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Zambezi River runoff Zambezi River runoff
A major impact of the construction of the Kariba and Cahora Bassa dams from 1950–1970 was the reduction in the Zambezi River runoff. Before the dam construction, the Zambezi River was torrential with high flows during the wet season from November to March and relatively low flows in the dry season from April to October. On average, the river discharged 60 to 80 per cent of its mean annual flow during wet season. Since the dams were built, the w...
14 Oct 2013 - by GRID-Arendal
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World ocean thermohaline circulation (alternative version) World ocean thermohaline circulation (alternative version)
The global conveyor belt thermohaline circulation is driven primarily by the formation and sinking of deep water (from around 1500m to the Antarctic bottom water overlying the bottom of the ocean) in the Norwegian Sea. When the strength of the haline forcing increases due to excess precipitation, runoff, or ice melt the conveyor belt will weaken or even shut down. The variability in the strength of the conveyor belt will lead to climate change in...
01 Jun 2007 - by Hugo Ahlenius, UNEP/GRID-Arendal
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Trends in continental river discharge Trends in continental river discharge
River flow is cyclical in nature, with alternating cycles of wet and dry years. This graphic uses bar graphs to show the average annual volumes of river runoff in cubic kilometres for each continent from 1921 to 1985. Over the time period the discharge there has been no obvious trends, but a certain degree of fluctation, the level dependent on the area of the basins and the precipitation over the area. The sizes of the bars in this graph reflect ...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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Global sediment loads Global sediment loads
Asia exhibits the largest runoff volumes and, therefore, the highest levels of sediment discharge. This graphic shows the amounts of suspended sediments discharged, in millions of tonnes per year, in the major regions of the world. The levels of suspended sediments are increased by soil erosion and land degradation, and may affect aquatic ecosystems negatively.
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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Aral Sea: trends and scenarios Aral Sea: trends and scenarios
The demise of the Aral Sea was caused primarily by the diversion of the inflowing Amu Dar'ya and Syr Dar'ya rivers to provide irrigation water for local croplands. This graphic shows the disappearance of the Aral Sea from 1957 to 2000 and three possible scenarios showing the relationship between future demand (and thus water abstraction) and future available runoff in cubic kilometres per year. The scenarios cover the time period from 2000 to 202...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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World's surface water: precipitation, evaporation and runoff World's surface water: precipitation, evaporation and runoff
The world's surface water is affected by different levels of precipitation, evaporation and runoff in different regions. This graphic illustrates the different rates at which these processes affect the major regions of the world, and the resulting uneven distribution of freshwater. It shows the amount of precipitation in cubic kilometres for each region, and the percentage of that amount which evaporates or becomes runoff. The text below the grap...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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World's water cycle: schematic and residence time World's water cycle: schematic and residence time
The water cycle consists of precipitation, evaporation, evapotranspiration and runoff. This graphic explains the global water cycle, showing how nearly 577 000 km3 of water circulates through the cycle each year. A table of estimated residence times of the world's water shows the estimated times that water resources exist as biospheric water; atmospheric water; river channels; swamps; lakes and reservoirs; soil moisture; ice caps and glaciers; oc...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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Long-term variations in river flow, by continent Long-term variations in river flow, by continent
River runoff is cyclical in nature, with alternating cycles of wet and dry years. Significant deviations from average values differ in duration and magnitude. For example, 1940-44, 1965-68 and 1977-79 are clearly low periods in terms of total runoff from the world's rivers. During these periods, the runoff was estimated at 1 600-2 900 km3 below the average value. By contrast, 1926-27, 1949-52 and 1973-75 saw much greater levels of river runoff (S...
17 May 2005 - by Delphine Digout, UNEP/GRID-Arendal
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River runoff by continent, 1921-1985 River runoff by continent, 1921-1985
River runoff represents the accumlated water, from preciptation and meltwater, that feeds into rivers that feeds into seas. The estimated annual figures in this graphic is what volume of water that ends up in the World Ocean. Factors influencing this volume is primarily the area that drains (e.g. area of continent), precipitation and evaporation. Tropical regions typically exhibit greater river runoff volumes. The Amazon carries 15% of all the wa...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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Iceland, topographic map Iceland, topographic map
Iceland is located in Northern Europe, is an island between the Greenland Sea and the North Atlantic Ocean, northwest of the UK, comprising 103,000 sq km. It has an estimated population of 296,737 (2005). Major environmental concerns are: water pollution from fertilizer runoff; inadequate wastewater treatment.
04 Oct 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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World ocean thermohaline circulation World ocean thermohaline circulation
The global conveyor belt thermohaline circulation is driven primarily by the formation and sinking of deep water (from around 1500m to the Antarctic bottom water overlying the bottom of the ocean) in the Norwegian Sea. When the strength of the haline forcing increases due to excess precipitation, runoff, or ice melt the conveyor belt will weaken or even shut down. The variability in the strength of the conveyor belt will lead to climate change in...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
4
Freshwater stress and risk Freshwater stress and risk
One study suggests that although global water conditions may worsen by 2025 due to population pressure, climate change could have a net positive impact on global water resources. NB! Note that other studies indicate that with present consumption patterns, 2 of every 3 persons on Earth will experience water stress by 2025. The diagram on the left side shows the result of this particular study, indicating the water availability for the population...
17 May 2005 - by Philippe Rekacewicz, UNEP/GRID-Arendal
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World ocean thermohaline circulation World ocean thermohaline circulation
The global conveyor belt thermohaline circulation is driven primarily by the formation and sinking of deep water (from around 1500m to the Antarctic bottom water overlying the bottom of the ocean) in the Norwegian Sea. When the strength of the haline forcing increases due to excess precipitation, runoff, or ice melt the conveyor belt will weaken or even shut down. The variability in the strength of the conveyor belt will lead to climate change in...
01 Jun 2007 - by Hugo Ahlenius, UNEP/GRID-Arendal
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Annual snowfall pattern for a typical mountain environment, Columbine Pass, Colorado 1971-2000 Annual snowfall pattern for a typical mountain environment, Columbine Pass, Colorado 1971-2000
Mountain snow cover typically develops in the autumn and grows to a maximum depth in early spring. As day length and sun angles increase, so do air temperatures, causing snow cover to warm and begin to melt. Snow cover balances the availability of water in mountain environments. Where winter precipitation falls as rain, surface runoff occurs almost immediately. In contrast, snow stores water during the winter and then melts in the spring and ear...
01 Oct 2007 - by Hugo Ahlenius, UNEP/GRID-Arendal
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Example of enhanced water levels produced from river ice, Liard River, Canada Example of enhanced water levels produced from river ice, Liard River, Canada
The lower curve shows the correspondence between river flow and water levels under open-water conditions. The much greater maximum water levels possible under ice-jam conditions are illustrated by the upper curve. The transition in break-up severity from dynamic to thermal break-up effects (see text) is depicted by the gradually shaded area between the two curves. Dots are observed annual maximum water levels during the spring break up. The 1990...
01 Jun 2007 - by Hugo Ahlenius, UNEP/GRID-Arendal
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Arctic delta pond ecosystems, seasonal flooding and adaptation Arctic delta pond ecosystems, seasonal flooding and adaptation
The physical development and ecosystem health of river deltas in cold regions are strongly controlled by ice processes and thus are highly susceptible to the effects of climate change. As an example, the photograph shows a typical lake/pond and river network in the Peace-Athabasca Delta (Canada), one of the largest freshwater deltas in the world. The water budget and sediment-nutrient supply for the multitude of lakes and ponds that dot the ripar...
01 Nov 2007 - by Hugo Ahlenius, UNEP/GRID-Arendal
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Time series of intercepted continental runoff and large reservoir storage, 1900–2000 Time series of intercepted continental runoff and large reservoir storage, 1900–2000
The years 1960–2000 have shown a rapid move toward flow stabilization, which has slowed recently in some parts of the world due to the growing social, economic, and environmental concerns surrounding large hydraulic engineering works.
30 Nov 2007 - by Philippe Rekacewicz, Emmanuelle Bournay, UNEP/GRID-Arendal
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Mean changes in runoff Mean changes in runoff
Changes in precipitation and temperature influence changes in runoff and the availability of water. Results from models of changes in runoff are consistent with predictions for precipitation. For 2090-2099, in areas for which increases in the rainfall regimen are expected, increases in runoff are also projected. The anticipated changes in runoff are based on the A1B climate change scenario which assumes future rapid demographic and economic growt...
22 Nov 2010 - by Nieves López Izquierdo, Associate Consultant UNEP/GRID-Arendal
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The water cycle The water cycle
Environment water transport system. The water cycle consists of precipitation, vapour transport, evaporation, evapo-transpiration, infiltration, groundwater flow and runoff. Figure 1 explains the global water cycle, illustrating how nearly 577,000 km3 of water circulates through the cycle each year. A table of estimated residence times shows the approximate times that water resources exist as biospheric water, atmospheric water and so on. The w...
01 Feb 2008 - by Philippe Rekacewicz , UNEP/GRID-Arendal
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Nitrate levels: concentrations at river mouths Nitrate levels: concentrations at river mouths
Nutrients are essential to life. In aquatic systems, nitrogen and phosphorus are the two nutrients that most commonly control the growth of aquatic plants, algae and bacteria. Nitrogen and phosphorus are considered to be the primary drivers of eutrophication of aquatic ecosystems, where increased nutrient concentrations lead to increased primary productivity. Some systems are naturally eutrophic, whereas others have become eutrophic as a result o...
26 Jan 2009 - by Philippe Rekacewicz
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