ABSTRACT
The role of humanity in altering the hydrological cycle may be minuscule compared with the range of natural climatic change over geological time. Even before civilization began deliberately harnessing water, prehistoric Man probably had a significant indirect effect upon hydrological processes simply by altering the vegetation. The overwhelming effect of the destruction of natural vegetation, from deforestation to desertification, must have been to slow down the hydrological cycle by reducing evaporative losses. The effects of urbanization on evaporation rates are less clearly established and may be more variable than on precipitation. In other words, the area of increased runoff was not correlated with the area of increased precipitation; it was more closely related to the amount of impermeable cover and perhaps the density of drains. Falling water tables are contributing to the expansion of desertification, which begins with individual zones of impoverished vegetation above the cone of depression around each well which subsequently expand and coalesce as the cones join.
