Contents of work on river basin water level monitoring using remote sensing technology in Vietnam

Contents of work on river basin water level monitoring using remote sensing technology in Vietnam (Internet image)

On October 30, 2023, the Minister of Natural Resources and Environment issued Circular 16/2023/TT-BTNMT regulating techniques for monitoring water levels in river basins using remote sensing technology in Vietnam.

Contents of work on river basin water level monitoring using remote sensing technology in Vietnam

(1) Collect and enter data

- Satellite altimeter data is collected at global data storage, processing, and supply centers.

- The product type of I/GDR data used in river basin water level monitoring is selected to suit applications in continental hydrological research.

- Remote sensing data collected has an acquisition time compatible with the acquisition time of satellite altimeter data and is corrected for influence due to the Earth's curvature, receiver errors, and atmospheric interference. It is referenced to the WGS-84 Global Coordinate System and Reference System, Geographic projection, and point position errors are corrected using physical models, image control points, and digital elevation models.

- Documents related to the natural characteristics, hydro-climate, and operating regime of water reservoirs in the river basin (if any).

- Satellite altimetry data is collected in a compatible way with the reading and data processing software of popular altimeter satellites and stored in NetCDF format.

(2) Determine the coordinates of the satellite altitude measurement

- Information about the coordinates of satellite altitude measurements is obtained from high-frequency latitude and longitude data fields in data records; Measured values within the research scope are processed to reduce calculation time and volume.

- Coordinates of high-frequency satellite elevation measurements are exported as a data file in ASCII format to facilitate data display in a graphical environment.

(3) Determine the exact geographical coordinates of the virtual station

- The virtual station location is limited by a rectangular window with coordinates determined by the longitude and latitude of the upper left corner and lower right corner, displayed as a decimal number, to ensure all measurements on the water surface at the intersection are contained and limit measurements on the land surface. The virtual station position is calculated from the average longitude and latitude of all satellite elevations located above the water surface.

- Determining the window of satellite altitude measurements is done simultaneously with displaying measurement points along the satellite orbital track in all data cycles on the remote sensing data base, which have been referenced to the WGS-84 Global Coordinate System and Reference System. This is carried out at locations where the width of the river in the direction of the satellite orbit has a minimum size of 500 m for data from the traditional generation of radar altimeter satellites with low spatial resolution and 100 m for data from the generation of altimeter satellites applying synthetic aperture radar technology with high spatial resolution.

- The parameters of the satellite altimeter window that must be collected include: River/Lake, Hydrological station, Satellite, Satellite orbit, Virtual measuring station (Code, Longitude, Latitude), River/lake width, Distance to hydrological station/Dam (if any), shown according to the form in the Appendix attached to this Circular, data file format is *.docx (Microsoft Word Document).

(4) Calculate water level height

The water level height is determined by the difference between the satellite orbit altitude (Alt) and the satellite altimeter distance measurement value R and various correction numbers (including: time delay when radar signal pulses pass through the atmospheric environment and the influence of Earth's tides). Apply the formula below:

WSH = Alt - R + [DTC + WTC + IC + Ts ] - GC

In there:

- DTC is the refractive correction number in the dry troposphere

- WTC is the refraction correction number in the wet troposphere

- IC is the ionospheric refraction correction number

- Ts is the correction number due to the tidal influence of the solid Earth

- GC is the Geo-oir correction number

The atmospheric refraction correction number used is the standard model correction number found in the I/GDR data record. The water level elevation after calculation will be referenced to the EGM 2008 global geo-atmosphere.

(5) Export data to ASCII format

The results of calculating the water level height of the river basin are exported as a data file in ASCII format, in decimal form, with up to 4 numbers after the comma.

(6) Extract measurement values according to water surface objects

- Extraction of measurement values according to water surface objects at the intersection positions of the satellite orbital track with the river surface is performed on the basis of satellite data and vectorized with a polygon as the boundary. At that time, satellite measurements located at the limits of the polygon are used to calculate the water level elevation in the river basin.

- For rivers with medium and small widths (≤ 500 m), the river bank can be expanded to create a buffer zone that satisfies the conditions so that for each passing satellite orbit, at least 2–3 points in the polygon can be obtained to calculate the average water level height.

(7) Calculate the average water level with accuracy.