Watershed delineation

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Watershed delineation is the process of identifying the boundary of a watershed, also referred to as a catchment, drainage basin, or river basin. It is an important step in many areas of environmental science, engineering, and management, for example to study flooding, aquatic habitat, or water pollution.

The activity of watershed delineation is typically performed by geographers, scientists, and engineers. Historically, watershed delineation was done by hand on paper topographic maps, sometimes supplemented with field research. In the 1980s, automated methods were developed for watershed delineation with computers and electronic data, and these are now in widespread use.

Map of the watershed of the Lost Creek Reservoir in Morgan County, Utah, United States, showing the major streams in blue and the watershed boundary in red. Lost Creek Reservoir Watershed.jpg
Map of the watershed of the Lost Creek Reservoir in Morgan County, Utah, United States, showing the major streams in blue and the watershed boundary in red.

Computerized methods for watershed delineation use digital elevation models (DEMs), datasets that represent the height of the Earth's land surface. Computerized watershed delineation may be done using specialized hydrologic modeling software such as WMS, geographic information system software like ArcGIS or QGIS, or with programming languages like Python or R.

Watersheds are a fundamental geographic unit in hydrology, the science concerned with the movement, distribution, and management of water on Earth. Delineating watersheds may be considered an application of hydrography, the branch of applied sciences which deals with the measurement and description of the physical features of oceans, seas, coastal areas, lakes and rivers. It is also related to geomorphometry, the quantitative science of analyzing land surfaces. Watershed delineation continues to be an active area of research, with scientists and programmers developing new algorithms and methods, and making use of increasingly high-resolution data from aerial or satellite remote sensing.

Manual watershed delineation

The conventional method of finding a watershed boundary is to draw it by hand on a paper topographic map, or on a transparent overlay. The watershed area can then be estimated using a planimeter, by overlaying graph paper and counting grid cells, or the result can be digitized for use with mapping software. The same process can be done on a computer, sketching the watershed boundary (with a mouse or stylus) over a digital copy of a topographic map. [1] This is referred to as "heads up digitizing" or "on-screen digitizing." [2]

Example of an idealized watershed boundary, drawn on a topographic map with elevation contours. Any precipitation that falls inside the watershed boundary will flow toward the watershed outlet at the bottom. Watershed nrcs.jpg
Example of an idealized watershed boundary, drawn on a topographic map with elevation contours. Any precipitation that falls inside the watershed boundary will flow toward the watershed outlet at the bottom.

For "manual" watershed delination, one must know how to read and interpret a topographic map, for example to identify ridges, valleys, and the direction of steepest slope. [3] Even in the computer era, manual watershed delineation is still a useful skill, in order to check whether watersheds generated with software are correct. [1]

Instructions for manual watershed delineation can be found in some textbooks in geography or environmental management, in government pamphlets, [4] [5] or in online video tutorials. [6]

According to the US Geological Survey, there are 5 steps to manual watershed delineation: [6]

  1. Find the point of interest along a stream on the map. This is the "watershed outlet" or "pour point."
  2. Imagine or draw surface water flow lines that point downhill perpendicular to the topographic contours (this is the steepest direction).
  3. Mark the location of topographical high points (peaks) around the stream.
  4. Mark the points along contours that divide flows towards or away from the stream (ridges).
  5. Connect the dots to delineate the watershed.

General Rules:

One disadvantage to manual watershed delineation is that it is subject to errors and the individual judgment of the analyst. The Illinois Environmental Protection Agency wrote, "bear in mind that delineating a watershed is an inexact science. Any two people, even if both are experts, will come up with slightly different boundaries." [5]

Especially for smaller watersheds and when accurate results are important, field reconnaissance may be needed to find features that are not shown on maps. "Going out into the field allows you to identify human alterations, such as road ditches, storm sewers and culverts that could change the direction of waters flow and thus change the watershed boundaries." [5]

Automated or computerized watershed delineation

Using computer software to delineate watersheds can be much faster than manual methods. It may also be more consistent, as it removes analyst's subjectivity. Automatic methods of watershed delineation have been in use since the 1980s, and are now in widespread use in the science and engineering communities. Researchers have even used computer methods to delineate watersheds on Mars. [7] [8]

Automated watershed delineation methods use digital data of the earth's elevation, a Digital Elevation Model, or DEM. Typically, algorithms use the method of "steepest slope" to calculate the flow direction from a grid cell (or pixel) to one of its neighbors. [9]

It is possible to use DEMs in different formats for watershed delineation, such as a Triangular Irregular Network (TIN), [10] or Hexagonal tiling [11] however most contemporary algorithms make use of a regular rectangular grid. [12] In the 1980s and 1990s, digital elevation models were often obtained by scanning and digitizing the contours on paper topographic maps, which were then converted to a TIN or a gridded DEM. [13] More recently, the DEM is obtained by aerial or satellite remote sensing, using stereophotogrammetry, lidar, or radar. [14]

To use a rectangular grid DEM for watershed delineation, it must first be processed or "conditioned" in order to return realistic results. [9] The result is sometimes referred to as a "hydro-enforced" DEM or a "HydroDEM." Most of the software packages listed below can perform these functions on a "raw" DEM, or analysts can download hydrologically-conditioned DEMs such as the near-global HydroSHEDS, [15] MERIT-Hydro, [16] or EDNA [17] for the continental United States. The usual steps for hydrologic conditioning of a DEM are:

  1. Fill sinks.
  2. "Burn in" the stream channels.
  3. Calculate flow direction.
  4. Calculate flow accumulation.
3D rendering of a hydrologically-conditioned digital elevation model. This example from the U.S. Geological Survey shows "walling" or "fencing" using data from the Watershed Boundary Dataset (WBD) and "burning" of streams from the National Hydrography Dataset (NHDPlus). DEM conditioning example.jpg
3D rendering of a hydrologically-conditioned digital elevation model. This example from the U.S. Geological Survey shows “walling” or “fencing” using data from the Watershed Boundary Dataset (WBD) and “burning” of streams from the National Hydrography Dataset (NHDPlus).

Additionally, some methods allow for "fencing ridgelines" and burning in flow pathways through lakes. [18] Some methods also enforce a small slope onto flat areas so that flow will continue to move toward the outlet. [19] The step of "burning in" stream channels involves artificially deepening the channel, by subtracting a large elevation value from pixels that represent the channel. This ensures that once flow has entered the channel, it will stay there rather than jumping out and flowing overland or into another channel. Some algorithms infer the location of channels automatically from the DEM. Better results are usually obtained by burning in mapped stream channels, or channels derived from satellite or aerial imagery. [20]

There are several different algorithms available for calculating flow direction from a DEM. The first method, introduced by Australian geographers O'Callaghan and Mark in 1984, is referred to as D8. [12] Water flows from a pixel to one of 8 possible directions to a neighboring cell (including diagonally), based on the direction of steepest slope. There are disadvantages to this method as water flow is limited to 8 directions, separated by 45°, which may result in unrealistic flow patterns. Also, because all of the flow is routed in one direction, the D8 method is unable to model situations where the flow diverges, such as on convex hillsides, in a river delta, or in branched or braided rivers. Alternative algorithms have been proposed and implemented to overcome this limitation, such as D∞. [21] Nevertheless, the D8 algorithm remains in widespread use, and has been used to create important datasets such as HydroBasins [15] and MERIT-Basins. [16]

Computerized watershed delineation is not always correct. Some errors stem from incorrectly placing the watershed outlet on the digital river network, or "snapping the pour point." [22] Another class of errors stems from inaccuracies in the digital terrain data, or where its resolution is too coarse to capture flow pathways. [2] In general, DEMs with higher spatial resolution can more realistically describe topography of the land surface and flow direction. However, there is a tradeoff, as a finer grid with more pixels increases computing time. [16] Nevertheless, even high-resolution data may not adequately capture flow pathways in complex environments like cities and suburbs, where flow is directed by curbs, culverts, and storm drains. [23] Finally, some errors can result from the algorithm or the choice of parameters. [24]

Because errors are common, some authorities insist that the results of automated delineation must be carefully checked. The US Geological Survey's standards for the US Watershed Boundary Dataset allow the use of software "to generate intermediate or “draft” boundary lines," which then must be verified by the analyst by overlaying them on a computer display over basemaps (scanned topographic maps, aerial photographs) to verify their accuracy. [1]

Software for watershed delineation

Some of the first watershed delineation software was written in FORTRAN, such as CATCH [25] and DEDNM. [19] Watershed delineation tools are a part of several Geographic Information System software packages such as ArcGIS, QGIS, and GRASS GIS. There are standalone programs for watershed delineation such as TauDEM. Watershed delineation tools are also incorporated into some hydrologic modeling software packages.

Software developers have also published libraries or modules in several languages (see list below). Many of these packages are free and open source, which means they can be expanded or adapted by those willing and able to write or modify code. Finally, there are web applications for delineating watersheds. Some of these web apps have extra features for science and engineering like calculating flow statistics or watershed land cover types (e.g.: StreamStats, Model My Watershed).

Standalone watershed delineation software

Hydrologic Modeling Software with Watershed Delineation Capability

GIS-based software

Web Applications

Vector datasets of pre-delineated watersheds

There are a number of vector datasets representing watersheds as polygons that can be displayed and analyzed with GIS or other software. In these datasets, the entire land surface is divided into "subwatersheds" or "unit catchments." Individual unit watersheds can be combined or merged to find larger watersheds. The unit catchments have linked hydrological code data or similar metadata to create a flow network, so flow pathways and connections can be determined via network analysis. [34]

Watershed Boundary Dataset Subregions Map for the United States, created by the US Geological Survey HUC subregions.png
Watershed Boundary Dataset Subregions Map for the United States, created by the US Geological Survey

This list is non-exhaustive, as many organizations and territories have produced their own watershed map data and have published via the web. Notable datasets include:

Related Research Articles

<span class="mw-page-title-main">Geographic information system</span> System to capture, manage and present geographic data

A geographic information system (GIS) consists of integrated computer hardware and software that store, manage, analyze, edit, output, and visualize geographic data. Much of this often happens within a spatial database, however, this is not essential to meet the definition of a GIS. In a broader sense, one may consider such a system also to include human users and support staff, procedures and workflows, the body of knowledge of relevant concepts and methods, and institutional organizations.

<span class="mw-page-title-main">Digital elevation model</span> 3D computer-generated imagery and measurements of terrain

A digital elevation model (DEM) or digital surface model (DSM) is a 3D computer graphics representation of elevation data to represent terrain or overlaying objects, commonly of a planet, moon, or asteroid. A "global DEM" refers to a discrete global grid. DEMs are used often in geographic information systems (GIS), and are the most common basis for digitally produced relief maps. A digital terrain model (DTM) represents specifically the ground surface while DEM and DSM may represent tree top canopy or building roofs.

<span class="mw-page-title-main">Topography</span> Study of the forms of land surfaces

Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps.

<span class="mw-page-title-main">Drainage basin</span> Land area where water converges to a common outlet

A drainage basin is an area of land where all flowing surface water converges to a single point, such as a river mouth, or flows into another body of water, such as a lake or ocean. A basin is separated from adjacent basins by a perimeter, the drainage divide, made up of a succession of elevated features, such as ridges and hills. A basin may consist of smaller basins that merge at river confluences, forming a hierarchical pattern.

<span class="mw-page-title-main">Terrain</span> Vertical and horizontal dimension and shape of land surface

Terrain or relief involves the vertical and horizontal dimensions of land surface. The term bathymetry is used to describe underwater relief, while hypsometry studies terrain relative to sea level. The Latin word terra means "earth."

<span class="mw-page-title-main">Elevation</span> Height of a geographic location above a fixed reference point

The elevation of a geographic location is its height above or below a fixed reference point, most commonly a reference geoid, a mathematical model of the Earth's sea level as an equipotential gravitational surface . The term elevation is mainly used when referring to points on the Earth's surface, while altitude or geopotential height is used for points above the surface, such as an aircraft in flight or a spacecraft in orbit, and depth is used for points below the surface.

<span class="mw-page-title-main">Shuttle Radar Topography Mission</span> Project to create a digital topographic database of Earth

The Shuttle Radar Topography Mission (SRTM) is an international research effort that obtained digital elevation models on a near-global scale from 56°S to 60°N, to generate the most complete high-resolution digital topographic database of Earth prior to the release of the ASTER GDEM in 2009. SRTM consisted of a specially modified radar system that flew on board the Space Shuttle Endeavour during the 11-day STS-99 mission in February 2000. The radar system was based on the older Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR), previously used on the Shuttle in 1994. To acquire topographic data, the SRTM payload was outfitted with two radar antennas. One antenna was located in the Shuttle's payload bay, the other – a critical change from the SIR-C/X-SAR, allowing single-pass interferometry – on the end of a 60-meter (200-foot) mast that extended from the payload bay once the Shuttle was in space. The technique employed is known as interferometric synthetic aperture radar. Intermap Technologies was the prime contractor for processing the interferometric synthetic aperture radar data.

Geomorphometry, or geomorphometrics, is the science and practice of measuring the characteristics of terrain, the shape of the surface of the Earth, and the effects of this surface form on human and natural geography. It gathers various mathematical, statistical and image processing techniques that can be used to quantify morphological, hydrological, ecological and other aspects of a land surface. Common synonyms for geomorphometry are geomorphological analysis, terrain morphometry, terrain analysis, and land surface analysis. Geomorphometrics is the discipline based on the computational measures of the geometry, topography and shape of the Earth's horizons, and their temporal change. This is a major component of geographic information systems (GIS) and other software tools for spatial analysis.

<span class="mw-page-title-main">MapWindow GIS</span> Open-source GIS desktop application

MapWindow GIS is a lightweight open-source GIS (mapping) desktop application and set of programmable mapping components.

Geographic information systems (GISs) have become a useful and important tool in the field of hydrology to study and manage Earth's water resources. Climate change and greater demands on water resources require a more knowledgeable disposition of arguably one of our most vital resources. Because water in its occurrence varies spatially and temporally throughout the hydrologic cycle, its study using GIS is especially practical. Whereas previous GIS systems were mostly static in their geospatial representation of hydrologic features, GIS platforms are becoming increasingly dynamic, narrowing the gap between historical data and current hydrologic reality.

The National Elevation Dataset (NED) consists of high precision topography or ground surface elevation data for the United States. It was maintained by the USGS and all the data is in the public domain. Since the 3D Elevation Program came online, the NED was subsumed into The National Map as one of its layers of information.

The stream order or waterbody order is a positive whole number used in geomorphology and hydrology to indicate the level of branching in a river system.

GSSHA is a two-dimensional, physically based watershed model developed by the Engineer Research and Development Center of the United States Army Corps of Engineers. It simulates surface water and groundwater hydrology, erosion and sediment transport. The GSSHA model is used for hydraulic engineering and research, and is on the Federal Emergency Management Agency (FEMA) list of hydrologic models accepted for use in the national flood insurance program for flood hydrograph estimation. Input is best prepared by the Watershed Modeling System interface, which effectively links the model with geographic information systems (GIS).

Green Kenue is an advanced data preparation, analysis, and visualization tool for hydrologic modellers. It is a Windows/OpenGL-based graphical user interface, integrating environmental databases and geo-spatial data with model input and results data. Green Kenue provides complete pre- and post-processing for the WATFLOOD and HBV-EC hydrologic models. Also included is a 1D "reach scale" unsteady hydrodynamic flow solver, Gen1D.

<span class="mw-page-title-main">United States Geological Survey</span> Scientific agency of the United States government

The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the United States government whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March 3, 1879, to study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The agency also makes maps of extraterrestrial planets and moons based on data from U.S. space probes.

<span class="mw-page-title-main">Birch Creek (Umatilla River tributary)</span> River in Oregon, United States of America

Birch Creek is a 16-mile (26 km) tributary of the Umatilla River in eastern Oregon in the United States. It rises at the confluence of East and West Birch creeks south of Pilot Rock, Oregon, at the base of the Blue Mountains and flows north, slightly west of the city of Pendleton. It enters the Umatilla River about 49 miles (79 km) from the larger stream's confluence with the Columbia River.

<span class="mw-page-title-main">Vflo</span>

Vflo is a commercially available, physics-based distributed hydrologic model generated by Vieux & Associates, Inc. Vflo uses radar rainfall data for hydrologic input to simulate distributed runoff. Vflo employs GIS maps for parameterization via a desktop interface. The model is suited for distributed hydrologic forecasting in post-analysis and in continuous operations. Vflo output is in the form of hydrographs at selected drainage network grids, as well as distributed runoff maps covering the watershed. Model applications include civil infrastructure operations and maintenance, stormwater prediction and emergency management, continuous and short-term surface water runoff, recharge estimation, soil moisture monitoring, land use planning, water quality monitoring, and water resources management.

DPHM-RS is a semi-distributed hydrologic model developed at University of Alberta, Canada.

<span class="mw-page-title-main">Hydrologic unit system (United States)</span> Watershed tracking system

For the use of hydrologists, ecologists, and water-resource managers in the study of surface water flows in the United States, the United States Geological Survey created a hierarchical system of hydrologic units.

<span class="mw-page-title-main">Effects of lakes on floods in Canada</span>

The abundance of lakes in Canada is unique in the world, with nearly 900,000 lakes covering more than 10 hectares. This unique abundance is due to Canada’s glacial history, with the vast majority of the country covered by a massive ice sheet during the last ice age. Canadian lakes represent approximately 62% of the world's 1.42 million lakes. Lake levels influence many aspects of our lives, such as water resource management, and environmental sustainability. Water levels in lakes are highly susceptible to climatic fluctuations, which have a significant impact on both the volume and purity of available water resources, as well as the ecological health of the watershed. Accurate lake level predictions have therefore become critical for effective water resource management in an era of increasing climate variability and changing hydrological patterns. Indeed, water levels in lakes are highly susceptible to climatic fluctuations, which have a significant impact on both the volume and purity of available water resources, as well as the ecological health of the watershed. The expected increase in both the frequency and intensity of extreme weather events may threaten the natural quality of water, emphasising the critical need for well-planned strategies for managing water resources and maintaining water quality.

References

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  42. ↑ Amatulli, Giuseppe; Garcia Marquez, Jaime; Sethi, Tushar; Kiesel, Jens; Grigoropoulou, Afroditi; Üblacker, Maria M.; Shen, Longzhu Q.; Domisch, Sami (2022-10-17). "Hydrography90m: a new high-resolution global hydrographic dataset". Earth System Science Data. 14 (10): 4525–4550. Bibcode:2022ESSD...14.4525A. doi: 10.5194/essd-14-4525-2022 . ISSN   1866-3508 . Retrieved 2023-03-08.
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