Most of us understand "water flow" to mean the speed or rate at which water is moving. However, hydrologists and other scientists that study the movements of water refer to this phenomenon as "current", since "flow" actually means not only the rate but also the amount, or volume, of water moving past a point. Are you ready for another term? Discharge. "Discharge" and "flow" are similar in that both refer to the rate as well as the volume of water moving past a point. There are times when one of these terms is better to use than the other, but for the purpose of our discussion on "water flow", we can use them interchangeably. Flow (or discharge) is reported in either cubic feet per second (cfs) or cubic meters per second (cms).
Measuring water flow began at the turn of the 19th Century, largely in response to the demand for water as large populations centers developed around streams. As in current times, these communities were using, and over-using, vast amounts of water for drinking, industry, transportation and waste disposal. The first federally-funded gaging station (flow monitoring station) was established on the Rio Grande in 1889, and the first flow gaging network (seven stations) was established in Kansas in 1895. Since this time, hundreds of gaging stations have been established by federal, state, county and municipal agencies, and by private organizations for purposes of water resource planning, flood control, irrigation, environmental restoration and navigation. In fact, stream gaging has become the backbone of virtually every water resource and water quality management program worldwide.
Discharge measurements are made using a variety of methods and technologies. Handheld sensors can be used to measure flow while wading across streams between specific points on the banks. In many watersheds, permanent stations are constructed on stream banks or over streams such that sensors can be deployed permanently and discharge can be measured and monitored as often as desired, including via remote sensing methods. Scientists refer to this continuous monitoring of flow as "gaging". No, that is not a misspelling; "gaging" is a mode of flow measurement, and the devices used to make those measurements are referred to as gauges.
Although methods, techniques, and technologies vary, all gaging first involves developing a clear understanding of the physical features of the stream or canal such as its width, water depth, the amount and degree of meandering, the slope and "roughness" (takes into account the composition of the bottom and how it affects water movement) of the channel, locations of log jams or dams, and the location of confluences with other waterbodies. If any historic data exist, these are studied to learn more about important factors such as temperature extremes, floods and drought, all of which are useful for determining where to place a gaging station and how it should be equipped.
Flow (or discharge) at one point in a stream can be calculated using the following formula:
Flow = (rate) x (channel width x water depth)
Comparing discharge measurements from season-to-season, year-to-year, provides valuable insight regarding water quantity trends in streams and their watersheds. When combined with water quality and land use data, for example, scientists are able to determine the positive and negative impacts of various activities in the watershed.
Flow data are most valuable when collected over long periods of time and at the same locations, and when analyzed with extensive knowledge of land use and meteorological conditions. Accurate representation of flow regimes can only be accomplished with frequent measurements, over long periods of time, using accurate, well calibrated gaging instruments.