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Probabilities of future climate states can be estimated by fitting distributions to the members of an ensemble of climate model projections. The change in the ensemble mean can be used as an estimate of the change in the mean of the real climate. However, the level of sampling uncertainty around the change in the ensemble mean varies from case to case and in some cases is large. We compare two model-averaging methods that take the uncertainty in the change in the ensemble mean into account in the distribution fitting process. They both involve fitting distributions to the ensemble using an uncertainty-adjusted value for the ensemble mean in an attempt to increase predictive skill relative to using the unadjusted ensemble mean. We use the two methods to make projections of future rainfall based on a large data set of high-resolution EURO-CORDEX simulations for different seasons, rainfall variables, representative concentration pathways (RCPs), and points in time. Cross-validation within the ensemble using both point and probabilistic validation methods shows that in most cases predictions based on the adjusted ensemble means show higher potential accuracy than those based on the unadjusted ensemble mean. They also perform better than predictions based on conventional Akaike model averaging and statistical testing. The adjustments to the ensemble mean vary continuously between situations that are statistically significant and those that are not. Of the two methods we test, one is very simple, and the other is more complex and involves averaging using a Bayesian posterior. The simpler method performs nearly as well as the more complex method.

Probabilities of future climate states can be estimated by fitting distributions to the members of an ensemble of climate model projections. The change in the ensemble mean can be used as an estimate of the unknown change in the mean of the distribution of the climate variable being predicted. However, the level of sampling uncertainty around the change in the ensemble mean varies from case to case and in some cases is large. We compare two model averaging methods that take the uncertainty in the change in the ensemble mean into account in the distribution fitting process. They both involve fitting distributions to the ensemble using an uncertainty-adjusted value for the ensemble mean in an attempt to increase predictive skill relative to using the unadjusted ensemble mean. We use the two methods to make projections of future rainfall based on a large dataset of high resolution EURO-CORDEX simulations for different seasons, rainfall variables, RCPs and points in time. Cross-validation within the ensemble using both point and probabilistic validation methods shows that in most cases predictions based on the adjusted ensemble means show higher potential accuracy than those based on the unadjusted ensemble mean. They also perform better than predictions based on conventional Akaike model averaging and statistical testing. The adjustments to the ensemble mean vary continuously between situations that are statistically significant and those that are not. Of the two methods we test, one is very simple, and the other is more complex and involves averaging using a Bayesian posterior. The simpler method performs nearly as well as the more complex method.

© 2019 Elsevier Ltd Climate change is expected to alter European floods and associated economic losses in various ways. Here we investigate the impact of precipitation change on European average winter and summer financial losses due to flooding under a 1.5 °C warming scenario (reflecting a projected climate in the year 2115 according to RCP2.6)and for a counterfactual current-climate scenario where the climate has evolved without anthropogenic influence (reflecting a climate corresponding to pre-industrial conditions). Climate scenarios were generated with the Community Atmospheric Model (CAM)version 5. For each scenario, we derive a set of weights that when applied to the current climate's precipitation results in a climatology that approximates that of the scenario. We apply the weights to annual losses from a well-calibrated (to the current climate)flood loss model that spans 50,000 years and re-compute the average annual loss to assess the impact of precipitation changes induced by anthropogenic climate change. The method relies on a large stochastic set of physically based flood model simulations and allows quick assessment of potential loss changes due to change in precipitation based on two statistics, namely total precipitation, and total precipitation of very wet days (defined here as the total precipitation of days above the 95th percentile of daily precipitation). We compute the statistics with the raw CAM precipitation and bias-corrected precipitation. Our results show that for both raw and bias-corrected statistics i)average flood loss in Europe generally tend to increase in winter and decrease in summer for the future scenario, and consistent with that change we also show that ii)average flood losses have increased (decreased)for winter (summer)from pre-industrial conditions to the current day. The magnitude of the change varies among scenarios and statistics chosen.

Variable effects of backwaters complicate the development of rating curves at hydrometric measurement stations. In areas influenced by backwater, single-parameter rating curve techniques are often inapplicable. To overcome this, several authors have advocated the use of an additional downstream level gauge to estimate the longitudinal surface level gradient, but this is cumbersome in a lowland meandering river with considerable transverse surface level gradients. Recent developments allow river flow to be continuously monitored through velocity measurements with an acoustic Doppler current profiler (H-ADCP), deployed horizontally at a river bank. This approach was adopted to obtain continuous discharge estimates at a cross-section in the River Mahakam at a station located about 300 km upstream of the river mouth in the Mahakam delta. The discharge station represents an area influenced by variable backwater effects from lakes, tributaries and floodplain ponds, and by tides. We applied both the standard index velocity method and a recently developed methodology to obtain a continuous time-series of discharge from the H-ADCP data. Measurements with a boat-mounted ADCP were used for calibration and validation of the model to translate H-ADCP velocity to discharge. As a comparison with conventional discharge estimation techniques, a stage-discharge relation using Jones formula was developed. The discharge rate at the station exceeded 3250 m3 s−1. Discharge series from a traditional stage-discharge relation did not capture the overall discharge dynamics, as inferred from H-ADCP data. For a specific river stage, the discharge range could be as high as 2000 m3 s−1, which is far beyond what could be explained from kinematic wave dynamics. Backwater effects from lakes were shown to be significant, whereas interaction of the river flow with tides may impact discharge variation in the fortnightly frequency band. Fortnightly tides cannot easily be isolated from river discharge variation, which features similar periodicities.

Large rivers often present a river–lake–delta system, with a wide range of temporal and spatial scales of the flow due to the combined effects of human activities and various natural factors, e.g., river discharge, tides, climatic variability, droughts, floods. Numerical models that allow for simulating the flow in these river–lake–delta systems are essential to study them and predict their evolution under the impact of various forcings. This is because they provide information that cannot be easily measured with sufficient temporal and spatial detail. In this study, we combine one-dimensional sectional-averaged (1D) and two-dimensional depth-averaged (2D) models, in the framework of the finite element model SLIM, to simulate the flow in the Mahakam river–lake–delta system (Indonesia). The 1D model representing the Mahakam River and four tributaries is coupled to the 2D unstructured mesh model implemented on the Mahakam Delta, the adjacent Makassar Strait, and three lakes in the central part of the river catchment. Using observations of water elevation at five stations, the bottom friction for river and tributaries, lakes, delta, and adjacent coastal zone is calibrated. Next, the model is validated using another period of observations of water elevation, flow velocity, and water discharge at various stations. Several criteria are implemented to assess the quality of the simulations, and a good agreement between simulations and observations is achieved in both calibration and validation stages. Different aspects of the flow, i.e., the division of water at two bifurcations in the delta, the effects of the lakes on the flow in the lower part of the system, the area of tidal propagation, are also quantified and discussed.

Acoustic Doppler current profilers (ADCPs) are the current standard for flow measurements in large-scale open water systems. Existing techniques to process vessel-mounted ADCP data assume homogeneous or linearly changing flow between the acoustic beams. This assumption is likely to fail but is nevertheless widely applied. We introduce a new methodology that abandons the standard assumption of uniform flow in the area between the beams and evaluate the drawbacks of the standard approach. The proposed method strongly reduces the extent over which homogeneity is assumed. The method is applied to two field sites: a mildly curved bend near a junction featuring a typical bend flow and a sharply curved bend that features a more complex sheared flow. In both cases, differences are found between the proposed method and the conventional method. The proposed technique yields different results for secondary flow patterns compared with the conventional method. The velocity components estimated with the conventional method can differ over 0.2 m/s in regions of strong shear. We investigate the number of repeat transects necessary to isolate the mean flow velocity vector from the raw ADCP signal, discarding the influences of noise, positioning and projection errors, and turbulence. Results show that several repeat transects are necessary. The minimum number of repeat measurements needed for robust mean velocity estimates is reduced when applying the proposed method.

[1] Collocated profiles of the Reynolds stress tensor and eddy covariance fluxes are obtained to derive vertical profiles of turbulent momentum and sediment diffusivity in a tidal river, using coupled acoustic Doppler current profilers (ADCPs). Shear and normal stresses are obtained by combining the variances in radial velocities measured by the ADCP beams. The covariances between radial velocities and calibrated acoustic backscatter allow the determination of the three Cartesian components of the turbulent flux of suspended sediment. The main advantage of this new approach is that flow velocity and sediment concentration measurements are exactly collocated, and allowing for profiling over longer ranges, in comparison to existing techniques. Results show that vertical profiles of the inverse turbulent PrandtlSchmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity. Citation: Sassi, M. G., A. J. F. Hoitink, and B. Vermeulen (2013), Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers, Geophys. Res. Lett., 40, 5692–5697, doi:10.1002/2013GL058299.

[1] Tidal rivers feature oscillatory and steady gradients in the water surface, controlled by interactions between river flow and tides. The river discharge attenuates the tidal motion, and tidal motion increases tidal-mean friction in the river, which may act as a barrier to the river discharge. Time series of tidal water level amplitudes at five gauge stations along the River Mahakam in Indonesia, and tidal flow velocity amplitudes at a discharge monitoring station were obtained applying wavelet analysis. Temporal variations in tidal damping coefficients for quaterdiurnal, semidiurnal, and diurnal tidal species were quantified from the observed amplitude profiles. The analysis shows that tidal damping during the rising limb of a discharge wave differs from damping during the falling limb. Wavelet cross-correlations between surface levels yielded empirical estimates of wave numbers. An empirical relation between tidal damping and river flow is derived to describe subtidal bottom friction along an idealized tidal river resembling the Mahakam. The subtidal friction is decomposed into contributions from the river flow only, from river-tide interaction, and from tidal asymmetry. Even for high river flow and low tidal velocity, river flow enhances friction attributed to river-tide interaction, causing subtidal water level setup. A simple multilinear regression model using subtidal bottom friction is employed to predict subtidal water levels at locations upstream, with a relatively good agreement between predictions and observations. The explicit expression shows the nonlinear dependence of subtidal friction on river flow velocity, explaining the complex behavior of tidal-mean surface level profiles.

Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.

[1] We characterize and quantify the sediment discharge division at two tidally influenced river bifurcations in response to mean flow and secondary circulation by employing a boat-mounted acoustic Doppler current profiler (ADCP), to survey transects at bifurcating branches during a semidiurnal tidal cycle. The ADCP collecting flow velocity and acoustical backscatter data was used to quantify suspended sediment discharge, adopting a recently introduced calibration procedure. Measured profiles of flow velocity and sediment concentration allowed us to compute spatiotemporal distributions of the shear velocity, the roughness length and the Rouse number. Spatiotemporal distributions of the settling velocity were obtained by combining the Rouse number and shear velocity estimates with in situ measurements from a laser particle size analyzer. Bed-load transport rates were inferred from shear stress estimates. The concentration field shows a direct response to bed shear stress, stressing the alluvial context of the system. The flow in the bifurcation regions is characterized by counter rotating secondary-flow cells, which stretch over the full width and depth of the cross sections in the downstream branches, and persist throughout the entire tidal cycle. The pattern of secondary flow suggests the flow approaching the bifurcation is concentrated in two independent threads. A two-cell structure inhibits the exchange of sediment that would occur in case a single cell would stretch over the full channel width. The division of suspended sediment primarily depends on the upstream transverse profile of the suspended sediment concentration, which is in turn dependent on geometrical factors such as upstream curvature.

The Mahakam is a 980-km-long tropical river flowing in the East Kalimantan province (Borneo Island, Indonesia). A significant fraction of this river is influenced by tides, the modelling of which is the main subject of this study. Various physical and numerical issues must be addressed. In the upstream part of the domain, the river flows through a region of three lakes surrounded by peat swamps. In the lowland regions, the river is meandering and its hydrodynamics is mostly influenced by tides. The latter propagate upstream of the delta, in the main river and its tributaries. Finally, the mouth of the Mahakam is a delta exhibiting a high number of channels connected to the Makassar Strait. This article focusses on the flow in the delta channels, which is characterised by a wide range of time and space scales. To capture most of them, the depth-integrated and the section-integrated versions of the unstructured mesh, finite-element model Second-Generation Louvain-la-Neuve Ice-Ocean Model are used. Unstructured grids allow for a refinement of the mesh in the narrowest channels and also an extension of the domain upstream and downstream of the delta in order to prescribe the open-boundary conditions. The Makassar Strait, the Mahakam Delta and the three lakes are modelled with 2D elements. The rivers, from the upstream limit of the delta to the lakes and the upstream limit of the domain, are modelled in 1D. The calibration of the tidal elevation simulated in the Mahakam Delta is presented. Preliminary results on the division of the Eulerian residual discharge through the channels of the delta are also presented. Finally, as a first-order description of the long-term transport, the age of the water originating from the upstream limit of the delta is computed. It is seen that for May and June 2008, the time taken by the water parcel to cross the estuary varies from 4 to 7 days depending on the channel under consideration.

SLIM is an unstructured mesh, finite element model of environmental and geophysical fluid flows, which is being improved to simulate fine-grained sediment transport in riverine and marine water systems. A 2D depth-averaged version of the model is applied to the Mahakam Delta (Borneo, Indonesia), the adjacent ocean, and three lakes in the central part of the Mahakam River catchment. The 2D code is coupled to a 1D section-averaged model for the Mahakam River and four tributaries. The coupled 2D/1D model is mainly aimed at simulating fine-grained sediment transport in the riverine and marine water continuum of the Mahakam River system. Using the observations of suspended sediment concentration (SSC) at five locations in the computational domain, the modelling parameters are first determined in a calibration step, for a givenperiod of time. A validation step is then performed using data related to another period of time. It is concluded that the coupled 2D/1D modelreproduces very well the observed suspended sediment distribution within the delta. The spatial distribution of sediment concentration in the deltaand its temporal variation are also discussed.

At tidal inlets, large amounts of water are exchanged with the adjacent sea during the tidal cycle. The residual flows, the net effect of ebb and flood, are generally small compared with the gross flux; they vary in magnitude and sign from one tidal period to the other; and their long-term mean varies from year to year. Here, we focus on the temporal variability of the residual flows in the Marsdiep tidal inlet, which is the western-most inlet of the Wadden Sea, a tidal lagoon along the coasts of the Netherlands, Germany, and Denmark. We compare the transport from a high-resolution numerical model with the transport from velocity profile data collected beneath a ferry that crosses the inlet daily. The comparison works in two ways: for the areas and times covered by the measurements, the data serve to validate the model, and conversely, the model is employed to assess the consequences of spatial and temporal gaps in the data. Modeled and observed transports over the region of the flow that is covered by the acoustic Doppler current profiler are in good agreement for gross and residual quantities. Results indicate that uncertainties due to spatial gaps can be overcome with a simple extrapolation approach applied to the velocity profiles, whereas uncertainties due to temporal gaps are more problematic and leave large discrepancies in the residuals.

Morphological changes in the Pearl River Delta (PRD) have been investigated using bathymetric charts, underwater Digital Elevation Models, remote sensing data and Geographic Information Systems. Water depths were extracted from digitized charts to explore the accretion–erosion characteristics of three estuarine environments, and to provide quantitative estimates of changes in sediment volumes. Multi-temporal satellite images have been used, in combination with topographical data, to analyze the coastline changes. PRD has gained an abundant amount of sediment of almost 9.45 × 105 km3 above the 10-m isobath in the period roughly between 1970 and 2010; the average sedimentation rate was 3.15 × 104 km3/yr. Between 1976 and 2006, the coastline extended seaward by 579.2 m on average, with a mean net extension rate of 19.3 m/yr. The results suggest that the PRD experienced a major phase of accretion, with net erosion only in some local zones. Coastline extension, associated with major morphological changes, has accelerated in recent decades. Changes in boundary conditions, such as sea-level rise, seem to have relatively minor impacts on the dramatic changes in the morphology of the estuaries. The seaward extension of the coastlines shows an increasing trend whereas the sediment supply from the delta displays a decreasing trend. A detailed comparative analysis demonstrates that land reclamation in the PRD is the most significant factor that progressively alters the delta morphology, overwhelming the effects of subsidence and sediment supply.

In multiple tidal-inlet systems such as the Dutch Wadden Sea, the exchange of sediments between the coastal lagoon and the adjacent sea is controlled by the combined effect of the tides, wind-driven flows, and density-driven flows. We investigate the variability of residual (tidally averaged) fluxes of suspended sediment with the three-dimensional numerical model GETM in relation to forcing mechanisms and model parameters. Sediment transport is modeled with three sediment classes. A modified Partheniades-Krone formulation describes the erosion and deposition fluxes from a single-layer sediment pool. The model is initialized with a uniform sediment pool for each class and the spin up period amounts to six months. Simulations span 1 year. Comparisons with observations show that model results are fairly realistic. Residual fluxes of water and suspended sediment are episodic in nature and vary strongly throughout the year, mainly due to wind variability. The net balance between import and export of material is very sensitive to model parameters. Residual fluxes are sensitive to the geographical orientation and location of the inlets, and the effect of driving mechanisms on the residual fluxes and concentrations can be organized hierarchically, with wind forcing having the largest effect on concentration levels and variability.

Discharge predictions at tidally affected river reaches are currently still a great challenge in hydrological practices. In tidal rivers, water levels are not uniquely a function of streamflow. Here, the possibility to predict discharge from water level information from gauge stations at sea and in the river is explored. A hindcast model is established for a tide-dominated lowland site along the Mahakam River (East Kalimantan, Indonesia), using an artificial neural network (ANN) model. The input data for the ANN are gradually increased, by adding new input data in each step. The results show that the inclusion of data from tide predictions at sea leads to an improved model performance. The optimized ANN-based hindcast model produces a good discharge estimation, as shown by a consistent performance during both the training and validation periods. Using this model, discharge can be predicted from astronomical tidal predictions at sea plus water level measurements from a single station at an upstream location. Alternatively, the ANN model can be used as a tool for data gap filling in a disrupted discharge time-series based on a horizontal acoustic Doppler current profiler (H-ADCP). A forecast model is developed for the same river site that is located near the city of Samarinda. To this end, water level data, predicted tide levels, and at-site historical data are considered as input for the model. The discharge time-series derived from H-ADCP data are used for calibration and validation of the multistep ahead discharge forecast model. A good performance is obtained for predictions with a forecast lead time up to two days. Read More: http://ascelibrary.org/doi/10.1061/%28ASCE%29HE.1943-5584.0000970

[1] Channel geometry in tidally influenced river deltas can show a mixed scaling behavior between that of river and tidal channel networks, as the channel forming discharge is both of river and tidal origin. We present a method of analysis to quantify the tidal signature on delta morphology, by extending the hydraulic geometry concept originally developed for river channel networks to distributary channels subject to tides. Based on results from bathymetric surveys, a systematic analysis is made of the distributary channels in the Mahakam Delta (East Kalimantan, Indonesia). Results from a finite element numerical model are used to analyze the spatial variation of river and tidal discharges throughout the delta. The channel geometry of the fluvial distributary network scales with bifurcation order, until about halfway the radial distance from the delta apex to the sea. In the seaward part of the delta, distributary channels resemble funnel shaped estuarine channels. The break in morphology, which splits the delta into river- and tide-dominated parts, coincides with a break in the ratio between tidal to fluvial discharges. Downstream hydraulic geometry exponents of the cross-sectional area show a transition from the landward part to the seaward part of the delta. The numerical simulations show that the tidal impact on river discharge division at bifurcations increases with the bifurcation order, and that the variation of river discharge throughout the network is largely affected by the tides. The tidal influence is reflected by the systematic variation of downstream hydraulic geometry exponents.

[1] Although designed for velocity measurements, acoustic Doppler current profilers (ADCPs) are being widely used to monitor suspended particulate matter in rivers and in marine environments. To quantify mass concentrations of suspended matter, ADCP backscatter is generally calibrated with in situ measurements. ADCP backscatter calibrations are often highly site specific and season dependent, which is typically attributed to the sensitivity of the acoustic response to the number of scatterers and their size. Besides being a joint function of the concentration and the size of the scatterers, the acoustic backscatter can be heavily affected by the attenuation due to suspended matter along the two-way path to the target volume. Our aim is to show that accounting for sound attenuation in ADCP backscatter calibrations may broaden the range of application of ADCPs in natural environments. The trade-off between the applicability and the accuracy of a certain calibration depends on the variation in size distribution and concentration along the sound path. We propose a simple approach to derive the attenuation constant per unit concentration or specific attenuation, based on two water samples collected along the sound path of the ADCP. A single calibration was successfully applied at five locations along the River Mahakam, located up to 200 km apart. ADCP-derived estimates of suspended mass concentration were shown to be unbiased, even far away from the transducer.

Horizontal acoustic Doppler current profilers (H-ADCPs) can be employed to estimate river discharge based on water level measurements and flow velocity array data across a river transect. A new method is presented that accounts for the dip in velocity near the water surface, which is caused by sidewall effects that decrease with the width to depth ratio of a channel. A boundary layer model is introduced to convert single-depth velocity data from the H-ADCP to specific discharge. The parameters of the model include the local roughness length and a dip correction factor, which accounts for the sidewall effects. A regression model is employed to translate specific discharge to total discharge. The method was tested in the River Mahakam, representing a large river of complex bathymetry, where part of the flow is intrinsically three-dimensional and discharge rates exceed 8000 m3 s-1. Results from five moving boat ADCP campaigns covering separate semidiurnal tidal cycles are presented, three of which are used for calibration purposes, whereas the remaining two served for validation of the method. The dip correction factor showed a significant correlation with distance to the wall and bears a strong relation to secondary currents. The sidewall effects appeared to remain relatively constant throughout the tidal cycles under study. Bed roughness length is estimated at periods of maximum velocity, showing more variation at subtidal than at intratidal time scales. Intratidal variations were particularly obvious during bidirectional flow conditions, which occurred only during conditions of low river discharge. The new method was shown to outperform the widely used index velocity method by systematically reducing the relative error in the discharge estimates

[1 ] We introduce a new technique to measure profiles of each term in the Reynolds stress tensor using coupled acoustic Doppler current profilers (ADCPs). The technique is based on the variance method which is extended to the case with eight acoustic beams. Methods to analyze turbulence from a single ADCP rely on questionable assumptions on turbulence anisotropy ratios and on the requirement of perfect vertical alignment, which are no longer necessary with the coupled system. We apply the technique successfully to data collected in the Mahakam River, East Kalimantan, Indonesia. Citation: Vermeulen, B., A. J. F. Hoitink, and M. G. Sassi (2011), Coupled ADCPs can yield complete Reynolds stress tensor profiles in geophysical surface flows, Geophys. Res. Lett., 38, L06406, doi:10.1029/2011GL046684.

Bifurcations in tidally influenced deltas distribute river discharge over downstream channels, asserting a strong control over terrestrial runoff to the coastal ocean. Whereas the mechanics of river bifurcations is well-understood, junctions in tidal channels have received comparatively little attention in the literature. This paper aims to quantify the tidal impact on subtidal discharge distribution at the bifurcations in the Mahakam Delta, East Kalimantan, Indonesia. The Mahakam Delta is a regular fan-shaped delta, composed of a quasi-symmetric network of rectilinear distributaries and sinuous tidal channels. A depth-averaged version of the unstructured-mesh, finite-element model second-generation Louvain-la-Neuve Ice-ocean Model has been used to simulate the hydrodynamics driven by river discharge and tides in the delta channel network. The model was forced with tides at open sea boundaries and with measured and modeled river discharge at upstream locations. Calibration was performed with water level time series and flow measurements, both spanning a simulation period. Validation was performed by comparing the model results with discharge measurements at the two principal bifurcations in the delta. Results indicate that within 10 to 15 km from the delta apex, the tides alter the river discharge division by about 10% in all bifurcations. The tidal impact increases seaward, with a maximum value of the order of 30%. In general, the effect of tides is to hamper the discharge division that would occur in the case without tides.

Wageningen University Hydrology and Quantitative Water Management, Environmental Sciences Group (Atlas, Droevendaalsesteeg 4, 6708 PB, Wageningen, The Netherlands) Descriptors: Mesozooplankton, Acartia tonsa , Tidal variability, Estuaries, Argentina. Descritores: Mesozooplancton, Acartia tonsa , Ciclo de marea, Estuario, Argentina. In estuaries and coastal areas, planktonic organisms are subject to tidal, diurnal and seasonal environmental changes (MARQUES et al., 2009). Knowledge of the variability of zooplankton communities on different spatial and temporal scales is a prerequisite for the understanding of ecosystem dynamics. Tidal currents and estuarine circulation are well known phenomena that contribute to both vertical and horizontal redistribution of planktonic organisms (MORGADO et al., 2003. Moreover, biological mechanisms may also account for a significant part of the temporal variation in zooplankton community structure (MARQUES et al., 2009). Planktonic communities have presumably developed adaptations in order to compensate for or resist dispersive losses (LAM-HOAI et al., 2006), thus resulting in recurrent patterns of zooplankton distribution. The Bahia Blanca Estuary constitutes a mesotidal system with a semidiurnal tidal cycle, the tides being one of the most important sources of energy (PERILLO; PICCOLO, 1991). The temporal and spatial dynamics of the mesozooplankton in this estuary have been extensively investigated (e.g. HOFFMEYER 2004; HOFFMEYER et al., 2008) but none of these studies has considered the short-term variability associated with tidal cycles. The objectives of the present study were to describe the influence of the tidal cycle on mesozooplankton variability and to compare the mesozooplankton distribution of two zones in the main channel of the estuary: its northern margin and the deeper central zone. The Bahia Blanca Estuary is a coastal plain system (38°45´S; 62°22´W) on the Atlantic coast of Argentina (Fig. 1). It is formed by several NW to SE tidal channels separated by extensive intertidal flats, low marshes and islands (PICCOLO; PERILLO, 1990). The tides and winds have been considered the main factors controlling the water turbulence processes (PICCOLO; PERILLO, 1990), especially in the inner zone where the tidal height is maximal (tidal range of up to 3.6 m). Tidal currents are reversible with maximum surface velocities of about 1.3 m s