| 1. |
- Al-Kharusi, Enass Said, et al.
(author)
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Drought Offsets the Controls on Colored Dissolved Organic Matter in Lakes
- 2024
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In: Remote Sensing. - 2072-4292. ; 16:8
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Journal article (peer-reviewed)abstract
- The concentration of colored dissolved organic matter (CDOM) in lakes is strongly influenced by climate, land cover, and topographic settings, but it is not known how drought may affect the relative importance of these controls. Here, we evaluate the controls of CDOM during two summers with strongly contrasting values of the Palmer drought index (PDI), indicating wet vs. dry conditions. We hypothesized that lake CDOM during a wet summer season is regulated mainly by the surrounding land cover to which the lakes are hydrologically connected, while, during drought, the lakes are disconnected from the catchment and CDOM is regulated by climatic and morphometric factors that govern the internal turnover of CDOM in the lakes. A suite of climate, land cover, and morphometric variables was assembled and used to explain remotely sensed CDOM values for 255 boreal lakes distributed across broad environmental and geographic gradients in Sweden and Norway. We found that PDI explained the variability in CDOM among lakes in a dry year, but not in a wet year, and that severe drought strongly decreased CDOM during the dry year. Large lakes, especially, with a presumed high degree of catchment uncoupling, showed low CDOM during the dry year. However, in disagreement with our hypothesis, climate, land cover, and morphometry all showed a stronger impact on lake CDOM in wet vs. dry years. Thus, drought systematically weakened the predictability of CDOM variations at the same time as CDOM was offset toward lower values. Our results show that drought not only has a direct effect on CDOM, but also acts indirectly by changing the spatial regulation of CDOM in boreal lakes.
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| 2. |
- Al-kharusi, Enass Said., et al.
(author)
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Large-Scale Retrieval of Coloured Dissolved Organic Matter in Northern Lakes Using Sentinel-2 Data
- 2020
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In: Remote Sensing. - : MDPI AG. - 2072-4292. ; 12:1, s. 157-157
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Journal article (peer-reviewed)abstract
- Owing to the significant societal value of inland water resources, there is a need for cost-effective monitoring of water quality on large scales. We tested the suitability of the recently launched Sentinel-2A to monitor a key water quality parameter, coloured dissolved organic matter (CDOM), in various types of lakes in northern Sweden. Values of a(420)CDOM (CDOM absorption at 420 nm wavelength) were obtained by analyzing water samples from 46 lakes in five districts across Sweden within an area of approximately 800 km2. We evaluated the relationships between a(420)CDOM and band ratios derived from Sentinel-2A Level-1C and Level-2A products. The band ratios B2/B3 (460 nm/560 nm) and B3/B5 (560 nm/705 nm) showed poor relationships with a(420)CDOM in Level-1C and 2A data both before and after the removal of outliers. However, there was a slightly stronger power relationship between the atmospherically-corrected B3/B4 ratio and a(420)CDOM (R2 = 0.28, n = 46), and this relationship was further improved (R2 = 0.65, n = 41) by removing observations affected by light haze and cirrus clouds. This study covered a wide range of lakes in different landscape settings and demonstrates the broad applicability of a(420)CDOM retrieval algorithms based on the B3/B4 ratio derived from Sentinel-2A. View Full-Text
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| 3. |
- Kari, Elina, 1986-
(author)
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Light conditions in seasonally ice-covered waters : within the Baltic Sea region
- 2018
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Doctoral thesis (other academic/artistic)abstract
- Seasonal ice cover is a major driver of seasonality in aquatic ecosystems in the Baltic Sea region. Ice cover influences the underwater light conditions directly by limiting the light transfer and indirectly by modifying the mixing and circulation under the ice. Light conditions and stratification are key factors controlling the onset of the phytoplankton spring bloom. Therefore, the seasonal ice cover has an important role in setting the time frames for the primary production and in influencing the seasonality of the ecological processes. This thesis investigates the optical properties of the ice cover and the bio-optical substances in the water column.Bio-optical substances, suspended particulate matter (SPM), Coloured dissolved organic matter (CDOM) and Chlorophyll-a (Chl-a), determine the availability and spectral distribution of light. Measuring turbidity is quick and easy compared to the gravimetrical determination of the SPM concentration. Paper I provides a new model to estimate the concentration of SPM from turbidity. The new SPM-turbidity model predicts SPM concentrations well, despite the high CDOM absorption and the optical differences in the coastal northwestern and southeastern Baltic proper. Therefore, the new SPM-turbidity model offers a cost-effective and reliable method to monitor SPM concentration.The light transfer through the snow and ice cover was studied both in freshwater lake ice and in brackish sea ice (Papers II and III). Additionally, the seasonal evolution of light transmission through lake ice was investigated during spring. The crystal structure of the ice cover was analysed both in the coastal fast ice zone and in drift ice in the open Baltic Sea. The snow and ice cover was found not only to reduce the amount of light, but also to change its spectral and directional distribution. The light field under ice depended strongly on the snow cover. In addition, the bio-optical substances were analysed within sea ice and in the underlying water, as well as their effect on the light conditions.The seasonal sea ice cover also limits the wind-driven mixing of the water column. The development of stratification was investigated in a coastal bay in the northwestern Baltic proper (Paper IV). The preconditions for an under-ice plume development were defined along with the spatial and temporal dimensions of the stratification pattern. Furthermore, an under-ice plume was found to cause a delay in the onset of the phytoplankton spring bloom, but the timing of the Chl-a maximum was not affected. The results also show that although diatoms dominate the phytoplankton community with and without under-ice plume, the dynamic conditions without under-ice plume seem to favour the motile photosynthetic ciliate Mesodinium rubrum. Overall, this thesis contributes to better understanding of the current role of seasonal ice cover on the light conditions and consequently on to the ecosystem.
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| 4. |
- Kutser, Tiit, et al.
(author)
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Estimating lake carbon fractions from remote sensing data
- 2015
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 157, s. 138-146
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Journal article (peer-reviewed)abstract
- Issues like monitoring lake water quality, studying the role of lakes in the global carbon cycle or the response of lakes to global change require data more frequently and/or over much larger areas than the in situ water quality monitoring networks can provide. The aim of our study was to investigate whether it is feasible to estimate different lake carbon fractions (CDOM, DOC, TOC, DIC, TIC and pCO(2)) from space using sensors like OLCI on future Sentinel 3. In situ measurements were carried out in eight measuring stations in two Swedish lakes within 2 days of MERIS overpass. The results suggest that the MERIS CDOM product was not suitable for estimating CDOM in lakes Malaren and Tamnaren and was not a good proxy for mapping lake DOC and TOC from space. However, a simple green to red band ratio and some other MERIS products like the total absorption coefficient, turbidity index, suspended matter and chlorophyll-a were correlated with different carbon fractions and could potentially be used as proxies to map these lake carbon fractions (CDOM, DOC, TOC, DIC, TIC and pCO2) from space. (C) 2014 Elsevier Inc All rights reserved.
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| 5. |
- Kutser, Tiit, et al.
(author)
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Impact of iron associated to organic matter on remote sensing estimates of lake carbon content
- 2015
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 156, s. 109-116
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Journal article (peer-reviewed)abstract
- There is a strong need to develop remote sensing methods for mapping lake carbon content on regional to global scales. The use of in situ methods is impractical for monitoring lake water quality over large geographical areas, which is a fundamental requirement to understand the true role of lakes in the global carbon cycle. The coloured component of dissolved organic carbon (DOC), called CDOM, absorbs light strongly in the blue part of the visible spectrum and can be used as a proxy for mapping lake DOC with remote sensing. However, iron associated to organic matter can cause extra browning of waters. Consequently, the remote sensing signal we interpret as DOC may partially be attributed to the presence of iron associated to organic matter, potentially hampering our ability to estimate carbon concentrations. A thorough analysis of biogeochemical parameters was carried out on Lake Malaren on August 23, 2010, and a MERIS full resolution image was acquired simultaneously. MERIS standard, Case 2 Regional, and Boreal processors were used to calculate remote sensing products, which were compared with different lake water characteristics. The carbon to iron ratio was different from the rest of the lake in one of the basins. MERIS standard and Case 2 Regional processors were sensitive to this difference as the correlation between MERIS CDOM product and DOC was low (R-2 = 0.43) for all sampling stations and increased to 0.92 when the one basin was excluded. The Boreal Lakes processor results were less disturbed by the different carbon-iron ratios found in one basin and produced reasonably good results (R-2 = 0.65). We found MERIS products (e.g. total absorption) that provided good correlation (R-2 = 0.80) with DOC-specific absorbance at 254 nm, called SUVA, which is a metric commonly used to assess drinking water treatability. However, none of the MERIS products were suitable for mapping the total organic carbon in Lake Malaren.MERIS total suspended matter product was a good (R-2 = 0.73) proxy for particulate iron, meaning that the particulate iron content in Malaren can be mapped from space. (C) 2014 Elsevier Inc. All rights reserved.
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| 6. |
- Kutser, Tiit, et al.
(author)
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Remote Sensing of Black Lakes and Using 810 nm Reflectance Peak for Retrieving Water Quality Parameters of Optically Complex Waters
- 2016
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In: Remote Sensing. - : MDPI AG. - 2072-4292. ; 8:6
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Journal article (peer-reviewed)abstract
- Many lakes in boreal and arctic regions have high concentrations of CDOM (coloured dissolved organic matter). Remote sensing of such lakes is complicated due to very low water leaving signals. There are extreme (black) lakes where the water reflectance values are negligible in almost entire visible part of spectrum (400-700 nm) due to the absorption by CDOM. In these lakes, the only water-leaving signal detectable by remote sensing sensors occurs as two peaksnear 710 nm and 810 nm. The first peak has been widely used in remote sensing of eutrophic waters for more than two decades. We show on the example of field radiometry data collected in Estonian and Swedish lakes that the height of the 810 nm peak can also be used in retrieving water constituents from remote sensing data. This is important especially in black lakes where the height of the 710 nm peak is still affected by CDOM. We have shown that the 810 nm peak can be used also in remote sensing of a wide variety of lakes. The 810 nm peak is caused by combined effect of slight decrease in absorption by water molecules and backscattering from particulate material in the water. Phytoplankton was the dominant particulate material in most of the studied lakes. Therefore, the height of the 810 peak was in good correlation with all proxies of phytoplankton biomasschlorophyll-a (R-2 = 0.77), total suspended matter (R-2 = 0.70), and suspended particulate organic matter (R-2 = 0.68). There was no correlation between the peak height and the suspended particulate inorganic matter. Satellite sensors with sufficient spatial and radiometric resolution for mapping lake water quality (Landsat 8 OLI and Sentinel-2 MSI) were launched recently. In order to test whether these satellites can capture the 810 nm peak we simulated the spectral performance of these two satellites from field radiometry data. Actual satellite imagery from a black lake was also used to study whether these sensors can detect the peak despite their band configuration. Sentinel 2 MSI has a nearly perfectly positioned band at 705 nm to characterize the 700-720 nm peak. We found that the MSI 783 nm band can be used to detect the 810 nm peak despite the location of this band is not in perfect to capture the peak.
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| 7. |
- Kutser, Tiit
(author)
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The possibility of using the Landsat image archive for monitoring long time trends in coloured dissolved organic matter concentration in lake waters
- 2012
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In: Remote Sensing of Environment. - : Elsevier BV. - 0034-4257 .- 1879-0704. ; 123, s. 334-338
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Journal article (peer-reviewed)abstract
- Recent studies indicate that lakes are regulators of carbon cycling and climate. Therefore, it is important to know how the lake carbon content has changed over the last decades. In situ long time data series about the amount of dissolved organic carbon (DOC) in lakes are rare. The only potential way to study retrospectively the changes in lake carbon over the last decades is by means of remote sensing data provided there are sensors that can provide data about coloured dissolved organic matter (CDOM) in lakes over long periods. Landsat data archive contains images from 1984 to nowadays and covers the whole Earth. Although the sensors were not designed for remote sensing of aquatic environments it is still tempting to utilise the long data series. Landsat 4, Landsat 5 and Landsat 7 imagery available in free Landsat image archive was compared with time series of CDOM in situ data from 19 sampling stations available in the Swedish University of Natural Sciences lake monitoring database. There was no correlation between the image and in situ data when all the above mentioned data were used. Low radiometric resolution of the sensor, small size of many lakes (= large adjacency effects) and high concentration of CDOM (negligible water leaving radiation) could be the reasons. The results were more promising (R-2 = 0.62) when Lake Malaren stations were analysed separately. The lake is sufficiently large and with variable, but not extremely high. CDOM content. The Lake Malaren in situ data showed very different trends in CDOM concentrations in different basins of the lake over the last 45 years. Although the correlation between the image and in situ data was a bit low for accurate daily estimation of CDOM concentrations from Landsat data it could allow detecting general trends in lake CDOM content. Unfortunately, there is currently a gap in Landsat archive (for our study sites) between 1988 and 1998 which makes calculations of long time trends unreasonable for the time being. (C) 2012 Elsevier Inc. All rights reserved.
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| 8. |
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| 9. |
- Kutser, Tiit, et al.
(author)
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Variations in colored dissolved organic matter between boreal lakes studied by satellite remote sensing
- 2009
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In: Journal of Applied Remote Sensing. - : SPIE-Intl Soc Optical Eng. - 1931-3195. ; 3, s. 033538-
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Journal article (peer-reviewed)abstract
- Therehave been major improvements in our understanding of the roleof lakes and impoundments in global carbon cycle. Estimating thetrue role of lakes as sentinels, regulators and integrators ofclimate change requires analyzing carbon content of vast number oflakes. This is not realistic without using remote sensing methods.There are no satellite sensors at the moment that providefull global coverage and at the same time have spatialand radiometric resolutions suitable for lake color dissolved organic matter(CDOM) mapping. Therefore, the global study has to be performedusing more sensitive sensors to create regional lake CDOM statisticsfor as many sites as possible and extrapolating the resultson global lake map that can be created from lesssensitive sensor data with full global coverage (Landsat). As afirst step towards the global lake carbon mapping we showthat the Advanced Land Imager (ALI) allows to study regionalvariations in lake CDOM content and consequently estimate closely correlatedDOC (dissolved organic carbon) and CO2 saturation values. The resultsshow also that there may be regional differences in lakeCDOM content even if the study sites are geographically relativelyclose to each other and occupying zones with similar landcover and annual runoff. In one occasion the difference canbe explained with human impact that has caused acidification oflakes but the other occasion needs further studies.
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| 10. |
- Said Al-Kharusi, Enass, et al.
(author)
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Mapping conservation priorities in alpine and subartctic Swedish lakes affected by rapid climate change
- 2018
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Conference paper (other academic/artistic)abstract
- Swedish alpine and subarctic areas undergo rapid transitions due to climate change, especially in relatively humid areas where the forest vegetation is expanding. It has been suggested that lakes in these areas are in transition from clearwater into brownwater state, because of the humus layer building up in surrounding soils, with negative consequences, e.g., for the biomass production of zooplankton and fish. However, this idea remains a hypothesis to be tested at a large scale. We used the new high-resolution satellite Sentinel-2 to evaluate the relationship between lake color and forest succession in 250 widely distributed lakes across Sweden. Preliminary results show that lakes remain surprisingly clear in areas of recent forest expansion, suggesting that there is lag time before brownification occurs. Our study could be used to map regions that are sensitive to future water brownification.
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| 11. |
- Toming, Kaire, et al.
(author)
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Mapping Water Quality Parameters with Sentinel-3 Ocean and Land Colour Instrument Imagery in the Baltic Sea
- 2017
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In: Remote Sensing. - : MDPI AG. - 2072-4292. ; 9:10
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Journal article (peer-reviewed)abstract
- The launch of Ocean and Land Colour Instrument (OLCI) on board Sentinel-3A in 2016 is the beginning of a new era in long time, continuous, high frequency water quality monitoring of coastal waters. Therefore, there is a strong need to validate the OLCI products to be sure that the technical capabilities provided will be used in the best possible way in water quality monitoring and research. The Baltic Sea is an optically complex waterbody where many ocean colour products, performing well in other waterbodies, fail. We tested the performance of standard Case-2 Regional/Coast Colour (C2RCC) processing chain in retrieving water reflectance, inherent optical properties (IOPs), and water quality parameters such as chlorophyll a, total suspended matter (TSM) and coloured dissolved organic matter (CDOM) in the Baltic Sea. The reflectance spectra produced by the C2RCC are realistic in both shape and magnitude. However, the IOPs, and consequently the water quality parameters estimated by the C2RCC, did not have correlation with in situ data. On the other hand, some tested empirical remote sensing algorithms performed well in retrieving chlorophyll a, TSM, CDOM and Secchi depth from the reflectance produced by the C2RCC. This suggests that the atmospheric correction part of the processor performs relatively well while IOP retrieval part of the neural network needs extensive training with actual IOP data before it can produce reasonable estimates for the Baltic Sea.
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| 12. |
- Toming, Kaire, et al.
(author)
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Predicting lake dissolved organic carbon at a global scale
- 2020
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In: Scientific Reports. - : Springer Nature. - 2045-2322. ; 10:1, s. 1-8
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Journal article (peer-reviewed)abstract
- The pool of dissolved organic carbon (DOC), is one of the main regulators of the ecology and biogeochemistry of inland water ecosystems, and an important loss term in the carbon budgets of land ecosystems. We used a novel machine learning technique and global databases to test if and how different environmental factors contribute to the variability of in situ DOC concentrations in lakes. In order to estimate DOC in lakes globally we predicted DOC in each lake with a surface area larger than 0.1 km2. Catchment properties and meteorological and hydrological features explained most of the variability of the lake DOC concentration, whereas lake morphometry played only a marginal role. The predicted average of the global DOC concentration in lake water was 3.88 mg L−1. The global predicted pool of DOC in lake water was 729 Tg from which 421 Tg was the share of the Caspian Sea. The results provide global-scale evidence for ecological, climate and carbon cycle models of lake ecosystems and related future prognoses.
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| 13. |
- Tranvik, Lars J., et al.
(author)
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Lakes and reservoirs as regulators of carbon cycling and climate
- 2009
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In: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 54:6:2, s. 2298-2314
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Research review (peer-reviewed)abstract
- We explore the role of lakes in carbon cycling and global climate, examine the mechanisms influencing carbon pools and transformations in lakes, and discuss how the metabolism of carbon in the inland waters is likely to change in response to climate. Furthermore, we project changes as global climate change in the abundance and spatial distribution of lakes in the biosphere, and we revise the estimate for the global extent of carbon transformation in inland waters. This synthesis demonstrates that the global annual emissions of carbon dioxide from inland waters to the atmosphere are similar in magnitude to the carbon dioxide uptake by the oceans and that the global burial of organic carbon in inland water sediments exceeds organic carbon sequestration on the ocean floor. The role of inland waters in global carbon cycling and climate forcing may be changed by human activities, including construction of impoundments, which accumulate large amounts of carbon in sediments and emit large amounts of methane to the atmosphere. Methane emissions are also expected from lakes on melting permafrost. The synthesis presented here indicates that (1) inland waters constitute a significant component of the global carbon cycle, (2) their contribution to this cycle has significantly changed as a result of human activities, and (3) they will continue to change in response to future climate change causing decreased as well as increased abundance of lakes as well as increases in the number of aquatic impoundments.
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| 14. |
- Verpoorter, Charles, et al.
(author)
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A global inventory of lakes based on high-resolution satellite imagery
- 2014
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In: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 41:18, s. 6396-6402
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Journal article (peer-reviewed)abstract
- An accurate description of the abundance and size distribution of lakes is critical to quantifying limnetic contributions to the global carbon cycle. However, estimates of global lake abundance are poorly constrained. We used high-resolution satellite imagery to produce a GLObal WAter BOdies database (GLOWABO), comprising all lakes greater than 0.002 km(2). GLOWABO contains geographic and morphometric information for similar to 117 million lakes with a combined surface area of about 5 x 106 km(2), which is 3.7% of the Earth's nonglaciated land area. Large and intermediate-sized lakes dominate the total lake surface area. Overall, lakes are less abundant but cover a greater total surface area relative to previous estimates based on statistical extrapolations. The GLOWABO allows for the global-scale evaluation of fundamental limnological problems, providing a foundation for improved quantification of limnetic contributions to the biogeochemical processes at large scales.
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| 15. |
- Verpoorter, Charles, et al.
(author)
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Automated mapping of water bodies using Landsat multispectral data
- 2012
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In: Limnology and Oceanography. - : Wiley. - 1541-5856. ; 10, s. 1037-1050
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Journal article (peer-reviewed)abstract
- The assessment of the role of lakes and impoundments at regional and global scales, e.g., in biogeochemical cycles, requires good estimates of the areal extent and shape of water bodies. Upscaling to large regions, except in limited areas where precise maps are available, so far depends on statistical estimates of the number and size of lakes, which explains why estimates are poor. We present an automated procedure that allows mapping of the actual number, size, and distribution of lakes at large scale. Landsat 7 Enhanced Thematic Mapper Plus (ETM +) mosaics from the GeoCover Circa 2000 dataset covering the Earth land surfaces with 14.25 m spatial resolution were used as input data. We developed an approach called GWEM (GeoCoverTM Water bodies Extraction Method) that combines remote sensing and GIS to extract water bodies and study their abundance and morphometry. All water bodies greater than 0.0002 km2 were taken into account as lakes. The accuracy of the method was tested on Sweden, where detailed maps of lakes, based on in situ data and orthophotos, exist for the whole country. The proposed method produced accurate results. The largest sources of errors are shadows of mountains and clouds, since the GeoCover mosaics are not absolutely cloud free.
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