The dissolved load

The dissolved load of a river is the product of concentration and discharge, and usually is expressed as kilograms per day or tons per year. In comparing catchments or river basins it is helpful to present this as a yield per unit area, by dividing by drainage area. Streamflow is more variable than ionic concentration, and so be-tween-year variation in the export of ions depends strongly on interannual variation in discharge (Figure 4.3). Because discharge and ionic concentration often are inversely related, the range of the dissolved load of ions transported by the world's major rivers varies over only two orders of magnitude, from around three to as high as 5001 km 2 year1, with highest values observed in small alpine rivers (Meybeck 1977). It also varies less over time than does the solid load. The greater discharge of rivers in humid areas more than compensates for lower ionic concentrations, and so the dissolved load is less in arid areas and greater in areas of higher runoff.

FIGURE 4.3 Between-year variation in the gross output of calcium, sodium, magnesium, and potassium depends mainly on between-year variation in discharge for undisturbed catchments of the Hubbard Brook Experimental Forest. Data span 1963-1974. (Reproduced from Likens and Bormann 1995.)

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