Streamflow

The volume of flow moving past a point over some time interval is referred to interchangeably as discharge or flow. Usually it is calculated from measurements of width (w), depth (d), and current velocity (v), and expressed in m3 s 1 or ft3 s 1 (cfs).

(a)

FIGURE 2.6 (a) Cross section of a gaining stream, typical of humid regions, where groundwater recharges the stream. (b) Cross section of a losing stream, typical of arid regions, where streams can recharge groundwater. (Reproduced from Fetter 1988.)

FIGURE 2.6 (a) Cross section of a gaining stream, typical of humid regions, where groundwater recharges the stream. (b) Cross section of a losing stream, typical of arid regions, where streams can recharge groundwater. (Reproduced from Fetter 1988.)

In practice, discharge is estimated by dividing the stream cross section into segments, measuring area and velocity for each, and summing the discharge estimates for the segments (Figure 2.7).

Velocity is measured at the midpoint of the segment and (in shallow streams) at 0.6 depth below the surface. At least ten subsections are required, and none should have more than 10% of the total flow (Whiting 2003).

Current velocity varies considerably within a stream's cross section owing to friction with the bottom and sides, and to sinuosity and obstructions. Highest velocities are found where friction is least, generally at or near the surface and near the center of the channel. In shallow streams, velocity is greatest at the surface due to attenuation of friction with the bed, and in deeper rivers, it is greatest just below the surface because of friction with the atmosphere (Gordon et al. 2004). Velocity then decreases as a function of the logarithm of depth (Figure 2.8), approaching zero at the substrate surface. In streams with logarithmic velocity profiles, one can obtain an average value fairly easily by measuring current speed at 0.6 of the depth from the surface to the bottom. At depths >0.75 m, velocities measured at 0.2 and 0.8 beneath the surface can be averaged, and in very turbulent water it may be necessary to measure velocity

FIGURE 2.7 Estimation of discharge from the integration of point measurements of velocity and associated area of flow in subsections of the channel cross section. Velocity is measured at 0.6 depth from the surface in shallow streams. (Reproduced from Whiting 2003.)

Water surface

Water surface

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