ABSTRACT
The surface of the earth has been significantly modified by human activities over at least the past 6,000 years. This has affected local and even regional water balances and river regimes, either directly via manipulation of water resources or indirectly, for example through the effects of forest clearance and agriculture. The earliest modification probably occurred when Mesolithic peoples used fire to clear woodland for hunting. Although 43 million km2 of forest land remain, over the millennia at least 30 million km2, or more than 20 per cent of the land area of the globe, has been drastically changed by grazing and agriculture.
