As is known to all, China is a large agricultural country, agricultural water conservancy project has been the focus of the country's development of the object. The development of agriculture can not be separated from irrigation water, so it can not be measured by water conservancy. At present our country uses the most advanced measuring instrument is the ultrasonic flowmeter, uses the velocity area survey method: the measurement method is uses the supersonic flow meter according to the current velocity of the channel to multiply the channel cross section area, the water flow velocity is generally measured by the ultrasonic flow meter in the specific pipe. There are three methods for measuring the method. One is the velocity distribution method; the two is the 0.6H method; the three is the 0.2H ~ 0.8H method. Velocity distribution law is using ultrasonic flowmeter in surface drains and channels between the bottom of the choice different points to determine the velocity, then the measured vertical water velocity curve, according to the definition of velocity curve area divided by the depth, flow to calculate the average velocity; 0.6H measurement is below the surface 0.6H deep single point velocity, and assuming that value is the average of all speeds; 0.2h ~ 0.8h measurement is surface water under 0.2h and 0.8h depth two-point velocity, the two readings of the average value can be estimated value is the average flow velocity.

These three methods are used to measure the average velocity of the flow in a specific area, and the measurement results of different measurement points may be different. European countries require at least 5 points in the measurement, while the United States believes that 2 points are sufficient. The depth of the sounding is determined by the width of the channel. The number of times in Europe is 5 ~ 15, and the United States is from 8 to 18. The measurement error is determined by the number of times and the measurement time at each point, and the average error of the velocity area method can be less than 6%.

Water flow measurement can also be used to use electromagnetic flowmeter. Ultrasonic technology is to calculate the velocity of ultrasonic transmission across the channel, the transmitter and receiver are placed on both sides of the channel, so that the wave path and the direction of the flow between 45 degrees to 60 degrees. This geometry makes the time difference between the sound pulses in the upstream direction and the downstream direction. The use of electromagnetic flow meter is the use of electric current in the artificial magnetic field, and then measure the two sides of the flow across the synthetic measurement results. Electromagnetic flow meter is generally made up of the excitation coil and a pair of electrodes.

Although ultrasonic flowmeter and electromagnetic flowmeter can be applied to the measurement of the flow in the field of irrigation water, but the practical application of the use of the environment and other conditions, most of the use of ultrasonic flowmeter.