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Exploring the secrets of topography: What is the traditional method of manually demarcating watersheds?
Catchment delineation is the process of determining the boundaries of a catchment area, also known as a detention area, drainage basin or river basin. In several fields, including environmental science, engineering and management, it is an important step in studying problems such as flooding, aquatic habitats or water pollution. This activity is usually performed by geographers, scientists, and engineers.
Historically, catchment area delineation relied mainly on manual work on paper topographic maps, supplemented by field surveys. In the 1980s, with the development of computers and electronic data, automated watershed delineation methods began to appear and became widely used.
Computerized catchment delineation methods use digital elevation models (DEMs), which are data sets that represent the height of the Earth's surface.
Today, watershed delineation is usually done using specialized hydrological modeling software, such as WMS, geographic information system software (such as ArcGIS or QGIS), or using programming languages such as Python or R. As the basic geographical unit of hydrology, a catchment involves the movement, distribution and management of water on the Earth.
Manual delineation of catchment areas
The traditional method of calculating catchment boundaries is to draw them manually on paper topographic maps, sometimes assisted by the use of transparent films. You can then use a rangefinder to make an area estimate, or the results can be digitized for use with mapping software. Even in the electronic age, the skill of manual catchment delineation is still very useful because it can check the correctness of catchments generated using software.
According to the U.S. Geological Survey, the steps to manually delineate a watershed include: identifying watershed outlet points on a map, imagining or drawing surface water lines flowing to lower areas, marking the locations of high relief points, identifying points of flow to watersheds, and connecting these points to delineate catchment areas.
A few basic rules for delineating boundaries include: catchment boundaries should be perpendicular to contour lines as they cross them, and should not cross rivers or streams except at their exit. Catchment boundaries should follow ridge lines and connect high points.
The disadvantage of manual delineation is that it is subject to personal judgment and there is a certain error rate. The Illinois Environmental Protection Agency states, "Delimiting watersheds is an inexact science. Even two experts may arrive at slightly different boundaries." For smaller watersheds, especially when When accurate results are critical, field trips may be necessary to discover features not shown on the map.
Automated water catchment area delineation
Using computer software to delineate watersheds is often faster and more consistent than manual methods because it eliminates analyst subjectivity. Automated watershed delineation methods have been popular since the 1980s and are widely used in the scientific and engineering communities.
Researchers are even using computer methods to delineate water catchment areas on Mars.
Automated catchment delineation methods use digital elevation models (DEMs), typically using a "steepest slope" approach to calculate flow direction. This process is usually performed on a rectangular grid, as is the case with most contemporary algorithms. To obtain more accurate results, the data needs to be processed and "conditioned" so that realistic hydrological results can be produced.
In the process of delineating water catchment areas based on DEMs, the algorithm will consider low-lying areas in the terrain, flow paths and other factors, and then determine the direction of the water flow, which involves a variety of calculation methods. The more commonly used D8 algorithm has limitations. Water flows in eight directions, which may lead to unrealistic flow patterns.
Software used for watershed delineation
Some of the earliest watershed delineation software were written in FORTRAN, such as CATCH and DEDNM. Today, many geographic information system software (GIS) packages, such as ArcGIS, QGIS and GRASS GIS, include watershed delineation tools. Additionally, stand-alone catchment delineation applications such as TauDEM are also available.
Some software can be used for hydrological simulation and has the function of delineating watersheds. These include WMS (hydrological software), SWAT model and BASINS, etc.
Other available datasets and resources
There are many vector data sets representing watersheds that can be displayed and analyzed with GIS or other software. These datasets divide the entire land surface into "subcatchments" or "unit catchments". Individual unit catchments can be combined to produce larger catchments, and flow paths and connections can be determined through network analysis.
With the continuous advancement of science and technology, the technology and methods of watershed delineation are still developing. Those more precise data and new algorithms could further improve delineation accuracy. When considering the impact of environmental changes and human activities, how does the delineation of watersheds affect our lives and ecology?
