Proposed features/Reservoir

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Deprecation of landuse=reservoir
Status: Proposed (under way)
Proposed by: ZeLonewolf
Tagging: water=reservoir
Applies to: area
Definition: An artificially-created or enlarged body of water created using a dam to store water for human use.
Rendered as: Base layer water.png
Drafted on: 2020-12-16
RFC start: 2020-12-20

Proposal

This proposes that:

Summary of Tagging Changes
Deprecated Tag Replacement
landuse=reservoir natural=water + water=reservoir
reservoir_type=water_storage None. This tag can be safely removed.
reservoir_type=sewage natural=water + water=wastewater

Definitions

Mappers should tag lakes, ponds, and reservoirs (which often have similar appearance on imagery) as natural=water. If they are able to further determine the specific type of waterbody, water=* can also be applied. Definitions for three of these values (reservoir, lake, and pond) are described below.

Reservoir

A reservoir is an artificially-created or enlarged body of water created using a dam to store water for human use. Dam, in this context, specifically refers to a man-made structure which allows for human control of the reservoir water level and downstream water flow. (See: Reservoir).

Reservoirs are be tagged with natural=water + water=reservoir.

Reservoirs include the following types:

  • Dammed valleys rely on the natural topography to provide most of the body of the reservoir. A dam is typically located at a narrow part of a valley downstream of a natural basin. Sometimes additional side dams are required to contain the reservoir. In hilly regions, reservoirs may be constructed by enlarging existing lakes.
  • Coastal reservoirs are fresh water storage reservoirs located on the sea coast near the river mouth to store the flood water of a river. Many coastal reservoirs were constructed in Asia and Europe.
  • Bank-side reservoirs, where water is pumped or siphoned from a river to store the water. Such reservoirs are usually formed partly by excavation and sometimes by building an encircling levee or embankment.

Reservoirs are used for multiple purposes:

  • Water supply. Many reservoirs are used to provide the raw water feed to a water treatment plant which delivers drinking water through water mains, or irrigation water used by agriculture.
  • Hydroelectricity. A reservoir generating hydroelectricity includes turbines connected to the retained water body by large-diameter pipes. These generating sets may be at the base of the dam or some distance away.
  • Controlling watersources. Reservoirs can be used to control water flow in downstream waterways:
    • Water may be released from an upland reservoir so that it can be used downstream for drinking, irrigation, etc.
    • Flood control and flow balancing reservoirs collect water at times of high flow, then release it slowly during the following weeks or months.
    • Water may be released from a reservoir to supplement white water conditions for white-water sports, or to encourage natural migration behaviours in fish
  • Recreation. Many reservoirs often allow some recreational uses, such as fishing and boating. Recreation is usually a secondary use of reservoirs that are used for hydroelectric generation or irrigation, however there are some cases of reservoirs which are purely recreational.
  • Navigation. It is common on large navigable rivers (for example, the Mississippi River in the USA) to create a series of subsequent reservoirs with dams and locks to allow for waterborne navigation. The dam in this case removes rapids and creates a level surface for boats. In cases where these dam/lock systems meaningfully widen the river into a larger named water body, this should be tagged as a reservoir.
  • Cooling. Also known as a cooling pond, a cooling reservoir is a man-made body of water primarily formed for the purpose of cooling heated water and/or to store and supply cooling water to a nearby power plant or industrial facility such as a petroleum refinery, pulp and paper mill, chemical plant, steel mill or smelter.

Additional clarification:

  • Service reservoirs are used for storage of potable drinking water close to the point of distribution. While these are called "reservoirs" in water systems jargon, these are usually not open bodies of water, but rather a storage tank, cistern, basin or a water tower, and thus should be tagged instead as man_made=storage_tank, water=basin/landuse=basin or man_made=water_tower as appropriate.
  • Bodies of water with dams that do not fit the description above should not be tagged as reservoirs, and should instead be tagged with water=lake or water=pond instead. This includes:
    • Bodies of water with with beaver dams
    • Ponds created for decorative or aesthetic landscaping purposes
    • Structures which were historically used for water-powered machinery, such as a Mill pond
  • Bodies of water created by a weir (waterway=weir) are not reservoirs, as these structures do not have flow control. A reservoir may include weirs for grade separation between sections of a reservoir, however the most downstream outlet must be a dam with flow control.

Lakes and Ponds

A lake or a pond is a naturally-occurring or artificial inland body of water. There is no precise difference between a lake or a pond. The definitions below are provided as guidelines for mappers to make the determination of which tag to use. Note that some bodies of water may exhibit aspects of both lists; local mappers should make a determination which definition is the best fit.

Lakes are tagged natural=water + water=lake. Lakes, in the context of OpenStreetMap:

  • Are not a reservoir (see definition above)
  • Not a basin (see landuse=basin, water=basin, and basin=*)
  • Are usually larger than 8-20 hectares in size
  • Often named with "lake" (or local language equivalent) in the name
  • Have areas of deep water that receive no sunlight, preventing plant growth (see Aphotic zone on Wikipedia)

Ponds are tagged natural=water + water=pond. Ponds, in the context of OpenStreetMap, are:

  • Not a reservoir (see definition above)
  • Not a basin (see landuse=basin, water=basin, and basin=*)
  • Lacking wave action at the shoreline
  • Are usually no more than 8-20 hectares in size
  • Often named with "pond" (or local language equivalent) in the name
  • Shallow enough for sunlight to reach the bottom or to allow rooted plants to grow throughout it (see Photic zone on Wikipedia)

Note that:

  • Some naturally-occurring lakes or ponds may have a regulating dam or weir at its outlet which does not meaningfully change the size of the water body, but rather was built to minimize seasonal variations in water level on the lake or outlet stream, or to provide hydroelectric power generation from an existing natural lake. These lakes and ponds are not reservoirs merely because of the presence of outflow regulating infrastructure. Flathead Lake in Montana USA is one such example of this; while the added hydroelectric dam raised the lake level by 3 meters, this only represented a small increase in the size of the existing natural lake.
  • Stream pools and plunge pools, which are bodies of water that naturally occur along the course of the waterway, are covered by the existing tag water=stream_pool, and are not considered ponds.
  • Because the distinction between a lake and a pond is imprecise, "vanity naming" sometimes occurs. For example, a real estate developer may construct a pond as part of a housing development but name it as lake for marketing purposes.
  • The question of how to tag a quarry lake is unresolved. A mailing discussion on this topic showed that there is disagreement as to whether quarry lakes should be tagged with water=lake or with some other key such as water=quarry_lake. Commenters noted that quarry lakes had unique characteristics that made them distinct from other types of lakes. However, other commenters noted the difficulty of determining the quarry origin of a lake, particularly for older or even ancient quarries. Because there does not appear to be a firm consensus either way, the question of how to tag these bodies of water is deliberately left unresolved by this proposal.

Meaning of "Deprecated"

Specifically, in this context the term deprecated means:

Background

The 2011 proposal Water details approved the new key water=*. Specifically, it proposed the combination of natural=water + water=reservoir as the replacement for landuse=reservoir. The purpose of that proposal included the following rationale:

  • "For areas tagged with natural=water it should be possible to mark which type of water body it is."
  • "[...] reservoirs should be tagged consistently with other water bodies, using natural=water."

Since water=* was a new key, it was recommended in that proposal that mappers tag both water=reservoir and landuse=reservoir in order for data consumer to begin adopting the new tagging. Over time, a debate emerged as to whether landuse=reservoir was deprecated or whether this 2011 proposal intended to create a fully parallel tagging scheme for reservoirs. This proposal ends that ambiguity by affirming the intent of the 2011 proposal by declaring that landuse=reservoir is deprecated.

Rationale

  1. A land tag should not be used to tag a water feature. Map features describes the landuse=* key as describing "the purpose for which an area of land is being used."
  2. Mappers should be able to tag visible water features with natural=water without having to determine in advance whether or not it is a reservoir. With landuse=reservoir, a mapper must first determine whether or not a body of water is a reservoir before deciding which tag to use.
  3. Mappers have preferred the use of water=reservoir over landuse=reservoir in recent years. Since 2016, usage of water=reservoir has increased by 241,066 while landuse=reservoir has decreased by 2,403. (data as of 12/19/20) This suggests that mappers are using water=reservoir for mapping new reservoirs, while slowly replacing existing usages of landuse=reservoir.
  4. Mappers have preferred since water=reservoir over landuse=reservoir since mid-2016, 2.5 years before the introduction of changes to iD editor presets in favor of the water=* tag. The iD preset change merely accelerated an existing mapper preference for water=* over alternative schemes.
  5. There is a need for formal definitions in order to clarify what exactly a water=reservoir is, and how it differs from water=lake and water=pond.
  6. Despite the lack of a clear distinction between lakes and ponds in the real world, mailing list discussions have indicated that there is community support for maintaining a distinction between them, even if that distinction involves a degree of subjective judgement on the part of mappers.

Tagging Analysis

Data from taginfo API as of 18 Feb 2021

This table does not include nodes. There are an additional 50,000 nodes tagged landuse=reservoir resulting from the 2008 GNIS import in the United States which have not yet been traced into areas.

Area Tag Comparison landuse=reservoir vs water=reservoir (nodes are ignored)
Year landuse=reservoir Comparison water=reservoir
# Areas Change Preference Change Usage Ratio Change # Areas
2021 322,251 Red Arrow Down.svg7,580 25,728 TriangleArrow-Right.svg 18,386 TriangleArrow-Right.svg Green Arrow Up.svg10,806 347,979
2020 329,831 Red Arrow Down.svg17,597 7,342 TriangleArrow-Right.svg 104,053 TriangleArrow-Right.svg Green Arrow Up.svg86,456 337,173
2019 347,428 Red Arrow Down.svg14,851 TriangleArrow-Left.svg 96,711 83,022 TriangleArrow-Right.svg Green Arrow Up.svg68,171 250,717
2018 362,279 Green Arrow Up.svg14,575 TriangleArrow-Left.svg 179,733 26,685 TriangleArrow-Right.svg 1:2.83 Green Arrow Up.svg41,260 182,546
2017 347,704 Green Arrow Up.svg14,655 TriangleArrow-Left.svg 206,418 34,614 TriangleArrow-Right.svg 1:3.36 Green Arrow Up.svg49,269 141,286
2016 333,049 Green Arrow Up.svg49,463 TriangleArrow-Left.svg 241,032 TriangleArrow-Left.svg 18,394 1.59:1 Green Arrow Up.svg31,069 92,017
2015 283,586 Green Arrow Up.svg31,879 TriangleArrow-Left.svg 222,638 TriangleArrow-Left.svg 162 1.01:1 Green Arrow Up.svg31,717 60,948
2014 251,707 Green Arrow Up.svg39,788 TriangleArrow-Left.svg 222,476 TriangleArrow-Left.svg 24,527 2.61:1 Green Arrow Up.svg15,261 29,231
2013 211,919 Green Arrow Up.svg44,135 TriangleArrow-Left.svg 197,949 TriangleArrow-Left.svg 38,900 8.43:1 Green Arrow Up.svg5,235 13,970
2012 167,784 Green Arrow Up.svg46,950 TriangleArrow-Left.svg 159,049 TriangleArrow-Left.svg 41,642 8.85:1 Green Arrow Up.svg5,308 8,735
2011 120,834 Green Arrow Up.svg24,381 TriangleArrow-Left.svg 117,407 TriangleArrow-Left.svg 20,955 7.12:1 Green Arrow Up.svg3,426 3,427
2010 96,453 Green Arrow Up.svg78,517 TriangleArrow-Left.svg 96,452 TriangleArrow-Left.svg 78,516 78517.00:1 Green Arrow Up.svg1 1
2009 17,936 Green Arrow Up.svg14,427 TriangleArrow-Left.svg 17,936 TriangleArrow-Left.svg 14,427 ?:1 0 0
2008 3,509 Green Arrow Up.svg2,481 TriangleArrow-Left.svg 3,509 TriangleArrow-Left.svg 2,481 ?:1 0 0
2007 1,028 Green Arrow Up.svg1,028 TriangleArrow-Left.svg 1,028 TriangleArrow-Left.svg 1,028 ?:1 0 0

Graphical Analysis

These graphs shows the usage of the two tags over time. There is a clear trend towards the use of water=* tags. The first graph compares way tagging, and the second graph compares relation tagging.

Tagging of way water=reservoir vs way landuse=reservoir over time
Taghistory-reservoir-ways.svg
Tagging of relation water=reservoir vs relation landuse=reservoir over time
Taghistory-reservoir-relations.svg

iD 2.13.0 Preset Change

By mid-2016, mappers began consistently favoring the use of water=reservoir over landuse=reservoir. The graphic below shows the difference in monthly change in area tagging between the two tags.

On December 11, 2018, a new issue was opened in the iD editor's issue tracker to change that editor's presets to use the water=* scheme rather than competing alternatives. This change took effect with version 2.13.0, released Jan 23, 2019.

A few months after this iD editor preset change, mappers began consistently favoring the use of water=reservoir in much larger numbers.

Mapper preference.png

Examples

Tagging of the Loch Raven Reservoir, Baltimore, Maryland, USA
Lock Raven.jpg relation Loch Raven Reservoir

The following listing demonstrates the various types of reservoirs described above.

Dammed valleys

  • relation Lake Mead, a dammed valley and the largest reservoir in the United States.
  • relation Llyn Clywedog in Wales, a dammed valley in which additional side dams are required to contain the reservoir.
  • relation Llwyn-on, relation Cantref and relation Beacons Reservoirs are examples of a chain of reservoirs in the River Taff valley.

Coastal Reservoirs

Bank-side Reservoirs

Flood control Reservoirs

Reservoirs used for hydro-electric power generation

Reservoirs used for drinking water

Reservoirs used for recreation

  • relation Dry Falls Lake in the state of Washington, USA.
  • relation Oswego Lake in Oregon, USA. While this reservoir was originally created for industrial purposes, it is now used for recreation.

Cooling reservoirs

Applies to

area

Rendering

It is recommended that data consumers and renderers continue to support both landuse=reservoir and natural=water + water=reservoir until such time as landuse=reservoir use drops to insignificant levels.

Features/Pages affected

Page Action
landuse=reservoir Mark this tag as deprecated, and add it to Deprecated features
Update definitions on their respective wiki pages

External discussions

Comments

Please comment on the discussion page.