Water Level Time Series Research Articles

Development of an accurate and reliable hourly flood forecasting model using wavelet–bootstrap–ANN (WBANN) hybrid approach

Summary A hybrid wavelet–bootstrap–ANN (WBANN) model is developed in this study to explore the potential of wavelet and bootstrapping techniques for developing an accurate and reliable ANN model for hourly flood forecasting. The wavelet technique is used to decompose the times series data into different components which capture useful information on various resolution levels. Five years hourly water level data for monsoon season from five gauging stations in Mahanadi River basin, India are used in this study. The observed water level time series of a particular gauging station is decomposed to sub-series by discrete wavelet transformation and then appropriate sub-series are added up to develop new time series. The bootstrap resampling method is used to generate different realizations of the newly constructed datasets using discrete wavelet transformation to create a set of bootstrap samples that are finally used as input to develop WBANN model. Performance of WBANN model is also compared with three different ANN models: traditional ANNs, wavelet based ANNs (WANNs), bootstrap based ANNs (BANNs). The results showed that the hybrid models WBANN and BANN produced better results than the traditional ANN and WANN models. WBANN model simulated the peak water level better than ANN, WANN and BANN models, and in general, the overall performance of WBANN model is accurate and reliable as compared to the other three models. This study reveals that whereas wavelet decomposition improves the performance of ANN models, bootstrap resampling technique produces more consistent and stable solutions. WBANN model is also used to assess the predictive uncertainty in forms of confidence intervals (CI) to assess the predictive uncertainty for 1–10 h lead time forecasts. Results obtained indicate that WBANN forecasting model with confidence intervals can improve their reliability for flood forecasting.

Assessing coupling between lakes and layered aquifers in a complex Pleistocene landscape based on water level dynamics

The biosphere reserve Schorfheide-Chorin is a scenic region with many lakes. Hydraulic coupling between lakes and groundwater is difficult to assess due to the very heterogeneous Pleistocene deposits with a complex layering of different aquifers, part of them being confined. Thus, a principal component analysis of time series of groundwater and lake water levels was performed. The first two principal components provided a quantitative measure of damping of the input signal, i.e., the extent to which time series of groundwater pressure heads or lake water levels are smoothed and delayed with respect to the input signal, i.e., groundwater recharge or precipitation minus evapotranspiration, respectively. The lakes differed substantially with respect to damping behaviour, indicating different impacts of deep groundwater contribution. For most of the groundwater wells, damping increased linearly with mean depth to water table. In contrast, some wells exhibited nearly identical behaviour independent of depth. High-pass filtered data of water table level from these wells were strongly and inversely correlated with those of barometric pressure fluctuations, pointing to a confined aquifer which was evidently not connected to the adjacent lake.

Fine sediment dynamics in a shallow lake and implication for design of hydraulic works

Lake Markermeer is a large (680 km2), shallow body of water in the middle of the Netherlands, with a mean water depth of 3.6 m. One of the major problems in the lake is its decreasing ecological value which is, among other reasons, caused by a gradual increase of suspended sediment concentration and associated increase of light attenuation in the water column. A thorough understanding of fine sediment dynamics in the lake is a prerequisite for solving this problem. This paper addresses the 3D nature of near-bed sediment dynamics in Lake Markermeer, based on data sampled from a 1-month field experiment in autumn 2007. The campaign involved the collection of 71 bed samples across the lake. At each location, dual-frequency echo soundings were carried out to assess the thickness of the silt layer, and sediment concentration throughout the water column was measured with an Optical Backscatter Sensor (OBS). Moreover, 2-week time series of wave height, water level, current velocities, and near-bed sediment concentration were collected at a single location. The time series of sediment concentration were measured with a regular OBS and an Argus Surface Meter IV (ASM). During the measurement period, flow velocities ranged between 2 and 15 cm/s, wave heights up to 1.2 m were observed and turbidity levels varied between 40 mg/l to more than 300 mg/l. The ASM data generally showed uniform concentration profiles. However, profiles with steep concentration gradients near the bed were found for wave heights above 0.5 m. The field experiments further revealed pronounced 3D structures near the bed during discrete storms. The results are generalized for a wider range of conditions and across the full water depth through application of a 1DV point model, using a two-fraction representation of the grain size distribution. The fine and coarse fractions are found to resuspend rapidly for wind speeds above 5 m/s and 10–12 m/s, respectively, forming a uniform concentration profile if these wind conditions persists. High-concentration (?1 g/l) layers near the bed, containing the coarse sediment fraction, only occur at the onset and towards the end of a storm, when wind speed changes rapidly. It is under these conditions that horizontal gradients in layer density or thickness can transport considerable fine sediment. This transport provides an additional mechanism for the infill of, for instance, silt traps and navigation channels.

Multi-temporal monitoring of wetland water levels in the Florida Everglades using interferometric synthetic aperture radar (InSAR)

Interferometric synthetic aperture radar (InSAR) techniques can successfully detect phase variations related to the water level changes in wetlands and produce spatially detailed high-resolution maps of water level changes. Despite the vast details, the usefulness of the wetland InSAR observations is rather limited, because hydrologists and water resources managers need information on absolute water level values and not on relative water level changes. We present an InSAR technique called Small Temporal Baseline Subset (STBAS) for monitoring absolute water level time series using radar interferograms acquired successively over wetlands. The method uses stage (water level) observation for calibrating the relative InSAR observations and tying them to the stage's vertical datum. We tested the STBAS technique with two-year long Radarsat-1 data acquired during 2006–2008 over the Water Conservation Area 1 (WCA1) in the Everglades wetlands, south Florida (USA). The InSAR-derived water level data were calibrated using 13 stage stations located in the study area to generate 28 successive high spatial resolution maps (50 m pixel resolution) of absolute water levels. We evaluate the quality of the STBAS technique using a root mean square error (RMSE) criterion of the difference between InSAR observations and stage measurements. The average RMSE is 6.6 cm, which provides an uncertainty estimation of the STBAS technique to monitor absolute water levels. About half of the uncertainties are attributed to the accuracy of the InSAR technique to detect relative water levels. The other half reflects uncertainties derived from tying the relative levels to the stage stations' datum.

Water levels in the Amazon basin derived from the ERS 2 and ENVISAT radar altimetry missions

This study sets out to analyze the stages of water bodies in the Amazon basin derived from the processing of ERS-2 and ENVISAT satellite altimetry data. For ENVISAT, GDR measurements for both Ice-1 and Ice-2 tracking algorithms were tested. For ERS-2, the Ice-2 data produced by the OSCAR project was used. Water level time series over river segments of very different width, from several kilometers to less than a hundred of meters, were studied. The water level time series that can be derived from narrow riverbeds are enhanced by off-nadir detections. Conversely, the off-nadir effect may degrade the series over large bodies if not properly accounted for. Comparison at crossovers and with in situ gauges shows that the quality of the series can be highly variable, from 12cm in the best cases and 40cm in most cases to several meters in the worse cases. Cautious data selection is clearly a key point to achieve high quality series. Indeed, low quality series mostly result from inclusion of outliers in the data set finally retained for the computation of the series. Ice-2 and Ice-1 tracking algorithms in the ENVISAT data perform almost equally well. ENVISAT altimetry is clearly an improvement on ERS-2 altimetry.

Structure, variability, and salt flux in a strongly forced salt wedge estuary

The tidally varying circulation, stratification, and salt flux mechanisms are investigated in a shallow salt wedge estuary where fluvial and tidal velocities are large and the steady baroclinic circulation is comparatively weak. The study integrates field observations and numerical simulations of the Merrimack River estuary. At moderate to high discharge the estuary is short and highly stratified, while at lower discharges it shifts to a longer, more weakly stratified estuary; the transition occurs when the length of the salinity intrusion is similar to the tidal excursion. The Merrimack is highly variable at tidal time scales owing to the advection and mixing of a bottom salinity front. Salt flux is predominantly due to tidal processes rather than steady baroclinic or bathymetric shear. Tidal pumping is important near the mouth, but inside the estuary salt flux is due to tidal asymmetries in the elevation and thickness of the halocline that depend on the tidal amplitude and river discharge. Conditions in the Merrimack, including the salinity intrusion length and stratification, vary more with event to seasonal shifts in river discharge than with spring‐neap changes in tidal amplitude. An unstructured grid hydrodynamic model is used to simulate conditions in the Merrimack and model results are compared quantitatively against field observations. The model achieves a high skill against time series of water level, salinity, and velocity and captures the spatial structures of salinity, velocity, and salt flux observed in along‐ and across‐estuary transects. High model skills depend on accurate and well‐resolved grid bathymetry and low background vertical and horizontal diffusivities.

Fluvial modulation of hydrodynamics and salt transport in a highly stratified estuary

An oceanographic campaign was conducted in the Araranguá river estuary during the period from May 11th to 13th of 2006 in order to produce a first hydrographic characterization of this system. The campaign was carried out during the spring tide period, and coincidentally after an intense rain event in the region which produced a peak in river discharge. Water level, currents and salinity time series were recorded hourly during a 50-hour period, at a site nearly 7 km upstream from the estuarine mouth. Two longitudinal distributions of salinity along the estuary were also recorded. The hydrographic data time-series were used to compute the advective salt flux in order to investigate the changes in the transport terms as a function of the change in discharge. The results showed that the estuarine structure was strongly modulated by the river discharge. The drop in water level of about 0.5 m during the first 24 hours was directly related to the ebb phase of the river flood. The water column was highly stratified throughout the period, therefore the stratification increased during the last 24 hours. The currents were stronger, ebbing and uni-directional at the beginning and became weaker and bidirectional as the water level went down, assuming a tidal pattern. The total salt transport in the first 25 hours was of -13.6 kg.m-1.s-1 (seawards), decreasing to 3 Kg.m-1.s-1 during the last 25 hours (landwards). It was also noticeable that the pH in the estuary, recorded together with the salinity, was around 5, showing that the water quality in the estuary is affected by the coal mining activity in the hydrographic basin.

The influence of changes in tidal asymmetry on residual sediment transport in the Western Scheldt

This study fits into a wider research program from RWS RIKZ concerning the exchange of sediment between the coast and the tidal basins, Western Scheldt and Wadden Sea, which are the largest basins along the Dutch Coast, over different time-scales. For both basins, questions about the evolution of the import/export at the mouth recently arose. In case of the Western Scheldt, which is the subject of this study, mainly the uncertainty about the future developments after the change from import to export at the mouth was noticed in the 1990s, which necessitated a more detailed study of this area. To understand the sediment exchange between the Dutch coast and the tidal basin Western Scheldt, the mechanism responsible for this transport must be identified. This study focuses on only one possibly important mechanism: the interaction between the tides and bathymetry. An existing numerical, depth-averaged Delft3D model of the Western Scheldt has been applied. The bathymetry of the years 1970, 1983 and 2002 was selected based on previous sand balance studies. Thus three different situations at the mouth are represented: strong import, import and export of sediments. Each situation was forced by the same morphological tide, where a tide is selected in such a way that during ebb and during flood the computed sediment transport through a cross-section near Vlissingen is similar to the average ebb and flood transport for a complete spring–neap tidal cycle. A comparison was made of the vertical tidal asymmetry in the model with field data for calibration purposes. Then, both the vertical and horizontal tides were analysed. Time series of water levels, velocities and discharges were harmonically analysed to derive the M 2, M 4 and M 6 tidal constituents. The tidal asymmetry is expressed by the amplitude ratios and the phase differences. Furthermore the tide-driven sediment transport ascribed to a given bathymetry was determined. The residual transport patterns were analysed and a sand balance was derived from the modelled transports. This led to the evolution in time of the sediment exchange between, and the erosion/sedimentation within, different macro cells ( Winterwerp et al., 2000) of the Western Scheldt. The human interventions, the changes in tidal asymmetry and the evolution in the sediment transport interact. The increased landward transport inside the river is reproduced in the model; however, the change from sediment import to sediment export at the mouth could not be explained by tidal asymmetries only. Other processes such as winds and waves should also be studied to determine the mechanisms, which influence most of the observed sediment exchanges.

Water exchange and pressure transfer between conduits and matrix and their influence on hydrodynamics of two karst aquifers with sinking streams

summary Karst aquifers are heterogeneous media where conduits usually drain water from lower permeability volumes (matrix and fractures). For more than a century, various approaches have used flood recession curves, which integrate all hydrodynamic processes in a karst aquifer, to infer physical properties of the movement and storage of groundwater. These investigations typically only consider flow to the conduits and thus have lacked quantitative observations of how pressure transfer and water exchange between matrix and conduit during flooding could influence recession curves. We present analyses of simultaneous discharge and water level time series of two distinctly different karst systems, one with low porosity and permeability matrix rocks in southern France, and one with high porosity and permeability matrix rocks in north-central Florida (USA). We apply simple mathematical models of flood recession using time series representations of recharge, storage, and discharge processes in the karst aquifer. We show that karst spring hydrographs can be interpreted according to pressure transfer between two distinct components of the aquifer, conduit and matrix porosity, which induce two distinct responses at the spring. Water exchange between conduits and matrix porosity successively control the flow regime at the spring. This exchange is governed by hydraulic head differences between conduits and matrix, head gradients within conduits, and the contrast of permeability between conduits and matrix. These observations have consequences for physical interpretations of recession curves and modeling of karst spring flows, particularly for the relative magnitudes of base flow and quick flow from karst springs. Finally, these results suggest that similar analyses of recession curves can be applied to karst aquifers with distinct physical characteristics utilizing well and spring hydrograph data, but information must be known about the hydrodynamics and physical properties of the aquifer before the results can be correctly interpreted.

A Falling‐Head Method for Measuring Intertidal Sediment Hydraulic Conductivity

This paper presents an in situ falling-head method for measuring hydraulic conductivity of beach sediments in tidal environment. A polyvinyl chloride (PVC) standpipe was vertically pushed into the submerged beach sediments so that its lower part was filled by a sediment column. During the experiment, the sediments were submerged by sea water and the standpipe top was higher than the sea level. The pipe was fully filled with sea water at the beginning of the experiment. Then the water level time series inside and outside the standpipe were recorded. Analytical solutions were derived to describe the relation among the sediment's hydraulic conductivity and the water levels inside and outside the standpipe and used to analyze the experiment data obtained from the intertidal zone of Puqian Bay, Haikou, Hainan Province, China. The water levels predicted by the analytical solution agreed very well with all the experiment data. Experiments for horizontal hydraulic conductivity estimation were also conducted using L-shaped standpipe which bends from vertical to horizontal in the beach sediments. The averaged hydraulic conductivity anisotropy ratio at the study area is about 2.9. After each in situ experiment, the sediments in the standpipe were stored in a plastic box and transported to university laboratory to measure the hydraulic conductivity using falling-head method. It is found that the in situ hydraulic conductivity averages one order of magnitude greater than the laboratory one, indicating that the original beach surface sediments were loose due to tidal and wave actions and that the samples were significantly compacted during the transportation to laboratory.

Classification of hydrological conditions to assess water allocation schemes for Lake Baiyangdian in North China

In this study an assessment framework based on analysis of water level fluctuations (WLFs) is presented to classify hydrological conditions and investigate the effect of recent water allocation to the lake of Baiyangdian. Good, moderate, and bad hydrological conditions in dry and wet periods are identified respectively using this framework. Annual average water level (AAWL) of moderate hydrological conditions is considered as the annual minimum water level (AMWL). The trend and determined range of intra-annual WLFs are estimated based on monthly optimum water level (MOWL) and monthly minimum water level (MMWL). Both estimated AMWL and intra-annual WLFs are compared respectively with AAWL and intra-annual WLFs observed in the corresponding year in order to assess recent water allocation schemes. Results show that in Lake Baiyangdian, a 16-year periodic component with alternation of dry and wet periods is found in the time series of water levels for 55 years. In dry period, the AMWL is 7.50 m with the determined range of intra-annual WLFs from 0.93 m to 1.04 m. While in wet period, the AMWL is 7.76 m with the determined range varying from 1.35 m to 1.56 m. The trend of intra-annual WLFs deduced by MMWL indicates that in both periods the maximum and minimum values of water levels appear in September and June, respectively. Assessment results suggest that recent water allocations for Lake Baiyangdian are irrational. Although, in some years, AAWL is higher than AMWL, ignorance of trend and determined range of intra-annual WLFs undermine the effects of water allocation. This framework can be used as a preliminary management tool for water allocation in degraded lakes.

Lake Superior water level fluctuation and climatic factors: A dynamic linear model analysis

We use Dynamic Linear Models (DLM) to analyze the time series of annual average Lake Superior water levels from 1860 to 2007, as well as annual averages of climate drivers including precipitation (1900–2007), evaporation and net precipitation (1951–2007). Our results indicate strong evidence favoring the presence of a systematic trend over a random walk for Lake Superior water levels, and this trend has been negative in recent decades. We then show decisive evidence, in terms of improved predictive performance, favoring a model in which the trend component is replaced with regression components consisting of climatic drivers as predictor variables. Because these models use lagged values of precipitation or net precipitation as predictors, the models can be used to forecast water levels, with the associated uncertainty, several years into the future. We use several of the best fit models and compare one (2008) and two step-ahead (2009) forecasts. The 2008 forecasts compare very well with the observed 2008 water level; the two step-ahead 2009 forecasts are offered as testable hypotheses. The Bayesian context in which these models are developed provides a rigorous framework for data assimilation and regular model updating.

Effects of high winds on the circulation of the using a mixed open boundary condition: the Ría de Muros, Spain

The Ría de Muros is the northernmost of the Rías Baixas, four large coastal embayments on the north-western coast of the Iberian Peninsula. In this work a 3D baroclinic model with an unconventional mixed boundary condition at the ocean boundary is implemented and applied to understand how high winds affect the circulation of the ría. This boundary condition is particularly appropriate for a combined forcing, not least when its non-tidal part is significant. The model also has the capability to deal with the alternately drying and wetting areas of the inner ría. For calibration and validation, field data of current velocity and direction, water level, wind velocity and direction, and river discharge are used. The model is forced by the seven major tidal constituents, wind and river discharge data, and thermohaline conditions at the ocean boundary based on measurements. Simulated time series of water level and current velocity are found to be in close agreement with observations. Upon validation the model is applied to determine the effects of strong winds on the circulation. For this purpose three cases are considered, all with a combined forcing: a no-wind reference case in which the model is driven by the average river discharge and M 2 tide, and two wind cases, in which an NNE and a SSW wind (the prevailing directions) of 10 m s −1 is added to the previous forcing. In the no-wind case a relatively weak two-layer circulation exists, with inflow in the bottom layer and outflow in the surface layer (positive estuarine circulation). Among other effects, the NNE wind strengthens significantly this circulation pattern. Instead, the SSW wind turns the two-layer pattern into a three-layer circulation, with inflow in the surface and bottom layers and outflow in the intermediate layer.

Hydrological monitoring of poorly gauged basins based on rainfall–runoff modeling and spatial altimetry

The main objective of this study is to take advantage of spatial altimetry data to help monitoring river discharges in poorly gauged basins. A methodology is defined to obtain high accuracy relationships between water levels provided by spatial altimetry and water discharge computed by a hydrological model. The proposed method is based on a distributed rainfall–runoff model calibrated and validated against a few in situ data. Discharge is modeled in a daily time step at river locations tracked by the satellite and combined with water height observations to produce rating curves. The methodology is evaluated in the Northern Amazon basin, corresponding to the upper part of the Branco River basin, the major tributary of the Negro River. The MGB-IPH (an acronym from the Portuguese for Large Basins Model and Institute of Hydraulic Research) is run for a 10-year period, from January 1997 to December 2006, in a daily time step. Eight gauge stations are used to calibrate and validate the model, resulting in Nash–Sutcliffe coefficients from 0.47 (Maloca do Contão station – 5896 km 2 ) to 0.85 (Caracaraí station – 126,085 km 2 ) in the calibration process, and 0.42 (Posto Funai station – 9708 km 2 ) to 0.94 (also at Caracaraí station) in the validation process. Results suggest the existence of relations between model accuracy and the size of drainage areas, which can be firstly attributed to a poor rainfall monitoring and also to model restrictions, which was specially developed to large basins. Water level time series (35-day time step) at 12 virtual stations (VS) are deduced from ENVISAT tracks from October 2002 to November 2006. The rating curve parameters, including the zero-flow depth ( h ), are estimated by fitting a power-law relation between modeled discharges and satellite-derived water altimetry. Zero-flow depths are optimized for each VS in order to define the best fitted curve. Additionally, slopes along reaches where two or more VS’s are defined can be provided. Comparisons between daily water discharges provided by in situ observations and rating curve estimates at three VS’s resulted in RMS errors from 9.9% to 26.9%, which are in accordance with previous experiences and acceptable for most hydrological applications. River bed slopes derived from both ENVISAT and SRTM were compared. Results suggest that altimetric data provide more accurate slopes and it should be preferred whenever it is available.

Integrated analysis of PALSAR/Radarsat-1 InSAR and ENVISAT altimeter data for mapping of absolute water level changes in Louisiana wetlands

Interferometric Synthetic Aperture Radar (InSAR) has been used to detect relative water level changes in wetlands. We developed an innovative method to integrate InSAR and satellite radar altimetry for measuring absolute or geocentric water level changes and applied the methodology to remote areas of swamp forest in coastal Louisiana. Coherence analysis of InSAR pairs suggested that the HH polarization is preferred for this type of observation, and polarimetric analysis can help to identify double-bounce backscattering areas in the wetland. ENVISAT radar altimeter-measured 18-Hz (along-track sampling of 417 m) water level data processed with regional stackfile method have been used to provide vertical references for water bodies separated by levees. The high-resolution (~ 40 m) relative water changes measured from ALOS PALSAR L-band and Radarsat-1 C-band InSAR are then integrated with ENVISAT radar altimetry to obtain absolute water level. The resulting water level time series were validated with in situ gauge observations within the swamp forest. We anticipate that this new technique will allow retrospective reconstruction and concurrent monitoring of water conditions and flow dynamics in wetlands, especially those lacking gauge networks.

Sea level variability and the ‘ Milghuba’ seiche oscillations in the northern coast of Malta, Central Mediterranean

The characteristics of the water level variations in the northern coastal area of Malta are studied by a set of densely sampled data collected in the period 1993–1996 (43 months) at a permanent coastal sea level installation in Mellieha Bay. These measurements constitute the first digitised sea level recordings in the Maltese Islands and were collected as part of a research programme that produced a long time series of simultaneous water level and meteorological parameters in the Central Mediterranean. Tidal oscillations reach a maximum range of only 20.6 cm on average and are predominantly semi-diurnal. Important non-tidal signals are however found to span the whole spectral range of frequencies. Seiche oscillations in the form of large amplitude sea level fluctuations, known locally as the ‘ milghuba’, carry substantial energy in the range of long wave frequencies (0.2–2 cph) and often mask completely the tidal signal. These coastal seiches are believed to be the expression of shelf scale resonances; in the numerous embayments on the northern coastline of the Maltese Islands, these seiches are greatly amplified and have associated swift alternating currents that are useful for the mixing and exchange of the water body in the embayments with the adjoining open sea areas, but can constitute a nuisance to navigation especially at harbour entrances. In the synoptic and sub-synoptic time scales, variations in atmospheric pressure associated with mesoscale meteorological phenomena produce a predominant effect on the sea level, but the response of the sea is non-isostatic and carries the signature of oceanographic conditions in the region as well as that of non-local forcing resulting from intra-basin differences. Strong seasonal non-eustatic fluctuations in the mean sea level are characterised by a high sea level in December and is typically followed by a sharp fall to a minimum in February/March. This seasonal variability is a manifestation of the adjustments in the mass balance of the whole Mediterranean basin.

Residual circulation in the Ría de Muros (NW Spain): A 3D numerical model study

The residual circulation of the Ría de Muros, a large coastal embayment in NW Spain, are studied using a three-dimensional baroclinic finite-difference model. The driving forces considered by the model include the tide, winds, river inflows and density forcing at the open boundary. In situ data of current velocity and direction, water level, wind velocity and direction, river discharge, and temperature and salinity are used for model validation. Simulated and observed time series of water level and current velocity are in good agreement. Once validated, the model is applied to compute the residual circulation induced by the relevant agents of the ría hydrodynamics—the tide, an upwelling-favourable wind characteristic of spring and summer, a downwelling-favourable wind typical of winter, and freshwater inflows associated with high river runoff. The resulting residual circulation differ notably. The tide does not generate significant residual flows except in the inner ría. By contrast, winds and river discharges induce important residual flows throughout; in the middle and outer ría they generate a 3D residual circulation pattern which renders the conventional two-layer scheme of estuarine circulation too simplistic in this case. Thus, this first application of a 3D numerical model to the Ría de Muros sheds new light on its fundamental hydrodynamics.

Climate change and recent water level variability in Patagonian proglacial lakes, Argentina

A long series of lakes (~ 150) borders the Patagonian Andes (south of ~ 38°S), most of which are a geomorphologic relict of Pleistocene glaciations. Employing instrumental records, we inspected lake water level departures from seasonal variations in seven proglacial lakes: Lacar, Mascardi, Steffen, Escondido, Puelo, Vinter, and Argentino. Lakes north of ~ 42°S show maximum gage (water) level during austral winter months; lakes between ~ 42° and ~ 45°S appear transitional; the one lake south of ~ 50°S (Argentino) shows maximum water level in early autumn. Most lakes show moderate level fluctuation throughout yearly records and, in general, show heteroscedacity. Furthermore, Hurst exponents reveal persistent behavior (i.e., long-term memory effect) in all water level series. In most lakes there are no trends in deseasonalized mean and maximum water levels (Seasonal Kendall test). Lake Mascardi–Manso River system (mostly fed by melt water from the retreating Manso Glacier) is a contrasting example that shows a decreasing trend during summer months that we ascribe to the also declining ice volume. Harmonic analysis (Fourier and wavelet transform) of deseasonalized mean and maximum water level time series shows interannual and decadal periodicities that we link to the occurrence of El Niño and/or the Antarctic Oscillation. The associated phase spectrum indicates that there is a ~ 13-month lag between ENSO occurrences and its effect on anomalous lake water levels. Increased snow accumulation during austral winters usually follows summertime El Niño events, which normally result in increased melt water volume that occurs with about one-year delay during the following (austral) spring/summer.

A method for estimating pore water drainage from marsh soils using rainfall and well records

Rainfall events during low tide exposure cause the water table in marshes to rise. If one has long time series of both rain events and water levels in wells along transects from creek bank to marsh interior, one can correlate well response with rain amount. In cases examined so far the well response is found to be a linear function of rain amount. As it is reasonable to assume that the amount of tidal infiltration required to restore the water table to the elevation of the marsh surface is equal to the amount of rain that would be required to do so, one can estimate the annual drainage of pore water from a well site by dividing the mean drawdown of the water table at low tide by the slope of the response versus rain regression and then multiplying the result by the number of tidal drawdowns in a year. Integration of such results along the transect then gives an estimate of the total annual drainage. An example of the use of this method is given for two well transects in a Typha and a Spartina marsh at the Plum Island Estuary Long Term Ecological Research (PIE-LTER) site in Massachusetts, USA. Both transects yielded pore water drainage rates of about 160 m 3 year −1 per meter of channel length. Although the annual volume of pore water drainage is small compared to the annual volume of the tidal prism, its impact on nutrient budgets in the estuary could be large because of the high concentrations of nutrients in marsh pore waters. We also discuss the possible effects of the capillary fringe, air entrapment and tidal forcing during rain events on these results.

Nonlinear influences on ocean waves observed by X-band radar

The purpose of this study is to discuss the influence of signal nonlinearity upon X-band radar observations. A method for estimating the degree of nonlinearity by bispectral analysis was applied and discussed. We found that bispectral analyses from spatial radar backscatter series are similar to results obtained from water level time series. In addition, the average nonlinear degree from radar backscatter is related to wind speed. The accuracy of wave observations derived by consideration of the nonlinear effect from radar backscatter was also investigated. The estimated error in wave height from the radar data is also related to the degree of nonlinearity. In order to improve accuracy, the modulation transfer function method was applied in order to eliminate the influence of nonlinearity.