Abstract

Two small watersheds were studied to determine rates of dissolution of granite in Rhodesia. One watershed (0.91 km2) is hilly and has an annual rainfall of 1,220 mm, whereas the other (7.33 km2) is more nearly level and has an annual rainfall of 922 mm. Samples of drainage water were collected weekly for 2 yr. Water samples from the streams were analyzed for SiO2, A12O3, Fe2O3 (total Fe), K, Na, Ca, and Mg. Elemental levels were found to be relatively constant throughout the year. The flux of elemental removal was calculated from the streamflow rate and chemical concentrations.

The loss of each constituent as a result of weathering could be determined after the chemical composition was determined for the parent rock and the weathered product. Knowing the amount of an element lost and its rate of net removal from the watershed, a rate of weathering for granite was calculated as 400 kg·ha−1 · yr−1 for the higher rainfall area, and 150 kg·ha−1 · yr−1 for the lesser rainfall area. These rates can also be expressed as 15.4 and 5.8 mm of rock weathered per 1,000 yr for the two areas, to produce 11.0 and 4.1 mm of soil, respectively. Thus, slope reduction on granite in Rhodesia is 4.4 and 1.7 mm/1,000 yr for the higher and lower rainfall areas, respectively.

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