| 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. |
- Georganos, Stefanos, et al.
(author)
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Examining the NDVI-rainfall relationship in the semi-arid Sahel using geographically weighted regression
- 2017
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In: Journal of Arid Environments. - : Elsevier BV. - 0140-1963. ; , s. 64-74
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Journal article (peer-reviewed)abstract
- The Sahel of Africa is an eco-sensitive zone with complex relations emerging between vegetation productivity and rainfall. These relationships are spatially non-stationary, non-linear, scale dependant and often fail to be successfully modelled by conventional regression models. In response, we apply a local modelling technique, Geographically Weighted Regression (GWR), which allows for relationships to vary in space. We applied the GWR using climatic data (Normalized Vegetation Difference Index and rainfall) on an annual basis during the growing seasons (June–September) for 2002–2012. The operating scale of the Sahelian NDVI–rainfall relationship was found to stabilize around 160 km. With the selection of an appropriate scale, the spatial pattern of the NDVI-rainfall relationship was significantly better explained by the GWR than the traditional Ordinary Least Squares (OLS) regression. GWR performed better in terms of predictive power, accuracy and reduced residual autocorrelation. Moreover, GWR formed spatial clusters with local regression coefficients significantly higher or lower than those that the global OLS model resulted in, highlighting local variations. Areas near wetlands and irrigated lands displayed weak correlations while humid areas such as the Sudanian region at southern Sahel produced higher and more significant correlations. Finally, the spatial relationship of rainfall and NDVI displayed temporal variations as there were significant differences in the spatial trends throughout the study period.
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| 4. |
- Metcalfe, Daniel B., et al.
(author)
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Patchy field sampling biases understanding of climate change impacts across the Arctic
- 2018
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In: Nature Ecology and Evolution. - : Springer Science and Business Media LLC. - 2397-334X. ; 2:9, s. 1443-1448
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Journal article (peer-reviewed)abstract
- Effective societal responses to rapid climate change in the Arctic rely on an accurate representation of region-specific ecosystem properties and processes. However, this is limited by the scarcity and patchy distribution of field measurements. Here, we use a comprehensive, geo-referenced database of primary field measurements in 1,840 published studies across the Arctic to identify statistically significant spatial biases in field sampling and study citation across this globally important region. We find that 31% of all study citations are derived from sites located within 50 km of just two research sites: Toolik Lake in the USA and Abisko in Sweden. Furthermore, relatively colder, more rapidly warming and sparsely vegetated sites are under-sampled and under-recognized in terms of citations, particularly among microbiology-related studies. The poorly sampled and cited areas, mainly in the Canadian high-Arctic archipelago and the Arctic coastline of Russia, constitute a large fraction of the Arctic ice-free land area. Our results suggest that the current pattern of sampling and citation may bias the scientific consensuses that underpin attempts to accurately predict and effectively mitigate climate change in the region. Further work is required to increase both the quality and quantity of sampling, and incorporate existing literature from poorly cited areas to generate a more representative picture of Arctic climate change and its environmental impacts.
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| 5. |
- Njoku, Elijah A., et al.
(author)
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Quantitative assessment of the relationship between land use/land cover (LULC), topographic elevation and land surface temperature (LST) in Ilorin, Nigeria
- 2022
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In: Remote Sensing Applications: Society and Environment. - : Elsevier BV. - 2352-9385. ; 27
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Journal article (peer-reviewed)abstract
- The urbanization of landscapes and the increase in impervious land cover materials is known to cause significant changes in the landscape's thermal properties, typically leading to urban heat island (UHI). Although previous studies have investigated the impacts of land cover land use (LULC) and other factors on the urban land surface temperature (LST), the results of such studies are mixed. For instance, it is not yet clear what factors affect the spatial and temporal variation of the relationship between LULC and the LST, as well as the exact trajectory through which identified factors affect the thermal character of the urban landscape. In the current study, we examined the relationship between LULC, elevation and LST in Ilorin from the period of 2002–2020. The overriding aim was to understand the degree of association between LULC, elevation and LST, and the drivers of the spatial and temporal variation in the observed relationship. LST and NDVI were derived from Landsat data products. LST was derived using a mono-window algorithm and NDVI was used as proxy for LULC. The spatial pattern of LULC was analyzed using Moran's I spatial autocorrelation statistics. To investigate the relationship between LULC, elevation and LST, we adopted both ordinary least square (OLS) regression models and the geographically weighted regression (GWR) method to reveal both the linear and the geographically varying relationship between the LULC, elevation and LST. The results of the study show that the LULC pattern of Ilorin has been significantly altered during the 2000 to 2020 period. The urban proportion of the landscape has shown a significant increase, rising by more than 11% relative to 2002 figures, and the vegetation proportion, especially the forest component, has diminished within same temporal extent. LST values varied in both space and time, with high temperature clusters noticeable in the built-up areas and decreasing towards the urban periphery. The result of the autocorrelation analysis using Moran's I index reveals a significant clustering of LST in all the epochs investigated. Across the study area, minimum and maximum LST values of 0 °C and 41 °C were recorded in 2002 and 2020 respectively. Statistically significant relationships were observed between LULC, elevation and LST. However, the relationship between elevation and LST was very weak. Temporally, the strength of the relationship between the variables (as indicated in the variables' coefficient) as well as the overall model predictive performance (indicated by in the R2) fluctuated over the years. Spatially, the strength of relationship between LST and LULC and elevation varied significantly. LULC explained approximately between 26% and 64% of total LST variation in the city between 2002 and 2020. The study findings are relevant for efforts geared towards alleviating the degree of UHI or its impacts, generally, and in the city of Ilorin, in particular. An understanding of the spatial distribution of LULC and their impacts on the LST can be helpful in identifying areas needing attention. The observed relationship between relevant LULC classes can be incorporated into urban planning strategies to ensure a sustainable city development.
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| 6. |
- Rizinjirabake, Fabien, et al.
(author)
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Data for assessment of leached dissolved organic carbon in watersheds
- 2020
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In: Data in Brief. - : Elsevier BV. - 2352-3409. ; 32
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Journal article (peer-reviewed)abstract
- (“Dissolved organic carbon leaching flux in a mixed agriculture and forest watershed in Rwanda” [1]). This article presents data of leached dissolved organic carbon (LDOC), stream water dissolved organic carbon), rainfall amount (Ra), rainfall intensity (Ri), rainfall soil storage (S), runoff (Q), and soil properties such as total organic carbon (TOC), total nitrogen (TN), cation exchange capacity (CEC), and soil texture data collected in the Rukarara River Watershed (RRW), a tropical watershed. All these data were used to analyze leached dissolved organic carbon (LDOC) fluxes in the watershed and their relationship with stream DOC. LDOC and soil properties data were collected at three sites in multiple plots per site located in natural forest (NF), tea plantations (TP), plantation forests (PF), and croplands (CL). Twenty-three plots in total were sampled to collect LDOC data. Soil properties data were analyzed from soil samples collected nearby the plots. Soil texture elements data were used to calculate soil porosity and saturated hydraulic conductivity (Ks). Data of stream DOC were analyzed from water samples collected and analyzed in the laboratory using a TOC analyzer. Rainfall data were recorded within the RRW using tipping bucket rain gauges installed at three sites. These rainfall data were used to calculate rainfall intensity, potential surface runoff, and rainfall soil storage.
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| 7. |
- Rizinjirabake, Fabien, et al.
(author)
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Data for assessment of soil water extractable and percolation water dissolved organic carbon in watersheds
- 2019
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In: Data in Brief. - : Elsevier BV. - 2352-3409. ; 27
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Journal article (peer-reviewed)abstract
- (“Sources of soil dissolved organic carbon in a mixed agricultural and forested watershed in Rwanda”, [6]) This data article presents water extractable organic carbon (WEOC), percolation water dissolved organic carbon (pDOC), and mean antecedent precipitation indices (API) and mean antecedent temperature (MAT) data. The article also presents edaphic properties such soil texture elements, total organic carbon (TOC), total nitrogen (TN), cation exchange capacity (CEC), iron (Fe), and aluminum (Al). Additionally, the article presents topography attributes such including topographic position index (TPI) and curvature. All these data were used to analyze both WEOC and pDOC dynamics in the Rukarara River Watershed (RRW), Rwanda. WEOC and soil properties data were analyzed from sampled 52 soil composites samples collected during from October to December 2016 using 53 × 50 mm rings. Data of pDOC were analyzed from percolation water samples collected using a zero tension lysimeters on various dates during the period from Jun 2015 to Jun 2017. API and MAT data for various antecedent days were calculated on basis of rainfall and air temperature data recorded at three stations within the RRW using respectively tipping bucket rain gauges and sensors installed at three sites located representing the main land use land cover classes within the RRW.
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| 8. |
- Rizinjirabake, Fabien, et al.
(author)
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Data for dynamics analysis of riverine dissolved organic carbon in Rukarara watershed, Rwanda
- 2018
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In: Data in Brief. - : Elsevier BV. - 2352-3409. ; 20, s. 1252-1255
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Journal article (peer-reviewed)abstract
- This data article presents water stage, flow, and net primary productivity (NPP) data that were used to analyze the dynamics of the riverine dissolved organic carbon (DOC) dynamics in the Rukarara River watershed in Rwanda. We measured water stage data every 15 min and calculated daily averages used to estimate flow based on rating curves. The rating curves were produced using several measured contemporaneous water stage and flow data. Estimated flow data were used to calculate water dissolved organic carbon (DOC) loadings separate the total stream flow into quick and baseflow. Annual NPP data for a 15-year period were used to estimate the effect of proportion of stream DOC loading on carbon sequestration within the Rukarara River watershed.
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| 9. |
- Rizinjirabake, Fabien, et al.
(author)
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Dissolved organic carbon leaching flux in a mixed agriculture and forest watershed in Rwanda
- 2019
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In: Journal of Hydrology: Regional Studies. - : Elsevier BV. - 2214-5818. ; 26
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Journal article (peer-reviewed)abstract
- Study region: Rukarara River Watershed (RRW), Rwanda. Study focus: DOC leaching has important environmental consequences for both terrestrial and aquatic ecosystems. This paper measures leached dissolved organic carbon (LDOC) and the factors controlling its variation in the RRW, a mixed agriculture and forest watershed in Rwanda. The study describes the relationship of LDOC with land use/land cover (LULC), soil properties, rainfall characteristics, and stream DOC using linear regression and linear mixed effects models. New hydrological insights: The annual LDOC flux from topsoil to deeper soil horizons was 13.62 tonnes of carbon for the study area. This annual LDOC flux represents approximately 2% of the net primary productivity and 0.02% of the soil carbon stock. The plantation forest and natural forest sites showed higher LDOC fluxes than other LULC classes. Soil TOC, TN, rainfall intensity and amount positively affected LDOC flux (7 ≤ R2 ≤ 30%) in the RRW. Cation exchange capacity, runoff, and rainfall storage negatively affected the RRW (7 ≤ R2 ≤ 23%). LDOC flux explained 14% of the variation of stream DOC in the RRW. Our results imply that, under ongoing climate and LULC changes, an increase of the proportion of plantation forest and rainfall intensity throughout the watershed will increase LDOC flux, and will have a weak positive effect on stream DOC.
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| 10. |
- Rizinjirabake, Fabien, et al.
(author)
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Riverine dissolved organic carbon in Rukarara River Watershed, Rwanda
- 2018
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In: Science of the Total Environment. - : Elsevier BV. - 0048-9697. ; 643, s. 793-806
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Journal article (peer-reviewed)abstract
- Dissolved organic carbon (DOC) loading is rarely estimated in tropical watersheds. This study quantifies DOC loading in the Rukarara River Watershed (RRW), a Rwandan tropical forest and agricultural watershed, and evaluates its relationship with hydrological factors, land use and land cover (LULC), and topography to better understand the impact of stream DOC export on watershed carbon budgets. The annual average load for the study period was 977.80 kg C, which represents approximately 8.44% of the net primary productivity of the watershed. The mean daily exports were 0.37, 0.14, 0.075 and 0.32 kg C/m2 in streams located in natural forest, tea plantation, small farming areas, and at the outlet of the river, respectively. LULC is a factor that influences DOC loading. The quick flow was the main source of stream DOC at all study sites. Stream DOC increases with increasing water flow, indicating a positive relationship. Thus, the expectation is that a change in land cover and/or rainfall will result in a change of stream DOC dynamics within the watershed. Topography was also found to influence the dynamics of stream DOC through its effect on overland flow in terms of drainage area and total length of flow paths. Tea plantations were located in areas of high drainage density and projected increase of rainfall in the region, as a consequence of climate change, could increase stream DOC content and affect stream water quality, biodiversity, balance between autotrophy and heterotrophy, and bioavailability of toxic compounds within the RRW.
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| 11. |
- 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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| 12. |
- Yan, Yanzi, et al.
(author)
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The accuracy of drainage network delineation as a function of environmental factors : A case study in Central and Northern Sweden
- 2020
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In: Hydrological Processes. - : Wiley. - 0885-6087 .- 1099-1085. ; 34:26, s. 5489-5504
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Journal article (peer-reviewed)abstract
- Drainage networks delineated from Digital Elevation Models (DEMs), are the basis for the modelling of geomorphological and hydrological processes, biogeochemical cycling, and water resources management. Besides providing effective models of water flows, automatically extracted drainage networks based on topography can diverge from reality to varying degrees. The variability of such disagreement within catchments has rarely been examined as a function of the heterogeneity of land cover, soil type, and slope in the catchment of interest. This research gap might not only substantially limit our knowledge of the uncertainty of hydrological prediction, but can also cause problems for users attempting to use the data at a local scale. Using 1:100000 scale land cover maps, Quaternary deposits maps, and 2 m resolution DEMs, it is found that the accuracy of delineated drainage networks tends to be lower in areas with denser vegetation, lower hydraulic conductivity, and higher erodibility. The findings of this study could serve as a guide for the more thoughtful usage of delineated drainage networks in environmental planning, and in the uncertainty analysis of hydrological and biochemical predictions. Therefore, this study makes a first attempt at filling the knowledge gap described above.
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