Francisco J. Rueda

On the effects of topography on wind and the generation of currents in a large multi-basin lake

AbstractThe wind field over a lake surface is the key element in driving the exchange of momentum and energy at the free surface. It is rarely uniform, having a considerable degree of spatial variability, both on a synoptic and a local scale. Topographic features are one of the most common contributors to this variability. The aerodynamic effects of topography on the windfield, and its influence on the circulation patterns and interbasin exchange rates in a large multi-basin lake are documented. The complex landscape that surrounds this lake is dominated by the presence of a mountain peak rising over 900 m above the lake level. The circulation patterns in those basins of the lake located in the leeward side of the mountain reflect the spatial patterns of the wind field generated by the surrounding topography. The spatial variations in the wind field are shown to significantly alter the residence times of each basin and the exchange rates between basins, isolating one from the other two basins of the lake. The inter-basin exchange rates determined with wind field variability are consistent with annual contaminant loadings estimated for each of the three basins, which suggests a close link between chemical and hydrodynamic behavior.

The influence of external perturbations on the functional composition of phytoplankton in a Mediterranean reservoir.

The changes in abundance and composition experienced by phytoplankton communities in lakes and reservoirs occur in response to variations in the physical (light climate or energy) and the chemical (nutrient availability or resources) constraints for algal growth. Mediterranean reservoirs are very dynamic systems, subject to frequent changes in the physical environment as a result of water management operations, which suggests that phytoplankton communities might also undergo frequent changes. The phytoplankton community composition, abundance and seasonal dynamics of El Gergal, a medium-size Mediterranean reservoir, is analyzed and interpreted in terms of changes in the nutrient-energy balance. It is demonstrated that the seasonal scale changes in the physical environment trigger the seasonal predictable autogenic dynamics of the phytoplankton community. In addition, frequent short-term external perturbations of the physical environment may also induce allogenic shifts and reversions in the succession. The physical changes occur mainly as a result of variations in the outflows. Results are discussed in terms of phytoplankton functional groups life cycle strategies and water quality management.

A calibration strategy for dynamic succession models including several phytoplankton groups

A fundamental problem in water quality modeling is adequately representing the changing state of aquatic ecosystems as accurately as possible, but with appropriate mathematical relationships without creating a highly complex and overly parameterized model. A model more complex than necessary will require more input and results in unaffordable calibration times. In this work we propose and test a calibration strategy for a one-dimensional dynamic physical-ecological model (DYRESM-CAEDYM) to reproduce the seasonal changes in the functional composition of the phytoplankton community existing in El Gergal reservoir (Seville, Spain). The community is described as a succession of functional groups with different response to environmental conditions. First, we performed a sensitivity analysis to identify the parameters to include in the calibration process, and then applied a global optimization algorithm to fit the model for each algal group in a sequential fashion. Finally we simulated all the functional groups adopting parameter values established during the group-by-group calibrations. Our results show that the performance of this approach is strictly related with: (1) the level of system description (i.e. the model structure and the number of functional groups simulated); (2) the level of information included in the calibration process (i.e. the observations); and (3) the non-linear interactions among functional groups. Functional segmentation of the model should be minimized even though groups with different environmental requirements must be discriminated. Although magnitudes of biomass peaks were not always estimated correctly, the calibrated model was able to predict peak sequence and timing of dominant phytoplankton groups. Thus our study showed that: (1) model structure and nature of observations adopted have to be in agreement with the level of organization in the system; (2) integration of automatic calibration strategies is a useful approach in complex deterministic ecological models.

MECHANISMS OF CONTAMINANT TRANSPORT IN A MULTI-BASIN LAKE

Tracer studies are combined with a three-dimensional (3-D) numerical modeling study to provide a robust description of hydrodynamic and particle transport in Clear Lake, a multi-basin, polymictic lake in northern California, USA. The focus is on the mechanisms of transport of contaminants away from the vicinity of the Sulphur Bank Mercury Mine and out of the Oaks Arm to the rest of the lake and the hydraulic connection existing among the sub-basins of the lake. Under stratified conditions, the rate of spreading of the tracer was found to be large. In less than a week the tracer spread from the eastern end of the Oaks Arm to the other basins. Under non-stratified conditions, the tracer spread more slowly and had a concentration that gradually diminished with distance from the injection location. The numerical results showed that the mechanisms accounting for these observed patterns occur in pulses, with maximum rates coinciding with the stratified periods. Stratification acts first to enhance the currents by inhibiting vertical momentum mixing and decoupling the surface currents from bottom friction. The diversity of the flow structures that results from the interaction of the wind and the density fields in the lake is responsible for the high dispersion rates. Contaminants originating in the Oaks Arm are shown to be transported into the Lower Arm following the surface currents and into the Upper Arm mainly through the bottom currents. It was also shown that, under stratified conditions, both the baroclinic (density driven) gradients and the wind forcing act jointly to exacerbate the interbasin exchange.

Experimental observations of the splitting of a gravity current at a density step in a stratified water body

When a gravity current reaches the level of neutral buoyancy in a stratified water body it can separate from the sloping boundary as an intrusion. If there is a density gradient within the gravity current, then multiple intrusions can form in the stratified water body. Using a series of laboratory experiments in a two-layered ambient stratification, we document how a gravity current splits in two upon reaching the sharp density step. Our laboratory observations stress the significance of the densimetric Froude number of the gravity current (Fr), as well as a measure of the ambient stratification (density Richardson number, Riρ), on determining how a gravity current intrudes into a two-layered stratified ambient water. Gravity currents are more likely to detrain into two parts at a density step when they have a diffuse density interface (Fr > 1). However, gravity currents tend to intrude as a single intrusion when they have a sharp, more step-like density profile (Fr < 1). Using details of the internal structure of the gravity current, we develop a theory to predict the partition of the buoyancy flux into the interflow and underflow when the gravity current splits at the density step. Our predictions of buoyancy portions are in agreement with our experimental observations. We discuss when the application of our equations will be relevant for river intrusions into reservoirs, and for gravity currents in the stratified ocean.

Localized algal blooms induced by river inflows in a canyon type reservoir

The local response of the phytoplankton community to river inflow processes was investigated with modeling and field analyses in a long and narrow, stratified reservoir in mid-summer. The river water had high concentrations of phosphorus and nitrogen (ammonium and nitrate) and temperature had large variations at diurnal scales. As a consequence of the large variation in river temperature, the level of neutral buoyancy (the depth where the river water spreads laterally in the reservoir) oscillated between the surface (overflows) during the day, and the depth of the metalimnion (interflows) during the night. The reservoir remained strongly stratified, which favoured the presence of cyanobacteria. It is shown that under these conditions, nutrient-rich river water injected during overflows into the surface layers promoted the occurrence of localized algal blooms in the zones where the overflow mixed with the quiescent water of the reservoir. A series of hydrodynamic simulations of the reservoir were conducted both with synthetic and realistic forcing to assess the importance of river temperatures and wind-driven hydrodynamics for algal blooms. The simulations confirmed that the river inflow was the main forcing mechanism generating the localized bloom.

LOW PREDICTABILITY IN THE DYNAMICS OF SHALLOW LAKES: IMPLICATIONS FOR THEIR MANAGEMENT AND RESTORATION

This study was conducted in two eutrophic shallow lakes (Lake Honda LH and Lake Nueva LN) that share geographic proximity but have contrasting hydrology, meteorology, biogeochemistry, and geomorphology. Our objective was to explore the inter-annual, seasonal, and daily variability in selected biological, physical, and chemical variables of these two systems. Although the study lakes demonstrated a notable inter-annual and seasonal variation in nutrient concentrations, water transparency was the only variable that was consistently more variable in LH than LN. The reason for the greater temporal variability in water transparency of LH is its major susceptibility to wind and rain events. The impact of wind events in this lake is favored by its shallowness and by its silty surface sediment; the high ratio of catchment area to lake area is responsible for the relatively higher susceptibility of LH to rain events than LN. By contrast, in the younger and deeper LN, ground-water discharge buffers certain water chemistry parameters such as conductivity, turbidity, and alkalinity. Interestingly, differences in turbidity and ground-water discharge do not seem to affect the variability in nutrient concentrations, which was similar between the lakes, although these factors may explain differences between the lakes in nutrient concentrations. This paper reveals that the unpredictability and frequency of events in Mediterranean aquatic ecosystems makes it necessary to increase data collection frequency to obtain more accurate simulations in water quality models.

A hydrodynamics-based approach to evaluating the risk of waterborne pathogens entering drinking water intakes in a large, stratified lake

Pathogen contamination of drinking water lakes and reservoirs is a severe threat to human health worldwide. A major source of pathogens in surface sources of drinking waters is from body-contact recreation in the water body. However, dispersion pathways of human waterborne pathogens from recreational beaches, where body-contact recreation is known to occur to drinking water intakes, and the associated risk of pathogens entering the drinking water supply remain largely undocumented. A high spatial resolution, three-dimensional hydrodynamic and particle tracking modeling approach has been developed to analyze the risk and mechanisms presented by pathogen dispersion. The pathogen model represents the processes of particle release, transport and survival. Here survival is a function of both water temperature and cumulative exposure to ultraviolet (UV) radiation. Pathogen transport is simulated using a novel and computationally efficient technique of tracking particle trajectories backwards, from a drinking water intake toward their source areas. The model has been applied to a large, alpine lake - Lake Tahoe, CA-NV (USA). The dispersion model results reveal that for this particular lake (1) the risk of human waterborne pathogens to enter drinking water intakes is low, but significant; (2) this risk is strongly related to the depth of the thermocline in relation to the depth of the intake; (3) the risk increases with the seasonal deepening of the surface mixed layer; and (4) the risk increases at night when the surface mixed layer deepens through convective mixing and inactivation by UV radiation is eliminated. While these risk factors will quantitatively vary in different lakes, these same mechanisms will govern the process of transport of pathogens.

Wind‐driven nearshore sediment resuspension in a deep lake during winter

Ongoing public concern over declining water quality at Lake Tahoe, California-Nevada (USA) led to an investigation of wind-driven nearshore sediment resuspension that combined field measurements and modeling. Field data included: wind speed and direction, vertical profiles of water temperature and currents, nearbed velocity, lakebed sediment characteristics, and suspended sediment concentration and particle size distribution. Bottom shear stress was computed from ADV-measured nearbed velocity data, adapting a turbulent kinetic energy method to lakes, and partitioned according to its contributions attributed to wind-waves, mean currents, and random motions. When the total shear stress exceeded the critical shear stress, the contribution to overall shear stress was about 80% from wind-waves and 10% each from mean currents and random motions. Therefore, wind-waves were the dominant mechanism resulting in sediment resuspension as corroborated by simultaneous increases in shear stress and total measured sediment concentration. The wind-wave model STWAVE was successfully modified to simulate wind-wave-induced sediment resuspension for viscous-dominated flow typical in lakes. Previous lake applications of STWAVE have been limited to special instances of fully turbulent flow. To address the validity of expressions for sediment resuspension in lakes, sediment entrainment rates were found to be well represented by a modified 1991 Garcia and Parker formula. Last, in situ measurements of suspended sediment concentration and particle size distribution revealed that the predominance of fine particles (by particle count) that most negatively impact clarity was unchanged by wind-related sediment resuspension. Therefore, we cannot assume that wind-driven sediment resuspension contributes to Lake Tahoes declining nearshore clarity.

Spatial and temporal scales of transport during the cooling phase of the ice-free period in a small high-mountain lake

Abstract.We examine, by means of scaling analysis tools and three-dimensional numerical simulations, the time and spatial scales of transport and mixing processes during the cooling phase of the ice-free period in La Caldera, a small lake located at 3,050 m.a.s.l. in Sierra Nevada (Southern Spain). La Caldera is used here as a prototypical example of small high-mountain lakes. Our results demonstrate that transport and mixing in small high-mountain lakes are shaped by the severe changes exhibited at diurnal time scales by the heat fluxes through the air-water interface, strong winds of episodic nature (storms), and the limited horizontal and vertical length scale of the basin. The thermal structure during the cooling phase of the ice-free period undergoes episodic changes that occur at short-time scales, driven by the strong winds with speeds of 15 m s−1 and higher. During those storms, horizontal and vertical mixing and transport is predominantly driven by wind. During inter-storm periods, the oscillatory nature of the air-water heat exchange at diurnal time scales drives convective circulation, which for a small lake occurs at the basin scale and becomes the dominant mechanism of horizontal transport.