Partitioning carbon sources between wetland and well-drained ecosystems to a tropical first-order stream – implications for carbon cycling at the watershed scale (Nyong, Cameroon)

Tropical rivers emit large amounts of carbon dioxide (CO2) to the atmosphere, in particular due to large wetland-to-river carbon (C) inputs. Yet, tropical African rivers remain largely understudied, and little is known about the partitioning of C sources between wetland and well-drained ecosystems to rivers. In a first-order sub-catchment (0.6 km2) of the Nyong watershed (Cameroon 27 800 km2), we fortnightly measured C in all forms and ancillary parameters in groundwater in a well-drained forest (hereafter referred to as non-flooded forest groundwater) and in the stream. In the first-order catchment, the simple land use shared between wetland and well-drained forest, together with drainage data, allowed the partitioning of C sources between wetland and well-drained ecosystems to the stream. Also, we fortnightly measured dissolved and particulate C downstream of the first-order stream to the main stem of order 6, and we supplemented C measurements with measures of heterotrophic respiration in stream orders 1 and 5. In the first-order stream, dissolved organic and inorganic C and particulate organic C (POC) concentrations increased during rainy seasons when the hydrological connectivity with the riparian wetland increased, whereas the concentrations of the same parameters decreased during dry seasons when the wetland was shrinking. In larger streams (order > 1), the same seasonality was observed, showing that wetlands in headwaters were significant sources of organic and inorganic C for downstream rivers, even though higher POC concentration evidenced an additional source of POC in larger streams during rainy seasons that was most likely POC originating from floating macrophytes. During rainy seasons, the seasonal flush of organic matter from the wetland in the first-order catchment and from the macrophytes in higher-order rivers significantly affected downstream metabolism, as evidenced by higher respiration rates in stream order 5 (756 ± 333 gC-CO2 m−2 yr−1) compared to stream 1 (286 ± 228 gC-CO2 m−2 yr−1). In the first-order catchment, the sum of the C hydrologically exported from non-flooded forest groundwater (6.2 ± 3.0 MgC yr−1) and wetland (4.0 ± 1.5 MgC yr−1) to the stream represented 3 %–5 % of the local catchment net C sink. In the first-order catchment, non-flooded forest groundwater exported 1.6 times more C than wetland; however, when weighed by surface area, C inputs from non-flooded forest groundwater and wetland to the stream contributed to 27 % (13.0 ± 6.2 MgC yr−1) and 73 % (33.0 ± 12.4 MgC yr−1) of the total hydrological C inputs, respectively. At the Nyong watershed scale, the yearly integrated CO2 degassing from the entire river network was 652 ± 161 GgC-CO2 yr−1 (23.4 ± 5.8 MgC CO2 km−2 yr−1 when weighed by the Nyong watershed surface area), whereas average heterotrophic respiration in the river and CO2 degassing rates was 521 ± 403 and 5085 ± 2544 gC-CO2 m−2 yr−1, which implied that only ∼ 10 % of the CO2 degassing at the water–air interface was supported by heterotrophic respiration in the river. In addition, the total fluvial C export to the ocean of 191 ± 108 GgC yr−1 (10.3 ± 5.8 MgC km−2 yr−1 when weighed by the Nyong watershed surface area) plus the yearly integrated CO2 degassing from the entire river network represented ∼ 11 % of the net C sink estimated for the whole Nyong watershed. In tropical watersheds, we show that wetlands largely influence riverine C variations and budget. Thus, ignoring the river–wetland connectivity might lead to the misrepresentation of C dynamics in tropical watersheds.

Evaluation of morphometric characteristics of Betwa drainage basin

The morphometric analysis of any drainage basin is considered useful for water resource studies such as flood assessment, water quality sampling, water use reporting, watershed management etc. Drainage basin is generally defined as the areal extent of land from which the surface runoff flows to a defined drain, channel, stream or river. It is mainly governed by the topography of the terrain. Geographical Information System and Image Processing Techniques can be used for the identification of morphological features and analyzing properties of the basin. The morphometric parameters include linear, areal and relief aspects. ‘Watershed Atlas of India’ (2014) on 1:50,000 scale is an important digital database for planning and monitoring of development programs on a watershed basis. It serves as a uniform baseline for developing a hydrological unit-based data bank to be used for the management of water resources in the country. Run-off, sedimentation, water balance, evapotranspiration and several other catchment characterization related studies may be taken up on a watershed basis. The present study deals with morphometric parameters such as stream order (Nu), stream length (Lu), bifurcation ratio (Rb), drainage density (D) and stream frequency (Fs) of the Betwa drainage basin. Geographically the basin (77° 30′ to 80° 12′ east longitudes and 23°30′ to 25°55′ north latitudes) is located in two states i.e. Madhya Pradesh and Uttar Pradesh occupying an area of 43780 km2. The length of the stream segment is maximum for the first-order stream and decreases as the stream order increases. This study would help in understanding the hydrological behaviour of the basin. This, in turn, may enable the local people to utilize the resources of the basin for the sustainable development of the area.

Evaluation of Erosion-Prone Areas in Lamurde River Basin, Nigeria Using Morphometric Prioritization Method

Remote sensing and GIS techniques have been increasingly used in characterization of drainage basin and prioritization of erosion prone watershed. This study uses remote sensing and GIS to characterise drainage basin morphometry and prioritize soil erosion prone sub watershed in the Lamurde watershed in Taraba state Nigeria. The study adopted standard formulae and methods to compute the morphometric parameters. The Lamurde watershed was delineated to fifteen sub-watersheds with each coded as WS1 to WS15. The result of the findings reveals that Lamurde watershed has a dendritic to sub-dendritic drainage pattern with the smaller streams intersecting the main trunk at acute angles. The findings reveal that Lamurde is a ninth order stream with total area of 1,458.66 km2 and a perimeter of 395.93 km. The basin also has 258,493 total number of streams. The main soil types in the Lamurde basin are fluvisol, lithosol, ferric luvisols and humic nitosols. The surface soil texture of the area is mainly loamy type and particle size classes are fine loamy type. Depth of soil varies from shallow to very deep and having parent material derived from sandstones, mudstones and shales. The findings of the study reveals that watershed: WS7, WS8, WS5, WS11, WS15, WS14, WS2 and WS6 in ascending order are very highly vulnerable to soil erosion. Despite inherent limitation in the use of morphometric parameters to prioritize erosion prone sub watersheds, it is most suitable in the present circumstances because of inadequate information and lack of functional measurement station in the basin, since they have more stable and accessible data on which prioritization of the watersheds can be based on. This study contributes to the problem of dearth of information regarding the susceptibility to erosion in the Lamurde River Basin in Taraba State Nigeria. Based on this findings, these sub watersheds should be given higher priority on any soil conservation intervention measures in the study area. This will go a long way to help address the problem of soil erosion in the area.

Changes in the hydro-sedimentary balance: Impacts of the use of a borrow pit in a low-order stream

Construction of dams for hydroelectric power requires significant quantities of soil and rock, which are often extracted in borrow pits from adjacent regions. Although the effects of dams on stream processes has received significant attention, the effects of borrow pits has not. The main objective of this study was to analyze the geomorphological and sedimentological aspects of two second-order streams, one of which was directly affected by the borrow pit located upstream of its source (Pedra Branca stream). Flow rates were measured and cross-sections of 600m stretches in both streams were monitored over a hydrological year. At the same time, sediments from the bed of the channels and soils on their banks had their physical and chemical characteristics evaluated. Streams sediments differed in their chemical and organic matter composition. The mean particle size of the sediment particles was different between the reference and degraded streams. The water flow was very similar to both streams, only varying along the seasonal seasons. However, the fluvial channels presented great geomorphological differentiation, mainly downstream, due to the location of the Pedra Branca stream and its proximity to the borrow pit. Despite the great importance for the production of clean electric energy, the construction of hydroelectric plants promotes persistent impacts that affect structural and functional aspects of the adjacent aquatic habitats. Borrow pits used for the construction of projects become large sources of sediment for aquatic environments, affecting the drainage network of the hydrographic basin and the balance of river erosion, transport and deposition processes. The results show the need to review the intervention protocols in borrow pits and the environmental legislation that regulates their rehabilitation.

MORPHOMETRIC ANAYSIS OF THE MAJOR VALLEY SYSTEMS AROUND BENGALURU

The three major Valley systems of Bengaluru namely Vrishabhavathi Valley, Hebbal Valley and Kormangala-Challaghatta Valley houses many lakes and play a very important role in its hydrological processes. The morphometric analysis helps us to learn about the characteristics of the underlying rock type, pervious nature of soil, slope gradients, runoff behavior and water retention potential within the Valley systems. Morphometric analysis was carried out for Linear, areal and relief aspects. The Survey of India topographical maps and Digital Elevation Model data were used to prepare the base map and the drainage maps with the help of GIS software. The Strahler system of stream ranking was adopted. Among the three Valleys, Vrishabhavathi Valley is observed to be the largest Valley in terms of area and perimeter. Vrishabhavathi Valley basin has sixth order stream as the highest stream order where as the other two Valleys have fifth order stream as the highest order. The drainage pattern formed within the Valley systems was observed to be dendritic. The watershed shape factor showed that the Vrishabhavathi Valley is elongated in shape where as the K-C Valley and the Hebbal Valleys are less elongated in shape comparatively. The drainage density of the three Valleys revealed that they fall under coarse drainage density classification. The relief aspects of the three Valleys exhibit low reliefs indicating a flat surface. This helps in designing a sustainable management plan for the three major Valley systems in terms of their conservation and also ensure sustainable soil and water usage within the Valley systems.

Establishment of Geochemical Thresholds for Vanadium Throughout Korea and at Potential Development Sites Using Geochemical Map Data

Geochemical maps can be used for a variety of purposes, one of which is to establish regional or local geochemical thresholds for the analyzed elements. In the case of vanadium, as industrial demand and use increases, it is necessary to expand the development of vanadium in Korea. However, the environmental management standards are insufficient. Therefore, in this study, using geochemical data, we derived geochemical threshold values for the entire country and areas with potential for the development of vanadium deposits. The regional (country-wide) threshold value was derived using logarithmic transformation of raw data (N = 23,548) of the first- and second-order stream sediments collected across the country in the late 1990s and the early 2000s. The median + 2 median absolute deviation (MAD) and Tukey inner fence (TIF) values were 116 mg/kg and 200 mg/kg, respectively. Of these, the TIF standard, which showed 0.6% of data exceeding the threshold, was judged to be appropriate for distinguishing clear enrichment or contamination of vanadium. In the case of the Geumsan and Pocheon, areas with potential for vanadium development, the TIF and median + 2 MAD values of 259 mg/kg and 218 mg/kg, respectively, can be used as the criteria for evaluating the impact of environmental pollution before and after deposit development. Likewise, by deriving threshold values of the target elements using geochemical map data, it is possible to provide basic environmental information for geochemical evaluation and follow-up management in advance during large-scale site development.

Morphometric analysis of a drainage basin using geographical information system in Gilgel Abay watershed, Lake Tana Basin, upper Blue Nile Basin, Ethiopia

GIS and remote sensing approach is an effective tool to determine the morphological characteristics of the basin. Gilgel Abay watershed is stretched between latitude 10.56° to 11.22° N and longitude 36.44° to 37.03° E which is one major contributing river of Lake Tana which is the source of Blue Nile. The present study addressed linear and areal morphometric aspect of the watershed. The study deals with emphasis on the evolution of morphometric parameters such as stream order, stream length, bifurcation ratio, drainage density, stream frequency, texture ratio, elongation ratio, circularity ratio, and form factor ratio. The morphometric analysis of the basin revealed that Gilgel Abay is firth-order drainage basin with total of 662 drainage network, of which 511 are first order, 111 are second order, 30 are third order, 9 are fourth order, and 1 is fifth-order stream. The total length of stream is longer for first order and decrease with increasing stream order. The mean bifurcation ratio is 5.16 which is greater than the standard range, and it indicates that basin is mountainous and susceptible to flooding. Low drainage density is observed which is 0.6 km−2. It indicates that basin is highly permeable and thick vegetation cover. Areal aspect of the morphometric analysis of the basin revealed that the basin is slightly potential to flooding and soil erosion, indicating that runoff generated from the upland area of the watershed is significantly infiltrated at the gentle downstream part and contributing to groundwater potential. Further studies with the help of GIS and remote sensing with high-resolution remote sensing data integrating with ground control data in the field are more effective to formulate appropriate type of natural resource management system.

Simulating Preferential Flow by A Multi-dimensional Process-based HYDRUS Model

<p>Preferential flow<strong> </strong>(PF) processes are controlled by subsurface structures with a hierarchical organization across scales, but there is a lack of multiscale model validation using field data. In this study, a comprehensive dataset collected in the forested Shale Hills catchment was used to test and validate PF simulations with the 2-dimensional HYDRUS-2D model at the hillslope scale. The simulations were also compared with the 1-dimensional results at the pedon scale (HYDRUS-1D) and 3-dimensional results at the catchment scale (HYDRUS-3D). There was a good agreement between the 1D simulations and soil moisture measurements, which were mainly affected by the vertical change in porosity/permeability with depth and precipitation characteristics. However, short-term fluctuations due to PF were poorly captured. Notably, 2D and 3D simulations, accounting for PF controlled by slope position and shallow fractured bedrock, provided better results than the 1D simulations. The dual-porosity or anisotropic model provided more accurate soil moisture predictions than the single-porosity or isotropic model due to the more realistic representation of local soil and fractured shale. Consequently, our study shows the importance of multi-dimensional model approaches and the need to adequately represent the bedrocks' soil structure and fractured nature for the PF simulation. The multi-dimensional modeling approaches can represent PF pathways to the first-order stream and shows the benefits of the 3D simulation with detailed information to identify the dominant hydrological process.</p>

Periodic alterations of the hydrological exchange in hyporheic sediments: colmation, hyporheic fauna and abiotic parameters in a second order stream during one year

<p>In this study, the temporal variability of the hydrological exchange between stream water (SW) and groundwater (GW), colmation, hyporheic invertebrate fauna, organic matter (OM) and physicochemical parameters were examined for the period of one year. Sampling and measuring were conducted monthly from May 2019 to April 2020 at the Guldenbach river, a second order stream in Rhineland-Palatinate, Germany. All hyporheic samples were extracted from a depth of 15 cm below stream bottom. Colmation was measured quantitatively in the same depth.</p><p>Following the biotic and abiotic patterns found, three temporal stages of different hydrological conditions can be described:</p><ul><li>1) Strong floods, in February and March 2020 caused hydromorphological alterations of the river bed, leading to a decolmation of the hyporheic zone, a wash out of OM and hyporheic fauna. Due to high GW tables the vertical hydrological gradient (VHG) was positive indicating upwelling GW.</li> <li>2) In the months of Mai to August 2019 and April 2020, precipitation and stream discharge were lowest. Predominantly exfiltrating conditions were observed, while the amount of fine sediments (clay and silt) increased as well as colmation. High densities of hyporheic fauna, dominated by fine sediment dwelling taxa, were assessed.</li> <li>3) From September 2019 to January 2020 stream discharge was low. The VHG became increasingly negative, indicating downwelling SW. In accordance, colmation increased continuously, while densities of hyporheic invertebrates decreased and sediment dwellers became more dominant.</li> </ul><p>Precipitation, discharge events and GW table were found to be the driving factors for the annual dynamics of the hydrological exchange as well as for colmation, fauna and hydrochemistry. Electric conductivity seems a suitable indicator for the origin of water with high values in months of low precipitation and lower values after extensive precipitation events, respectively. Hyporheic fauna displayed a significant seasonality and the community structure was correlated with colmation and changes in the VHG.</p><p>This pronounced seasonality seems to be typical of many streams and should be considered for the monitoring of sediments and hyporheic habitats: Seasons with lower stream discharge are probably the most critical periods for sediment conditions.</p><p>We assume that the basic patterns of the dynamics observed basically reflect the natural situation in the catchment. However, the strength of surface run-off and the amount of fine sediments are mainly the result of anthropogenic activities and land use in the catchment.</p><p>These findings underline the significance of dynamical processes for the assessment and implementation of the Water Framework Directive.</p>