Observed Salinity Distribution Research Articles

Extension of the general unit hydrograph theory for the spread of salinity in estuaries

Abstract. From both practical and theoretical perspectives, it is essential to be able to express observed salinity distributions in terms of simplified theoretical models, which enable qualitative assessments to be made in many problems concerning water resource utilization (such as intake of fresh water) in estuaries. In this study, we propose a general and analytical salt intrusion model inspired by Guo's general unit hydrograph theory for flood hydrograph prediction in a watershed. To derive a simple, general and analytical model of salinity distribution, we first make four hypotheses on the longitudinal salinity gradient based on empirical observations; we then derive a general unit hydrograph for the salinity distribution along a partially mixed or well-mixed estuary. The newly developed model can be well calibrated using a minimum of three salinity measurements along the estuary axis and does converge towards zero when the along-estuary distance approaches infinity asymptotically. The theory has been successfully applied to reproduce the salt intrusion in 21 estuaries worldwide, which suggests that the proposed method can be a useful tool for quickly assessing the spread of salinity under a wide range of riverine and tidal conditions and for quantifying the potential impacts of human-induced and natural changes.

Revisiting the salinity condition in the Kuantan Estuary

The downstream region of the Kuantan River has a high potential of being affected by salinity issues according to research conducted nearly 40 years ago in studying the salinity effects on the freshwater intake in Kobat. The resulting results from the study then lead to the construction of a barrage in 1985 to prevent salt intrusion into the Kobat freshwater intake station area during low flow and high tides.However, salt intrusion problems were reported to occur in recent years. Thus, this research aims to: 1) determine salt intrusion extends in the Kuantan Estuary after the construction of the barrage; 2) calibrate and validate the Van der Burgh Coefficient K, and dispersion coefficient D in determining the salt intrusion pattern; and 3) compare the salinity pattern before and after the construction of the barrage. A 1-D analytical salt intrusion model was adopted in this study for the salinity analysis. Field measurements were conducted along the dry season at spring tide when the salinity travels furthest with a minimal amount of freshwater discharge. Salinity data was collected only up to the Kobat barrage by using the moving boat. The total salinity length at HWS obtained for 27th & 29th March 2017, 3rd October 2017, 7th March 2018 and 15th April 2018 are 21 km, 18.82 km, 21.05 km, 16.24 km, and 24 km, respectively. Results obtained from the salt intrusion simulation showed that the Van der Burgh Coefficient K for Kuantan Estuary is 0.45 and the dispersion ranged between 100 and 250 m2/s. Meanwhile, the comparisons of the newly surveyed salinity results with the previous research have indicated that the pattern of longitudinal salinity distribution has significantly changed after the construction of the barrage. The finding from the recent survey demonstrated the scatter distribution pattern compared to the gradually uniform pattern in the previous survey. This condition shows that the natural hydrodynamic process in the Kuantan Estuary has no longer in equilibrium condition and is unstable. The validation results obtained show good agreement between the simulated and observed salinity distribution in the estuary with low RMSE in the range 0.4 ppt–5.0 ppt and high NSE values ranges 0.97 to 1.00. This indicates that the model is reliable and can become an important tool for water managers in conducting salt intrusion studies for this area.

Testing thermodynamics method for salt intrusion during infrequent astronomical event

Knowledge of the processes governing salt intrusion in estuaries is important since it influences the eco-environment of estuaries as well as its water resource potential in many ways. The aim of this paper is to test the effectiveness of the new 1-D thermodynamic salt intrusion equation proposed by Zhang and Savenije (i.e., is an extreme power theory for the mixing of saline sea water and fresh river water in estuaries); and existing salt intrusion model (i.e., Van der Burgh method) at two previously unsurveyed estuaries in India and Morocco. To successfully perform the analysis using the 1-D models, real time data is required. Field measurements during infrequent astronomical event were recorded along the estuaries (31/01/2018 for Ashtamudi estuary (India); and during 31/01/2018 for Bouregreg estuary (Morocco)). Shape (geometry) and salinity analyses were performed on each of the estuaries. The results obtained show good agreement between the simulated and observed salinity distribution in the estuaries with high R 2 values. Finally, by utilizing the new predictive equation, we can estimate the minimum amount of river discharge needed, and the maximum allowable depth for dredging, to prevent saline water intruding further into the areas studied. Thus, the simple and effective approach presented in this study can describe the current state of salt intrusion in India and Morocco estuaries, and be used for future development.

Record-breaking salinities in the middle Adriatic during summer 2017 and concurrent changes in the microbial food web

Oceanographic measurements carried out in the middle Adriatic during summer 2017 revealed anomalous conditions in both physical and microbial properties. High salinities were observed throughout the entire water column, with an ‘inverse’ salinity profile in August and a maximum in the surface layer, recorded for the first time in the middle Adriatic. Surface salinity of 39.02 recorded in August was 2.5 standard deviations above the long-term average (1961–2016). The observed salinity distributions are the result of both local and remote drivers, whereby the North Ionian cyclonic gyre controlled by the Adriatic-Ionian Bimodal Oscillating System has been responsible for the overall above-average salinities since 2011. Yet, local factors present in 2017, such as strong evaporation caused by extremely high air temperatures, lack of precipitation and low river discharges, combined with a decrease in horizontal transport estimated from the Regional Ocean Modeling System simulations, contributed substantially to the observed surface salinity anomaly. The decrease in horizontal advection was conjoined with high values of repelling barriers in the fields of the finite-time Lyapunov exponent. Documented physical conditions were synchronized with considerably lower bacterial production and abundance of the most studied picoplankton groups in comparison to values during the last decade. The exception was the euryhaline organism Synechococcus, whose abundance was 88% higher than the average in the study area. Nutrient content and chlorophyll-a concentrations followed regular seasonal cycles during 2017, with typical low values pointing to salinity as a possible driver of the observed changes in the microbial food web. Following ongoing climate change and future projections, these documented anomalous physical and microbiological conditions may become more frequent in the Adriatic Sea.

Investigation of Salt Intrusion Condition in the Belat Estuary

Awareness on salt intrusion problem is still lacking in Malaysia due to high precipitation in the region. However, the El-Nino phenomenon that occurred recently has caused extremely low fresh water discharge in the Kuantan River which allowed the sea water to intrude further into its water intake region. Consequently, the Belat River may become potential water resources alternative to build new water intake station for the water supply in the Kuantan River Basin. The aims of this study are to: 1) investigate the salinity distribution in the Belat Estuary; 2) evaluate the applicability of the 1-D analytical salt intrusion solution; 3) determine the calibration parameters in the salt intrusion model. Salt intrusion measurements was conducted during the dry season at spring tide when the fresh water discharge is the minimum. Collection of data such as hydrological data, river cross section and salinity were collected to be used in the salt intrusion analysis. The results obtained show good agreement between the simulated and observed salinity distribution in the estuary with low RMSE and high NSE values. This indicates that the model is reliable and can become an important tool for water manager in conducting salt intrusion study for this area.

Satellite Observed Salinity Distributions at High Latitudes in the Northern Hemisphere: A Comparison of Four Products.

Global surface ocean salinity measurements have been available since the launch of SMOS in 2009 and coverage was further enhanced with the launch of Aquarius in 2011. In the polar regions where spatial and temporal changes in sea surface salinity (SSS) are deemed important, the data has not been as robustly validated because of the paucity of in situ measurements. This study presents a comparison of four SSS products in the ice-free Arctic region, three using Aquarius data and one using SMOS data. The accuracy of each product is assessed through comparative analysis with ship and other in situ measurements. Results indicate RMS errors ranging between 0.33 and 0.89 psu. Overall, the four products show generally good consistency in spatial distribution with the Atlantic side being more saline than the Pacific side. A good agreement between the ship and satellite measurements were also observed in the low salinity regions in the Arctic Ocean, where SSS in situ measurements are usually sparse, at the end of summer melt seasons. Some discrepancies including biases of about 1 psu between the products in spatial and temporal distribution are observed. These are due in part to differences in retrieval techniques, geophysical filtering, and sea ice and land masks. The monthly SSS retrievals in the Arctic from 2011 to 2015 showed variations (within ~1 psu) consistent with effects of sea ice seasonal cycles. This study indicates that spaceborne observations capture the seasonality and interannual variability of SSS in the Arctic with reasonably good accuracy.

Predicting the salt water intrusion in the Shatt al-Arab estuary using an analytical approach

Abstract. Longitudinal and vertical salinity measurements are used in this study to predict the extent of inland seawater intrusion in a deltaic river estuary. A predictive model is constructed to apply to the specific tidal, seasonal, and discharge variability and geometric characteristics of the Shatt al-Arab River (SAR) situated along the border of Iraq and Iran. Reliable hydrologic simulation of salinity dynamics and seawater intrusion was lacking prior to this study. Tidal excursion is simulated analytically using a 1-D analytical salt intrusion model with recently updated equations for tidal mixing. The model was applied under different river conditions to analyse the seasonal variability of salinity distribution during wet and dry periods near spring and neap tides between March 2014 and January 2015. A good fit is possible with this model between computed and observed salinity distribution. Estimating water abstractions along the estuary improves the performance of the equations, especially at low flows and with a well-calibrated dispersion–excursion relationship of the updated equations. Salt intrusion lengths given the current data varied from 38 to 65 km during the year of observation. With extremely low river discharge, which is highly likely there, we predict a much further distance of 92 km. These new predictions demonstrate that the SAR, already plagued with extreme salinity, may face deteriorating water quality levels in the near future, requiring prompt interventions.

Predicting of salt water intrusion in the Sebou river estuary (Morocco)

Salt water intrusion in estuary is an urgent environmental challenge across the world, because salinity influences water quality. The Sebou river estuary is an area with high agricultural potential becoming one of the most important industrial zones in Morocco. However, salt water intrusion along the estuary is a natural phenomenon that affects the economic development of the whole region. Analytical models of salinity distribution are simple and efficient tools to study salt intrusion in alluvial estuaries since they require minimum data for their application. In this paper, an analytical model based on Savenije theory is used to assess salinity variation in the Sebou river estuary. A two optimization key parameters of the model (Van Der Burgh's and dispersion coefficients) are estimated by comparing computed and observed salinity distribution for high water slack and low water slack. Additionally, statistical performance evaluation and sensitivity analysis were conducted on the model results. The results show good agreement with salinity observations, which suggests that the predictive model can be used to obtain a first-order estimation of salt intrusion along the Sebou estuary axis.

Mapping tidal residual circulations in the outer Xiangshan Bay using a numerical model

Xiangshan Bay, which is elongated and semi-enclosed, is characterized by strong tides. The original understanding of the tidal residual circulation in the bay was based on the Eulerian time-mean method. However, it has been theoretically proved that the Lagrangian time-mean method rather than the Eulerian one should be employed to detide. This knowledge motivated us to remap the tidal residual circulation in the bay. A three-dimensional numerical model with a Lagrangian particle-tracking module was used to simulate tides, from which the Lagrangian and Eulerian tidal residual circulations were produced. The Lagrangian residual circulation exhibited a two-branch pattern that connected the southern and northern outer seas of Xiangshan Bay, whereas the Eulerian residual circulation was characterized by eddies that are not favorable for water exchange. Through comparing to the observed salinity distribution, the Lagrangian residual current presented a reasonable spatial pattern which could explain the inter-tidal mass transport. Furthermore, the Lagrangian residual current performed better in sustaining the mass conservation as predicted by theory. A series of sensitivity experiments were conducted to investigate the effect of nonlinear mechanisms on the Lagrangian residual circulation. The results showed that nonlinear advection is dominant while the time-varying width and depth play a minor role, and the quadratic bottom friction has no influence with only M2 tide driving the model.

Testing a 1-D analytical salt intrusion model and its predictive equations in Malaysian estuaries

Little is known about the salt intrusion behaviour in Malaysian estuaries. Study of salt intrusion generally requires large amounts of data, especially if 2-D or 3-D numerical models are used; thus, in data-poor environments, 1-D analytical models are more appropriate. A fully analytical 1-D salt intrusion model, which is simple to implement and requires minimal data, was tested in six previously unsurveyed Malaysian estuaries (Kurau, Perak, Bernam, Selangor, Muar and Endau). The required data can be collected during a single day of observations. Site measurements were conducted during the dry season (June–August 2012 and February–March 2013) near spring tide. Data on cross-sections (by echo-sounding), water levels (by pressure loggers) and salinity (by moving boat) were collected as model input. A good fit was demonstrated between the simulated and observed salinity distribution for all six estuaries. Additionally, the two calibration parameters (the Van der Burgh coefficient and the boundary condition for the dispersion) were compared with the existing predictive equations. Since gauging stations were only present in some nested catchments in the drainage basins, the river discharge had to be up-scaled to represent the total discharge contribution of the catchments. However, the correspondence between the calibration coefficients and the predictive equations was good, particularly in view of the uncertainty in the river discharge data used. This confirms that the predictive salt intrusion model is valid for the cases studied in Malaysia. The model provides a reliable, predictive tool, which the water authority of Malaysia can use for making decisions on water abstraction or dredging.Editor D. Koutsoyiannis; Associate editor A. Fiori

Evaluating salinity distribution in soil irrigated with saline water in arid regions of northwest China

In arid and semi-arid regions, salinity is a serious and chronic problem for agriculture. A 3-year field experiment in the arid environment of Xinjiang, northwest China, was conducted to study the salinity change in soil resulting from deficit irrigation of cotton with non-saline, moderate saline and high saline water. The salinity profile distribution was also evaluated by an integrated water, salinity, and nitrogen model, ENVIRO-GRO. The simulated and observed salinity distributions matched well. Results indicated that after 3 years of cotton production, the average salinity in the 1.0-m soil profile was 336% and 547% of the original soil profile, respectively, for moderate saline and high saline water irrigation. If the practices continued, the average soil salinity (ECe) in the 1.0-m soil profile would approach a steady level of 1.7, 10.8, and 14.7 dS m−1, respectively, for the treatments receiving irrigation waters of 0.33, 3.62, and 6.71 dS m−1. It was concluded that deficit irrigation of saline water in this region was not sustainable. Model simulation showed that a big flood irrigation after harvest can significantly reduce the salt accumulation in the soil profile, and that this practice was much more efficient for salinity control than applying the same extra amount of water during the growing season.

Modelled response of Gulf St Vincent (South Australia) to evaporation, heating and winds

In Gulf St Vincent, Australia, the salinity of the head waters can exceed 42 in summer when evaporation is maximum and the rainfall is minimal. A depth-integrated implicit finite difference model is extended to simulate the summer–autumn evolution of salinity, temperature, and density distributions, with climatological evaporation, rainfall, air temperature, and wind stress as inputs. Advection of salt and heat by the density- and wind-driven circulation is modelled by the Quick scheme, whereas horizontal mixing by tidal circulation is parameterised by a dispersion coefficient related to the oscillatory vertical shear. Simulated distributions and seasonal variations compare well with available observations, which feature the flow of highly saline water along the eastern side of the gulf, while the western side is bathed by less saline shelf water. Model results show that, despite the increasing salinity gradients in summer, opposing temperature gradients can stifle the shelfward density currents in the southern parts of the region. Autumn cooling intensifies these density currents so that at the end of the season the flushing of highly saline water, accumulated in the gulf throughout the summer, is enhanced. It was found that the variability in the general circulation brought about by the directional variability in the prevailing winds is an important factor in maintaining the observed salinity distributions in the region.

An investigation of thermohaline circulation in Terra Nova Bay polynya

This paper addresses the study of the thermohaline circulation and convective processes induced by the presence of a coastal polynya in Terra Nova Bay (Ross Sea, Antarctica). These processes are analysed by means of oceanographic observations and numerical simulations. Time series of hydrological parameters and currents show the High Salinity Shelf Water (HSSW) formation with the mixing of the water column down to great depths, and a seasonal cycle of kinetic energy. A two-dimensional time dependent model for the numerical simulation of the effects of polynya on the circulation has been implemented. This model allows estimation of the contribution of thermohaline forcing on the circulation pattern, the amount of ice production consistent with the observed salinity distribution and annual cycle of currents and the amount of HSSW exported. The thermohaline forcing due to the polynya is able to induce strong horizontal currents up to 40 cm s−1 near the polynya area, as well as vertical movements of the order of 1 cm s−1. An annual ice production, ranging from 45 m to 90 m, mixes the water column at great depths, up to 750 m, and increases the salinity of the surface incoming waters up to 34.82. The estimated net north-eastward transport of HSSW ranges from 0.34 Sv to 1.23 Sv, depending on ice production.

Tropical Subsurface Salinity and Tritium Distributions in the Pacific: Their Differences and Formation Mechanisms*

While high salinity water extends to the equator in the upper thermocline of the Pacific in the Southern Hemisphere (SH), it hits the western boundary (WB) farther north of the equator in the Northern Hemisphere (NH), suggesting that no interior pathway exists to the equatorial region. By contrast, high tritium water appears on the equator in the central Pacific, apparently through a NH interior pathway within the thermocline. The mechanisms of forming these salinity and tritium distributions and the causes of their difference are investigated using a realistic ocean general circulation model (OGCM). The OGCM reproduces the properties of tropical salinity distribution quite well and displays interior pathways in the NH. Analysis indicates that the observed salinity distribution is compatible with the existence of a NH interior pathway. Key to the hemispheric difference in thermocline salinity is the sea surface salinity (SSS) distribution in relation to the so-called WB (interior) exchange window, from which subducted water goes to the equatorial region through the WB region (interior ocean). In the NH, high SSSs are found only in the WB exchange window, and high salinity water thus appears to turn onto the WB before reaching the equator. In the SH, on the other hand, high SSSs are found in both the WB and interior exchange windows, and, as a result, high salinity water extends to the equatorial region through both the WB region and interior ocean. The sea surface tritium field has high values near the eastern boundary within the interior exchange window in the midlatitude North Pacific. Thus, high tritium water takes the NH interior pathway to the equatorial region after the subduction. This is demonstrated by a passive tracer experiment with a sea surface distribution resembling that of tritium. This result suggests that the apparent differences between the isopycnal salinity and tritium distributions are largely due to differences in surface distribution, raising caution about interpreting ocean circulation with tracer fields alone.

The Circulation in the Upper and Intermediate Layers of the South China Sea

The circulation in the basin of the South China Sea is reproduced using a four-layer numerical model. Current fields in the second (upper) and third (intermediate) layers are emphasized. Three eddies coexist in the upper layer in summer. The circulation pattern in this layer is similar to that in the first (surface) layer. In winter, a cyclonic circulation occupies the entire basin of the South China Sea in the upper layer as in the surface layer. On the other hand, the circulation pattern in the intermediate layer is fairly different from that in upper two layers especially in winter. A double-gyre pattern appears in the intermediate layer during winter. The pattern is caused by the propagation of the baroclinic Rossby wave of the second mode. This wave is excited at onset of the winter monsoon wind. Such circulation pattern well explains the observed salinity distribution in the intermediate layer. Although the double-gyre pattern in the intermediate layer is revealed even in summer in this model, it is restricted in the western part of the basin. Besides, its current speed is small compared to that in winter.

Dinámica de las aguas en la parte interna del Golfo de Nicoya ante altas descargas del Río Tempisque

The water dynamics of the upper part of the Gulf of Nicoya were studied under high discharges of the Tempisque river. The salinity values close to freshwater found north of Chira island are the lowest reported for this lone. and have importance in mariculture planing. A salinity gradient is developed during low tide, indicating a strong stratification in the inner Gulf. High freshwater river discharge is coup led with an upstream bottom flow (sail wedge) that reacher; the head of the estuary. Under high river discharges, surface and bottom salinity values are correlated with depth, where vertical shear and tidal pumping must explain the high salinity values found in the bottom inward flow. A fish distribution pattern is proposed based on the observed salinity distribution and some field data. Salinity gradients appear to influence the concentration of fish in several relatively deep areas in the inner Gulf of Nicoya.

Modeling, measurements, and satellite remote sensing of biologically active constituents in coastal waters

Abstract We examine ways of addressing coastal environmental quality concerns through the use of modeling, measurements, and in the future, satellite remote sensing. In a summary of historical trace-metal concentrations in the waters of Narragansett Bay, we partitioned the estuary into sectors that reflect the morphology of the Bay and the transition between freshwater inputs and offshore coastal waters. We constructed a 24-box two-layer model of the Bay. A convenient summary of the chemical variations in the Bay was provided by a schematic diagram which for a constituent such as copper or other metals displays the average concentration, the range, the standard deviation, and the number of observations in our database for each sector. This diagram shows the spatial gradients through the Bay and the variability within a sector. Using a simple two-layer box model with seven transport terms we computed the physical exchanges between boxes using freshwater input and salinity data. The box model approach was applied in greater detail to the upper portion of Narragansett Bay to provide transport terms for use in an oxygen water quality evaluation. A digital bathymetric map of the estuary was compiled to enable volume-weighted calculations of physical and chemical properties. A seasonally variable data set was available to determine the effects of summer/winter and high-/low-runoff conditions on the oxygen concentrations of the waters. Using the freshwater input rates and the observed salinity distribution in the estuary we calculated the transport of waters between boxes and the residence times of water within each box. The model was applied to oxygen concentrations in the estuary incorporating estimates of the effects of air-sea exchange, of sediment oxidation demand, of photosynthetic production and respiratory consumption, and of biochemical oxygen demand from sewage treatment effluents. The model provides a basis to estimate the relative importance of various processes that may cause low oxygen conditions in the waters. An investigation of oxygen variations in coastal waters was conducted with an Endeco/YSI rapid-pulse dissolved oxygen electrode. A 30-day time series was obtained at a depth of 1–2 m in Narragansett Bay. Measurements of oxygen, temperature, and salinity were obtained every 30 min during October 1993. Fourier analyses were used to determine the frequencies in the oxygen, temperature, and meteorological (wind speed and sunlight levels) variables. There was a strong diel signal in oxygen with smaller amplitude variations at the semidiurnal tidal frequency and a large amplitude variation with a period of 3–5 days. High temporal resolution data are needed to detect the events in coastal waters that result in substantial chemical variations of biologically active constituents such as oxygen. In anticipation of possible applications of the next generation ocean color satellite sensor, SeaWiFS, we have been examining the historical CZCS data from the region off the northeastern U.S.A. We have worked with both the 4-km and the 1-km horizontal resolution CZCS data. The scales of variability that are evident are in the range of 5–50 km. For quantitative use of the ocean color data attention must be given to spatial variations in the atmospheric attenuation of the visible radiation, and to the separation of chlorophyll, suspended matter, and possibly blue-absorbing organic matter in the ocean color signal.

Estimating estuarine residence times in the Westerschelde (The Netherlands) using a box model with fixed dispersion coefficients

The residence time of the water masses in the Westerschelde estuary was determined using a simple compartment-model that simulates the advective-diffusive transport of a conservative dissolved substance (chlorinity). The residence time of a water parcel in the upstream part of the estuary (i.e. the time needed for this water parcel to leave the estuary) varied from about 50 days in winter to about 70 days in summer. The most seaward compartment had residence times of about 10-15 days. Dispersive coefficients that are fixed in time were able to reproduce the observed salinity distributions very well in the Westerschelde. They were obtained by calibration on observed chlorinities. It is argued that the apparent relationship of dispersive coefficients with freshwater flow, which is observed in certain studies, could (partly) reflect the deviation from steady state conditions which are required assumptions to calculate these dispersive coefficients directly from salinity profiles.

Vertical Diffusivity in Coastal Seas

Two distinct methods to estimate the vertical diffusion coefficient, Kz, in coastal seas are developed. One is an analytical method to calculate Kz from observed salinity distribution. The method o...

Transport and Water Quality Modelling in the Southern North Sea in Relation to Coastal Pollution Research and Control

Mathematical modelling techniques are used to quantify the transport in the southern part of the North Sea of pollutants originating from various inputs. Special attention was given to the anthropogenic increase in local concentrations of nutrients (N, P) and heavy metals (Cd, Hg, Cu, Pb, Zn, Cr) and their potential impact on marine organisms. A depth-averaged hydrodynamic model is used to calculate tidal and wind driven velocities and water levels. By averaging, residual flows are calculated, forming the basis for advective transports in a water quality model. Dispersive transports are derived from a comparison of simulated and observed salinity distributions. Water mass distributions and age functions for various inflowing water types are determined with the model. Transports of nutrients and heavy metals in the southern part of the North Sea are calculated using annual pollution inputs for 1980. Although interactions with bottom sediments are not considered, the calculated and measured concentrations show good similarities. The water quality in the Dutch coastal zone and German Bight area is shown to be highly determined by local pollution loads from the rivers Rhine, Weser and Elbe respectively. Comparison of simulated concentrations for 1980 with those resulting from simulations with estimated natural river inputs, shows that more than 50% of nutrients and heavy metal concentrations originate from human activities in large coastal zone areas. From toxicological information and standards, it is concluded that Cd, Hg and Cu are substances that need special attention in pollution research and control for the Dutch coastal waters.