River Basin.docx

River Basin: A river basin is an area of land drained by a river and its tributaries. River basins have typical features, these include: Tributaries - smaller rivers flowing into a larger river. A Watershed - an area of highland surrounding the river basin. A confluence - where a river joins another river. Source - the start of a river. Mouth - Where a river meets a lake, the sea or an ocean. The diagram below shows these features.

River Basin Characteristics: The watershed characteristics (shape, length, topography,) control the response of the river basin, shape of the hydrograph, soil water dynamics can be used to identify similarity of response among different basins extrapolation and regionalization of models. Following are the River Basin characteristics:

Topography: Generally, topography plays a big part in how fast runoff will reach a river. Rain that falls in steep mountainous areas will reach the primary river in the drainage basin faster than flat or lightly sloping areas (e.g., > 1% gradient).

Shape: Shape will contribute to the speed with which the runoff reaches a river. A long thin catchment will take longer to drain than a circular catchment.

Size: Size will help determine the amount of water reaching the river, as the larger the catchment the greater the potential for flooding. It is also determined on the basis of length and width of the drainage basin.

Soil type: Soil type will help determine how much water reaches the river. Certain soil types such as sandy soils are very free-draining, and rainfall on sandy soil is likely to be absorbed by the ground. However, soils containing clay can be almost impermeable and therefore rainfall on clay soils will run off and contribute to flood volumes. After prolonged rainfall even free-draining soils can become saturated, meaning that any further rainfall will reach the river rather than being absorbed by the ground. If the surface is impermeable the precipitation will create surface run-off which will lead to higher risk of flooding; if the ground is permeable, the precipitation will infiltrate the soil.

Land use: Land use can contribute to the volume of water reaching the river, in a similar way to clay soils. For example, rainfall on roofs, pavements, and roads will be collected by rivers with almost no absorption into the groundwater.

Drainage Density: It measures the efficiency of the basin drainage (i.e. of how well or how poorly a watershed is drained by rivers). It depends upon both climate (e.g. rainfall regime) and physical characteristics (e.g. geology, slope, soil, land cover) of the drainage basin. For equal climatic characteristics it can be used as proxy information for permeability.

Effects of Precipitation on River Discharge: The annual mean precipitation, evaporation, and runoff in the future increase in high latitudes of the Northern Hemisphere, southern to eastern Asia, and central Africa. In contrast, they decrease in the Mediterranean region, southern Africa, southern North America, and Central America. The change ratio of the global mean runoff is larger than the precipitation change. Nevertheless, the area where reliable runoff change is projected is smaller than that of the changes in precipitation and evaporation. Although the spatial distribution of the changes in the precipitation and runoff tends to coincide with that in the river discharge, it should be emphasized that the change of runoff in the upstream region affects the river flow in the downstream region. In the high-latitude Rivers the discharges increase, and the peak timings shift earlier due to the earlier snowmelt caused by global warming.

Effects of Infiltration and Interception on River Discharge: Anywhere in the world, a portion of the water that falls as rain and snow infiltrates into the subsurface soil and rock. How much infiltrates depends greatly on a number of factors. Some water

that infiltrates will remain in the shallow soil layer, where it will gradually move vertically and horizontally through the soil and subsurface material. Some of the water may infiltrate deeper, recharging groundwater aquifers. If the aquifers are porous enough to allow water to move freely through it, people can drill wells into the aquifer and use the water for their purposes. Water may travel long distances or remain in groundwater storage for long periods before returning to the surface or seeping into other water bodies, such as streams and the oceans. Infiltration and runoff are lower in an area with a dense vegetation cover. Leaves, stems and branches can catch falling rainfall- this is called interception. Some of the water will be stored in the leaves, and evaporate back into the atmosphere. Interception can be high enough for light summer rainfall not to reach the ground. If the rain is heavy, or lengthy, water will run down the leaves (drip), stems (stem flow) and branches, and will either infiltrate or moves as overland flow. As the forest is deciduous, the percentage of rainfall intercepted will vary. In winter, the deciduous trees will be bare of leaves and less water will be intercepted and more will reach the channel. Rainfall intercepted by deciduous woodland: in summer: 25-40%, in winter: 12-15%. Interception usually results in a net loss of water available to the basin hydrological cycle, a decrease of the speed at which water gets to the river and a lower river discharge.

Effects of excess rainfall on river discharge: Weather patterns determine the amount and location of rain and snowfall. Unfortunately the amount and time over which precipitation occurs is not constant for any given area. Overall, the water cycle is a balanced system. Water flowing into one part of the cycle (like streams) is balanced by water flowing back to the ocean. But sometimes the amount flowing in to one area is greater than the capacity of the system to hold it within natural confines. The result is a flood. Combinations of factors along with exceptional precipitation can also lead to flooding. For example, heavy snow melts, water saturated ground, unusually high tides, and drainage modifications when combined with heavy rain can lead to flooding.

How River Basin characteristic affect the river flow: River flow regime change is strongly influenced by the hydrological characteristics of the river basin. The main characteristics of a river basin are rainfall and land cover types. Rainfall in a river basin generates runoff, which eventually joins the river. During this movement, the factor of land cover type influences the amount of water that will finally join the river and become available for use. Land cover type is determined by land use type. Activities that change land use types can enhance or reduce the water yield. Global climate change has affected river flow regimes (as has land use conversion, and the land cover changes associated with it. Land use changes that cause an increase in impervious surfaces reduce infiltration and groundwater recharge and increase surface runoff. Water table decline directly affects river discharge, which is felt more during the dry season.

Amazon River main stem stretches 6500 km across South America, draining nearly one third of the continent and depositing almost 20% of the global river discharge onto the continental shelf of the equatorial Atlantic Ocean. The magnitude of this freshwater source is unique in the global oceans; the Amazon River discharges as much freshwater as the next 8 largest rivers in the world combined. The Amazon River serves as an important connection between continental hydrology and the ocean. Typically, river discharge is a small component of the open ocean salinity balance but the Amazon discharge volume is twice the evaporation minus precipitation budget integrated over the region of 0°–10°N, 70°–20°WThus, the fate of Amazon River water has a large impact on the tropical Atlantic freshwater budget and its variability. In addition to the physical climate impacts of the Amazon River on the region, the river also injects terrestrially derived sediments, nutrients, and colored as well as transparent dissolved organic matter (CDOM, DOM) which can be traced thousands of kilometers from the Amazon River mouth. Biological community structure is strongly influenced by these dissolved organic matter and nutrient inputs as well as by the plume's role in stratifying the upper ocean. This has been shown to lead to globally significant uptake of atmospheric carbon dioxide in the river plume. Because the Amazon discharges at the equator and on the western boundary of the ocean, its waters are entrained in energetic boundary currents associated with the North Brazil Current (NBC), North Equatorial Counter Current (NECC), and coastal Guyana Current (GC).Strong seasonal variations in these currents occur in response to the annual migration of the atmospheric Intertropical Convergence Zone (ITCZ) between its southern position in winter, and its northern position in summer. This leads to northward transport of Amazon water in winter, and eastward transport of Amazon water in the NECC in spring through fall northward pathway is interrupted in summer and fall.

Amazon Basin

Indus Basin: The Trans boundary Indus river basin has a total area of 1.12 million km2 distributed between Pakistan (47 percent), India (39 percent), China (8 percent) and Afghanistan (6 percent).The Indus river basin stretches from the Himalayan Mountains in the north to the dry alluvial plains of Sindh province in Pakistan in the south and finally flows out into the Arabian Sea. In Pakistan, the Indus river basin covers around 520 000 km², or 65 percent of the territory, comprising the whole of the provinces of Punjab and Khyber Pakhtunkhwa and most of the territory of Sindh province and the eastern part of Baluchistan. The drainage area lying in India is approximately 440 000 km2, nearly 14 percent of the total area of the country, in the States of Jammu and Kashmir, Himachal Pradesh, Punjab, Rajasthan, Haryana and Chandigarh. Only about 14 percent of the total catchment area of the basin lies in China, covering just 1 percent of the area of the country, and Afghanistan, where it accounts for 11 percent of the country’s area. Very roughly, at least 300 million people are estimated to live in the Indus basin. Climate is not uniform over the Indus river basin. It varies from subtropical arid and semiarid to temperate sub humid on the plains of Sindh and Punjab provinces to alpine in the mountainous highlands of the north. Annual precipitation ranges between 100 and 500 mm in the lowlands to a maximum of 2 000 mm on mountain slopes. Snowfall at higher altitudes (above 2 500 m) accounts for most of the river runoff .The Upper Indus river basin is a high mountain region and the mountains limit the intrusion of the monsoon, the influence of which weakens northwestward. Most of the precipitation falls in winter and spring and originates from the west. Monsoonal incursions bring occasional rain to trans-Himalayan areas but, even during summer months, not all precipitation derives from monsoon sources. Climatic variables are strongly influenced by altitude. Northern valley floors are arid with annual precipitation from 100 to 200 mm. Totals increase to 600 mm at 4 400 m, and glaciological studies suggest accumulation rates of 1 500 to 2 000 mm at 5 500 m. Winter precipitation (October to March) is highly spatially correlated across the Upper Indus basin, north and south of the Himalayan divide. From 1961 to 1999 there were significant increases in winter, summer and annual precipitation and significant warming occurred in winter whilst summer showed a cooling trend. These trends will impact upon water resource availability (. The climate in the Indus plains is arid to semi-arid.

Indus basin