Temporal and spatial variability of streamflow
Streamflow occurs as a result of the interaction of the many components of the hydrologic cycle, as shown in Figure 5 of Riggs and Wolman (this volume). The streamflow part of the cycle is driven by precipitation in the form of rain or snow. The complex processes of interception, infiltration, evaporation, and transpiration (Saxton and Shiau, this volume) serve to reduce the amount of water available for runoff at any one time and at any one location. These proceses are influenced by various climatic factors, such as precipitation, temperature, wind, and solar radiation; and by physiographic factors related to geology and topography. As a result, streamflow varies considerably both in time and space. For example, see the variation of daily streamflows in Figure 8 of Riggs and Wolman (this volume). Lins and others (this volume) and Saxton and Shiau (this volume) describe the climatic and physiographic variations throughout the continent.
This chapter describes the variation of runoff (the volume of flow for a given time) in North America and explains the major climatic and physiographic factors underlying this variation. Following a description of common terminology related to runoff, the variation of runoff in various parts of the continent is presented on maps and graphs. The next section describes the effects of climatic, topographic, and geologic factors on runoff using appropriate examples. A final section documents the integrated effects of climate and physiography within a specific watershed as runoff drains from the headwaters to the basin outlet.