agriculture and continue with intensive mechanical working practices, fertilisation, use of pesticides and erosion on agricultural land, and last not least by construction. Changes in land use influence not only the amount, but also the quality of water (Fig. 5.2.3 A; WBGU

• Changes in runoff caused by dam building and irrigation (partially from fossil ground water storage). Well-known examples of the effects of irrigation and dam building on surface runoff are the drastically reduced flow of the Colorado in Arizona and the drying out of the Aral Sea (Fig. 5.2.3 B,C; WBGU 1999). Changes in water levels of the lake caused by the Aswan Dam (Egypt) and Lake Chad (Chad) are further impressive examples of global changes in the water cycle (Evans 1996).

• Changes in the distribution of precipitation (Fig. 5.2.4A; Toon 2000; Fig. 5.2.4 B; Cerveny and Balling 1998). Because of production of aerosols, more nucleation bodies reach the atmosphere so delaying precipitation (Toon

2000). This explains the statistically established periodicity in weekly precipitation (Cerveny and Balling 1998) in the eastern USA where aerosol concentration increases rapidly at the beginning of the week and reaches a maximum on Wednesday/Thursday.

This delays rainfall from the atmosphere. Precipitation in the first half of the week is low, increases only in the second half of the week, and reaches its statistical maximum on Saturday. Statistically, least precipitation falls on Monday.

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