ABSTRACT
The hydrological characteristics of a catchment, however, are not constant, and vary naturally over different time scales for many reasons. The physical landscape in many parts of the world reflects the legacy of different hydrological processes operating at different periods in the past (ranging from the dry valleys of the chalk of northern Europe, formed under periglacial hydrological conditions, to the massive erosional and depositional features in many rivers draining the Himalaya caused by the sudden failure of ice-dammed lakes during deglaciation). For several thousand years human activities have been affecting hydrological behaviour. These effects were initially local, but during the latter part of the twentieth century these human activities have been having effects over sufficiently large spatial scales to merit the term “global environ mental change”.
