Surface waters, rivers and lakes, are the most easily accessible water source for economic needs of humans, being of paramount importance for water ecosystems. The term 'renewable' means not only quantitative recharge, but also the possibility of in-river restoration of water quality (self-purification).
The distribution of river water in space is quite uneven in space and time; some locations have plenty of it while others have very little. River discharges in continents vary in time, following a periodic rule. For instance, a major part of river runoff in Europe occurs from April to June, in Asia from June to October, while in Australia and Oceania from January to April.
The global river runoff is estimated, on average, as about 40 000 km yr~ , but it varies in time. The largest river discharges are in Asia and South America (respectively, 13 500 and 12 000 km3 yr-1), while the smallest are in Europe and Oceania (respectively, 2900 and 2400 km3 yr_1).
The year-to-year variability of water resources can be quite significant and considerably departs from the average values. This especially pertains to the arid and semiarid regions, where the water resources themselves are generally low. Here, in individual years, values of river discharges can be only half of the long-term averages, whereas for humid regions this difference is lower. Apart from between-year variability, important is the within-year variability, and seasonal and monthly patterns of discharge. Often, river runoff distribution is not uniform in time: a flooding season, which may last 3-4 months, is responsible for even 80% of annual discharge total, while during the low flow period, lasting 3-4
months, the river runoff may amount to a small portion (below 10%) of annual total.
The highest river runoff potential is concentrated in six countries: Brazil, Russia, Canada, the USA, China, and India, where nearly half of the total annual river runoff is formed. The greatest river of the world, Amazon, carries about 7000 km3 of water, that is, 16% of annual global river runoff, while 11% of the total runoff is due to the four other large river systems: Ganges with Brahmaputra, Congo, Yangtze, and Orinoco.
Many river basins belong to the so-called endorheic (drainless) runoff regions that are not connected to oceans. The total area of endorheic runoff regions is about 30 million km2 (20% of the total land area). However, only 2.3% (^1000 km yr~ ) of annual global river runoff is formed in these regions, much of whose area is covered by deserts and semi-deserts with a very low precipitation. The largest endorheic regions include the Caspian Sea basin, much of Central Asia, northeastern China, Australia, Arabian Peninsula, and North Africa. In endorheic regions, much of water is lost for evaporation and does not reach river mouths.
Approximately half of the total river water inflow to the world ocean (19 800 km) feeds into the Atlantic, where four of six largest rivers of the world flow into (Amazon, Congo, Orinoco, and Parana). The smallest amount of river water (5000 km3 yr_1) flows into the Arctic Ocean; however, river waters are of most importance for the regime of this ocean. While containing only 1.2% of total oceanic water storage, the Arctic Ocean receives 12.5% of global river runoff. On average, much of the total river runoff (about 42%) enters the ocean in the equatorial region between 10° N and 10° S.
The values given above describe the average situation for a long-term period. For shorter time intervals (e.g., for an individual year), the values of water amounts in different stores in the hydrosphere may considerably depart from the long-term average.
All the lakes on Earth store approximately 91 000 km3 of freshwater - much more than the rivers. Most lakes are young in geological terms (being 10-20 ky old), except for much older lakes of tectonic or volcanic origin (such as the Lake Baikal or lakes of the East African Rift). The Lake Baikal is the largest (by volume) and the oldest freshwater lake in the world, containing 91 000 km3 of freshwater. Similar volume of freshwater is stored in the North American Great Lakes.
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