The need to quantify the flow of liquids has been recognized since the the beginning of civilization. The first efforts were probably directed toward survival during floods and to waterborne transportation.

As civilization became more advanced, the demands for water supply, irrigation, navigation, and water power all contributed to the development of techniques to measure liquid level, flow rates, and quantities. It is known that the ancient Egyptians and Babylonians used some form of water accounting as a basis for levies to individual land holders for their usage of water from extensive irrigation systems.

The River Nile of Egypt has probably been studied longer than any other river in the world. The crop yields in the lower Nile Valley are dependent upon the annual flooding of the river, and thus the annual yields are a function of the river’s level. Because of this, taxes were based on the maximum level of the river. Mention of the annual rises of the Nile dates back between 3000 and 3500 B.C., and known flood marks extend as far back as 1800 B.C. More than 3000 years ago, the Egyptian Pharaoh Menes developed a flood control system for the Nile River.

This system included at least 20 recording stations along the Nile. These stations used a crude form of a staff gauge to measure the level of the river. These levels were recorded daily, and compared with previous years’ records to predict the future levels of the river.

One of the earliest and most complete records of an attempt to measure water flow volume is that of Sextus Julius Frontinus, who was the Water Commissioner of Rome in 52 A.D. He attempted to determine the quantity of water delivered to each user in the Roman system by measuring the cross-sectional area of the spouts through which the water was discharged. Since Frontinus ignored the velocity of the flow, his efforts were not entirely successful. However, it presents an interesting record of an early flow measurement system.

The techniques of flow measurement have advanced through the centuries. Much of the theoretical background for the science of hydraulics, which is the basis for modern flow measurement, was developed in the 17th and 18th centuries by researchers such as Torricelli, Pitot, Woltman, and Venturi. Advances in the art and science of liquid flow measurement continued into the 19th and 20th centuries, paralleling the general advancement of technology. However, many methods of open channel flow measurement are simply sophisticated adaptations of the level measurements practiced by the Egyptians on the Nile over 4000 years ago. ​

The rapid growth of urban areas throughout the world and advances in technology and industry to meet society’s ever increasing demands for more goods, energy, etc. have greatly increased the potential for environmental pollution. They have also contributed to an increasing awareness of and concern for the environment. Population density and advanced technology continue to place increasing demands on society to control the quality and conserve the supply of water. 

Increasingly strict legislation and continuing public interest in conservation and environmental matters have emphasized the importance of flow measurement. Uniform and reliable measurement data are needed to identify the resource levels and quality of bodies of water, to determine the results of conservation and quality control efforts, and to enforce water conservation and quality regulatory requirements. 

The majority of recent interest in flow measurement has centered on water quality regulatory requirements. For example, in the United States, federal law states that “…the purpose of self-monitoring and reporting effluent data is to permit federal and state regulating agencies to follow on a continuing basis, the discharger’s effluent quality trends as well as specific variation from established limitations.” 

Local agencies in the U.S. are required by the same legislation to establish a local surcharge on industrial waste to insure that these users pay their “fair share” of the cost of existing and new treatment facilities. Their “fair share” entails the measurement of both the quality and the quantity of industrial discharge. Thus, an economic value has been placed on industrial waste, and it is important for both industrial dischargers and municipalities to be able to measure and record flow data. 

There are variations of environmental monitoring regulations throughout the world. There are often layers from national to regional, and a number of local levels for regulatory law as well as enforcement agencies. The purpose of this handbook, however, is not to define the protocols of environmental flow monitoring, but to address the field in general. You should always consult your local regulatory agency regarding the standard practices and requirements for your specific monitoring location. 

Of course, flow measurement still is of great importance in more traditional areas such as irrigation, stream measurement, and sewage treatment plants. It also has other applications, for example, in storm and combined sewer flow studies, in sedimentation work, in runoff studies, and inflow and infiltration isolation. As Kirkpatrick and Shelley state: “Measurements of quantity of flow, usually in conjunction with sampling for flow quality, are essential to nearly all aspects of water pollution control. Research, planning, design, operation and maintenance, and enforcement of pertinent laws—all are activities which rely on flow measurement for their effective conduct.” 

Thus in the context of modern society, there is an ever increasing need for simple, accurate, and reliable methods of flow measurement. These needs are usually dictated by legislation, but in a larger sense are dictated by society’s desire to reverse the trend of increasing environmental pollution, and to ensure a clean, livable planet for this and future generations. ​

​Frequently Asked Questions

What is the unit of measurement fo​r the flow of water?

Flowing water is measured in units of volume per unit of time – gallons per minute (gpm), cubic feet per second (cfs), acre-inches per hour, or acre-feet per day.

How do you quantify the flow of wat​​er?

To calculate the water flow (in m3) multiply the average water velocity (in m/s) by the average width (in m) and by the average depth (in m). 

How do you measure the flow rate of wa​ter in a pipe?

A simple formula is: Q = V/t, where: Q = Flow rate (typically measured in liters per minute or gallons per minute). V = Volume of fluid (in liters or gallons). T = Time (in seconds).​​​​​​​​​