Surface Return Flow Research Articles

An observational analysis of the eastern tropical Pacific Shallow Meridional Circulation using YOTC data and pilot balloons from Isla del Coco, Costa Rica

The low-level circulation of the atmosphere over Isla del Coco has been studied and the presence of a northerly wind at low levels of the atmosphere in the eastern tropical Pacific, in addition to the deep Hadley circulation cell, has been confirmed. Using data from pilot balloons (May 1997 through January 1999, October 2007 through April 2008), the northern flow is between 1 and 5 km high, depending on the time of year, with a maximum speed located between 2 and 3 km above the surface. The generating mechanism of the surface return flow in the Hadley circulation cell has been formulated as a sea breeze, Ekman pumping of the boundary layer, and it could even be a response to the Rossby wave generated by warming in the Chocó region. These results agree with those obtained from the ERA5 reanalysis (January 1979 through December 2020), which show that the southern return cell varies in position and height during the course of the year, with a poorly organized circulation in March and strengthening from July to February. The incorporation of data of low, medium and high cloud cover from Year of the Tropical Convection (boreal summer 2009) evidenced the presence of high-level clouds in the ITCZ region and low-level clouds to the south of the ITCZ, latitudes where the south surface circulation cell is located.

Modeling Surface Water Availability for Irrigation Development in Mbarali River Sub-Catchment Mbeya, Tanzania

Although Tanzania has a large land suitable for irrigation development, only 4.2% of the arable land which is potential for irrigation has been developed. Mbarali District is characterized by commercial and small-scale irrigation activities for paddy production. Currently, surface water availability for irrigation in Mbarali District is dwindling due to high water demands. Inadequate studies that estimate water availability for irrigation is one of the underlying factors to the lack of irrigation development in many parts of Tanzania including in Mbarali District. This study, therefore, aimed to model surface water availability for irrigation development in Mbarali River sub-catchment Mbeya, Tanzania. The Soil and Water Analysis Tool (SWAT) model and field observations were used to accomplish the study. The model estimates that Mbarali River sub-catchment receives about 631 mm of total mean precipitation annually. About 53% of received precipitation is lost through evapotranspiration, 12% recharged to deep aquifer and the remaining 35% discharged to the stream flow through surface runoff, lateral flow and return flow from unconfined aquifer. Discharge to the steam flow contributes to the total annual means of river discharge ranging from 0 - 10 cubic meters per second at upper catchment to 120 - 140 cubic meters per second at lower catchment. The study recommends that the lower reach of the Mbarali River sub-catchment is potential for irrigation than the upper reach as it has potential river flow that can support irrigation activities. The study also notes the urgent need for water reallocation plan to meet competing water needs in the lower reach of Mbarali River sub-catchment. Moreover, the study addresses the potentiality of irrigation in upper catchment under sustainable water management practices including excavation of small ponds to capture and store surface runoff for dry season use or to supplement irrigation as the rainfall declines.

Reducing pesticide transport in surface and subsurface irrigation return flow in specialty crop production

This study investigated the transport of 9 pesticides in surface and subsurface irrigation return flow from a container-plant production system over 3 16-day monitoring periods. Pesticides were applied 3 times during the year and the pesticides were selected to provide a range of physiochemical properties. Pesticides are most commonly transported in water and irrigation return flow is a driving factor, therefore, irrigation methods were investigated to determine effects on irrigation return flow volume and transport of pesticides. A control overhead irrigation and two treatments irrigating based on substrate volumetric moisture content (θ) were used. Pesticide transport in irrigation return flow was related to days after application, physiochemical properties and irrigation method. Additionally, pesticide transport was disproportionately reduced, in respect to physiochemical properties, in subsurface irrigation return flow in response to irrigation methods. θ-based irrigation treatments reduced irrigation volume applied by 49% and 77% compared to the control leading to a reduction in the volume of surface irrigation return flow by 71% and 92%. The θ-based treatments reduced the total surface and subsurface volume of irrigation return flow by 52% and 78% versus the control. In most cases, pesticides movement in surface irrigation return flow exhibited a linear or quadratic decrease for the control, while pesticide movement via subsurface irrigation return flow was related more to physiochemical properties limiting mobility rather than irrigation return flow volume or irrigation practice. This study demonstrates pesticide movement in irrigation return flow can be substantially reduced by selecting pesticides with low solubility/high adsorption coefficients whenever possible, and reducing the volume/application of irrigation to non-target areas.

Irrigation return flow and nutrient movement mitigation by irrigation method for container plant production

The production of nursery crops demands substantial irrigation, with overhead irrigation the most common method of application; however, this method is inefficient with respect to water used and the precision with which it is applied, resulting in the generation of irrigation return flow and concomitant agrochemical export. Microirrigation systems such as individual container spray stakes provide water directly to crops thus applying water more efficiently than overhead systems but may be more costly in terms of installation (smaller pipes and components; however, a greater quantity of pipes and components) and maintenance. The study was conducted at the Michigan State University Research Nursery, where four ornamental shrub taxa were produced in #3 (11.3 L) containers using a control with 19 mm overhead irrigation per day and a conventional phosphorus fertilizer (Conv) (19-2.16-6.64), compared with four treatments: a static, daily (2 L per container) spray stake irrigation (SS2Lpd) and conventional phosphorus fertilizer; a static daily (2 L per container) spray stake irrigation and low phosphorus fertilizer (LowP) (19-1.62-6.64); spray stake irrigation based on substrate volumetric water content (θ) (up to 2.4 L per container) (SSθ) and conventional fertilizer; and spray stake irrigation based on θ (up to 2.4 L per container) and low phosphorus fertilizer. Spray stakes reduced irrigation by 76–80% compared to the overhead control, and reduced the generation of both surface and subsurface irrigation return flow (IRF), mitigating the movement of both N and P (over 98% reduction in surface IRF). Plant growth index (GI) was measured on 12 June 2017 and 6 October 2017, followed by a destructive harvest to measure shoot dry weight, and shoot nutritional content. For all four taxa, microirrigation systems were capable of producing plants of equivalent GI and shoot nutritional concentration; however, plants receiving the low phosphorus fertilizer produced less shoot biomass. Microirrigation is effective in reducing water use, water lost to IRF (particularly surface IRF), and associated fertilizer movement, while maintaining crop size.

A semi-distributed drainage model for monthly drainage water and salinity simulation in a large irrigation district in arid region

The complex and dense irrigation and drainage networks and diverse irrigation scheduling result in uncertainties and complexities in the simulation of drainage water and salinity at different regional scales. We propose a semi-distributed model to simulate monthly drainage water and salinity, considering both the surface return flow and groundwater drainage to drainage ditches. The model uses sub-drainage command areas (SDCAs) controlled by main irrigation canals and drainage ditches as simulation units and uses a conceptual model to estimate monthly drainage water and salinity from each SDCA. The proposed model is applied in the Hetao Irrigation District (HID) in North China with 19 SDCAs, and calibrated and validated using monitoring data during 2008–2010 and 2012–2013, respectively. The results reveal better model performance in estimating the drainage water and salinity at SDCA, sub-irrigation district and irrigation district scales. The average annual drainage is 47,021 * 104m3 in HID, accounting for 10.5 % of the annual irrigation water amount. The surface return flow and groundwater drainage account for 32.7 % and 67.3 % of total drainage, respectively. The ratio of surface return flow to groundwater drainage is higher in the upstream area and lower in the downstream area. Higher surface return flow ratio (>5 %) is observed in the western Wulanbuhe, middle and southern Jiefangzha, and northern Wulate sub-irrigation areas, indicating the necessity to improve irrigation water management in these areas. Lower groundwater drainage capability is observed in western and southern HID, while enhanced groundwater drainage capability is observed in northern HID. Average annual salt leaching through the drainage system is 126.5 * 104 t, which is only 43 % of the total salt introduced from irrigation. The Wulate sub-irrigation district performs best on salt leaching, while other sub-irrigation district should improve the drainage system and irrigation water use efficiency to control the salt accumulation.

Impact of seawater equation of state on the simulation of Atlantic Meridional Overturning Circulation

The Atlantic Meridional Overturning Circulation (AMOC) plays a central role in the decadal variability of global and regional climate through changing poleward transport of heat. However, realistic simulation of the AMOC, i.e., its strength and spatial structure, remains a challenge for ocean general circulation models (OGCMs) and coupled climate models. Here, we investigate how the simulated AMOC could be affected by improved accuracy of the seawater equation of state (EOS) with an OGCM. Two EOSs used in this study: the UNESCO EOS80, and the “stiffened” EOS derived from the compressibility of sea water and the UNESCO EOS80. Compared to the model using the UNESCO EOS80, the model using the “stiffened” EOS yields stronger deep convection in the Labrador Sea, the Irminger-Iceland-Scotland Basin, and the Greenland-Iceland-Norwegian (GIN) seas, which leads to an improvement in the simulation of the AMOC: Along 26.5°N, the maximum transport is increased from 14.9 to 17.4 Sv and the interface between the upper clockwise cell and lower counterclockwise cell is deepened from 2.8 to 3.3 km, both matching the observations better. Taken the Labrador Sea as an example, the processes, including both direct and indirect causes, that in part responsible for the improved AMOC are as follows. The use of “stiffened” EOS increases the density throughout the water column and weakens the stability of sea water. Moreover, the enhanced cabbeling and thermobaric effect strengthen the vertical advection, intensifying the deep convection and increasing formation of deep water, which eventually improves the simulation of the AMOC. The intensified AMOC, in turn, speeds up the surface return flow, transporting more warm and saline water to the high latitudes in the North Atlantic, which contributes to the densification of surface water. Similar analyses can be applied to the Iceland-Scotland Basin and GIN seas. Thus, the enhanced deep convection and formation of deep water in the Labrador Sea, as well as in the Iceland–Scotland Basin and GIN seas, improve the simulated AMOC.

Simulation and analysis of return flow at the field scale in the northern rice irrigation area of China

To study variations in the field return flow, a field water balance model was established according to the water balance principle, and the field return flow was simulated using system dynamics and tested with field-measured data collected in a trial district in the Hu Lanhe irrigation area in 2016. The main conclusions are as follows: trends of the simulated and measured field water storage and field runoff were consistent. The daily error range between the simulated and measured field water storage values was (−0.1996, 0.2034) mm, and the daily error range between the simulated and measured field runoff values was (−0.2183, 0.1853) mm. Comparison of the trends among the water inflow, the surface return flow, the underground return flow and the total return flow revealed that the greater the water inflow was, the more similar the trends of the surface return flow and total return flow were, with no obvious change in the underground return flow. The total return flow increased with increasing field ridge length and decreased with increasing field ridge height and width. The range of the return flow coefficient of irrigation in 2011˜2016 was (0.5825, 0.6395), and the range of the return flow coefficient of irrigation and precipitation was (0.2734, 0.3386); the impact of irrigation water on the return flow was greater than that of precipitation. The research results are significant to the evaluation and analysis of field return flow and the management of agricultural irrigation water and provide a comparative reference for the study of return water in other areas.

Timing of Deglacial AMOC Variability From a High‐Resolution Seawater Cadmium Reconstruction

AbstractA new, high‐resolution record of benthic seawater Cd (Cdw) was generated from a Florida Straits sediment core at 546 m water depth. The record provides additional evidence for Cdw below modern values in this channel during the Younger Dryas and Heinrich Stadial 1—climatological periods associated with ice sheet melt. Lower Cdw values are interpreted as a weakening of the Atlantic Meridional Overturning Circulation (AMOC), reflecting a decreased northward transport of southern sourced higher‐nutrient intermediate waters by the surface return flow of AMOC. Comparison of this new Cdw record with previously published neodymium isotope and δ18O records from the same core shows synchronous transitions, further illustrating the connection between Cdw levels and AMOC strength in the Florida Straits. An increase in Cdw near 16 ka bolsters existing evidence for a resumption of upper branch AMOC strength approximately midway through Heinrich Stadial 1.

Constructing Comprehensive Datasets for Understanding Human and Climate Change Impacts on Hydrologic Cycle

Climate change and human activities altered the hydrologic systems and exerted global scale impacts on our environment with significant implications for water resources [1]. To secure a complete picture of future water resources, it will be necessary to consider climate change and variability, hydrology, water engineering, and human systems in an integrated framework [2]. Climate change can be characterized by the change of precipitation and temperature, and both precipitation pattern change and global warming are associated with the increase in frequency of flooding or drought and the decrease of low flows. With increasing water demand from domestic, agricultural, commercial, and industrial sectors, humans are increasingly becoming a significant component of the hydrologic cycle. Human activities have transformed hydrologic processes at spatial scales ranging from local to global as shown in figure 1. Human activities affecting watershed hydrology include land use change, dam construction and reservoir operation, groundwater pumping, surface water withdrawal, irrigation, return flow, and others e.g., Wang and Cai [3], Vogel [4]. The land use change impacts have been studied in many watersheds around the world. However, the fundamental role of human water use in the hydrologic cycle is not yet well understood [5].

Atmospheric Moisture Transports from Ocean to Land and Global Energy Flows in Reanalyses

Abstract An assessment is made of the global energy and hydrological cycles from eight current atmospheric reanalyses and their depiction of changes over time. A brief evaluation of the water and energy cycles in the latest version of the NCAR climate model referred to as CCSM4 is also given. The focus is on the mean ocean, land, and global precipitation P; the corresponding evaporation E; their difference corresponding to the surface freshwater flux E–P; and the vertically integrated atmospheric moisture transports. Using the model-based P and E, the time- and area-average E–P for the oceans, P–E for land, and the moisture transport from ocean to land should all be identical but are not close in most reanalyses, and often differ significantly from observational estimates of the surface return flow based on net river discharge into the oceans. Their differences reveal outstanding issues with atmospheric models and their biases, which are manifested as analysis increments in the reanalyses. The NCAR CCSM4, along with most reanalysis models, the exception being MERRA, has too-intense water cycling (P and E) over the ocean although ocean-to-land transports are very close to observed. Precipitation from reanalyses that assimilate moisture from satellite observations exhibits large changes identified with the changes in the observing system, as new and improved temperature and water vapor channels are assimilated and, while P improves after about 2002, E–P does not. Discrepancies among hydrological cycle components arise from analysis increments that can add or subtract moisture. The large-scale moisture budget divergences are more stable in time and similar across reanalyses than model-based estimates of E–P. Results are consistent with the view that recycling of moisture is too large in most models and the lifetime of moisture is too short. For the energy cycle, most reanalyses have spurious imbalances of ~10 W m−2 within the atmosphere, and ~5–10 W m−2 in net fluxes into the surface and to space. Major improvements are needed in model treatment and assimilation of moisture, and surface fluxes from reanalyses should only be used with great caution.

Multidecadal variability of the North Brazil Current

The Atlantic meridional overturning circulation (AMOC), which carries warm water to high northern latitudes near the surface and returns cold water in the deep ocean to the Southern Hemisphere, affects and is affected by global climate change. There has been debate as to whether the AMOC has begun slowing down due to global warming, but research on AMOC variability based on instrumental records is limited. One possible indicator of change in the AMOC is the North Brazil Current (NBC), a strong current that flows northward in the tropical South Atlantic, connecting the North and South Atlantic oceans, and plays an important role as a major pathway for surface return flow in the AMOC. Zhang et al. calculated how NBC varies on multidecadal time scales based on a record of 50 years of observations off the coast of Brazil. They found that NBC transport changes are correlated with Labrador Sea deep convection (important for deepwater formation) and with a broad pattern of sea surface temperature anomalies in the Atlantic (sometimes referred to as the Atlantic Multidecadal Oscillation or Atlantic Multidecadal Variability), both of which have previously been linked to AMOC fluctuations. The researchers therefore suggest that observed NBC variability is a useful indicator of AMOC variations. They confirm this using a climate model simulation. Furthermore, the authors note that although some studies have suggested that the AMOC is slowing down due to global warming, NBC shows multidecadal variability but no significant slowing trend over the past 50 years. (Journal of Geophysical Research‐Oceans, doi:10.1029/2010JC006812, 2011)

Multidecadal variability of the North Brazil Current and its connection to the Atlantic meridional overturning circulation

[1] The North Brazil Current (NBC) connects the North and South Atlantic and is the major pathway for the surface return flow of the Atlantic meridional overturning circulation (AMOC). Here, we calculate the NBC geostrophic transport time series based on 5 decades of observations near the western boundary off the coast of Brazil. Results reveal a multidecadal NBC variability that lags Labrador Sea deep convection by a few years. The NBC transport time series is coherent with the Atlantic Multidecadal Oscillation in sea surface temperature, which also has been widely linked to AMOC fluctuations in previous modeling studies. Our results thus suggest that the observed multidecadal NBC transport variability is a useful indicator for AMOC variations. The suggested connection between the NBC and AMOC is assessed in a 700 year control simulation of the Geophysical Fluid Dynamics Laboratory's CM2.1 coupled climate model. The model results are in agreement with observations and further demonstrate that the variability of NBC transport is a good index for tracking AMOC variations. Concerning the debate about whether a slowdown of AMOC has already occurred under global warming, the observed NBC transport time series suggests strong multidecadal variability but no significant trend.

Use of strontium isotopes in detrital sediments to constrain the glacial position of the Agulhas Retroflection

The Agulhas Leakage represents a significant portion of the warm, surface return flow of the global overturning circulation and thus may be an important feedback in the ocean climate system. Models indicate that reduced leakage could be caused by a stronger Agulhas Current and/or a more upstream (eastward) Agulhas Retroflection, while a weaker Agulhas Current would result in a more westward retroflection and increased leakage. However, data for the Last Glacial Maximum support both a weaker Agulhas Current and less leakage, implying a possible displacement of the retroflection. We present new 87Sr/86Sr results for modern sediments within this region, confirming that the modern pathway of the Agulhas Current, Retroflection, and Leakage can be traced by terrigenous sediment provenance using Sr isotopes. New 87Sr/86Sr data from sediments deposited during the Last Glacial Maximum suggest that the glacial Agulhas Current and Retroflection followed nearly their modern trajectory. The provenance data appear to rule out both a stronger Agulhas Current and a more upstream Agulhas Retroflection. We conclude that the reduced glacial leakage was caused by the weakened Agulhas Current, with no significant change in the retroflection position. This is inconsistent with the model predictions and thus emphasizes the need for further work in this region.

EXCHANGE FLOW IN A SHALLOW LAKE EMBAYMENT

Convectively driven currents can arise in the littoral zones of lakes, as a result of either differential heating or differential cooling of the shallow water. The result of these flows is to produce a surface flow away from shore with a bottom return flow or a bottom flow away from shore with a surface return flow. Measurements taken in a shallow embayment of Clear Lake, California, USA, show the presence of both kinds of convectively driven flows under a large variety of summer conditions. The magnitude of these flows is sufficient to transport material a distance on the order of 0.5 km during such events. Through both advection and dispersion the net result of this process would be to reduce the accumulation of particles and particle-associated contaminants such as mercury in the littoral zone and to move them offshore where they are more prone to permanent burial or further transport.

The mesoscale variability in the Caribbean Sea. Part II: Energy sources

The processes which drive the production and the growth of the strong mesoscale eddy field in the Caribbean Sea are examined using a general circulation model. Diagnostics of the simulations suggest that: (1) The mean currents in the Caribbean Sea are intrinsically unstable. The nature of the instability and its strength vary spatially due to strong differences of current structure among basins. (2) The greatest and most energetic eddies of the Caribbean Sea originate in the Venezuela Basin by mixed barotropic-baroclinic instability of an intense jet, formed with waters mostly from the surface return flow of the Meridional Overturning Circulation and the North Equatorial Current which converge and accelerate through the Grenada Passage. The vertical shear of this inflow is enhanced by an eastward undercurrent, which flows along the south American Coast between 100 and 250 m depth. The shallow eddies (less than 200 m depth) formed in the vicinity of the Grenada Passage get rapidly deeper (down to 1000 m depth) and stronger by their interaction with the deep interior flow of the Subtropical Gyre, which enters through passages north of St. Lucia. These main eastern Caribbean inflows merge and form the southern Caribbean Current, whose baroclinic instability is responsible for the westward growth and strengthening of these eddies from the Venezuela to the Colombia Basin. (3) Eddies of lesser strength are produced in other regions of the Caribbean Sea. Their generation and growth is also linked with instability of the local currents. First, cyclones are formed in the cyclonic shear of the northern Caribbean Current, but appear to be rapidly dissipated or absorbed by the large anticyclones coming from the southern Caribbean. Second, eddies in the Cayman Sea, which impact the Yucatan region, are locally produced and enhanced by barotropic instability of the deep Cayman Current. (4) The role of the North Brazil Current (NBC) rings is mostly to act as a finite perturbation for the instability of the mean flow. Their presence near the Lesser Antilles is ubiquitous and they appear to be linked with most of the Caribbean eddies. There are some evidences that the frequency at which they form near the Grenada Passage is influenced by the frequency at which the NBC rings impinge the Lesser Antilles. But large Caribbean eddies also form without a close influence of any ring, and comparison between simulations shows that mean eddy kinetic energy and eddy population in the Caribbean Sea are not substantially different in absence or presence of NBC rings: their presence is not a necessary condition for the generation and growth of the Caribbean eddies.

Three-Dimensional Simulation on the Water Flow Field and Suspended Solids Concentration in the Rectangular Sedimentation Tank

A 3D computational fluid dynamics model for describing the water flow and suspended solids (SS) concentration distribution in a rectangular sedimentation tank is presented. The interfacial momentum transfer, buoyant forces, and the effect of sediment-induced density currents are considered. A convection-diffusion equation, which is extended to incorporate the sedimentation of activated sludge in the field of gravity, governed the mass transfer in the clarifier. The double-exponential law is used to describe the dependence of the settling velocity on the concentration. The results show that during the dynamic settling process of the sludge, the mud surface rose slowly, and a period of time later, the mud surface kept stability and reached dynamic equilibrium in the tank. The distribution of velocity along the z axis in the rectangular tank is not uniform, and the surface return flow is found. The turbulent kinetic energy is larger and dropped drastically in the inlet zone, while in the settling zone the tu...

Deglacial variability in the surface return flow of the Atlantic meridional overturning circulation

Benthic foraminiferal Cd/Ca from a Florida Current sediment core documents the history of the northward penetration of southern source waters within the surface return flow of the Atlantic meridional overturning circulation (AMOC). Cd seawater estimates (CdW) indicate that intermediate‐depth southern source waters crossed the equator and contributed to the Florida Current during the Bølling‐Allerød warm period of the last deglaciation, consistent with evidence of only a modest AMOC reduction compared to today. The CdW estimates also provide the first paleoceanographic evidence of a reduction in the influence of intermediate‐depth southern source waters within the Florida Current during the Younger Dryas, a deglacial cold event characterized by a weak North Atlantic AMOC. Our results reveal a close correspondence between the northward penetration of intermediate‐depth southern source waters and the influence of North Atlantic Deep Water, suggesting a possible link between intermediate‐depth southern source waters and the strength of the Atlantic AMOC.

Indepth diagnosis of a secondary clarifier by the application of radiotracer technique and numerical modeling

To make an indepth diagnosis of a full-scale rectangular secondary clarifier, an experimental and numerical study has been performed in a wastewater treatment facility. Calculation results by the numerical model with the adoption of the SIMPLE algorithm of Patankar are validated with radiotracer experiments. Emphasis is given to the prediction of residence time distribution (RTD) curves. The predicted RTD profiles are in good agreement with the experimental RTD curves at the upstream and center sections except for the withdrawal zone of the complex effluent weir structure. The simulation results predict successfully the well-known flow characteristics of each stage such as the waterfall phenomenon at the front of the clarifier, the bottom density current and the surface return flow in the settling zone, and the upward flow in the exit zone. The detailed effects of density current are thoroughly investigated in terms of high SS loading and temperature difference between influent and ambient fluid. The program developed in this study shows the high potential to assist in the design and determination of optimal operating conditions to improve effluent quality in a full-scale secondary clarifier.

Modeling of Rectangular Settling Tanks

A mathematical model for predicting the velocity field and suspended solids transport in secondary rectangular settling tanks is presented. The model is applied to investigate the effects of density stratification on the hydrodynamics in the settling tank, such as the behavior of the bottom current and surface return flow and solids concentration distribution. The model can be used to simulate the so‐called density waterfall phenomenon at the front end of the settling tank. With the application of a double exponential relationship to describe the settling process of suspended solids with nonuniform particle sizes, the simulations of the solids concentration distribution in the low‐concentration environment can be significantly improved. By comparing the predictions with field measurements, this model is verified and generally good agreement is obtained.

Mathematical modelling of a circular clarifier

A numerical model has been developed to predict the velocity distribution and the concentration distribution for a nonuniform flocculated particle suspension for turbulent density stratified flow in secondary clarifiers. This model consists of a set of conservation equations for fluid mass and momentum and sediment concentration as well as a relationship for solids settling velocity. The turbulent stresses are calculated using the eddy-viscosity concept and the k-ε turbulence model. Since the strong local numerical instabilities near the reaction baffle lip, where the low-solids concentration surface return flow joins the high solids concentration bottom current, was successfully overcome in the iterative solution procedure, the calculation zone in this investigation can be extended to take into account the inlet zone of the clarifier. The model gave a satisfactory description of the influence of the density variations on the hydraulic regime and the solids concentration distribution in the clarifier. A good agreement was obtained between the experimental data and the numerical simulation. Key words: clarifier, numerical model, density stratification, turbulence.