3Supp%203%20Quantifying%20streamflow

3Supp%203%20Quantifying%20streamflow - HYDROGEOLOGY LECTURE...

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HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC PRINTED ON 8/2/2005 100 stream. This flow of water is called interflow . Streams Streams are generally hydraulically connected to the underlying groundwater. When groundwater is discharging into the stream, we call the stream a gaining stream. The amount of water that flows into the stream from the groundwater is called baseflow . If infiltration causes the water table to rise, the hydraulic gradient in the groundwater will increase and the amount of baseflow will also increase. For many streams, baseflow is the source of water to the stream except during storms, when precipitation in the watershed exceeds the infiltration capacity and the depression storage is filled. In this case, runoff or overland flow will occur, and this runoff will flow into the stream. Natural runoff is usually a small component of the total volume of water flowing through a watershed; only in arid regions where the streams are losing streams and are not receiving baseflow, is natural runoff a significant contributor. In heavily urbanized areas, with a lot of impervious cover (parking lots, roads, buildings), runoff is a much more significant contribution to the surface water system. Discharge Discharge is defined as the volume of water moving past a point on a stream in a given period of time. In the simplest terms, stream discharge (Q) is equal to the velocity of the water (v) times the cross sectional area of the stream (A), or Q = vA. Figure 15-9. Cross-section of stream showing velocity profile (© Uliana, 2001). The complication is that water in the stream is flowing fastest in the middle of the stream and is slowest at the edges and along the bed of the stream. Figure 15-9A shows a cross section of a stream flowing out of the page. Figure 15-9B shows contours indicating the changes in velocity (in units of, say, cm/sec) throughout the stream. The line a-a’ shows the velocity profile – the arrows represent velocity vectors.
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HYDROGEOLOGY LECTURE NOTES HYDROGEOLOGYLECTURENOTES(SPRING2004).DOC PRINTED ON 8/2/2005 101 To measure discharge, we have to determine the average velocity of the water in the stream. We can measure
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3Supp%203%20Quantifying%20streamflow - HYDROGEOLOGY LECTURE...

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