Increased River Discharge Research Articles

An assessment of residence and influence times in Xiamen Bay, China

Water transport time scales are important indicators to assess the renewal of substance inside coastal embayments. Estimates of water renewal efficiency represented by residence and influence times in Xiamen Bay were made via a series of numerical experiments, using the instantaneous tracer injection method. It was found that the average e-folding threshold residence time in the bay is approximately 16.5 days, and that the residence time increases near the entrances to land because water exchange between the bay and the Taiwan Strait is the dominant water-renewal process. The seawater influence time has a similar spatial distribution to the residence time. The freshwater influence time is much shorter in the western region of the bay than in the eastern region, which shows that the influence of Jiulong River freshwater is largely restricted to the western region. The changes in the residence and influence times reveal that increased river discharge mainly accelerates water renewal in the western region of the bay, whereas increased tides mainly accelerate water renewal in the eastern region and the northern part of the western region. The drivers of water renewal were examined using effective transport velocities that were defined based on the decomposition of tracer flux, and showed that tidal residual flow determines the extent of water renewal in Xiamen Bay. The estimation of residence time is sensitive to the operation of instantaneous method. The release of tracer at spring tide results in a shorter residence time and the use of a small threshold can improve the consistency of estimated threshold residence times.

Numerical Modeling for Region of Freshwater Influence by Han River Discharge in the Yeomha Channel, Gyeonggi Bay

This study estimates the region of freshwater influence (ROFI) by Han River discharge in the Yeomha channel, Gyeonggi Bay. A 3-D numerical model, which is validated for reproducibility of variation in current velocity and salinity, is applied in Gyeonggi Bay. Distance of freshwater influence (DOFI) is defined as the distance from the entrance of Yeomha channel to the point where surface salinity is 28 psu. Model scenarios were constructed by dividing the Han River discharge into 10 categories (200~10,000 m3/s). The relation equation between freshwater discharge and DOFI was calculated based on performing a non-linear regression analysis. ROFI in Yeomha channel expands from the southern sea area of Ganghwa-do to the northern sea area of Yeongheung- do as the intensity of Han River discharge increases. The discharge and DOFI are a proportional relationship, and the increase rate of DOFI gradually decreases as discharge increases. Based on the relation equation calculated in this study, DOFI in the Yeomha channel can be estimated through the monthly mean Han River discharge. Accordingly, it will be possible to respond and predict problems related to damage to water quality and ecology due to rapid freshwater runoff.

Water balance response of permafrost-affected watersheds to changes in air temperatures

Observations show increases in river discharge to the Arctic Ocean especially in winter over the last decades but the physical mechanisms driving these changes are not yet fully understood. We hypothesize that even in the absence of a precipitation increase, permafrost degradation alone can lead to increased annual river runoff. To test this hypothesis we perform 12 millennium-long simulations over an idealized hypothetical watershed (1 km2) using a distributed, physically based water balance model (Water flow and Balance Simulation Model, WaSiM). The model is forced by both a hypothetical warming defined by an air temperature increase of 7.5 ∘C over 100 years, and a corresponding cooling scenario. To assess model sensitivity we vary soil saturated hydraulic conductivity and lateral subsurface flow configuration. Under the warming scenario, changes in subsurface water transport due to ground temperature changes result in a 7%–14% increase in annual runoff accompanied by a 6%–20% decrease in evapotranspiration. The increase in runoff is most pronounced in winter. Hence, the simulations demonstrate that changes in permafrost characteristics due to climate warming and associated changes in evapotranspiration provide a plausible mechanism for the observed runoff increases in Arctic watersheds. In addition, our experiments show that when lateral subsurface moisture transport is not included, as commonly done in global-scale Earth System Models, the equilibrium water balance in response to the warming or cooling is similar to the water balance in simulations where lateral subsurface transport is included. However, the transient changes in water balance components prior to reaching equilibrium differ greatly between the two. For example, for high saturated hydraulic conductivity only when lateral subsurface transport is considered, a period of decreased runoff occurs immediately after the warming. This period is characterized by a positive change in soil moisture storage caused by the soil moisture deficit developed during prior cooling.

Late Holocene climate variation on the northern Tibetan Plateau inferred from Lake Ayakum

Understanding late Holocene climate variation on the northern Tibetan Plateau is essential for understanding regional climate dynamics, especially the potential response to mid-latitude westerlies. A continuous lacustrine sediment record covering the past 1500 years was recovered from Lake Ayakum on the northern margin of the Tibetan Plateau, in which a typical cold-phase mineral (mirabilite) was detected. Assisted by the simulation results in the Pitzer model, the preservation of mirabilite in the sediment core as well as its relative proportion was interpreted in terms of past temperature variations, in which a high mirabilite content corresponded to a relatively cold environment. Grain size analysis and subsequent end-member modeling results revealed potential correlations with regional moisture conditions, that more coarse materials were transported by increased river discharge under relatively wet conditions. The results showed that cold and humid conditions dominated the Dark Age Cold Period and Little Ice Age, while warm and dry conditions dominated the Medieval Warm Period. Following the increase in regional temperature, more coarse materials were deposited during the last 200 years owing to probable increased fluvial transport supplied by glacier meltwater. The overall temperature and moisture variations were broadly consistent with records from western China and central Asia, indicating that the northern Tibetan Plateau was predominately influenced by the mid-latitude westerlies during the late Holocene. In addition, the temperature change showed a close correlation with solar irradiance, pointing to a potential response to solar activity. On the other hand, the moisture change responded sensitively to the North Atlantic Oscillation and the migration of mid-latitude westerlies.

Groundwater-Surface Water Interaction in the Nera River Basin (Central Italy): New Insights after the 2016 Seismic Sequence

The highest part of the Nera River basin (Central Italy) hosts significant water resources for drinking, hydroelectric, and aquaculture purposes. The river is fed by fractured large carbonate aquifers interconnected by Jurassic and Quaternary normal faults in an area characterized by high seismicity. The 30 October 2016, seismic sequence in Central Italy produced an abrupt increase in river discharge, which lasted for several months. The analysis of the recession curves well documented the processes occurring within the basal aquifer feeding the Nera River. In detail, a straight line has described the river discharge during the two years after the 2016 seismic sequence, indicating that a turbulent flow characterized the emptying process of the hydrogeological system. A permeability enhancement of the aquifer feeding the Nera River—due to cleaning of fractures and the co-seismic fracturing in the recharge area—coupled with an increase in groundwater flow velocity can explain this process. The most recent recession curves (2019 and 2020 periods) fit very well with the pre-seismic ones, indicating that after two years from the mainshock, the recession process recovered to the same pre-earthquake conditions (laminar flow). This behavior makes the hydrogeological system less vulnerable to prolonged droughts, the frequency and length of which are increasingly affecting the Apennine area of Central Italy.

Linking ocean colour features in the Western Iberian margin to wave-induced sediment resuspension and coccolithophore patches

Two ocean colour features in the western Iberian margin (western Europe) were described in present work. They were identified in 10 years (2002–2012) of MODIS-AQUA normalized water-leaving radiance (nLw) at 555 nm and their causes were investigated using complementary satellite, in-situ and model data. One feature was a band of high nLw555 (>0.8 mW cm−2 μm−1 sr−1) with exceptionally large widths (>20 km) along the coast. Over the 10-year period, this wide coastal band of high nLw555 only occurred during winter, following better the increases in wave height, than the increases in precipitation and river discharge. The band seemed particularly associated with episodes of large wave heights (>6 m), under weak thermal stratification and northerly winds. Findings suggest that the band is the signature of exceptional offshore extensions of the more reflective turbid coastal waters, as a result of wave-induced sediment resuspension along the coast. The other feature consisted in areas of equally high nLw555, but separated from the coast by more than 40 km. These offshore areas of high nLw555 shared spectral signatures of phytoplankton, namely coccolithophores, typically formed during spring following water column re-stratification and reached larger areas after winters with deeper oceanic mixed layers. Less commonly, they also occurred from summer to autumn. Results indicate they are mainly the signature of enhanced coccolithophore abundances that also create turbid reflective waters due to their calcite plates. The used methodology separated the two features in time and therefore seems to allow for satellite monitoring of sediment resuspension and coccolithophore blooms off west Iberia and similar coastal environments, which is particularly relevant for detecting events of sediment transport, potentially linked to water quality problems, and long-term changes in phytoplankton composition.

Dependence of tides and river water transport in an estuarine network on river discharge, tidal forcing, geometry and sea level rise

Estuaries are often characterised by a complex network of branching channels, in which the water motion is primarily driven by tides and fresh water discharge. For both scientific reasons and management purposes, it is important to gain more fundamental knowledge about the hydrodynamics in such networks, as well as their implications for turbidity and ecological functioning. A generic 2DV estuarine network model is developed to study tides and river water transport and to understand the dependence of their along-channel and vertical structure on forcings, geometry characteristics and sea level changes. The model is subsequently applied to the Yangtze Estuary to investigate tides and the distribution of river water over channels during dry and wet season, spring tide, as well as prior to and after the formation of Hengsha Passage and the construction of the Deep Waterway Project and sea level rise. Increasing river discharge enhances the friction for tides by increasing both internal and bottom stresses. Changes in tidal forcing are correlated with the friction for both tide and river. A shortcut channel reduces the water level difference in adjacent channels, as well as tidal amplitudes difference. Sea level rise results in larger friction parameters and faster propagation of tides. The distribution of river water transport is hardly affected by above mentioned changes. Model results and current vertical structure are consistent with observations.

Integrative Assessment of Stormwater Infiltration Practices in Rapidly Urbanizing Cities: A Case of Lucknow City, India

The lack of strategic planning in stormwater management has made rapidly urbanizing cities more vulnerable to urban water issues than in the past. Low infiltration rates, increase in peak river discharge, and recurrence of urban floods and waterlogging are clear signs of unplanned rapid urbanization. As with many other low to middle-income countries, India depends on its conventional and centralized stormwater drains for managing stormwater runoff. However, in the absence of a robust stormwater management policy governed by the state, its impact trickles down to a municipal level and the negative outcome can be clearly observed through a failure of the drainage systems. This study examines the role of onsite and decentralized stormwater infiltration facilities, as successfully adopted by some higher income countries, under physical and social variability in the context of the metropolitan city of Lucknow, India. Considering the 2030 Master Plan of Lucknow city, this study investigated the physical viability of the infiltration facilities. Gridded ModClark rainfall-runoff modeling was carried out in Kukrail river basin, an important drainage basin of Lucknow city. The HEC-HMS model, inside the watershed modeling system (WMS), was used to simulate stormwater runoff for multiple scenarios of land use and rainfall intensities. With onsite infiltration facilities as part of land use measures, the peak discharge reduced in the range of 48% to 59%. Correlation analysis and multiple regression were applied to understand the rainfall-runoff relationship. Furthermore, the stormwater runoff drastically reduced with decentralized infiltration systems.

Upstream extension of a bottom-advected plume and its mechanism: The case of the Yellow River

AbstractHydrographic surveys have revealed that the Yellow River plume propagates in the direction opposite to that of a Kelvin wave (upstream) under a low river discharge condition, but turns downstream as the river discharge increases. A numerical model reproduced the upstream extension of the plume under the low river discharge condition and the transition to the downstream direction under the high river discharge condition, and confirmed that the summer wind is not the necessary condition for upstream extension of the plume. With the condition of low river discharge, the model also indicated the dependence of the upstream extension of the plume on the tidal range: extending upstream in spring tide but turning downstream in neap tide. The upstream movement of the plume results from the upstream transport of freshwater that depends on the upstream tide-induced residual current around the river mouth and the downstream density-driven current around the offshore plume front. With the condition of high river discharge, the upstream tide-induced residual current cannot compete with the downstream density-driven current and the plume turns downstream. Momentum analysis confirms the important roles of advection term and viscosity term in the condition of low river discharge and the shift to a Coriolis force-dominated system under high river discharge condition. An idealized model study suggests a dimensionless number for the river discharge changing the river plume extension from upstream to downstream under a specific upstream ambient current around the river mouth.

Identifikasi Perubahan Penggunaan Lahan dan Karakteristik Hidrologi DAS Citarum Hulu

The Citarum watershed is one of the priority watersheds due to problems of critical land, flooding, erosion, and sedimentation which continue to increase every year. As the main catchment area that contributes to maintaining the availability of water resources, the upstream Citarum watershed continues to experience a reduction in forest and an increase in built-up land. A research aimed to analyze land use changes to the hydrological characteristics was carried out at the Watershed. The methods used include interpretation of SPOT image data, analysis of land use changes, and determining the values of KRA and KAT. The results of the analysis of land use change conditions in 2009-2018 saw a reduction in forest area (-5.5%), rice fields (-17.4%), and shrubs (-60.8%). Land use increased in built up land (39.7%), dry land agriculture (13%), plantations (6.4%), and open land (95.5%). The highest discharge occurred in 2010 at 606,3 m3/second and the lowest discharge in 2012 was 4,3 m3/second. The KRA and KAT values of the Upper Citarum watershed were moderate and very high. The multiple linear regression model of the relationship between land use changes that affect river flow fluctuations is Y = 2380.5 + 0.9 rainfall -206.5 forest + 6.1 build-up area -31.8 dryland agriculture + e. Other factors that influence the increase in river discharge are the slope of land slopes, rainfall patterns, and inadequate land use methods.
 Keywords: hydrological characteristics, landuse change, Upper Citarum watershed

Environmental factors influencing annual sucker (Catostomus sp.) migration into a Great Lakes tributary

Fish migration in rivers is a growing area of concern as mounting anthropogenic influences, particularly fragmentation from dams and barriers, constitute major threats to global river species diversity. Barriers can impede the movement of fishes between areas critical to the completion of their lifecycle, affecting both population and ecosystem viability. In response, fish passage solutions have been identified as a critical need to maintain fisheries viability in the Laurentian Great Lakes, and around the world. Pivotal to the success of these fish passage solutions is a more complete understanding of the movement phenology and environmental cues that instigate migration. We used a dual-frequency identification sonar (DIDSON) to evaluate environmental triggers of river entry during spring and summer for three size classes of migratory fishes in the Boardman River, a Lake Michigan tributary. Our results indicate that medium size fish (>30 cm and < 50 cm), primarily composed of white sucker Catostomus commersonii and longnose sucker Catostomus catostomus were 21% more likely to enter the river at sunset and 25% less likely at midnight in comparison to midday. Entry rates of medium fish increased 6% for every 1 °C increase in river temperature, 4% for every 1 m3/s increase in river discharge from the day prior, and were reduced by 1% for every 10 cm increase in lake level. Understanding these processes in the tributaries of the Great Lakes is important to inform the fish passage solutions currently being developed for the Boardman River, and to inform management regulations for Great Lakes migratory fishes.

Increased Amazon Basin wet-season precipitation and river discharge since the early 1990s driven by tropical Pacific variability

The Amazon Basin, the largest watershed on Earth, experienced a significant increase in wet-season precipitation and high-season river discharge from the early 1990s to early 2010s. Some studies have linked the increased Amazon Basin hydrologic cycle to decadal trends of increased Pacific trade winds, eastern Pacific sea surface temperature (SST) cooling, and associated strengthening of the Pacific Walker circulation. However, it has been difficult to disentangle the role of Pacific decadal variability from the impacts of greenhouse gases and other external climate drivers over the same period. Here, we separate the contributions of external forcings from those of Pacific decadal variability by comparing two large ensembles of climate model experiments with identical radiative forcing agents but imposing different tropical Pacific wind stress. One ensemble constrains tropical Pacific wind stress to its long-term climatology, suppressing tropical Pacific decadal variability; the other ensemble imposes the observed tropical Pacific wind stress anomalies, simulating realistic tropical Pacific decadal variability. Comparing the Amazon Basin hydroclimate response in the two ensembles allows us to distinguish the contributions of external forcings common to both simulations from those related to Pacific trade wind variability. For the 1992–2012 trend, the experiments with observed tropical Pacific wind stress anomalies simulate strengthening of the Walker circulation between the Pacific and South America and sharpening of the Pacific–Atlantic interbasin SST contrast, driving increased Amazon Basin wet-season precipitation and high-season discharge. In contrast, these circulation and hydrologic intensification trends are absent in the simulations with climatological tropical Pacific wind stress. This work underscores the importance of Pacific decadal variability in driving hydrological cycle changes and modulating the hydroclimate impacts of global warming over the Amazon Basin.

Picophytoplankton dynamics in a large temperate estuary and impacts of extreme storm events

Picophytoplankton (PicoP) are increasingly recognized as significant contributors to primary productivity and phytoplankton biomass in coastal and estuarine systems. Remarkably though, PicoP composition is unknown or not well-resolved in several large estuaries including the semi-lagoonal Neuse River Estuary (NRE), a tributary of the second largest estuary-system in the lower USA, the Pamlico-Albemarle Sound. The NRE is impacted by extreme weather events, including recent increases in precipitation and flooding associated with tropical cyclones. Here we examined the impacts of moderate to extreme (Hurricane Florence, September 2018) precipitation events on NRE PicoP abundances and composition using flow cytometry, over a 1.5 year period. Phycocyanin-rich Synechococcus-like cells were the most dominant PicoP, reaching ~ 106 cells mL−1, which highlights their importance as key primary producers in this relatively long residence-time estuary. Ephemeral “blooms” of picoeukaryotic phytoplankton (PEUK) during spring and after spikes in river flow were also detected, making PEUK periodically major contributors to PicoP biomass (up to ~ 80%). About half of the variation in PicoP abundance was explained by measured environmental variables. Temperature explained the most variation (24.5%). Change in total dissolved nitrogen concentration, an indication of increased river discharge, explained the second-most variation in PicoP abundance (15.9%). The short-term impacts of extreme river discharge from Hurricane Florence were particularly evident as PicoP biomass was reduced by ~ 100-fold for more than 2 weeks. We conclude that precipitation is a highly influential factor on estuarine PicoP biomass and composition, and show how ‘wetter’ future climate conditions will have ecosystem impacts down to the smallest of phytoplankton.

Deep Thermohaline Circulation Across the Closure of the Central American Seaway

AbstractThe closure of the Central American Seaway (CAS) resulted in changes of ocean‐climate dynamics since the late Miocene following the uplift of northern Andes. Reconstructing the bottom‐water temperatures (BWTs) of the Caribbean Sea illustrates feedbacks of the closure on the ocean‐climate system including deep‐water dynamics of the Caribbean Sea. Here, Mg/Ca‐derived BWTs of the Plio‐Pleistocene Caribbean Sea from the benthic foraminifer Cibicidoides wuellerstorfi are presented for the first time and interpreted along with Na/Ca and Sr/Ca as proxies of salinity and continental input, respectively. Our results highlight several warm (93, Gi15‐19, and N1) and cool (92, M2, Gi20, and CN4) marine isotope stages (MISs). Accordingly, changes in the circulation of deep‐water masses during the CAS closure developed in four main time intervals: (I) between 5.2 and 4.1 Ma (million years ago) BWT was ~1.1°C, (II) 4.1–3.2 Ma ~2.1°C, (III) 3.2–2.7 Ma ~2.7°C, and (IV) 2.7–2.2 Ma ~2.1°C. Relatively higher, gradually increased temperatures between 3.2 and 2.7 Ma correspond to late Pliocene warmth and restricted inflow of Pacific waters into the Caribbean due to shoaling of the CAS. In addition, Sr/Ca values reveal gradually escalating terrigenous input until 2.7 Ma most likely related to the increased river discharge in response to the Andean uplift. The gradual decrease of the BWTs from 2.7 Ma may have resulted from the onset of Northern Hemisphere Glaciation. Overall, BWTs match with previous sea surface temperatures from the planktic foraminifer Neogloboquadrina dutertrei. The BWTs presented here confirm intensified thermohaline circulation during the overall Pliocene warmth with increased bottom‐water Na/Ca values indicating enhanced salinity.

Effects of Hurricane Irma on residual flows and saltwater intrusion in a subtropical estuary

The variability of residual flows and saltwater intrusion was evaluated for the duration of a hurricane-related freshwater pulse in a subtropical estuary, the Suwannee River. Increased rainfall affected the river’s watershed when Hurricane Irma skirted the west coast of Florida in September 2017. The river remained short of reaching flood stages, but the increased river discharge was enough to modify the residual circulation and restrict saltwater intrusion. Current velocity profiles and hydrographic variables were collected in five tidal-cycle experiments to study the lower estuary’s response to the peak river discharge and its subsequent relaxation. Data were recorded in three cross-estuary transects. In addition, continuous salinity measurements were recorded for 48 days over the shallow portion of the seaward transect. Results showed that the subtidal water-level slope inside the estuary, between the mouth and a river station at Wilcox, modified the residual circulation and restricted the intrusion of salt water. During high tide, salt water entered the estuary due to tidal pumping even though residual circulation was seaward in the cross-section. Residual seaward flows persisted for the duration of the hurricane-related freshwater pulse. After relaxation of the freshwater pulse, saltwater intrusion was observed in the time series over the shallow portion. This saltwater intrusion responded to changes in wind forcing, with largest intrusions when the wind blew into the estuary, i.e., toward the N and NW.

Dynamic interaction between tidal current and upstream discharge at the Kapuas River mouth

This paper explores the dynamic interaction between tidal current and upstream discharge at the Kapuas River mouth. Long term time series data of tidal current and river discharge obtained from software Windwave 05 and hydrograph data, respectively. Three prominent parameters, i.e., river discharge, tides, and tidal current were analyzed using the wavelet method, Hilbert function, and low pass filter to evaluate the tidal characteristics and phase difference of M2, M4, K1, O1. The relationship between the tidal current and water level shows a mimic standing wave with the phase difference around ∼ -180 – 900, indicating the tidal current leads the water level. The low-pass filter results indicate that the increasing river discharge hampers the tides and reduces the tidal current at the mouth. The Hilbert function result reveals the asymmetric tide and flood dominant flow which has the opposite direction with the seaward direction of the current. Moreover, the subtidal water level flows in a seaward direction. This happened because the net seaward current modulated by the river discharge. The currents at the mouth are affected significantly by the swell wave.

Evaluation of integrated impacts of climate and land use change on the river flow regime in Wamkurumadzi River, Shire Basin in Malawi

Abstract Climate and land use change (CC and LUC hereafter) are interlinked factors that can lead to river flow regime changes, as well as affecting hydrological extremes such as floods and drought. There is now considerable evidence of CC and LUC in many catchments in Malawi but without corresponding evaluations of their impacts on river flow regimes. Therefore, this study assessed how both factors affect the flow regime of Wamkurumadzi River, a key tributary of the major Shire River in southern Malawi. Land use and hydroclimatic data for the basin were first analyzed for spatial–temporal trends in the historical period between the years 1984 and 2015. The Soil and Water Assessment Tool (SWAT) model was then applied with different LUC and CC scenarios in order to assess their sole and combined impacts on the river flow regime. The model was calibrated and validated using the split sample method from the year 1984 to 1999 and from the year 2000 to 2015. Model performance was acceptable according to the selected evaluation criteria, with the Nash–Sutcliffe (NSE) coefficient of 0.78 and coefficient of determination (R2) of 0.96 during calibration and NSE of 0.93 and R2 of 0.98 during validation. Results of the integrated impacts of LUC and CC suggest a slight increase in river discharge of 0.05 m3/s for the period between the 1980s and 2000s. During the 1980s–1990s, both CC through rainfall decreased and LUC resulted in decreases in the mean river discharges by 1.58 and 0.37 m3/s, respectively. The study also found that CC through increased rainfall in the 1990s–2000s decades saw an overall increase of 1.39 m3/s in mean river discharge, while LUC shows the increase of mean river discharge by 0.25 m3/s. However, the study observed that reforestation efforts in the basin were greatly responsible for the alteration of the river flow regime in the later period.

Simulation of fluvial sediment dynamics through strategic assessment of stream gaging data: A targeted watershed sediment loading analysis

Near-channel sediment loading (NCSL) is localized and episodic, making it difficult to accurately quantify its cumulative contribution to watershed sediment loading, let alone predict the effects from changes in river discharge due to climate change or land management practices. We developed a methodological framework, using commonly available stream gaging data, for estimating watershed-scale NCSL, a feature generally absent in most watershed models. The method utilizes a network of paired gages that bracket the incised river corridors of 15 tributaries to the Minnesota River, in which near-channel sources are often the dominant contributors of sediment loading. For each set of paired gages, we calculate NCSL as the difference between the upstream and downstream sediment loading minus the field contribution between the gages. NCSL generally increases with river discharge when it exceeds the observed threshold benchmark in the tributaries of Minnesota River Basin; accordingly, we developed a predictive model for quantifying NCSL using river discharge as the independent variable. This approach provides a predictive basis for evaluating the impacts on near-channel sediment supply from increases in runoff and river discharge. Application of this approach includes evaluation of watershed-scale conservation trade-offs, where benefits of landscape management practices, such as wetlands and reservoirs are measured in terms of reduction in downstream near-channel sediment loading in the incised river corridors.

Manila River Mouths Act as Temporary Sinks for Macroplastic Pollution

The Philippines is suggested to be one of the world's main contributors to global marine plastic pollution. Several rivers in the Manila metropolitan area are assumed to be main pathways of land-based plastic waste into the ocean. However, these model estimates remain uncertain due to a lack of field data. The main goal of this study was therefore to collect field data on floating macroplastic ($>$0.5 cm) flux and polymer category in three of Manila's main rivers: the Meycauayan, Tullahan and Pasig. We measured plastic flux, item polymer category, and flow velocity at two locations per river during an 11-day period. Each river was measured close to the river mouth, and several kilometer upstream. The results showed no significant difference between the plastic flux in upstream and downstream flow direction at the three river mouths. The Meycauayan and Pasig rivers did have significantly higher plastic flux at the river mouth compared to the upstream location. The observations suggest accumulation of macroplastics in the river mouths during periods of low freshwater discharge. In this case, instantaneous plastic flux is mainly determined by the tidal dynamics. It is hypothesized that plastics are temporarily retained in estuaries, and may be flushed out during neap tide or increased river discharge. Due to the retention capacity of the estuaries, net plastic export into the ocean could not be estimated. Future research is needed to further investigate the role of tidal dynamics on the transport and net export of riverine macroplastics.

Future Water Use Planning by Water Evaluation and Planning System Model

Assessment of future water availability is a challenging task under changing climatic conditions and anthropogenic interventions. The current research focuses on future water resources scenario generation for contributing areas of proposed hydraulic structures generated from the Water Evaluation and Planning (WEAP) System model. The proposed methodology was implemented for the Dwarakeswar-Gandherswari river basin (India) which needs a long-term future water use plan. Bias-corrected Representative Concentration Pathways (RCPs) data were used for climate change analysis through a hydrological model. Different simulation model outputs [e.g. Dynamic Conversion of Land-Use and its Effects (Dyna-CLUE), Soil and Water Assessment Tool (SWAT), Modular Finite-Difference Flow Model (MODFLOW)] were utilized in water evaluation model for a generation of future water resources scenarios. Four scenarios (2010–2030–2050-2080) were generated for the sustainability of limited water resources management strategies. SWAT simulated results show an increase in river discharge for 2030 or 2080 and a decrease for 2050. MODFLOW simulated results show a visible groundwater storage change for 2030 but minimal change for 2050 and 2080 scenarios. The results also show a decrease in agricultural land and an increase in population for the contributing areas of three hydraulic structures during 2010–2030–2050-2080. These results provide a piece of valuable information for decision-makers in future water management plan preparation.