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DISCHARGE MEASUREMENTS AT GAGING STATIONS By Thomas j. Buchanan and William P. Somers Abstract The techniques used in making discharge measurements at gaging stations are described in this report. Most of the report deals with the current-meter method of measuring dis- charge, because this is the principal method used in gaging streams. The use of portable weirs and flumes, floats, and volumetric tanks in measuring discharge are briefly described. Introduction The U.S. Geological Survey makes thousands of streamflow measurements each year. Dis- charges measured range from a trickle in a ditch to a flood on the Amazon. Several methods are used, but the Geological Survey makes most streamflow measurements by current meter. The purpose of this report is to describe in detail the procedures used by the Geological Survey for making current- meter measurements and to describe briefly several of the other methods of measuring streamflow. Streamflow, or discharge, is defined as the volume rate of flow of the water including any sediment or other solids that may be dissolved or mixed with it. Dimensions are usually expressed in cubic feet per second. Other common units are million gallons per day and acre-feet per day. Current-Meter Measurements A current-meter measurement is the sum- mation of the products of the partial areas 326-258 O-OO-~ of the stream cross section and their respective average velocities. The formula Q=2:(av) (1) represents the computation where Q is total discharge, a is an individual partial cross- section area, and v is the corresponding mean velocity of the flow normal to the partial area. In the midsection method of making a current-meter measurement it is assumed that the velocity sample at each location represents the mean velocity in a partial rectangular area. The area extends laterally from half the distance from the preceding meter location to half the distance to the next and vertically, from the water surface to the sounded depth. (See fig. 1.) The cross section is defined by depths at locations 1, 2, 3, 4, . . . n. At each location the velocities are sampled by current meter to obtain the mean of the vertical distribution of velocity. The partial discharge is now computed for any partial section at location x as _ [(bz-b'z-,,)+(b'Z+U-bZ)]d qz-vz 2 2 % - [b<z+u-b'z-uJ.l -Oz 2 11hz, (2) where qz= discharge through partial section x, vz=mean velocity at location x, bz= distance from initial point to location x, b,z_u=distance from initial point to preced- ing location, b,z+,,=distance from initial point to next location, dz=depth of water at location x. 1