ABSTRACT
Hydrologic models attempt to represent various components of the precipitation-runoff process in a watershed (Figure 5.1). The processes illustrated begin with precipitation, which can fall on the watershed’s vegetation, land surface, and water bodies. Precipitation as snow would require additional accounting for snow-melt processes (not shown). In a natural hydrologic system, much of the water that falls as precipitation returns to the atmosphere through evaporation and transpiration from vegetation. At the land surface, the water may pond, and depending upon the soil type, ground cover, antecedent moisture, and other land surface properties, a portion may infiltrate into the soil. Impervious land surfaces restrict infiltration and increase runoff. Excess precipitation that does not pond or infiltrate moves by overland flow to a stream channel. Infiltrated water is stored temporarily in the upper layers of soil. From there, it may rise back to the surface by capillary and vegetative action, move horizontally as interflow, or percolate vertically to the groundwater aquifer. Interflow eventually moves into the stream channel. Water in the aquifer moves slowly, but eventually some returns to the channels as base flow. The stream-channel network collects and routes the overland flow, interflow, and base flow to the watershed outlet.
