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GCOS Terrestrial ECV T4
Lakes
Introduction: Information on changes in lake level and area (which are surrogates for lake volume) is required on a monthly basis for climate assessment purposes. Approximately 95% of the volume of water held globally in approximately 4 000 000 lakes is contained in the world’s 150 largest lakes. Most of these large lakes are hydrologically open. Closed-basin lakes are more sensitive to changes in regional water balance and therefore better indicators of changes in regional climate.
Large open lakes cannot be neglected in designing the monitoring programme because they are important sources of water for consumption, and because large expanses of water can have a regional impact on climate through albedo feedback and evaporation. Furthermore, in some regions (e.g., in the semi-arid interior of Australia or the Great Basin of the USA) highly ephemeral lakes (which contain water only every few years) provide a record of extreme events and also have potential feedback effects on regional climate.
The approach is to focus on the largest, primarily closed-basin, lakes but including major ephemeral lakes and a selection of the largest open lakes. Measurements at these lakes would form a new GCOS/GTOS baseline network, the Global Terrestrial Network – Lakes (GTN-L). For this purpose, lake level and area need to be measured weekly (ideally) or at least monthly, with a horizontal resolution of 10 m and a vertical resolution of at least 10 cm.
Satellite-based observations can substantially contribute to the monitoring of lake level and area, using appropriate visible and near-infrared imagery, radar imagery, and altimetry, especially in areas without in situ monitoring capability. In addition, observing the surface temperature of lakes (using mainly high-resolution infrared imagery) can serve as an indicator for regional climate monitoring, since it is linked to lake freeze-up and break-up dates (cf. Action T10).
The International Data Centre on the Hydrology of Lakes and Reservoirs (HYDROLARE) was established at the State Hydrological Institute in St. Petersburg, Russian Federation. HYDROLARE operates under the auspices of WMO. An agreement on the establishment of HYDROLARE was signed between ROSHYDROMET and WMO on May 2008.
HYDROLARE is working with National Hydrological Services (through WMO CHy) and other institutions and agencies providing and holding data on lakes and reservoirs to continue or, where necessary, initiate monitoring of these priority lakes and provide data to HYDROLARE. As satellite altimeter measurements can provide additional data, particularly in more remote areas, those Parties with space observing capacities will be asked to contribute to monitoring the GTN-L lakes and provide the measurements to HYDROLARE. TOPC, in collaboration with HYDROLARE, is maintaining the list of GTN-L sites.
The publication of measurement time series for the 19th and 20th centuries would considerably enhance the value of ongoing monitoring by allowing baseline measurements to be extended to cover most of the post-industrial period. There are only isolated measurements for the period prior to the 19th century, however, so obtaining these earlier measurements is not a priority.
There are a number of other lake-specific variables needed by the climate modelling community (e.g., surface water temperature) or for climate monitoring purposes (e.g., surface and sub-surface water temperature, date of freeze-up, and date of lake ice break-up). Whenever possible, these variables should be measured by National Hydrological Services and other responsible agencies holding data, in association with measurements of lake level and area, and be submitted to HYDROLARE.
The initial target of monitoring GTN-L lakes world-wide will be of immediate benefit to climate modellers. However, it will be important in the future to extend monitoring to as many of the 500 largest lakes as possible to ensure adequate regional coverage and a sufficient number of sites to enable the replication of derived records.
(Source: WMO/IOC Implementation Plan for the Global Observing System for Climate in Support of the UNFCCC (2010 Update) GCOS-138/GOOS-184/GTOS-76/WMO-TD/No. 1523)
Satellite Observations: Satellite techniques offer a unique, cost-effective and comprehensive source of landscape topography data. At present, most information is obtained primarily from multi-band optical imagers and synthetic aperture radar (SAR) instruments with stereo image capabilities. The pointing capability of some optical instruments allows the production of stereo images from data gathered on a single orbit (e.g. by ASTER) or multiple orbits (e.g. by SPOT series).These are then used to create digital elevation maps, which give a more accurate depiction of terrain.
Since SARs can also be used in interferometric mode to detect very small changes in topography, they have important applications in monitoring of volcanoes, landslides, earthquake displacements and urban subsidence. Current missions include Envisat, RADARSAT-2, TerraSAR-X and ALOS (which carries both high precision optical and SAR topographic mapping instruments). In future, ESA’s Sentinel-1 mission will also contribute to such information.
Radar altimeters can also provide coarse topographic mapping over land. They have been supplemented by a new generation of laser altimeters, such as GLAS (on ICESat) which can provide landscape topography products with height accuracies of order 50–100 cm, depending on slope.
The role of these satellites and their importance in mitigating geo-hazards, such as earthquakes, landslides, and volcanic eruptions, is the focus of the IGOS Geo-hazards Theme. The Geo-hazards Theme report is available from www.igospartners.org.
GCOS notes that measurements of lake area and lake level give an indication of the volume of the lake, an integrator variable that reflects both atmospheric (precipitation, evaporation-energy) and hydrological (surface water recharge, discharge and ground water tables) conditions. GCOS threshold requirements for these variables are currently met by existing missions.
(Satellite Missions) (Source: CEOS EO Handbook - Earth Observations Plans by Measurement) References:
Data, Product, Metadata and Information Access
[ECV Matrix Main Page] [About the ECV Matrix] [Reference Documents] [Contact] [Updated June 6, 2010]
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Non-satellite or in-situ
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Satellite
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