How are the water level, velocity and discharge of rivers measured?

The water regime of large and small rivers on land is unstable Some flow all the year round, some are cut off in dry season, and some are often flooded in flood season. In order to make rational use of river resources, it is necessary to master the changing law of rivers. The change of river regime is mainly manifested in the rise and fall of water level, the speed of flow, the increase and decrease of flow, the amount of sediment, and the change of water temperature and ice condition of the river. Water level refers to the height of the river at a certain place and time. It takes a certain point as the water level datum (that is, the water level zero) to measure. Water level datum is generally divided into absolute datum and measuring point datum: the absolute datum is calculated according to the average sea level of an estuary, and the current river water level in China is based on Qingdao zero in the Yellow Sea; The base surface of the measuring point is for the convenience of on-site observation and calculation on the river course, and the zero point is usually taken as the place half a meter to one meter below the lowest low tide surface of the measuring point.

However, when applying this observation data, it must be converted into a unified absolute elevation according to the relationship between the datum plane and the absolute datum plane of the measuring point. The fluctuation of water level is generally observed at the observation point with a water gauge or a self-recording water level gauge. Water level observation is one of the most important projects in hydrology, and the calculation of a series of other hydrological elements is influenced by water level data. According to the water level records at different times, we can draw a water level curve of a river, from which we can clearly see the water level change at this point throughout the year: velocity refers to the distance that the water flows forward in unit time. The velocity is not uniform in the river reach, and the velocity of bottom water is small due to the friction of river bed. From the bottom of the water to the surface of the water, the speed increases, but the water surface is rubbed by air, and the speed decreases, and the maximum speed is slightly lower than the water surface. From the horizontal distribution, the velocity on both banks is the smallest, and the velocity in the center of the river is the largest. The longitudinal velocity is usually observed by a current meter (rotor or paddle).