Hydrological Measurement

the aggregate of methods used to determine the quantities characterizing the movement and state of a liquid and the cycle of bodies of water.

Problems of hydrological measurement include the measurement of the levels, depths, bottom relief, and free surface of flows; of heads and pressures; of the velocities and directions of currents of a liquid; of the pulsations of velocities and pressures; of the elements of waves; of hydraulic gradients; of the turbidity of currents (alluvial concentrations); of the discharge of water, alluvium, and mixtures; and of the elements that characterize the thermal and glacial conditions of flowing streams. Hydrological measurement is used extensively in studying physical phenomena, especially in experimental hydroaerodynamics; in industry (aviation, petroleum, gas, chemical, and food-processing); in geophysics (land hydrology, hydrogeology, and oceanology); and in the planning, construction, and operation of river and maritime hydraulic-engineering installations, hydroelectric power plants, irrigation and drainage systems, and water pipelines. In addition to the quantities mentioned above, geophysical research also measures evaporation and precipitation.

Water levels under natural conditions are measured at gauging stations, where limnographs and marigraphs are utilized for recording them; data on water levels are transmitted over considerable distances by means of remote-control level gauges. Automatic level recorders or a measuring needle whose point coincides with the liquid surface are used in the laboratory and in industry. The head and pressure of a liquid are measured by piezometers and pressure gauges. Under natural conditions, water depth is measured by poles, depth gauges, and plumb lines. Depths are automatically recorded by water-measuring profilographs, which may be mechanical, hydrostatic, or acoustic (fathometers). Bottom relief and the shape of the free surface of a flow are photographed simultaneously by a stereophotogrammetric unit.

The flow velocity of water currents is measured locally (at certain points in the flow) by current meters, tubes, thermal measurement devices, vanes, floats, and electronic and mechanical instruments. In studying flow turbulence, the readings of many instruments are recorded on an oscillograph. The average vertical velocities of a headless flow are measured by integrator floats, water-measuring rods, and current meters (if they are moved vertically in the flow). Under laboratory conditions the kinematographic method is used for measuring a velocity field with a visualization of the flow by means of hydrokinematic indicators.

The discharge of liquid is determined by various means, which depend mainly on the type of movement of the liquid (with or without head), as well as on the amount of the discharge. The most precise methods are by weight and volume, but they are suitable for determining only small-scale discharges. Diaphragms, Venturis, and flow meters are used to measure the discharge of pressured flows. For river flows, the method used most frequently is based on measurement of the local velocities and depths, from which the discharge may be calculated. In water currents with a high degree of turbulence the use of the mixture method, which consists in the introduction of an indicator solution into the flow and the measurement of its concentration on a range with a complete mix, is appropriate. Water-measuring installations such as spillways, water-measuring chutes, man-made control cuts, and measuring nozzles are set up in small water currents. Automatic measuring devices are used in irrigation. The hydraulic-engineering installations themselves are utilized to determine the amount of discharges—for example, discharges at hydroelectric power plants may be ascertained according to the operational characteristics of the turbines.

The quantity of all alluvium transported by the flow is measured by bathometers. The concentration of pulp (hydraulic fluid) may be measured by a gamma-ray density meter. The runoff of water (that is, the volume of water that flows during a day, month, or year) is recorded by water meters in a water-supply system and by runoff recorders in irrigation and river hydrology (when there is a stable relationship between discharges and levels). To determine the runoff of a river, the levels are measured each day and, according to the correlation established between discharge and level, the runoff is calculated for any given time interval.