Water Replacement Rates Research Articles

Assessment of the development of Lactuca sativa Batavia Aficion in hydroponic and aquaponic systems

The need to improve the methods of growing plants in hydroponic systems to ensure optimal conditions for their growth and achieve high yields is urgent. The purpose of this study was to compare the hydroponic production of Lactuca sativa Batavia Aficion using a conventional Knop nutrient solution compared to aquaponics using nutrient-rich fish water. Laboratory, potentiometric, and photometric methods were used for this purpose. The yield, biometric, and qualitative indicators of lettuce leaves were investigated. Despite the lower nutrient concentration in the aquaponic solution, the nutritional status of Lactuca sativa Batavia Aficion was within the optimal range. The nitrate content of lettuce grown in the aquaponics system was higher than in hydroponics, but there were no significant differences in the content of total N (3.24% and 2.97%), Mg (1,973 mg/kg and 1,943 mg/kg), Fe (93.91 mg/kg and 93.83 mg/kg), K (73.7 mg/kg and 73.6 mg/kg), and Ca (19.5 mg/kg and 20.1 mg/kg). The yield of Lactuca sativa Batavia Aficion on aquaponics was 2.8 kg / m2 and 3.2 kg m2 – on hydroponics, with a density of 36 plants per square metre. Water monitoring in the aquaponic system showed low concentrations of nitrates, phosphorus (P), potassium (K), and magnesium (Mg), but the proportion of mineral nutrients and pH were stable throughout the lettuce growing period. Lettuce leaves in the aquaponics system reached a fresh weight of 80 g in 34 days, which is on average 13% less than lettuce leaves in the hydroponics system. The EC (electrical conductivity) values recorded in this study in a hydroponic system were between 1.2 and 1.5 cm/m. In the aquaponic system, EC has higher values due to the low rate of water replacement, contributing to greater growth and accumulation of solution ions. However, due to continuous recirculation in the water, the conditions become satisfactory for growing plants. The results obtained can contribute to the creation of more efficient and sustainable agricultural systems, reducing resource consumption and improving the resistance of cultivated crops to various stressful conditions

Effects of Water Deficit on Growth and Productivity in Tomato Crops Irrigated with Water Treated with Very Low-Frequency Electromagnetic Resonance Fields.

The tomato crop is very sensitive to stress conditions. A water deficit is defined as when precipitation is less than the evapotranspiration (ETc) of the crop in a given period, and in this scenario of climate change, it is identified as responsible for global productivity losses. The use of potential technologies for better irrigation management, such as electromagnetically treated water, remains controversial. Thus, the objective of the present work was to investigate the effects of very low-frequency electromagnetic resonance field treatment on water for tomato crops submitted to different irrigation rates. For this, an experiment was carried out under controlled conditions with different types of water: electromagnetically treated water (WTVLF) and untreated water (UNW), as well as four water replacement rates: 40, 60, 80, and 100% ETc. The electromagnetic treatment of the water was carried out using the commercial equipment AQUA4D®. The experiment was carried out in pots with five replications per treatment. Lower activity of SOD, POD, CAT, and APX enzymes was observed in plants irrigated with water treated with very low-frequency electromagnetic resonance fields (WTVLF), indicating less oxidative stress caused by water deficit. Water deficit reduced chlorophyll content, but the effects were less harmful with WTVLF water. The water deficit resulted in less accumulation of dry matter and less productivity in a linear relationship. However, plants irrigated with WTVLF showed increments of about 20% in dry matter accumulation and 20% in fruit production concerning plants irrigated with untreated water, independent of the irrigation rates. We can conclude that irrigation with WTVLF can be a solution to reduce the damage caused by water deficits and increase the productivity of tomato crops.

Simulation Study on the Impact of Water Flow Regulation Based on the MIKE 21 Model in a River Water Environment

Inadequate hydrodynamics can cause deterioration of the water environment within rivers. Regulating water conservancy projects can effectively improve the state of the water environment of rivers and promote sustainable regional development. The river plain in Zhejiang Province, China, suffers from severe hydrodynamic deficiencies, which have a significant impact on the state of the regional water environment. To investigate the changing state of the water environment of the river plain under water conservancy project control, in this study we constructed a two-dimensional hydrodynamic–water quality model based on the AD and ECOLAB modules in MIKE 21 software 2014 Edition. Combined with conservative tracers, the changes in the water displacement rate, flow rate, and water environment quality of the river plain were simulated under different regulation schemes over 7 days. A quantitative analysis of the effects of improving the state of the water environment in the river plain was carried out using a cluster analysis and a cloud model. (1) The water replacement rate of the outer river reached 90% after 3 days and approached 100% after 7 days. The water replacement rates of the inner pond were 51.2, 49.6, and 55.8%. This indicated that the engineering control measures effectively improved the replacement capacity of the river. (2) The contents of DO and BOD5 in the river have increased from class V to above class 3. The overall water quality is in the range of classes 2 to 3, and in some parts it can reach class 1. This indicates that the regulatory plan played a certain role in improving the river water environment. (3) The water pollution in the study area showed a fluctuating and decreasing trend over 7 days. There was a positive correlation between the flow velocity, water replacement rate, DO, and BOD5.

Optimization of water replacement during leachate recirculation for two-phase food waste anaerobic digestion system with off-gas diversion

An integrated two-phase AD with acidogenic off-gas diversion from a leach bed reactor to an upflow anaerobic sludge blanket was developed for improving methane production. However, this system had its own technical limitation such as mass transfer efficiency for solid-state treatment. In order to optimize the mass transfer in this two phase AD system, leachate recirculation with various water replacement rates regulating the total solids contents (TS) at 12.5%, 15%, and 17.5% was aim to investigate its effect on methane generation. The solubilization of food waste was increased with decreasing TS content, while the enzymatic hydrolysis showed the opposite trend. A TS contents of 15% presented the best acidogenic performance with the highest hydrogen yield of 30.3 L H2/kg VSadded, which subsequently resulted in the highest methane production. The present study provides an easy approach to enhance food waste degradation in acidogenic phase and energy conversion in methanogenic phase simultaneously.

White Plastic and Deficit Irrigation Improve Pomegranate Production

ABSTRACT Developing improved water management strategies and on-farm cultural practices are important in reducing pomegranate (Punica granatum L. cv Wonderful) production costs. A pomegranate orchard established in 2010 was irrigated at water replacement rates of 35, 50 and 100% of crop evapotranspiration from 2013 to 2016. A durable white plastic tarp was installed in one half of each water replacement plot to evaluate the added benefits to soil moisture retention, weed control, and plant growth and yield. Edaphic orchard floor conditions such as light reflectance into the lower tree canopy, mid-day soil surface temperature, soil water content, and average daily soil temperatures were moderated or improved by the plastic cover. Phenological changes such as leaf greenness, trunk basal suckering, and trunk diameter were generally enhanced by increased water application rates and plastic use. Marketable and total fruit yields increased by an average of 20 to 29% from 2013 to 2016 with the use of plastic cover compared to bare soil. Total fruit yield increased linearly with increasing water replacement rates. Neither the white plastic nor irrigation rates improved external fruit color. Fruit raw product attributes were little changed by treatments. The research clearly demonstrated the benefit of plastic cover on improving pomegranate production.

Physiology and yield of piel de sapo melon (Cucumis melo L.) under water deficit in semi-arid region, Brazil

Melon (Cucumis melo L.) is a crop of great socioeconomic importance in regions with semiarid climate, as found in Northeast Brazil. In this region water deficit is a usual condition as well as the main reduction factor of plant productivity. Due to this phenomenon, irrigation techniques are used to make available the amount of water needed for plant production. Therefore, this research evaluated the physiology and fruit production changes of ‘Juazeiro’ Piel de sapo melon plants under different water replacement rates. ‘Juazeiro’ melon was grown under four reference evapotranspiration replacement rates (40, 60, 80 and 100% of ETo) distributed randomly in blocks with 5 replicates. The plants were cultivated in the field, in an experimental area located in the Brazilian Northeast region, under hot and dry semiarid climatic conditions, and during the growing cycle the ‘Juazeiro’ hybrid melon plants were evaluated for leaf area, biomass accumulation, gas exchange, photochemical efficiency of photosystem PSII and production of fruits per plant. Application of water replacement rates of 100% ETo leads to highest growth, net photosynthesis, and fruit yield of 'Juazeiro' melon. Water replacement at 80% ETo can be used for ‘Juazeiro” melon under semiarid climatic conditions, but with small losses in fruit yield. Water replacement below 80% ETo leads to drastic reductions in growth, gas exchanges, quantum efficiency of photosystem II and fruit yield, which impairs melon production in semi-arid climate environments.

Molecular detection of acute hepatopancreatic necrosis disease (AHPND) in Penaeus vannamei shrimps in Costa Rica

Acute hepatopancreatic necrosis disease (AHPND) is a bacterial disease associated to severe mortality in farmed shrimps, and caused by Vibrio parahaemolyticus containing plasmid pVA-1 encoding pirA and pirB toxins. This study investigates the presence of Vibrio spp. carrying plasmid pVA-1 in post larvae and juveniles Penaeus vannamei from farms located in Costa Rica. Moreover, a possible corelation between Vibrio spp. presence, management parameters, and water quality was also investigated. Between 2017 and 2018, post larvae, the first water pumped into ponds, and juvenile shrimp (6 to 7 weeks after stocking) were collected from 15 farms located in the Gulf of Nicoya and the country's Central Pacific region. On the day when the juvenile shrimp were collected, a survey was applied to farmers to obtain information about management conditions, finally physical-chemical parameters of pond water were measured. Plasmidic pVA-1 pirA and pirB genes were detected in hepatopancreas of juvenile shrimp in 5 (33.3%) farms, while Vibrio spp. were found in 6 (40.0%) farms. Sequencing of pVA-1, pirA and pirB genes showed 99–100% similarity to pathogenic Vibrio parahemolyticus XN89 homologous genes identified in Vietnamese shrimps. Statistically significant differences were found in the water volume (p < .03), rate of water replacement (p < .04), and farms disease history (p < .05). A correlation between presence of Vibrio spp. and water quality was not established. The molecular diagnosis of Vibrio spp., the plasmid and the genes encoding toxins that are associated with AHPND are reported for the first time in Costa Rica. Further studies aimed to isolate AHPND-causing Vibrio spp. from ponds, to generate histopathological data, and to establish economic losses due to AHPND mortalities in Penaeus vannamei farms, are needed to clarify the role and pathogenic features of Vibrio spp. in AHPND.

PHOTOCHEMICAL AND PRODUCTIVE PERFORMANCE OF YELLOW PASSION FRUIT IRRIGATED IN THE BRAZILIAN SEMIARID

Drought stress is one of the greatest problems facing agriculture in the semiarid regions and affects immediately the physiological processes and plant production. For that, irrigation is an essential practice to minimize effects of water deficit. Its appropriate management can to increase the farm income. This study aimed to evaluate to analyze the photochemical and productive performance of passion fruit (Passiflora edulis) hybrid ‘BRS Gigante Amarelo’ irrigated in a Brazilian semiarid region. It was studied four water replacement rates (33, 66, 100 and 133% of ETo) at the field conditions. A randomized blocks experimental design it was used with five replications. The fluorescence of chlorophyll a, yield and fruit quality it were studied. The photochemical efficiency of photosystem II have been changed along the day at high temperatures. Occurred vidential damage in PHS II. Water replacement between 100 and 133% of ETo (1032 mm year-1) promotes the best production rates and quality of the fruit, but the fruit size and productivity are reduced in replacements over 100% of ETo.

Physiological, biochemical and productive changes in sesame genotypes subjected to different rates of water replenishment

ABSTRACT The sesame crop has stood out due to the high nutritional content of its seeds, in addition to being able to be cultivated in the tropical and subtropical regions such as Northeast Brazil. Thus, it is necessary to identify the physiological, biochemical and productive changes related to the tolerance to stress. Objective of this study was to evaluate the physiological, biochemical and productive aspects of sesame genotypes as a function of different rates of water replenishment. The experiment was carried out at the Embrapa Cotton Experimental Unit, located in the municipality of Barbalha-CE, Brazil, under field conditions. The experiment was carried out in a randomized complete block design, with treatments in a factorial scheme (4 × 6), corresponding to four irrigation depths (40, 70, 100 and 130% ETo) and six sesame genotypes (G1 = T3-EGSGO3; G2 = T7-EGSGO7; G3 = T5-EGSGO5; G4 = T2-EGSGO2; G5 = T6-EGSGO6; G6 = T4-EGSG04), with three replicates. Data corresponding to the following variables were collected: leaf area, photosynthetic pigments, relative water content in leaf, electrolyte leakage, catalase, peroxidase, yield and oil content. The genotypes did not differ statistically and there were differences in the variables between the water replacement rates. Increments in growth and, consequently, in production, enzymatic activity, oil content and maximum production potential were observed with water depths between 75 and 90% ETo. Contents of chlorophyll a and b, total chlorophyll and carotenoids, were increased with the application of 80% ETo.

The Study of Appropriate Strategy in Order to Avoid Economic losses Caused by the ICHthyophthirius Parasite on Ornamental Goldfish Species

This study aimed to provide the Appropriate Strategy in order to Avoid Economic losses Caused by the ICHthyophthirius Parasite on Ornamental Goldfish Species in the city of Shiraz in the four cardinal directions. In this study Pearl scale, Calico, Black Moor, Red Cap and Lionhead species, were examined and the prevalence of parasites in different conditions such as different temperatures, different turbidity, algal growth, feeding rate and etc. were reviewed. The main result is that management is the most important factor in this disease. The most resistant and most sensitive species are Calico and Pearl scale, respectively. In the absence of management prevalence is associated with increasing temperature and reaches its peak at 27 ° temperature. The proper management reduces the disease at high temperatures. Increased water turbidity, the amount of aeration, algal growth, water replacement rates are very important factors that make fish more susceptible to ICH disease. The results show that water temperature should be high for better and faster treatment of the disease and fish parasites more quickly reach the stage tomite. Goldfish lives at low temperatures, so it is better water temperature gradually raised from fish early life until goldfish reaches maturity to help fish adapt to high temperatures. This not only provides better management of ICH disease but also against other diseases and the disease can be treated quickly.

Influence of water replacement on Diflubenzuron duration effect in the control of Aedes aegypti in simulated field conditions, in Northeastern Brazil

Introduction: Theoretically, it is possible to control dengue by reducing breeding sites. However in most countries, it has not been possible to obtain sustainable dengue control using only the reduction of potential breeding sites thus use of larvicides continues to be necessary. This paper presents an estimate of the rate of water replacement in residential water containers in the city of Fortaleza, northeast Brazil, and the respective duration of the effect of Diflubenzuron on Aedes aegypti immatures. Methods: Several trials were performed to estimate the duration of the effect of Diflubenzuron in simulated field conditions with replacement daily with 20% by volume of water. Eighteen containers with 20 litres of water treated with Diflubenzuron, and 6 containers with well water were used. The initial concentration of Diflubenzuron in the treated containers was 2.875 x 10-4 grams/litre. Third stages larvae were added to the containers, the first day of experiment. All the containers were inspected daily and dead larvae and pupae were removed and counted. Each trial was terminated when all larvae added to one of the groups (treated or control) had died or pupated. Results: From the 1st to the 19th day, the Diflubenzuron achieved 100% control, adjusting for the emergence of pupae in untreated containers not allowing any larvae to develop into pupae. From the 20th to the 70th day, the control ranged from 97.9% to 36.6%. Conclusions: This study reports a good larvicide effect of Diflubenzuron in the doses used by the Brazilian Programme of Dengue Control.

Meteorological influences on algal bloom potential in a nutrient‐rich blackwater river

Summary1. The effect of variability in rainfall on the potential for algal blooms was examined for the St Johns River in northeast Florida. Water chemistry and phytoplankton data were collected at selected sites monthly from 1993 through 2003. Information on rainfall and estimates of water turnover rates were used in the analyses of trends in phytoplankton biomass.2. Major trends in rainfall and runoff within the lower St Johns River catchment over the 10‐year study period were marked by both significant drought and flood periods. Autumn and winter rainfall patterns were strongly correlated with the range of Pacific sea surface temperature anomalies associated with El Niño events and La Niña periods. The effect of these major shifts in rainfall was evident in the strong relationship to replacement rates for water within the lower St Johns River.3. The eutrophic status of the river was reflected in the high concentrations of nitrogen and phosphorus observed at all sampling sites, with total nitrogen concentrations up to 3100 μg L−1 and total phosphorus concentrations up to 180 μg L−1.4. While it is clear that the high phytoplankton biomass and frequent blooms that characterize the freshwater portions of the lower St Johns River are fundamentally based on nutrient status, the expression of that potential was strongly correlated to water replacement rates, as revealed by the inverse relationship between phytoplankton biovolume increase and water turnover rate, with an R2 of 0.80 for the major bloom season. The sensitivity of algal blooms to rainfall patterns over the 10‐year study period suggest that longer‐term temporal and spatial shifts in rainfall, such as multi‐decadal cycles and the global‐warming phenomenon, will also influence the frequency and intensity of algal blooms.

Effects of Water Level Management on Autumn Staging Waterbird and Macrophyte Diversity in Three Danish Coastal Lagoons

Many Danish wetlands frequently experience algae blooms and oxygen deficiencies because of eutrophication caused by enhanced nitrogen and phosphorous levels. As a consequence, wetland managers have focused on improving water quality, but often without considering the suitability of wetlands for waterbirds. In this study, managers improved water quality in two Danish lagoons by opening a floodgate to the sea. We studied the responses of autumn staging waterbirds and submerged vegetation, to resultant higher water replacement rates and water levels. A third lagoon with no change in management was studied for comparisons. Lagoons with rise in water levels experienced declines in bird species diversity, a decline in benthivore species abundances and an increase in herbivore species abundances. Macrophyte biomass increased but seagrass diversity was low due to high salinity. The lagoon with no changes in management control had high and stable waterbird diversity and bird-days spent was increasing during the study period. Explanations for this were diverse typography and low water levels. Furthermore, the macrophyte community was more diverse due to low salinity. In order to improve both water quality and to increase waterbird diversity in the managed lagoons, we suggest water levels should be managed actively during peak migration in autumn. This could secure more shallow-water areas to waders and create better congruence between appropriate water levels and timing of peak bird migration in autumn. The rest of the year the floodgate should be left open in order to secure a high water quality.

Conformations, Protonation Sites, and Metal Complexation of Benzohydroxamic Acid. A Theoretical and Experimental Study

A theoretical and experimental study on the structure and deprotonation of benzohydroxamic acid (BHA) has been performed. Calculations at the RHF/cc-pVDZ level, refined by the B3LYP/AUG-cc-pVDZ method, indicate that, in the gas phase, Z amide is the most stable structure of both neutral and deprotonated BHA. (1)H-(1)H nuclear Overhauser enhancement spectroscopy and (1)H-(1)H correlation spectroscopy spectra in acetone, interpreted with ab initio interatomic distances, reveal that BHA is split into the Z and E forms, the [E]/[Z] ratio being 75:25 at -80 degrees C. The formation of E-E, Z-Z, and E-Z dimers has been detected; in the presence of water, the dimers dissociate to the corresponding monomers. The rates of proton exchange within the Z and E forms and between E and Z were measured by dynamic (1)H NMR in the -60 to 40 degrees C temperature range; an increase in water content lowers the rate of exchange of the E isomer. The effect of D(2)O on the NMR signals indicates a fast hydrogen exchange between D(2)O and the E and Z amide forms. The sequence of the acid strength at low temperatures is (N)H(E)) approximately (O)H(E) < (O)H(Z) approximately (N)H(Z). The kinetics of complex formation between BHA and Ni(2+), investigated by the stopped-flow method, show that both neutral BHA and its anion can bind Ni(2+). Whereas the anion reacts at a "normal" speed, the rate of water replacement from Ni(H(2)O)(6)(2+) by neutral BHA is about 1 order of magnitude less than expected. This behavior was interpreted assuming that, in aqueous solution, BHA mainly adopts a closed (hydrogen-bonded) Z configuration, which should open (with an energy penalty) for the metal binding process to occur.

Estimation of water renewal times for the middle and lower sections of the Yellow River

AbstractReplacement of water in a given hydrological component is one of the effects of the hydrological cycle. We introduce the concept of renewal time to express the rate of water replacement in a specific reach of a river. Based on stream‐flow data from the 1990s, a simple formula is used to estimate renewal times for several reaches in the middle and lower sections of the Yellow River (Huanghe) in China. These estimates reveal the following trends: (i) renewal times are shortest during the May–October period and (ii) annual renewal times tend to decrease when moving downstream from the middle to lower reaches. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Influence of water volume, surface area, and water replacement rate on weight gain of juvenile freshwater prawns, Macrobrachium rosenbergii

The effects of combinations of water volume, bottom surface area, and water replacement rate on weight gain of juvenile freshwater prawns, Macrobrachium rosenbergii, were evaluated in two experiments. Prawns were fed a semi-purified diet and individually cultured in clear plastic containers, which were circular and slightly tapered. Each container received an independent water supply in a flow-through system. This design was used to evaluate weight gain in the absence of potentially confounding effects of tactile, chemical and visual stimuli. The first experiment was a 2×2×2 factorial design consisting of two levels of each of the three factors, water volume (1375 and 700 ml), bottom surface area (70.9 and 35.4 cm 2), and water replacement rate (86 and 44 min). After 60 days, weight gain of juvenile prawns (initially 221.8 to 227.0 mg) was significantly greater (232%) under conditions of both greater water volume and bottom surface area. Higher water replacement did not compensate for lower water volume. There was significantly more weight gain (149%) with greater bottom surface area at the lower rate of water replacement. However, at the higher rate of water replacement, greater bottom surface area did not result in a significant increase in weight gain. The second experiment was designed to determine at what point the reduction in growth identified in the first experiment became operative. It consisted of two treatments in which weight gain at low water volume and bottom surface area (750 ml and 78.5 cm 2) was compared to that at high water volume and bottom surface area (1300 ml and 147.4 cm 2) at equal rates of water replacement. Mean stocking weight of the prawns was 7.9 mg and the duration of the experiment was 143 days. Reductions in weight gain began to occur when a critical biomass density of approximately 500 mg/l was attained as reflected in significant differences in weight-frequency distributions between treatments at 90 days and thereafter. The response also reflects a previously identified relationship between maximum density and body size for aquatic animals. A culture volume that allows for maximum, density-independent growth of animals for all treatments is needed for experiments in which growth (weight gain) of crustaceans is used as a response variable to compare performance of different diets. Knowledge of the biomass density that causes growth reduction can be practically applied to establish selective harvest or stock manipulation strategies whereby overall production per unit of time or space can be increased. Alternatively, knowledge of this density threshold can be used to reduce growth rates and thereby assist in distributing production through time.

The biological effects of nitrate fertilization and water replacement in an oligotrophic cold water pond

Plastic enclosures (limnocorrals) were used to assess the impact of adding NO3 - to an oligotrophic pond. The pond, Mare's Egg Spring, is spring fed at a constant temperature of 4.5 °C (the main portion of the pool never exceeded 6 °C) and a molar ratio of N:P of 0.5. The pool is dominated by the cyanobacterium (blue green alga), Nostoc pruniforme, which is capable of nitrogen fixation. This, in combination with the low N:P ratio, suggested that non-nitrogen fixing primary producers were limited by nitrogen. Over a period of one month, growth and nitrogen fixation of Nostoc, suspended chlorophyll, sediment denitrification, and benthic diatom diversity were not dependent upon nitrate concentration. However, sedimentary chlorophyll levels increased slightly when NO inf3 sup- levels were increased from 0.71 µM (ambient) to 214 µM. Limnocorrals with slit sides gave water replacement rates between those for complete enclosure and those in open water. In the open channel, Nostoc growth was highest, and suspended chlorophyll the lowest. In the closed corrals, Nostoc growth was the lowest and suspended chlorophyll the highest, with intermediate values in the slit corrals. Therefore, short term increases of NO inf3 sup- levels in nitrogen poor aquatic systems do not necessarily affect the biological community, but slowing water replacement in small ponds may have significant effects.

Time scales of circulation and mixing processes of San Francisco Bay waters

Conceptual models for tidal period and low-frequency variations in sea level, currents, and mixing processes in the northern and southern reaches of San Francisco Bay describe the contrasting characteristics and dissimilar processes and rates in these embayments: The northern reach is a partially mixed estuary whereas the southern reach (South Bay) is a tidally oscillating lagoon with density-driven exchanges with the northern reach. The mixed semidiurnal tides are mixtures of progressive and standing waves. The relatively simple oscillations in South Bay are nearly standing waves, with energy propagating down the channels and dispersing into the broad shoal areas. The tides of the northern reach have the general properties of a progressive wave but are altered at the constriction of the embayments and gradually change in an upstream direction to a mixture of progressive and standing waves. The spring and neap variations of the tides are pronounced and cause fortnightly varying tidal currents that affect mixing and salinity stratification in the water column. Wind stress on the water surface, freshwater inflow, and tidal currents interacting with the complex bay configuration are the major local forcing mechanisms creating low-frequency variations in sea level and currents. These local forcing mechanisms drive the residual flows which, with tidal diffusion, control the water-replacement rates in the estuary. In the northern reach, the longitudinal density gradient drives an estuarine circulation in the channels, and the spatial variation in tidal amplitude creates a tidally-driven residual circulation. In contrast, South Bay exhibits a balance between wind-driven circulation and tidally-driven residual circulation for most of the year. During winter, however, there can be sufficient density variations to drive multilayer (2 to 3) flows in the channel of South Bay. Mixing models (that include both diffusive and dispersive processes) are based on time scales associated with salt variations at the boundaries and those associated with the local forcing mechanisms, while the spatial scales of variations are dependent upon the configuration of the embayments. In the northern reach, where the estuarine circulation is strong, the salt flux is carried by the mean advection of the mean salt field. Where large salinity gradients are present, the tidal correlation part of the salt flux is of the same order as the advective part. Our knowledge of mixing and exchange rates in South Bay is poor. As this embayment is nearly isohaline, the salt flux is dominated entirely by the mean advection of the mean salt field. During and after peaks in river discharge, water mixing becomes more dynamic, with a strong density-driven current creating a net exchange of both water mass and salt. These exchanges are stronger during neap tides. Residence times of the water masses vary seasonally and differ between reaches. In the northern reach, residence times are on the order of days for high winter river discharge and of months for summer periods. The residence times for South Bay are fairly long (on the order of several months) during summer, and typically shorter (less than a month) during winter when density-driven exchanges occur.

Rate of water replacement in a bay of Lake Mendota, Wisconsin

Rate of water replacement in a bay of Lake Mendota, Wisconsin