Streamflow

A second distinguishing feature of the channel is streamflow. As part of the water cycle, the ultimate source of all flow is precipitation. The pathways precipitation takes after it falls to earth, however, affect many aspects of streamflow including its quantity, quality, and timing. It is useful to recognize flow components based on these pathways.

The two basic components are:

• Stormflow, from precipitation that reaches the channel over a short time frame through overland or underground routes.
• Baseflow, from precipitation that percolates to the ground water and moves slowly through substrate before reaching the channel. It sustains streamflow during periods of little or no precipitation.

Streamflow at any one time might consist of water from one or both sources. If neither source provides water to the channel, the stream goes dry.

A storm hydrograph is a tool used to show how streamflow changes with time. The portion of the hydrograph that lies to the left of the peak is called the rising limb, which shows how long it takes the stream to peak following a precipitation event. The portion of the curve to the right of the peak is called the recession limb.

Changes in hydrology after urbanization
The hydrology of urban streams changes as sites are cleared and natural vegetation is replaced by impervious cover such as rooftops, roadways, parking lots, sidewalks, and driveways. One of the consequences is that more of a stream's annual flow is delivered as storm water runoff rather than baseflow. Depending on the degree of watershed impervious cover, the annual volume of storm water runoff can increase by up to 16 times that for natural areas (Schueler 1995). In addition, since impervious cover prevents rainfall from infiltrating into the soil, less flow is available to recharge ground water. Therefore, during extended periods without rainfall, baseflow levels are often reduced in urban streams (Simmons and Reynolds 1982).

Storm runoff moves more rapidly over smooth, hard pavement than over surfaces with natural vegetation cover. As a result, streamflow (or discharge, Q) undergoes changes in timing and magnitude. The rising limbs of storm hydrographs become steeper and higher in urbanizing areas. Recession limbs also decline more steeply in urban streams. The increased runoff and decreased infiltration recharge less ground water, which also results in lower baseflow after the storm runoff is over.