9 Hydrologic Processes

A catchment is a basic unit of landscape particularly for investigations of hydrologic processes. Typically, the topographic boundary of a catchment coincides with the hydrologic boundary causing any precipitation falling on to the catchment to be routed to a stream where it is transported out of the catchment. Fundamental components of the hydrologic cycle, such as precipitation, runoff and evapotranspiration (computed by difference between precipitation and runoff over long periods), have been documented from water balance studies on small catchments. Observations and time series data collected from small catchments provide a basis for the development of hydrologic models, and many such models have been used for flood forecasting. However, one of the more recent goals of hydrologic investigations in small catchments is to understand better how streamflow is generated and how this process relates to water quality genesis. Prior to the last few decades, studies of the sources of streamflow during storms or snowmelt were concerned primarily with the physics of the processes involved. Horton (1933) developed a hypothesis stating that the source of runoff during storms is the excess rainfall over infiltration capacity of basin surficial materials and that the water infiltrated would become groundwater which was the source of the baseflow part of the hydrograph. Horton's thesis is effectively a two-component mixing model. However, Hewlett (1961) showed that water draining from the soil, i.e. unsaturated flow, also contributed to baseflow. Betson (1964) suggested that only certain parts of drainage basins contributed runoff during most storms (the partial-area concept), which was supported by a study by Dunne and Black (1970) in the humid northeastern USA. In addition, Hewlett and Hibbert (1967) proposed that during storms, ephemeral streams expand upstream by collecting overland flow and shallow subsurface runoff along their channels (the variablesource area concept). On the whole, these physically based models came to the quite reasonable conclusion that new rainwater was the dominant source of runoff and several techniques, graphical and mathematical, were developed to subdivide the hydrograph into corresponding source waters (e.g. see Hewlett and Hibbert, 1967). Recently, the use of environmental tracers, such as naturally-occurring isotopes (180, D), solutes (Cl-, Br-) and other physical and chemical characteristics (tem-