Influence on a River Rates of Discharge

Introduction

The analysis of river ‘flow regimes’ has evolved into a fundamental aspect of the Geographical discipline. The contemporary geographer is aware of how a complex system of factors can influence a river’s rate of discharge.

Factors Influencing Flow Regimes

The six constituents of the hydrological cycle effect river flow regime. These are precipitation, infiltration, evaporation, transpiration, surface run-off and ground water flow.^[1] Veissman and Lewis have noted the complexity of this cycle by stating: ‘paths taken by precipitated droplets of water are many and varied before the sea is reached.’^[2]

Precipitation levels vary in accordance with a variety of factors; such as latitude, altitude, vegetation cover and micro-climatic particularities. However, precipitation is relatively uncomplicated to measure and thus grants the most data.^[3] Ingle Smith and Stopp have highlighted that a river’s discharge is ‘related to precipitation but complex.’^[4] Indeed, there is always a ‘time lag’ from the commencement of heavy periods of rainfall to a marked increase in river discharge.^[5] The duration of time lag will depend upon the infiltration capacity of the soil in the river’s catchment area, as well as local topography and the presence of vegetation. Rivers obtain the majority of their water supply via the process of ‘through flow.’^[6] Through flow occurs when water held within the soil gradually travels to the river channel and explains why, even during dry periods, ‘few rivers or streams cease to flow.’^[7]

Vegetation plays a vital role in the character of river regimes. Surrounding plants and trees intercept precipitation and influence the amount of water which will ultimately pass to the water way.^[9] Rain water may land on foliage or dead organic matter and evaporate, or be absorbed by roots in the soil. These processes constitute the phenomenon known as ‘evapotranspiration.’^[10] Forests have a balancing effect on the hydrological cycle and restrict a superfluous quantity of water entering the river channel during periods of heavy rainfall.^[11] In colder climes, such as the Tiaga region, the shade from trees can prevent rapid snow melt causing flash floods.^[12] Deforestation has had a marked influence on the nature of river regime characteristics.

Comparison of Discharge Values

The correlation of the mean monthly discharge values of specific river regimes, in different locations, imparts much fascinating information. The discharge value of a river is measured in cubic metres per second (m/sec).^[13]

The River Suir of the Republic of Ireland and the River Thames of Great Britain both exhibit very similar flow regimes of a uni-modal nature. The two rivers are situated in the north western European temperate zone and flow levels peak in January and ebb in July in tandem with the wet and dry seasons. On an annual basis the Suir has a mean monthly flow rate of 54.250 m/sec and the Thames has a rate of 61.583 m/sec. The greatest deviation from mean rate occurs in January. During this month the Thames exhibits a discharge rate of 110m /sec whilst the Suir’s rate is 92m /sec. The month of July provides the lowest discharge rate for both water ways: 2.98% of total annual discharge passes through the Thames, whilst it is 4.45% for the Suir.

The Mediterranean zone offers a distinct form of river regime flow pattern. The Vinalopo River, which is situated in South East Spain near the town of Alicante, illustrates this actuality. Unlike the rivers of northern temperate regions, the Vinalopo’s range of discharge is more extreme, ranging from an average level of only 25m /sec in September to a substantial 410m /sec in January. This represents a range of 385m /sec. Indeed, the greatest deviations from the monthly mean value of 197.417m /sec take place in the winter, during January (410 m /sec) and February (380 m /sec), and in late summer, in August (30 m /sec) and September (25 m /sec).

The mighty Yenisey River of Russia runs from the town of Kyzl in Southern Siberia and traverses theWest Siberian lowlands before entering the Kara Sea 388km away to the north.^[14] Mean monthly discharge is 17,916.667 m /sec, and peak discharge of 76,000 m /sec occurs in June, which constitutes 35.35% of total annual discharge in one month.. From November to April average discharge is only 4,750 m /sec and this six month period provides only 13.25% of total annual flow. Discharge rate increases abruptly in spring, peaks in June, and ebbs dramatically from July (28,000 m /sec) to October (15,000 m /sec). Thus, the Yenisey displays an entirely different regime to that of the Thames, the Suir and the Vinalopo. Base flow occurs during April and peak flow during June, and represents an astonishing range of 72,000m /sec throughout the year.

South East Asia is home to the Brahamaputra River which flows from high in the Himalayan Mountains before meeting the Ganges River in the delta of southern Bangladesh.^[15] Like the Yenisey River, the Brahmaputra also crosses an array of climatic regions. A peak discharge rate of 43,120 m /sec occurs in August and constitutes 18.64% of total annual discharge. This represents the largest deviation from a mean monthly discharge of 19,277.50 m /sec. Base flow is recorded at a rate of 4,190 m /sec in February and represents a flow range of 38,930 m /sec. The Bramhaputra is comparable to the Yennisey as flow trend ebbs in winter and increases in spring and summer. However, flow peaks later in the year and the period of November to April constitutes 17.58 % of total annual discharge, which is slightly higher than that of the Yenisey. The range of discharge rate of this water way is also not as extreme as that of the Yenisey River and is more dissipated throughout the months of May to October.

The Congo (Zaire) River is the fifth largest river in the world and is situated in central Africa.^[16] The river flows through zones of tropical rainforest and savannah. The flow regime of the river is bi-modal due to its situation in the tropics. Discharge peaks at 73,600 m /sec in December which constitutes 15.73% of total annual discharge. The secondary May peak of 62,100 represents 13.27% of yearly discharge. July witnesses a base flow level of 21,600 m /sec. Thus, the range of flow discharge is 52,000m/sec. Discharge follows an entirely different pattern to the aforementioned water ways. The volume increases and decreases twice on an annual basis. It rises from March to May and from July to December, respectively, and ebbs during the interim periods.

Climatic and Regional Influences

Each of the six river regimes exhibit these particular annual flow patterns due to the specific climatic and environmental factors which prevail within their catchment areas.

The Thames and Suir regimes are situated in the temperate forest biome which experiences ‘warm moist summers and mild winters.’^[17] Precipitation occurs throughout the year and peaks during the winter.^[18] This is why both rivers experience the highest levels of discharge in January. Evapotranspiration peaks during the summer, but the heavy peaty soils continue to provide supplies of stored water to the rivers via the process of through flow.^[19]

The Alicante Mountains north and west of the town of Alicante rise to a height of almost 1600 metres and influence the flow regime of the Vinalopo River. Precipitation levels increase with altitude and, during the autumn and winter rains, the river discharge rate rises as water enters the main channel via mountain tributaries. Evapotranspiration rates far outbalance rainfall in the hot months of July and August when temperatures around the town of Elche can reach as high as 26ï‚°C. Thus, discharge is severely reduced to only 2.33% of total annual discharge in August and September. Indeed, during this period of base flow the river benefits little from through flow as moisture in the arid Mediterranean soil is quickly evaporated upwards. The local demands of the population and vegetation also decreases the water table in the lower lying regions of the Vinalopo catchment. For example, the large incidence of palm trees surrounding the town of Elche naturally reduces the water budget in the region during the dry Mediterranean summer.

The flow regime of the Yenisey river is influenced by the continental climate of the Asiatic land mass, which experiences great extremes of temperature.^[20] As temperatures gradually rise in the spring time, after the bitterly cold winter, snow melt in the mountains, and ablation of glaciers, causes a surge in discharge. The water from the melting precipitation and ice cannot be absorbed by the permafrost, which underlies the soil, and thus runs off directly to the river and its tributaries.^[21] Permafrost will also melt as temperatures rise providing an additional source of water via through flow.

The discharge rate of the Brahmaputra River also increases in March and April due to Himalayan snow melt entering the river regime.^[22] The monsoon rains commence in April and continue until October. During this period up to 200cm of precipitation can fall and the Brahmaputra is ‘swollen by June or July.’^[23] Such a high influx of water explains why discharge increases rapidly. Non-equatorial tropical river systems experience higher rates of precipitation during the summer months and a considerable reduction in winter.^[24] Indeed, only 9.54% of total annual discharge flows through the Brahmaputra from December to March.

The flow regime of the Congo River is unique amongst the six river regimes as it is of a bi-modal nature, ebbing and flowing twice annually. Precipitation levels are bi-modal^[25] and peak at the time of the equinoxes in March and April.^[26] This is due to the tropical equatorial location of the river, and discharge rate exhibits a marked increase after these months. Indeed, rainfall is continuous throughout the year and the annual level in Kasangani, Democratic Republic of Congo, is approximately 170cm. This factor highlights why the monthly discharge rate never drops below 4.7% of the total annual volume. Temperature remains practically constant at 25ï‚°C along the route of the Congo throughout the year due to the absence of seasonality in this biome.^[27]

Conclusion

The discharge levels of the Suir, Thames and Vinalopo rivers all correspond with increased levels of precipitation in the winter and reduced precipitation in the summer. The range of discharge in the Thames and Suir is much less than that of the Vinalopo. This is due to the fact that they are situated in a temperate climate and do not experience the extremes of high temperature, altitude and rainfall which exist in the Mediterranean environment. Increased autumn and winter precipitation rates on the Alicante mountains, and the dry hot summers, are responsible for a flow discharge rate which ranges from 410m /sec in January to only 25m /sec in August.

Mountain ranges also influence the discharge rates of the Brahmaputra and Yennisey Rivers profoundly. Snow melt and glacial ablation at high altitudes cause a surge in discharge during the spring months in both rivers. However, discharge reduces to 6,000 m /sec on the Yennisey by November and this is due to the onset of the severely cold continental winter. Precipitation in the Siberian mountains is now frozen, and overland and through flow to the river channel is severely reduced. The monsoon climate of south east Asia ensures that the discharge of the Brahmaputra remains high for a longer period of time. In November average discharge is still 11,735 m /sec; almost double that of the Yennisey. Thus, total annual flow is dissipated over a longer time period than the more extreme ‘freeze/thaw’ trend of the Yennisey regime.

The Congo has a very different annual discharge trend to the other rivers due to its situation in equatorial Africa. The bi-modal rain season ensures that discharge rises and falls twice on an annual basis. The Congo has the highest total annual discharge of all the rivers. It is almost double that of the Brahmaputra and this is testimony to the incessant precipitation of the equatorial tropics.

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1

Footnotes

^[1] W Viessman, G L Lewis, Introduction to Hydrology, Pearson, 2003) 2

^[2] W Viessman, G L Lewis) 3

^[3] D Ingle Smith, P Stopp, The River Basin, An introduction to the Study of Hydrology, Cambridge, 1978) 15

^[4] D Ingle Smith, P Stopp) 15

^[5] D Ingle Smith, P Stopp) 15

^[6] J Holden, An Introduction to Physical Geography and the Environment, Pearson, 2005) 312

^[7] D Ingle Smith, P Stopp) 15

^[9] D Ingle Smith, P Stopp) 9

^[10] J Holden) 39

^[11] D S G Thomas, A Goudie, The Dictionary of Physical Geography, Blackwell, 2000) 209

^[12] D S G Thomas, A Goudie) 209

^[13] W Viessman, G L Lewis) 9

^[14] National Geographic Atlas of the World, National Geographic Society, 1995) 74

^[15] National Geographic Atlas of the World) 83

^[16] National Geographic Atlas of the World) 94

^[17] P Ganderton, Mastering Geography, MacMillan, 2000) 314

^[18] J Holden) 328

^[19] J Holden) 328

^[20] J C Dewdney, A Geography of the Soviet Union, Pergamon, 1979) 7

^[21] P Ganderton) 311

^[22] B A Weightman, Dragons and Tigers, A Geography of South, East and South East Asia, Wiley, 2006) 195

^[23] B A Weightman) 195

^[24] J Holden) 328

^[25] J Holden) 328

^[26] R White, Africa Geographical Studies, Heinemann, 1984) 169

^[27] R White) 167