Deals with watershed geomorphology and characteristics, including hierarchical structure of a draiange basin, morphological parameters, hypsometry, stream order, Horton’s laws, stream power, longitudinal stream profile, hydraulic geometry, drainage density, drainage pattern, lag time, and time of concentration.

For some purposes, the physical processes relating current runoff to precipitation can best be assumed to take place at the scale of the catchment, without consideration of the detailed subscale processes or for the intricate flow paths inside the watershed. The most common implementation of this idea has been the unit hydrograph (UH), which is based on the assumptions of linearity and stationarity. A UH is characterized by the duration of its precipitation input; this allows the definition of the instantaneous UH, that is the response of a catchment to a delta function precipitation input, or its Green’s function. The UH of a catchment can be identified from available data using the method of least squares. To facilitate the concise parameterization of UH functions for identification and prediction purposes, various conceptualizations have been proposed consisting of different combinations of linear translation elements and linear storage elements. Attempts have been made to extend the UH concept by allowing for nonstationarity and nonlinearities in the response. Long-term streamflow response to mean annual precipitation has also been the subject of many studies.