Contributors: B. Calmettes (Collecte Localisation Satellites), G. Calassou (Collecte Localisation Satellites), N. Taburet (Collecte Localisation Satellites), R. Kidd (EODC GmbH), C. Briese (EODC GmbH), A. Dostalova (EODC GmbH)
Issued by: EODC GmbH/Alena Dostalova
Date: 27/09/2023
Ref: C3S2_312a_Lot4.WP3-TRGAD-LK-v2_202304_LK_TR_GA_i1.0
Official reference number service contract: 2021/C3S2_312a_Lot4_EODC/SC1
History of modifications
Related documents
Acronyms
General definitions
Level 3 uncollated/collated/super-collated (L3U/L3C/L3S): this is a designation of satellite data processing level. "Level 3" indicates that the satellite data is a geophysical quantity (retrieval) that has been averaged where data are available to a regular grid in time and space. "Uncollated" means L2 data granules have been remapped to a regular latitude/longitude grid without combining observations from multiple source files. L3U files will typically be "sparse" corresponding to a single satellite orbit. "Collated" means observations from multiple images/orbits from a single instrument combined into a space-time grid. A typical L3C file may contain all the observations from a single instrument in a 24-hour period. "Super-collated" indicates that (for those periods where more than one satellite data stream delivering the geophysical quantity has been available) the data from more than one satellite have been estimated together into a single value, where relevant.
Target requirement: ideal requirement which would result in a considerable improvement for the target application.
Threshold requirement: minimum requirement to be met to ensure data are useful.
Brokered dataset: dataset that has been generated and validated generally with Research and Development (R&D) efforts in an external initiative/project, and then tailored to be made available via this Copernicus service.
Single-track lake: is a lake observed by a single track of an altimetry mission satellite. Those lakes are usually small, with a mean surface area of 13.44 km2 in the LWL-S dataset v1.0
Lake Water Level (LWL) refers to the water level above the geoid. The product for this service consists of one value of the water level for each lake considering geoid variation along the track and bias between tracks. The unit of the water level is in metre.
Lake Water Level – Single-track (LWL-S) refers to water level above the geoid for lakes being observed by a single track of an altimetry mission. The processing on those lakes does not consider geoid variations. Additionally, given that the size of the surface of those lakes is usually small, this dataset is also called Lake Water Level Small-lakes
Scope of the document
This document aims to provide users with the relevant information on requirements and gaps for each of the given products for Lakes within the Land Hydrology and Cryosphere service. The gaps in this context refer to data availability to enable the ECV products to be produced, or in terms of scientific research required to enable the current ECV products to be developed in response to the specified user requirements.
The Lakes Service provides two products streams. The first concerns Lake Water Level: a Lake Water Level (LWL) product for medium to large lakes (that considers the geoid variation along the tracks and the estimation of bias between tracks), and a new product containing the water level of lakes being observed by a single track (LWL-S), derived using a based a methodology used for rivers. Given that those lakes are usually small, this dataset is also called LWL-Small lakes The Lakes service also distributes a Lake Surface Water Temperature (LSWT) product. This has been provided up until the end of 2022. However, this product line has been discontinued since April 2023 and will no longer be updated on Climate Data Store (CDS). Given this discontinuation, this document focuses on the Lake Water Level products.
The first section contains an overview of each product is provided, including the required input data and auxiliary products, a definition of the retrieval algorithms and processing algorithms versions; including, where relevant, a comment on the current methodology applied for uncertainty estimation. The target requirements for each product are then specified in the user requirement section. They generally reflect the Global Climate Observing System (GCOS) ECV requirements. In the last section, the result of a gap analysis is provided that identifies the envisaged data availability for the next 10-15 years, along with the requirements for the further development of the processing algorithms, and the opportunities to take full advantage of current, external, research activities. Finally, where possible, areas of required missing fundamental research are highlighted, and a comment on the impact of future instrument missions is provided.
Executive Summary
The Lakes Service provides two water level products of the Lake Essential Climate Variable (ECV): Lake Water Level (LWL) and Lake Water Level for Small lakes (LWL-S).
The first LWL product, both brokered and generated in the ECV Lakes Service, is an estimate of the mean surface height of the lake, wherever at least three valid observations have been made within the intersect between the satellite ground track and a given lake. The most current (version 4.0) LWL product targets 229 lakes worldwide, from September 1992 to December 2022, with daily to decadal monitoring (in Climate Data Record (CDR) v4.0 with a status date of January 2023). The satellites contributing to the time series are: TOPEX-Poseidon, Jason-1/2/3, Sentinel-6A, and Sentinel-3A/3B.
The new product line (LWL-S), is an estimate of the surface height of lakes being monitored by a single track from a single mission. The methodology for the level estimation is based on an algorithm currently in use for rivers and uses the same altimetry missions as LWL.
The data format for both LWL and LWL-S products is netCDF4 classic, adopting relevant CF (Climate Forecast) conventions1. Initially CF was developed for gridded data from climate and forecast models (hence "CF") of the atmosphere and ocean, but its use has subsequently been extended to other geosciences, and to observations as well as numerical model outputs.
The requirements for the Lakes Service products are largely reliant upon the statements from GCOS, published literature and experience from other CDR projects. For LWL, either the target or the threshold target has already been reached. Section 1 of this document briefly presents the Lake ECV products provided in the service - LWL and LWL-S as background to the remainder of the report.
Section 2 presents known statements of requirements directly relevant to the products in the context of the C3S, in terms of definitional, coverage, resolution, uncertainty, format and timeliness requirements. The C3S team's view and interpretation of these statements of requirement and their relevance to the C3S service is stated.
Section 3 presents an analysis of gaps and opportunities in terms of:
- current observational constraints and additional/future sources of satellite data
- known areas for improvement of LWL estimation methods
- known areas for improvement of LWL uncertainty estimation methods
- lake ECV components not presently delivered by the Hydrology service within the C3S 312b LHC service
Reliance on External Research
Since the C3S programme only supports the implementation, development and operation of the CDR processor, any scientific advances of the C3S products entirely rely on funding provided by external programmes, (such as CCI-Lakes, THEIA/Hydroweb service, amongst others). Thus, the implementation of new scientific improvements can only be implemented if external funding allows for it. This depends both on the availability of suitable programmes to support the R&D activities and the success of the C3S contractors in winning potential suitable calls.
1. Product description: Lake ECV Service
1.1. Introduction
The Lake ECV Service delivers two lake water level product lines, which are generated using different methodologies. First method is currently used and is applied to lakes usually being observed by one or several missions/tracks. The second one, specially implemented for C3S, is based on the operational methodology being used for the estimation of water level over rivers (Halicki and Niedzielski, 2022) and will be applied to lakes that are overpassed by single missions and single tracks. Both datasets, presented as a Climate Data Record (CDR), are updated once a year. The CDRs are created as an adaptation of the state-of-the-art products specifically for the C3S service where scientific advancements have been investigated and applied. For these versions of the CDRs, the latest European Space Agency (ESA) Climate Change Initiative (CCI) Lakes datasets are employed. The C3S extends every year the CDR and, if available, includes new advancements of the datasets.
1.2. The Lake ECV products
1.2.1. LWL V4.0: Brokered and Generated CDR
The LWL CDR, brokered to the C3S, is a timeseries product derived from observations of one or more satellites. The reported LWL is an estimate of the mean surface height of the lake, wherever at least three valid observations have been made within the intersect between the satellite ground track and a given lake.
In addition to the lake mean LWL data, the timeseries contains:
- the Universal Time Coordinate (UTC) time of acquisition.
- uncertainty estimates for the mean LWL.
- metadata, including lake name in English, location, country, funder and citation instructions.
The data format is netCDF4 classic model2, adopting relevant CF conventions3.
The LWL - V4.0 CDR covers the period 1992 to 2022. The measurement sensors contributing to these time-series are radar altimeters onboard the following satellite platforms: TOPEX/Poseidon, Jason-1/2/3, Sentinel-6A, Sentinel-3A, and Sentinel-3B.
1.2.2. LWL-S: Generated CDR
The LWL-S products are generated for lakes that are observed by only one satellite mission with a single track. Because of this constraint, the lakes studied are smaller in size (from 1 to 10 km2) than the lakes present in the LWL product. The LWL-S product is an estimate of the median lake level when the lake has at least two valid measurements in the average spatial footprint of the lake provided by the Hydrolake4 database (Messager et al. 2016). This approach, using measurements from a single mission, avoids the time-consuming calculation of geoid undulations and relies on an a priori geoid grid.
The period covered by the LWL-S CDR product depends on the period of activity of the satellite mission providing information for a given lake. The sensors contributing to these time series are radar altimeters onboard the following satellite platforms: Jason-3 (01-2016 - 11-2020), Sentinel-3A (02-2016 - present), Sentinel-3B (04-2018 - present) and Sentinel-6A (11-2020 - present). In addition, Jason-3 products are used to complement the Sentinel-6A measurements, when the Jason-3 mission was on its operational phase.
The LWL-S product format is similar to that used for the LWL products.
2. User requirements
Based upon the precursor ESA CCI project addressing the Lake ECVs survey of user requirements for satellite-derived lake products, this section reports updates on the GCOS-2005, reflecting the Implementation Plan 2016. This section relies also on the statements for the Lake ECV published literature, experience from other CDR projects, and the requirements emerging from the definition of the service. The requirements are continuously updated using perspectives that emerge from users of the service and their feedback, and from any user requirements survey that is undertaken in the ESA CCI Lakes project.
The requirements involve different aspects of the product such as the definition, coverage, the spatial and temporal resolution, uncertainty, format and timeliness and they are similar for both products: LWL and LWL-S.
2.1.1. Definitional requirements
Property | Threshold | Target | Comments | Source |
|---|---|---|---|---|
LWL | Provide | - | Satellite RADAR and Doppler altimeters are used for computing lake levels. | GCOS (RD.1) |
Time base | UTC | - | Based on experience in the Hydroweb6. service. | Experience |
2.1.2. Coverage
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Spatial coverage | Global | Global | Based on experience in the Hydroweb service and the list of lakes defined in the Lakes CCI7. Project. | |
Temporal coverage | 10 years | >25 years | Based on experience in the Hydroweb service. | Experience |
2.1.3. Spatial and temporal resolution
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Spatial | area: 400 km² | area: 1 km² | The spatial resolution refers to the minimum lake area needed to estimate a water level value. | Experience |
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Temporal | 1-10 days | Daily | Threshold comes from experience in the Hydroweb service. This resolution depends on the altimetric missions overpassing the lake | GCOS (RD.1), Experience |
2.1.4. Data uncertainties
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Standard uncertainty of LWL | 15 cm | 3 cm for large lakes, 10 cm for the remainder | Threshold comes from experience in the Hydroweb service. | GCOS (RD.1), Experience, CCI target requirements |
Trend uncertainty (stability) | - | 1cm/decade | Target comes from GCOS. | GCOS (RD.1) |
2.1.5. Format requirements
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Format | NetCDF, CF Convention | NetCDF, CF Convention | This is a service requirement. | C3S |
2.1.6. Timeliness requirements
Property | Threshold | Target | Comments | Sources |
|---|---|---|---|---|
Ongoing timely updates | Annually | Annually | Driver of this timescale is to make an annual state-of-the-climate assessment. | C3S |
3. Analysis of gaps and opportunities
3.1. Satellite observational constraints and opportunities
3.1.1. Lake water level (LWL)
Table 1: LWL Gap Analysis Summary.
Property | Threshold | Target | Currently Achieved | Gap analysis |
|---|---|---|---|---|
Spatial coverage | Global | Global | Global coverage (229 Lakes in V4.0) | The number of Lakes being monitored is constantly increasing. |
Temporal coverage | 10 years | >25years | almost 30 years for some lakes (Sept 1992 - Dec 2022) | Target has been reached. New historic altimetry missions could be considered to improve the temporal resolution (ERS-1/2, Envisat, SARAL). |
Spatial resolution | area: 400km² | area: 1 km² | Lake area > 38 km² | Threshold reached. New algorithms must be implemented to improve the resolution. New missions/altimeters must be launched to reach target (e.g. SWOT: Surface Water & Ocean Topography). |
Temporal resolution | 1-10 days | Daily | 1-10 days | Threshold reached for bigger lakes |
Standard uncertainty of LWL | 15 cm | 3 cm for large lakes, 10 cm for the remainder | 10cm for large lakes, 20cm for medium lakes, small lakes not processed. | Threshold reached for most lakes in the product. New algorithms must be developed to reach target. New missions/altimeters will help to reach the target (e.g. SWOT). |
Trend uncertainty (stability) | - | 1cm/decade | Not estimated. For comparison, on oceanic surfaces, the trend uncertainty has been estimated up to 5cm/decade locally. | - |
Format | NetCDF, CF Convention | NetCDF, CF Convention | NetCDF, CF Convention | Target Reached |
Ongoing timely updates | Annually | Annually | Annually | Target Reached |
3.1.2. Lake water level small lakes (LWL-S)
Table 2: LWL-S Gap Analysis Summary.
Property | Threshold | Target | Currently Achieved | Gap analysis |
|---|---|---|---|---|
Spatial coverage | Global | Global | Global coverage | The number of Lakes being monitored must be increased (ongoing activity). |
Temporal coverage | 10 years | >25 years | Depends on the mission time activity | Target will be reached when the operational time of the mission will be long enough. |
Spatial resolution | Area: 400 km2 | area: 1km² | Lake area > 1km² | Threshold reached. |
Temporal resolution | 1 – 10days | Daily | Depends on the revisit time of the mission (10 or 27 days) | New missions/altimeters will help to reach the target (e.g. SWOT). |
Standard uncertainty of LWL | 15 cm | 3 cm for large lakes, 10 cm for the remainder | 20 cm for small and medium lakes. | Threshold reached for most lakes in the product. New algorithms must be developed to reach target. New missions/altimeters will help to reach the target (e.g. SWOT). |
Trend uncertainty (stability) | - | 1cm/decade | Not estimated. For comparison, on oceanic surfaces, the trend uncertainty has been estimated up to 5cm/decade locally. | - |
Format | NetCDF, CF Convention | NetCDF, CF Convention | NetCDF, CF Convention | Target Reached |
Ongoing timely updates | Annually | Annually | Annually | Target Reached |
3.2. Improvement of retrieval algorithms
3.2.1. Lake water level
The current state-of-the-art research and development (R&D) leading to the V4.0 CDR relies partly on a manual approach to estimate the geographic extraction zone of altimetry measurements. An automated version of this R&D has been implemented within the frame of the present project to ramp-up the products and be able to provide water level for a wider network of lakes. This has enabled the increase in the number of lakes monitored between the first version (V1.0 – 64 lakes) and the last version (v4.0 – 229 lakes) of the C3S dataset. Increase of the number of monitored lakes is a regular activity in the project. The method relies on a database of lake delineations and a land/water mask (from Hydrolakes9, Pekel et al. 2016), intersected with the theoretical ground-track of the satellites and the lakes polygons defined in the CCI Lakes project.
Then, the extracted data must be corrected for various propagations (e.g. corrections for ionosphere, wet troposphere, and dry troposphere amongst others) and geophysical corrections (e.g. geoid, pole tide, solid earth tide amongst others) based on models and with limitations. The geoid model, in particular, does not include small wavelengths of the geoid, and this must be estimated based on altimetry data and a posteriori corrected. The algorithm has been improved to cover both simple (cf Figure 1 left panel) and complex (cf Figure 1, right panel) cases.
Figure 1: Automatic extraction of altimetry measures over specific lakes. The left panel shows a simple case of automatic intersection between satellite ground tracks and the polygon defining a lake. The right panel shows a more complex case including some land zones in the target lake that need to be excluded in the processing.
These two implementations are performed to improve the number of lakes monitored in the LWL product (see Section 3.1.1). Additionally, other R&D algorithms should be developed within the CCI-Lakes project and then be implemented for operational use to improve the quality of the product.
3.2.2. Lake water level small lakes
The extraction of median water heights uses the same approach as that implemented in the Hydroweb service for rivers10. The method considers L2 measurements within polygons defined in Hydrolakes, a database of delineation and land/water masks (Messager et al. 2016), and their intersection with the Open-Loop Tracking Command (OLTC) footprint of the different altimetry missions (see Figure 2).
As for the LWL products, the LWL-S products are corrected using the same geophysical corrections, except for the geoid. The geoid is corrected using a geoid grid that is more appropriate for lakes with a surface area smaller than 10 km2 having a lower variability.
Figure 2: Intersection between the valid OLTC of the Sentinel-3B (S3B) altimetry mission and the water mask of Lake Biel (Switzerland) from the Hydrolake database.
3.3. Improvement of uncertainty estimation
The uncertainty variable distributed in the LWL and LWL-S products, associated to the Water Level variable, is currently estimated as the Median Absolute Deviation of selected water level measurements along-track (at level 2). The median value of the selected measurements at level 2 provides the median water level (level 3). It estimates the precision of the measurements but not their accuracy. The improvement of this uncertainty variable depends on achievements in the CCI Lakes project, but no strategy is currently foreseen to improve this variable. Nevertheless, new missions present a better performance, as observed by the dispersion of L2 measurements.
The ongoing offline validation exercise will provide global statistics on the LWL and LWL-S products and a characterisation of the global uncertainty based on:
- Comparison to other altimetry products (e.g. Global Reservoirs and Lakes Monitor: REALM11)
- Comparison to in situ data (e.g. Water Office of Canada12)
3.4. Lake ECV components not presently in the service
The GCOS definition (RD.1) of the Lake ECV includes, in addition to water level, the elements of lake surface water temperature, surface reflectance, lake area and lake ice cover and thickness. A review of the opportunity to broker datasets addressing these gap areas is ongoing and is not included in this report.
References
Pekel, J.F, Cottam A., Gorelick N. et al. High resolution mapping of global surface water and its long-term changes. Nature 540, 418-422 (2016).
Taburet et al. (2020) Lake and river water level, 300m, Version 2.1: Product User Manual. Copernicus Global Land Operations: Cryosphere and Water - CGLOPS-2 (JRC Framework Service Contract N° 199496). Available at: https://land.copernicus.eu/global/sites/cgls.vito.be/files/products/CGLOPS2_PUM_LakeAndRiverWaterLevel-V2.1_I2.11_0.pdf
(last accessed 1st April, 2023).
Messager, M.L., Lehner, B., Grill, G., Nedeva, I., Schmitt, O. (2016). Estimating the volume and age of water stored in global lakes using a geo-statistical approach. Nature Communications, 7: 13603. https://doi.org/10.1038/ncomms13603 (last accessed 1st April, 2023)
Michał Halicki, Tomasz Niedzielski, The accuracy of the Sentinel-3A altimetry over Polish rivers, Journal of Hydrology, Volume 606, 2022, 127355, ISSN 0022-1694.
This document has been produced in the context of the Copernicus Climate Change Service (C3S).
The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Contribution agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose.
The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view.
