River Catchment Research Articles

Quantitative Analysis of Planform Changes in the Lower Mahaweli River, Sri Lanka

ABSTRACT The development of hydraulic structures has impacted the river discharge and sediment transportation, thus highlighting the river planform changes. Among 103 river catchments in Sri Lanka, the Mahaweli River is the longest river with the largest basin. Many development projects over the years diversely impacted the changes in river masks. However, no study has been conducted to quantify the planform changes in the lower Mahaweli River. Therefore, a comprehensive study was conducted to analyse the river planform changes over 30 years (1991-2021) from Damanewewa to Trincomalee. Freely available remotely sensed satellite data with 30 m resolution were used in the analysis. These images were processed using the QGIS mapping tool and RivMAP toolbox in MATLAB. Major changes were identified at the downstream part of the river and an oxbow lake formation was also observed. The average width for the entire reach (Wra) was identified as 14.83 m and channel width average (Wavg) was noted as 18.09 m. In addition, erosion and accretion rates were calculated, and the cumulative sequence of these rates was increased over the years affecting the change in channel width. Furthermore, the migration rates were also computed with generated river centerline. Highest migration rate reached about 400 m/yr, in the downstream which finally leads to severe meandering. Results revealed that this methodology can be applied to similar river planform analysis. Further, these results showcase the potential importance of analyzing channel stability as well as for water resource management.

Identifying and testing adaptive management options to increase river catchment system resilience using a Bayesian Network model

AbstractThe cumulative impacts of future climatic and socioeconomic change threaten the ability of freshwater catchments to provide essential ecosystem services. Stakeholders who manage freshwaters require decision-support tools that increase their understanding of catchment system resilience and support the appraisal of adaptive management options to inform decision-making. Our research aims to test the ability of a Bayesian Network model to identify adaptive management scenarios and test their effectiveness across future pathways to 2050. Using the predominantly arable river Eden catchment (320 km2) in eastern Scotland as a case study, we invited stakeholders from multiple sectors to participate in a series of workshops aimed at addressing water quality issues and achieving good ecological status in the catchment both now and in the future. Our participatory methods helped stakeholders overcome multiple layers of complexity and uncertainty associated with future-focused water management. Outputs of a Bayesian Network model simulated both current and future catchment resilience to inform the identification of six management scenarios. The effectiveness of each management scenario was tested using the Bayesian Network model. Two adaptive management scenarios increased catchment resilience and helped achieve good ecological status; a ‘Best Available Technology’ scenario, including aerobic granular sludge treatment, and a management scenario focused on ‘Resource Centre’, including phosphorus recovery from wastewater treatment works and constructed lagoons for crop irrigation. Stakeholders were interested in a 'Nature Based' management scenario including options such as wetland wastewater treatment methods and rural sustainable drainage systems, which improved water quality in the catchment, but had lower certainty in achieving desired outcome. Findings led to a recognition that innovative and collaborative action was required to improve current and future freshwater conditions.

How effective are ecological metrics in supporting conservation and management in degraded streams?

AbstractBiodiversity loss is increasing worldwide, necessitating effective approaches to counteract negative trends. Here, we assessed aquatic macroinvertebrate biodiversity in two river catchments in Switzerland; one significantly degraded and associated with urbanisation and instream barriers, and one in a near-natural condition. Contrary to our expectations, environmental heterogeneity was lower in the near-natural stream, with enhanced productivity in the degraded system resulting in a greater range of environmental conditions. At face value, commonly employed alpha, beta and gamma biodiversity metrics suggested both catchments constituted healthy systems, with greater richness or comparable values recorded in the degraded system relative to the near-natural one. Further, functional metrics considered to be early indicators for anthropogenic disturbance, demonstrated no anticipated differences between degraded and near-natural catchments. However, investigating the identity of the taxa unique to each river system showed that anthropogenic degradation led to replacement of specialist, sensitive species indicative of pristine rivers, by generalist, pollution tolerant species. These replacements reflect a major alteration in community composition in the degraded system compared with the near-natural system. Total nitrogen and fine sediment were important in distinguishing the respective communities. We urge caution in biodiversity studies that employ numerical biodiversity metrics alone. Assessing just one aspect of diversity, such as richness, is not sufficient to track biodiversity changes associated with environmental stress. We advocate that biodiversity monitoring for conservation and management purposes must go beyond traditional richness biodiversity metrics, to include indices that incorporate detailed nuances of biotic communities that relates to taxon identity.

Sediment source tracing in a Brazilian subtropical catchment using diffuse reflectance: Effect of spectral ranges, pre-processing techniques, and multivariate model

ABSTRACT Agriculture intensification in Southern Brazil’s subtropical regions combined with the frequent occurrence of erosive rainfall has rendered the area a global water erosion hotspot. In this scenario, understanding and regulating erosion processes at the river catchment scale is critical for mitigating soil and water resource degradation. Traditional methods for tracing sediment sources are expensive and time-consuming and justify the development of alternative approaches. Therefore, in this study, we employed diffuse reflectance spectroscopy analyses in the ultraviolet-visible (UV-VIS), near-infrared (NIR), and mid-infrared (MIR) ranges, combined with multivariate models and spectral pre-processing techniques to estimate sediment source contributions in a homogeneous subtropical catchment (Conceição River, 804 km²). Soil samples (n = 181) were collected to characterize the four potential sediment sources, including: cropland (n = 78), stream bank (n = 36), unpaved road (n = 40) and pasture (n = 27). Moreover, 44 sediment samples were collected, including suspended sediment (n = 8), fine sediment deposited on the riverbed (n = 15), and suspended sediment samples collected in the water column during storm events (n = 21). Vector machine (SVM) model outperformed the others, with better accuracy and reliability. While UV-VIS spectra proved less effective due to soil homogeneity across the catchment, NIR and MIR spectra provided valuable information for discriminating sediment sources. Furthermore, reducing the number of potential sources (from four to three or two) improved model predictions, especially when distinguishing between surface sources (cropland and pasture) and subsurface sources (unpaved roads and stream banks). The study’s findings shed light on the power of efficient and cost-effective alternative methods for assessing sediment sources, which are vital for promoting effective erosion control and sustainable land management in similar regions.

The impacts of climate change, early agriculture and internal fluvial dynamics on paleo-flooding episodes in Central China

Floods clustered in episodes are the most prevalent natural disaster worldwide, causing substantial economic and human losses. Although these events are often linked to time-periods of extreme rainstorms and unique atmospheric circulation patterns, the river basin characteristics affected by anthropogenic land use changes could exert a strong influence. However, the way and extent of how land use changes across different time scales affect flooding periods are still unclear, especially considering the historical land use changes. This study uses the Landlab landscape evolution model, coupled with an evapotranspiration model, to investigate the forcing factors for the paleo-flooding trends in the Wei River catchment over the last 5000 years. The results indicate that the flooding period from 4400 to 4000 BP was caused by an increase of 28 % in antecedent moisture content as well as a decrease of 28 % in its spatial variability, which are primarily due to climate change, and that the contribution of land-use change is less than 5 %. The increases of about 14 % and 8 % in main channel sedimentation rate play a leading role in flood generation during the time periods from 3400 to 2800 BP and 2000–1400 BP, respectively. These two periods of increased flooding are primarily caused by the erosional effects of increasing anthropogenic land use, whose contributions range from 33 % to 64 %. Furthermore, based on our modelling results, we suggest that the downstream propagation of the main flooding locations, from the Wei River to the lower reaches of the Yellow River, can be explained by the downstream migrating sediment wave. In conclusion, our simulation results give new insights into the causes of Holocene flooding periods in the middle Yellow River from the perspective of dynamic changes in catchment characteristics, which is helpful to improve regional flood risk management under future climate change and anthropogenic activities.

Origin and composition of dissolved and particulate matter in mountain rivers of the Nepal Himalayas: Insights from Sr and Nd isotopes and elemental ratios

The Ganga River exhibits elevated concentrations of dissolved strontium (Sr) and a higher radiogenic Sr (87Sr/86Sr) among global rivers, which impacts oceanic Sr composition. This has implications for understanding weathering and sediment flux into open oceans. The Nepal Himalayan section is a significant part of the upstream Ganga River catchment. We analyzed Sr and Nd isotopes and major element concentrations in water and sediments to trace compositional and seasonal variability in dissolved and particulate matter. The Sr and Nd isotopes, in particular, are crucial in this study as they provide unique signatures that can be used to trace the origin and composition of the dissolved and particulate matter. Representative rivers draining monolithological terrains were selected to investigate the isotopic and elemental provenance and weathering intensity. The elemental ratios (Ca/Na, Mg/Na) indicate the watersheds are carbonate-rich terrains. The Lesser Himalaya (LH) rivers transport radiogenic Sr with high values compared to rivers draining the Tethyan Sedimentary Series (TSS), Higher Himalayan Crystalline (HHC), Mahabharat, and Siwalik Hills. The dry season records a higher 87Sr/86Sr ratio than the monsoon. Due to the faster dissolution of carbonates compared to silicates, monsoon waters transport less radiogenic Sr. The significant correlation of 87Sr/86Sr in dissolved and particulate phases signifies that short-term interactions between sediment and water may alter the 87Sr/86Sr composition. Notably, sediments originating from continental rocks exhibit an inverse correlation between 87Sr/86Sr and εNd. In conclusion, the Sr and Nd isotopic records in this study categorize the fluvial catchments into geological clusters aligned with the TSS, LH, and Siwaliks, which advance our understanding regarding the provenance besides providing crucial insights into geological processes, weathering, landscape evolution, and sediment flux.

Dioxins, PFOS, and 20 other Persistent Organic Pollutants in Eggs of Nine Wild Bird Species from the Vaal River, South Africa

The Vaal River catchment drains the largest and most populated industrial and mining region in Southern Africa. Heron, ibis, cormorant, egrets, and darter eggs, representing three habitats and four feeding guilds, were collected at four locations in 2009/10 to identify hotspots and hazards associated with persistent organic pollutants (POPs). The POPs included 21 organochlorine pesticides, five polybrominated diphenyl ether (PBDE) classes, 18 polychlorinated biphenyls (PCBs including six non-dioxin-like PCBs; NDL-PCB), and 12 dioxin-like PCBs (DL-PCBs), 17 polychlorinated dibenzo-p-dioxins and dibenzo-p-furans (PCDD/Fs), and perfluorooctane sulfonate (PFOS). Aquatic predators had higher PFOS and PCDD/F concentrations, while PCBs dominated in terrestrial eggs. Organochlorine pesticides, PBDEs, and PCBs were strongly associated with eggs from the industrial regions, while PCDD/F concentrations were evenly distributed. PCDD/F and PCB toxic equivalency quotient concentrations were low with no adverse effects expected. PFOS peaked at Bloemhof Dam with a maximum of 2300 ng/g wm in an African Darter egg, indicating an unexpected PFOS hotspot, the source of which is unknown. Despite order of differences in compound class concentrations, there was no association with egg size. To the best of our knowledge, this is the only study that analysed all 2010 POPs in bird eggs on a large geographic scale. This study highlighted the importance of multi-species studies sampling from multiple locations to assess the risk that POPs pose to avian populations as hotspots and species at risk may be missed by studies looking at one or few species.

Asymmetry Factor application based on the sectorization of watersheds: Morphostructural analysis of the Chopim River catchment, Paraná State (Brazil)

Asymmetry Factor application based on the sectorization of watersheds: Morphostructural analysis of the Chopim River catchment, Paraná State (Brazil)

Land-Use Pattern-Based Spatial Variation of Physicochemical Parameters and Efficacy of Safe Drinking Water Supply along the Mahaweli River, Sri Lanka

Exploration of the pollution status of river-based water sources is important to ensure quality and safe drinking water supply for the public. The present study investigated physicochemical parameters of surface water in the upper segment of River Mahaweli, which provides drinking water to the Nuwara Eliya and Kandy districts of Sri Lanka. River surface water from 15 intakes and treated water from 14 Water Treatment Plants (WTPs) were tested for pH, water temperature, turbidity, EC, COD, 6 anions, 21 cations, 3 pesticides, and 30 antibiotics once every 3 months from June 2022 to July 2023. Except for turbidity and iron concentrations, all other parameters were within the permissible range as per the Sri Lanka Standard Specification for Potable Water (SLS 614:2013). The uppermost Kotagala WTP raw water had a high concentration of iron due to runoff from areas with abundant iron-bearing minerals. Turbidity increased as the river flowed downstream, reaching its highest value of 13.43 NTU at the lowermost Haragama. Four intakes had raw surface water suitable for drinking as per the Water Quality Index (WQI). Pollution increased gradually towards downstream mainly due to agricultural runoff, industrial effluents, and urbanization. Poor water quality at the upstream Thalawakale-Nanuoya intake was due to highly contaminated effluent water coming from Lake Gregory in Nuwara Eliya. Cluster analysis categorized WTP locations in the river segment into 3 clusters as low, moderate, and high based on contaminations. Principal component analysis revealed that the significance of the 41.56% variance of the raw water was due to the pH and the presence of heavy metals V, Cr, Ni, Rb, Co, Sr, and As. All treated water from 15 WTPs had very good to excellent quality. In general, heavy metal contamination was low as indicated by the heavy metal pollution index (HPI) and heavy metal evaluation index (HEI). The treatment process could remove up to 94.7% of the turbidity. This is the first attempt to cluster the river catchment of the Mahaweli River based on physicochemical parameters of river water. We present here the land-use pattern-based pollution of the river and efficacy of the water treatment process using the Mahaweli River Basin as a case study. Regular monitoring and treatment adjustments at identified points are recommended to maintain the delivery of safe drinking water.

Anuran occupancy varies with stream characteristics and flow across Arizona wilderness areas

Abstract Riparian ecosystems comprise less than 2% of the landscape in the arid western United States, yet they provide habitat and resources to over half of arid‐land wildlife species, including a broad diversity of anurans (frogs and toads). Despite this, relatively few studies describe riparian herpetofaunal communities in the Southwest, and remote wilderness areas are particularly undersurveyed. We employed passive acoustic monitoring to capture anuran advertisement calls in wilderness area tributaries of the Verde River, Arizona, USA. In the spring and summer of 2021 and 2022, 13–29 autonomous recording units (ARUs) were deployed along perennial, intermittent and ephemeral tributary stream reaches. We characterized sites based on the per cent of pool, riffle, run and side channel habitat within 100 m of each ARU and quantified substrate, wetted channel width and canopy cover. To relate anuran occupancy to environmental and hydrological variables, we evaluated acoustic detection data using single‐species, single‐season occupancy models. Four species were detected in this study: canyon treefrog (Hyla arenicolor), red‐spotted toad (Anaxyrus punctatus), Woodhouse's toad (Anaxyrus woodhousii) and non‐native American bullfrog (Lithobates catesbeianus), with canyon treefrog being the most ubiquitous species observed. Vocal activity of canyon treefrog was higher at perennial and intermittent sites compared to ephemeral sites, and the presence of pools was most strongly associated with canyon treefrog occupancy. Persistent, slow‐moving water best predicted the presence of breeding canyon treefrogs. Notably, this study did not detect several species with historical records in the middle Verde River catchment, including Arizona toad (Anaxyrus microscaphus) and northern leopard frog (Lithobates pipiens). Given climate change‐related flow declines and intensifying demands for water in arid lands globally, maintaining stream flows that provide consistent and suitable hydroregimes for anuran breeding and larval development is of increasing importance. Determining habitat use and flow regimes necessary to support anuran populations can aid in prioritization of conservation actions related to water management and predict how changes in water availability may impact stream‐breeding anurans.

Far-future hydrology will differentially change the phosphorus transfer continuum

Climate change is likely to exacerbate land to water phosphorus (P) transfers, causing a degradation of water quality in freshwater bodies in Northwestern Europe. Planning for mitigation measures requires an understanding of P loss processes under such conditions. This study assesses how climate induced changes to hydrology will likely influence the P transfer continuum in six contrasting river catchments using Irish national observatories as exemplars. Changes or stability of total P (TP) and total reactive P (TRP) transfer processes were estimated using far-future scenarios (RCP4.5 and RCP8.5) of modelled river discharge under climate change and observed links between hydrological regimes (baseflow and flashiness indices) and transfer processes (mobilisation and delivery indices). While there were no differences in P mobilisation between RCP4.5 and RCP8.5, both mobilisation and delivery were higher for TP. Comparing data from 2080 (2070–2099) with 2020 (2010–2039), suggests that P mobilisation is expected to be relatively stable for the different catchments. While P delivery is highest in hydrologically flashy catchments, the largest increases were in groundwater-fed catchments in RCP8.5 (+ 22% for TRP and + 24% for TP). The inter-annual variability of P delivery in the groundwater-fed catchments is also expected to increase. Since the magnitude of a P source may not fully define its mobility, and hydrological connections of mobilisation areas are expected to increase, we recommend identifying critical mobilisation areas to target future mitigation strategies. These are hydrologically connected areas where controls such as soil/bedrock chemistry, biological activity and hydrological processes are favourable for P mobilisation.

Relationships between benthic macroinvertebrates and environmental variables of Nyamuhinga River, Lake Kivu southwest basin, Sud-Kivu, Democratic Republic of the Congo.

Benthic macroinvertebrate abundance and taxa richness associated with environmental variables were monitored monthly from September 2015 to August 2016 in the Nyamuhinga River. The benthic macroinvertebrate samples were collected using a D-frame aquatic net employing the man-time method. Environmental variables including pH, temperature, conductivity, and total dissolved solids (TDS) were measured using a Combo HI 98129 and GREISIN-GER 020 multimeters, and water flow was assessed using the Float Method procedure. The Shannon-Wiener and equitability indices were then calculated to assess diversity and richness, facilitating the comparison of diversity within sites or stations. A total of 35,142 macroinvertebrate individuals were collected from the 12 sampling sites belonging to 29 genera, 27 families, and 9 orders. Diptera were the most abundant (71.83%) followed by Odonata (9.13%) and Ephemeroptera (7.11%). The findings showed that taxa richness decreased from upstream to downstream. At the same time, absolute abundance increased from downstream to upstream due to riparian vegetation, substrate type, plant debris, and organic matter which are habitats for benthic macroinvertebrates in the river. Environmental variables such as flow, pH, temperature, conductivity, and TDS varied between sites and stations because of habitat disturbances, contaminant discharges into the catchment, and inflow of tributary waters into the river. The canonical correspondence analysis (CCA) results displayed that Eristalis and Chironomus were strongly associated with the flow, conductivity, TDS, temperature, and pH at the downstream sites related to anthropogenic activities from the catchment. From our results, the Nyamuhinga River needs conservation master plan/guidelines and increased awareness to reduce environmental impacts in Bukavu River catchments in the Lake Kivu basin.

Review on Conservation Approach Study of Largest Mangrove Ecosystem as the Mitigation of Climate Change

Bangladesh’s complicated socioeconomic background serves as an example of the 21st-century worldwide environmental challenges. With a land area of 10,200 km2, the ‘Sundarbans’ is the world’s biggest mangrove forest, a hotspot for biodiversity, and home to several rare or endangered birds, reptiles, and mammals. Issues with biodiversity and climate change are linked here. Bangladesh and India share this important carbon sink ecosystem downstream of the Ganges Brahmaputra Meghna River complex. The Sundarbans are extremely beneficial to the local population because they produce beeswax, honey, and an abundance of seafood. They also prevent saltwater intrusion onto arable land, accumulate sediment as sea levels rise, and lessen the effects of seasonal droughts and monsoon floods. The mangrove is the main source of food and money for a large number of the impoverished residents that live in and around this forest, mostly from farming and fishing. The negative effects of climate change are becoming more and more evident. Preserving this worldwide treasure is of utmost importance; nevertheless, how can this forest be sustained over the long haul? In order to ensure the effective conservation of this globally significant ecosystem in the context of climate mitigation, this paper evaluates the advantages of the Sundarbans ecosystem’s good ecosystem function and lays out an integrated approach at the national (Bangladesh), bi-national (India-Bangladesh), and regional (with all countries of the Sundarbans river catchment system) levels.

Processing time and precision of aging structures for Bighead Carp and Silver Carp in the lower Red River catchment in the southern Great Plains

AbstractObjectivePopulation demographics of invasive species are commonly evaluated to better develop management actions that are useful for reducing their abundance or controlling the population. Bighead Carp Hypophthalmichthys nobilis and Silver Carp H. molitrix are emblematic invaders in the United States, where they continue to expand their range. There is currently no consensus about which hard structure from these species is best for age estimation. Our study objective was to compare the processing time and precision of five hard structures used for age estimation of both species.MethodsWe sampled fish in the lower Red River catchment of Oklahoma, Arkansas, and Texas during summer and autumn 2021–2022 and removed the lapillus otolith, left primary pectoral fin ray, postcleithrum, urohyal bone, and anterior‐most pterygiophore of the dorsal fin from both Bighead and Silver carp. The structures (n = 1204) were either embedded in epoxy or thin‐sectioned and mounted on slides. Two readers estimated the age of the fish by using each structure and came to a consensus. Processing time was recorded from the onset of laboratory processing to the termination of polishing the cross sections for age estimation.ResultProcessing of otoliths was comparable to or faster than processing of the other structures and resulted in the highest between‐reader agreement. The lowest coefficients of variation in age estimation were represented using lapillus otoliths for Bighead Carp and postcleithra for Silver Carp. Our age bias plots indicated that all other structures underestimated age relative to the lapillus otoliths.ConclusionOur results indicated that using lapillus otoliths for age estimation of these species would have the highest between‐reader agreement and would incur no additional laboratory processing time. However, validation is needed to assess whether lapillus otoliths correctly age these species. From a management perspective, use of this structure would facilitate improved population comparisons.

Contaminant of Emerging Concerns in Modder River Catchment of Free State: Implication for Environmental Risk and Water Sources Protection

This study was aimed at monitoring the occurrence and potential sources of emerging contaminants in water sources within the Modder River catchment. Selected water quality indicators were analysed by Hanna multi-parameter meters. Emerging contaminants such as acetaminophen, carbamazepine, ibuprofen, atrazine, simazine, metolachlor, terbuthylazine, 17-alpha-ethinyl-estradiol, estradiol, progesterone, and testosterone were analysed by high performance liquid chromatography mass spectrometry. The sources of emerging contaminants were determined by statistical methods such as Pearson correlation and hierarchical cluster analysis. Results showed that all the sampled water sources have some level of questionable drinking water quality and necessitate some amount of treatment to reduce the contamination before consumption, especially DO, EC, and pH. The 17-alpha-ethinyl-estradiol mean values in rivers (7.79 and 31.55 µg/L), dams (1.83 and 6.90 µg/L), and treated drinking water (0.2 and 0.73 µg/L) were the highest in summer and autumn seasons, respectively. Wastewater effluents, domestic sewage, urban surface runoff, agricultural runoff, and illegal dumping were identified as the possible sources of emerging contaminants pollution. Waste management education, proper application of herbicides, and advance wastewater treatment methods were some of the suggested mitigation strategies. The outcomes may be relevant for environmental protection and water sustainability in the catchment.

Effect of Land Use Land Cover on the Hydrology of Rwizi River Catchment located in Southwestern Uganda

Changes in land use and land cover usually affect the hydrological processes of river flows, sediment transport, and water quality on a global scale. Hence, the objective of this paper was to investigate the effect of Land Use Land Cover (LULCC) on the Hydrology of Rwizi River Catchment located in South-western Uganda using appropriate standard techniques and procedures including Soil and Water Assessment Tool (SWAT). Model calibration and validation demonstrated good agreement (R² = 0.76-0.86, NSE = 0.75-0.78). LULC analysis revealed significant increases in built-up areas and forestland, with declines in grazing land and wetlands. These changes were driven by socio-economic factors and hydro-climatic influences. The study found strong correlations between LULC types and sediment yield, highlighting the implications for water quality and erosion control. The results show that urban growth raises surface runoff and peak flows as a result of a rise in impervious surfaces, whereas agricultural intensification raises water demand and lowers base flow, especially during dry spells. Deforestation has led to increased sedimentation rates, degrading water quality. These findings underscore the significant anthropogenic and climatic impacts on LULCC dynamics and hydrological processes in the catchment. The study emphasizes the critical need for integrated watershed management strategies to mitigate these impacts and ensure sustainable water management practices. By advancing understanding in hydrological sciences, this research informs policy and management decisions for sustainable development in tropical river basins globally.

Evaluating input data sources for isotope‐enabled rainfall‐runoff models

AbstractIsotope‐enabled models provide a means to generate robust hydrological simulations. However, daily isotope‐enabled rainfall‐runoff models applied to larger spatial scales (>100 km2) require more input data than conventional non‐isotope models in the form of precipitation isotope time series, which are difficult to generate even with point station measurements. Spatially distributed isotope data can be circumvented by isotope‐enabled climate models. Here, we evaluate the hydrological simulations of the J2000‐isotope enabled hydrological model driven with data from corrected and un‐corrected isotope‐enabled global and regional climate models (isotope‐enabled global spectral model [IsoGSM] and isotope‐enabled regional spectral model [IsoRSM], respectively) compared with 1 year of measured reference station and a yearly average precipitation isotope input for a pilot site, the data‐scarce sub‐humid Eerste River catchment in South Africa. The models driven by all input products performed well for upstream and downstream discharge gauges with Nash Sutcliffe efficiency (NSE) from 0.58 to 0.85 and LogNSE of 0.66 to 0.93. The simulated δ2H stream isotopes using the reference J2000‐iso and J2000‐isoRSM were good for the main river with a stream Kling Gupta efficiency (KGE) of between 0.4–0.9 and the top 100 Monte Carlo simulations varying by around 5‰ for δ2H. For smaller tributaries the model was unable to capture the measured stream isotopes due to biased precipitation isotope inputs. Adjusting the J2000‐iso with a bias corrected IsoRSM improved the stream and groundwater isotope simulation and outperformed the model driven by an average yearly precipitation isotope input. Differences in simulated hydrological processes were only evident between the models when evaluating percolation with unrealistic simulations for the standard J2000 model. While the regional climate model is computationally more intensive than its global counterpart, it provided better stream isotope simulations and improvements to simulated percolation. Our results indicate that isotope‐enabled climate models can provide useful input data in data scarce regions for hydrological models, where improved water management to address climate change impacts is needed.

Environmental DNA Metabarcoding Details the Spatial Structure of a Diverse Tropical Fish Assemblage in a Major East African River System

ABSTRACTManagement and conservation of species‐rich tropical freshwater systems require reliable information on the diversity and distribution of species present. Here, we used environmental DNA metabarcoding to reveal the diversity of the fishes in the Rufiji River catchment of central Tanzania. Across 174 samples from 49 sites, and using a newly developed reference library, we mapped the presence of 66 fish species from an estimated 91 that we are confident are present in the system. We found clear evidence of community structuring of the assemblage linked to key environmental gradients—elevation, temperature, and turbidity. We also identified core distributions of rare or threatened taxa, including migratory species such as the anguillid eels. With a focused analysis of 50 samples collected over a small spatial scale (<2 km) from the Kilombero River, we showed that each single sample can capture an average of 23.1 species, while three samples can capture 39.4 species, from a total of 56 species encountered in the 50 samples. Collectively the results help to identify species vulnerable to ongoing change in the catchment, including dam construction and agricultural intensification. The results clearly demonstrate how eDNA‐based metabarcoding can reliably describe the diversity and distributions of riverine fish species across a catchment, providing standardized information that will be valuable for environmental management.

A Simple Model of Flow Reversals in Florida’s Karst Springs

AbstractNorth Florida's karst springs are among the largest and most abundant in the world. Despite relatively stable spring discharges, flow reversals can episodically occur in some springs when river waters backflow into the aquifer during flood events. Reversals are normal features of the springs along the Suwanee River, but the changing incidence of these reversals in response to anthropogenic activities or climate change remains unclear and the mechanisms responsible for these reversals remain poorly described. Here we develop a reduced‐complexity hydrogeological model of the Suwannee River catchment to explore conditions needed to induce spring flow reversals. Our model demonstrates that reversals require two conditions: (a) a hydrogeological setting that combines an upstream catchment with rapid hydrological responses to meteorological drivers, which freely drains to a downstream catchment containing the karst aquifer (i.e., the spring‐fed river segment); and (b) meteorological conditions that create sufficient temporal variability in recharge. Given both conditions, recharge events can propagate from the upstream catchment and fill the downstream river segment faster than it can drain, causing river stage to rise above the aquifer head, resulting in temporary spring flow reversal (or bank storage). Our model accurately predicts significant post‐flood increases in spring flow as bank storage recedes, and using measured electrical conductivity at a major river‐adjacent spring we also quantify the enhancement of limestone dissolution (cave enlargement) due to reversal events. A comprehensive assessment of the incidence and duration of reversal events shows a predominant influence of climate and vegetation changes over that of groundwater pumping.

Simulating streamflow in a transboundary river catchment: The implications of hybrid rainfall data

The lack of observed data for the hydrological modelling of catchments across borders has hindered the management of transboundary water resources. This study investigated the implications of different degrees of hybridization of observed rainfall data using ERA5-Land reanalysis precipitation data in simulating streamflow in the Ruvubu River catchment across the Burundi-Tanzania border. The hydrological model of the Ruvubu River catchment was set up and iteratively updated using 0%–100% hybrid rainfall data, parameterized, simulated, and then evaluated against observed streamflow data at the catchment outlet. The findings show that the performance of the hydrological model decreased as the degree of rainfall data hybridization increased. However, model parameters satisfactorily compensated for input uncertainty when the model had hybrid rainfall data not exceeding 15% hybridization. Subsequently, errors in the simulated flow were also minimal. This implies that the simulated flow from the model when it has hybrid rainfall data not exceeding 15% hybridization can represent the simulated flow when it has 0% hybrid rainfall data. These findings show the need to develop thresholds of rainfall data hybridization for different global precipitation datasets in data-scarce transboundary river catchments.