Geospatial aspects of catchment hydrology

The catchment is a fundamental unit of study in hydrology. It is normally well defined topographically, can be studied as a series of nested units (larger catchments are made of many smaller sub-catchments), and is an open system for measuring inputs and outputs of mass and energy. Catchments are usually delineated by land-surface topography and are made of hillslopes and channels. The proportion of hillslope area to channel density or total channel length may determine how efficiently water can be removed from a catchment since water in channels tends to move much more quickly than water across and through hillslopes. Thus the spatial layout of hillslopes and channels is important. This article describes some basic principles of catchment hydrology and illustrates how determining spatial factors involved is fundamental for understanding how environmental change may impact on runoff production and resulting river flow.