River global runoff

FIGURE 3 Relationships between annual runoff and watershed export of DOC in streams and rivers reported in the literature. The respective lines extend only over the range of runoff values included in the dataset. The slopes of each line are approximately equivalent to the mean annual DOC concentration for that group (in parentheses). Sources for each relationship are as follows streams with wetlands, temperate (Mulholland, 1997) streams with wetlands, N. Carolina (Mulholland and Kuenzler, 1979) large rivers, global (Spitzy and Leenheer, 1991) large rivers, N. America (Mulholland and Watts, 1992) streams, tropical (McDowell and Asbury, 1994) streams, N. America (Mulholland, 1997). 

Once in the atmosphere, the water evaporates and some of the sea salt falls back to the sea surfece. The rest is transported considerable distances by winds imtil it is washed out of the atmosphere by rainfall. The salts that are transported back to the continents by this process are termed cyclic salts. After having been rained out onto the continents, the salts are carried back into the ocean by river runoff On short time scales, the global cycling of chlorine and sodium are dominated by this process. The cyclic salts are discussed further in Chapter 21. 

In the Broecker Box model, the total amount of water in the ocean is assumed to remain constant over time. In other words, the evaporation rate and burial of water in the sediments is equal to the rate of water input from river runoff and precipitation. The sizes of the surface- and deep-water reservoirs are also assumed to remain constant over time. This requires the global rate of upwelling to equal the global rate of downwelling. 

This estimate of the suspended plus the bed load is from Syvitski, J. P. M., et al (2005), Science 308, 376-380. The total input of dissolved solids in natural river water is estimated from the global mean river water TDS in Table 21.2 (99.6 mg/L) and the river runoff rate from Figure 2.1. 

First, the application of fertilizers to the land surface has led to increased runoff of N and P from the land surface. Discharge of human wastes has augmented this flux. It appears that the total global nutrient flux today is about 2.5 times greater than the long-term geologic flux, and results in excess accumulation of organic carbon in the ocean (Meybeck, 1982 Wollast, 1983). Furthermore, because of land use activities, the flux of POC, DOC (Likens et al., 1981), and DIC (Meybeck, 1982) from land via rivers to the ocean has been enhanced. 

Miller, J., and G. Russell. 1992. "The impact of global warming on river runoff." Journal of Geophysical Research 97 2757-2764. 

Oki, T., K. Musiake, H. Matsuyama, and K. Masuda. 1995. "Global atmospheric water balance and runoff from large river basins." Hydrological Processes 9 655-678. 

Russell, G., and J. Miller. 1990. "Global river runoff calculated from a global atmospheric general circulation model." Journal of Hydrology 117 241-254. 

A key feature of the above chemical-weathering scenarios is that relatively little atmospheric or biogenic CO2 is involved. Hence, whereas —23% and —77% of solutes, excluding recycled sea salt, found in global mean river water are derived from the atmosphere and rock, respectively (Holland, 1978), atmospheric sources account for a maximum of 3 -11 % of solute in glacial runoff (after Hodson et al., 2000). 

The 20 largest rivers on Earth carry about 40% of the total continental runoff, with the Amazon alone accounting for about 15% of the total. These rivers give the best indication of global average riverwater chemical composition, which can be compared with average continental crust composition (Table 5.1). Three features stand out from this comparison ... 

The surface runoff from the World s land plays an important role in the global carbon mass exchange. The continental runoff supply of HCO is 2.4 x 10 tons/year, that is, 0.47 x 10 tons/year for carbon. Besides, the stream water contains dissolved organic matter at 6.9 mg/L, which makes up to an annual loss of 0.28 x 10 tons/year. The average carbon concentration of suspended insoluble organic matter in the stream discharge is 5 mg/L, which gives the loss of about 0.2 x 10 tons/year. Most of this mass fails to reach the open ocean and becomes deposited in the shelf and the estuarine delta of rivers. We can see that equal amounts of Cc and Co (0.5 x 10 tons for each) are annually lost from the World s land surface (Dobrovolsky, 1994). 

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