration
S	=	Surface and subsurface storage of water
C	=	Anthropogenic consumption of water which is not returned to the river

Of all the properties of a large river, a high discharge is the one most expected (Potter 1978). This implies that at least part of the river basin lies in an area of high precipitation, or the area of the river basin is so huge that the cumulative flow in the trunk stream reaches a large volume, or both. The Amazon, the river with the biggest discharge, has a huge basin. It rains and snows heavily over its headwater basins in the Andes Mountains. Large volumes of discharge are contributed by its many tributaries draining the enormous basin. Its neighbour, the Orinoco, exhibits a similar pattern. The Orinoco is fed by drainage from the Andes, the Andean foreland (Llanos), and the Guiana Shield, a craton (Figure 3.1). On average, every square kilometre of its drainage area carries more water than even the basin of the Amazon.

A large river is presumed to have a huge discharge, and large rivers commonly are found in areas of high precipitation, usually determined by the pattern of global climate, and often on the windward slopes of high mountains. In comparison, precipitation over the north-flowing rivers of Eurasia, the Ob, Yenisei, and Lena, are not high but these still have become large rivers because of the collective precipitation falling over their huge catchment areas. The evapotranspiration is low due to prevailing low temperature.

There are variations from such simple explanations. The basin of the Indus is mostly arid, but the water collected in the headwater-mountains from local high annual precipitation maintains its large channel across the dry lower part of the drainage basin. In brief, discharge of a river is derived from both climate and size. By plotting basin area against mean annual discharge for 1100 rivers, Milliman and Farnsworth (2011) demonstrated that 68% of the variance in discharge for the same drainage area can be explained by their climatic characteristics.

We should note that the discharge given for a river refers to the discharge at a specific measurement station. The last gauging station of a large river is located not at its mouth but usually several hundred kilometres upstream, near the end of its tidal limit. The last discharge station on the Amazon is at Óbidos, about 1000 km above its mouth. The last station on the Changjiang is at Datong, 600 km from the sea. Our knowledge about the discharge of water and sediment over the last few hundred kilometres on large rivers is limited.