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1.
The time derivative (d H/dt) of the geomagnetic field horizontal component (H) for seven intervals of high geomagnetic activity in 2003–2005 has been calculated, based on the data of Alma-Ata, Novosibirsk, and Irkutsk observatories, in order to estimate the probability of appearance of geomagnetically induced current (GIC), the value of which is linearly dependent on d H/dt, in power lines on the territory of Kazakhstan. The distributions of the H and d H/dt directions have been constructed; in this case it was most interesting that these distributions were narrow and extended along the magnetic meridian for Alma-Ata and were wider angular for Novosibirsk and Irkutsk. It has been indicated that large H values, determining significant GIC values, took place at a sudden commencement of strong storms, which had a character of a pulsed disturbance of the geomagnetic field, and during large-amplitude geomagnetic field pulsations. The duration of the periods of large |d H/dt| values exceeding the threshold (30 nT/min), when GIC could cause unwanted consequences in power circuits, has been determined.  相似文献   

2.
Rock masses contain ubiquitous multiscale heterogeneities, which (or whose boundaries) serve as the surfaces of discontinuity for some characteristics of the stress state, e.g., for the orientation of principal stress axes. Revealing the regularities that control these discontinuities is a key to understanding the processes taking place at the boundaries of the heterogeneities and for designing the correct procedures for reconstructing and theoretical modeling of tectonic stresses. In the present study, the local laws describing the refraction of the axes of extreme principal stresses T 1 (maximal tension in the deviatoric sense) and T 3 (maximal compression) of the Cauchy stress tensor at the transition over the elementary area n of discontinuity whose orientation is specified by the unit normal n are derived. It is assumed that on the area n of discontinuity, frictional contact takes place. No hypotheses are made on the constitutive equations, and a priori constraints are not posed on the orientation on the stress axes. Two domains, which adjoin area n on the opposite sides and are conventionally marked + and ?, are distinguished. In the case of the two-dimensional (2D) stress state, any principal stress axis on passing from domain ? to domain + remains in the same quadrant of the plane as the continuation of this axis in domain +. The sign and size of the refraction angle depend on the sign and amplitude of the jump of the normal stress, which is tangential to the surface of discontinuity. In the three-dimensional (3D) case, the refraction of axes T 1 and T 3 should be analyzed simultaneously. For each side, + and ?, the projections of the T 1 and T 3 axes on the generally oriented plane n form the shear sectors S + and S ?, which are determined unambiguously and to whose angular domains the possible directions p + and p ? of the shear stress vectors belong. In order for the extreme stress axes T 1 + ,T 3 + and T 1 ? , T 3 ? to be statically compatible on the generally oriented plane n, it is required that sectors S + and S ? had a nonempty intersection. The direction vectors p + and p ? are determined uniquely if, besides axes T 1 ? , T 3 ? and T 1 + , T 3 + , also the ratios of differential stresses R + and R ? (0 ≤ R ± ≤ 1) are known. This is equivalent to specifying the reduced stress tensors T R + and T R ? The necessary condition for tensors T R + and T R ? being statically compatible on plane n is the equality p + = p ?. In this paper, simple methods are suggested for solving the inverse problem of constructing the set of the orientations of the extreme stress axes from the known direction p of the shear stress vector on plane n and from the data on the shear sector. Based on these methods and using the necessary conditions of local equilibrium on plane n formulated above, all the possible orientations of axes T 1 + , T 3 + are determined if the projections of axes T 1 ? , T 3 ? axes on side — are given. The angle between the projections of axes T 1 + , T 1 ? and/or T 3 + , T 3 ? on the plane can attain 90°. Besides the general case, also the particular cases of the contact between the degenerate stress states and the special position of plane n relative to the principal stress axes are thoroughly examined. Generalization of the obtained results makes it possible to plot the local diagram of the orientations of axes T 1 + , T 3 + for a given sector S ?. This diagram is a so-called stress orientation sphere, which is subdivided into three pairs of areas (compression, tension, and compression-extension). The tension and compression zones cannot contain the poles of T 3 + and T 1 + axes, respectively. The compression-extension zones can contain the poles of either T 1 + or T 3 + axis but not both poles simultaneously. In the particular case when the shear stress vector has a unique direction p ? on side ?, the areas of compression-extension disappear and the diagram is reduced to a beach-ball plot, which visualizes the focal mechanism solution of an earthquake. If area n is a generally oriented plane and if the orientation of the pairs of the statically compatible axes T 1 ? , T 3 ? and T 1 + , T 3 + is specified, then, the stress values on side + are uniquely determined from the known stress values on side ?. From the value of differential stress ratio R ?, one can calculate the value of R +, and using the values of the principal stresses on side ?, determine the total stress tensor T + on side +. The obtained results are supported by the laboratory experiments and drilling data. In particular, these results disclose the drawbacks of some established notions and methods in which the possible refraction of the stress axes is unreasonably ignored or taken into account improperly. For example, it is generally misleading to associate the slip on the preexisting fault with the orientation of any particular trihedron of the principal stress axes. The reconstruction should address the potentially statically compatible principal stress axes, which are differently oriented on opposite sides of the fault plane. The fact that, based on the orientation of the intraplate principal stresses at the base of the lithosphere, one cannot make a conclusion on the active or passive influence of the mantle flows on the lithospheric plate motion is another example. The present relationships linking the stress values on the opposite sides of the fault plane on which the orientations of the principal stress axes are known demonstrate the incorrectness of the existing methods, in which the reduced stress tensors within the material domains are reconstructed without allowance for the dynamic interaction of these domains with their neighbors. In addition, using the obtained results, one can generalize the notion of the zone of dynamical control of a fault onto the case of the existence of discontinuities in this region and analyze the stress transfer across the system of the faults.  相似文献   

3.
A transient model, hereafter referred to as ROM-TM, was developed to quantify river ecosystem metabolic rates and reaeration rates from field observation of changes in dissolved O2 (DO) and the ratio of 18O to 16O in DO (δ18O-DO). ROM-TM applies an inverse modeling approach and is programmed using MATLAB. Parameters describing photosynthesis, ecosystem respiration, gas exchange, and isotopic fractionation, such as maximum photosynthetic rate (P m ), photosynthetic efficiency parameter (a), respiration rate at 20 °C (R 20 ), gas exchange coefficient (K), respiration isotopic fractionation factor (a R ), and photorespiration coefficient (β R ), can be abstracted by minimizing the sum of square errors between the fitted data and the observed field data. Then DO and δ18O-DO time series can be reconstructed using estimated parameters and input variables. Besides being capable of teasing apart metabolic processes and gas exchange to provide daily average estimates of metabolic parameters at the ecosystem scale, ROM-TM can be used to address issues related to light including light saturation phenomena at the ecosystem level, the effect of cloud cover on the metabolic balance, and photorespiration. Error and uncertainty analysis demonstrates that ROM-TM is stable and robust for the random errors of DO time series. The photosynthetic parameters P m and a are more sensitive than other parameters to lower-resolution time series data.  相似文献   

4.
The three-dimensional nonstationary theoretical model of the concentrations and temperatures of electrons and ions in the ionospheric F region and plasmasphere at low and middle latitudes is used to study variations in the concentration NmF2 and height hmF2 of the ionospheric F2 layer under the action of the plasma zonal drift in the direction geomagnetic west-geomagnetic east perpendicularly to the electric E and geomagnetic B fields. The calculated and measured values of NmF2 and hmF2 for 16 ionospheric sounding stations during the quiet geomagnetic period on March 28–29, 1964 at low solar activity are compared. This comparison made it possible to correct the input parameters of the model: [O] from the NRLMSISE-00 model and the meridional component of the neutral wind velocity from the HWW90 model. It is shown that the nighttime NmF2 values decrease up to twice at low solar activity in the low-latitude ionosphere, and the hmF2 values change by up to 16 km, if the plasma zonal E×B drift is not taken into account. Under the daytime conditions, the influence of the plasma zonal E×B drift on NmF2 can be neglected.  相似文献   

5.
The variations in the electron number density of the ionospheric F2 layer maximum (NmF2) under the action of the zonal plasma drift in the geomagnetic west-geomagnetic east direction perpendicularly to the electric (E) and geomagnetic (B) fields during a geomagnetically quiet period on December 7, 1989, at high solar activity have been studied based on a three-dimensional nonstationary theoretical model of electron number densities and temperatures in the ionospheric F region. Calculated and measured NmF2 values for 12 low-latitude ionospheric sounding stations have been compared. When the zonal E × B plasma drift is ignored, the NmF2 values become smaller by up to a factor of 3 under nighttime conditions in the low-latitude ionosphere. The average effect of the zonal E × B plasma drift on NmF2 in the low-latitude ionosphere is larger during winter nights than under summer nighttime conditions.  相似文献   

6.
We used CHAMP satellite vector data and the latest IGRF12 model to investigate the regional magnetic anomalies over mainland China. We assumed satellite points on the same surface (307.69 km) and constructed a spherical cap harmonic model of the satellite magnetic anomalies for elements X, Y, Z, and F over Chinese mainland for 2010.0 (SCH2010) based on selected 498 points. We removed the external field by using the CM4 model. The pole of the spherical cap is 36N° and 104°E, and its half-angle is 30°. After checking and comparing the root mean square (RMS) error of ΔX, ΔY, and ΔZ and X, Y, and Z, we established the truncation level at K max = 9. The results suggest that the created China Geomagnetic Referenced Field at the satellite level (CGRF2010) is consistent with the CM4 model. We compared the SCH2010 with other models and found that the intensities and distributions are consistent. In view of the variation of F at different altitudes, the SCH2010 model results obey the basics of the geomagnetic field. Moreover, the change rate of X, Y, and Z for SCH2010 and CM4 are consistent. The proposed model can successfully reproduce the geomagnetic data, as other data-fitting models, but the inherent sources of error have to be considered as well.  相似文献   

7.
Sensitivity study of the air–ice drag coefficient C Dai is presented with an ice–ocean coupled model for the Sea of Okhotsk. The C Dai?×?103 value is varied from 2 to 5 based on the direct measurements in the region. The maximum volume transport of the East Sakhalin Current and the mean sea ice velocity were intensified as C Dai increased. The sensitivity experiment with the ice–water drag coefficient C Diw showed that the East Sakhalin Current volume transport is hardly affected by C Diw but significantly intensified by C Dai. While the ice drift in the off-ice-edge direction was intensified by the increase in C Dai and the decrease in C Diw, the ice edge location was nearly unchanged. This was due to melting caused by the relatively warm water inflow from the North Pacific. That is, sea ice extent in the region is strongly influenced by melting caused by a large ice–ocean heat transfer. In the active melting regions, the ice–ocean heat transfer of more than 100 W/m2 occurred even in mid-winter. This is the same order as the cooling by air in winter, and a heat insulation capacity of sea ice is weakened in such regions.  相似文献   

8.
The goal of the present work is to identify high-seismicity (High) intersections of morphostructural lineaments within the Mediterranean mountain belts (the Alps, Apennines, Balkanides, Dinarides, and the Carpathians). The intersections of lineaments, with the lineaments being boundaries of crustal blocks, were determined by morphostructural zoning. The epicenters of M ≥ 6.0 earthquakes and the intersections were found to be related. We used the KORA-3 recognition algorithm to identify the High intersections, where M >- 6.0 earthquakes can occur, separately for each mountainous country. Most of the High intersections identified here are located on higher-rank lineaments that separate major crustal blocks. The High intersections typically involve contrasting neotectonic movements and an increased crustal fragmentation. The results of this study point to a high seismic potential for the regions studied: we have identified many High intersections where no large earthquakes have yet been recorded.  相似文献   

9.
Ground motion prediction equations (GMPEs) have a major impact on seismic hazard estimates, because they control the predicted amplitudes of ground shaking. The prediction of ground-motion amplitudes due to mega-thrust earthquakes in subduction zones has been hampered by a paucity of empirical ground-motion data for the very large magnitudes (moment magnitude (M) $>$ 7) of most interest to hazard analysis. Recent data from Tohoku M9.0 2011 earthquake are important in this regard, as this is the largest well-recorded subduction event, and the only such event with sufficient data to enable a clear separation of the overall source, path and site effects. In this study, we use strong-ground-motion records from the M9 Tohoku event to derive an event-specific GMPE. We then extend this M9 GMPE to represent the shaking from other M $>$ 7 interface events in Japan by adjusting the source term. We focus on events in Japan to reduce ambiguity that results when combining data in different regions having different source, path and site effect attributes. Source levels (adjustment factors) for other Japanese events are determined as the average residuals of ground-motions with respect to the Tohoku GMPE, keeping all other coefficients fixed. The mean residuals (source terms) scale most steeply with magnitude at the lower frequencies; this is in accord with expectations based on overall source-scaling concepts. Interpolating source terms over the magnitude range of 7.0–9.0, we produce a GMPE for large interface events of M7–M9, for NEHRP B/C boundary site conditions (time-averaged shear-wave velocity of 760 m/s over the top 30 m) in both fore-arc and back-arc regions of Japan. We show how these equations may be adjusted to account for the deeper soil profiles (for the same value of $\hbox {V}_\mathrm{S30})$ in western North America. The proposed GMPE predicts lower motions at very long periods, higher motions at short periods, and similar motions at intermediate periods, relative to the simulation-based GMPE model of Atkinson and Macias (2009) for the Cascadia subduction zone.  相似文献   

10.
Groundwater models need to account for detailed but generally unknown spatial variability (heterogeneity) of the hydrogeologic model inputs. To address this problem we replace the large, m-dimensional stochastic vector β that reflects both small and large scales of heterogeneity in the inputs by a lumped or smoothed m-dimensional approximation γθ∗, where γ is an interpolation matrix and θ∗ is a stochastic vector of parameters. Vector θ∗ has small enough dimension to allow its estimation with the available data. The consequence of the replacement is that model function f(γθ∗) written in terms of the approximate inputs is in error with respect to the same model function written in terms of β, f(β), which is assumed to be nearly exact. The difference f(β) − f(γθ∗), termed model error, is spatially correlated, generates prediction biases, and causes standard confidence and prediction intervals to be too small. Model error is accounted for in the weighted nonlinear regression methodology developed to estimate θ∗ and assess model uncertainties by incorporating the second-moment matrix of the model errors into the weight matrix. Techniques developed by statisticians to analyze classical nonlinear regression methods are extended to analyze the revised method. The analysis develops analytical expressions for bias terms reflecting the interaction of model nonlinearity and model error, for correction factors needed to adjust the sizes of confidence and prediction intervals for this interaction, and for correction factors needed to adjust the sizes of confidence and prediction intervals for possible use of a diagonal weight matrix in place of the correct one. If terms expressing the degree of intrinsic nonlinearity for f(β) and f(γθ∗) are small, then most of the biases are small and the correction factors are reduced in magnitude. Biases, correction factors, and confidence and prediction intervals were obtained for a test problem for which model error is large to test robustness of the methodology. Numerical results conform with the theoretical analysis.  相似文献   

11.
The structurally adequate model of the geomagnetosphere consists of the following modules: A. Bow shock as a source of power for magnetospheric processes. B. The generation of magnetospheric convection. C. The plasma pressure distribution in the magnetosphere and electron-proton precipitations into the ionosphere. D. The magnetosphere-ionosphere coupling. Field-aligned currents. Essentially, each module presented in an analytical form is a model of a particular process described in physical terms. It has an input and output to couple with other modules. When combined, the modules comprise a single large physically adequate model describing a phenomenon in such a way that we understand its essence and contribution of each physical process into the overall picture.  相似文献   

12.
The problem of estimating the time derivatives of the horizontal components of the geomagnetic field and forecasting the probability of the occurrence of perturbations that exceed a given threshold level (the over-threshold perturbations) arises in the applications concerned with the geomagnetically induced currents (GICs). In this work, we consider the temporal and spatial structure of the Pi3 pulsations with quasi-periods of 102 to 103 s during which the auroral and subauroral stations of the IMAGE network record over-threshold values in the derivatives of the meridional (along the longitudinal circle) BX component and latitudinal (along the latitudinal circle) BY component. The extreme |dBX/dt| values mainly develop against the background of the Pi3 pulsations with a complex frequency content, whereas the extreme |dBY/dt| values appear when the buildup (decay) phases of the bay-like disturbance associated with the evolution of a substorm coincide with the respective phases of the field of pulsations. The conditions under which the derivatives |dBX/dt| and |dBY/dt| reach their over-threshold values are studied for subauroral latitudes by the technique of superposed epoch analysis. The extreme values of the derivatives most frequently occur during the main phase of moderate magnetic storms or beyond the storm—during high substorm activity under the conditions of a negative vertical component of the interplanetary magnetic field. The probability of the occurrence of over-threshold values increases at high amplitudes of the Pi3 pulsations and depends on their spectral content. The problem of analyzing and forecasting the over-threshold |dBY/dt| perturbations is complicated by the fact that the scale of the perturbations is small along the lines of latitude and large along the meridians. This can result in GIC excitation in the North–South oriented electric power lines by the geomagnetic perturbations localized within a narrow band in longitude which can be missed during the measurements.  相似文献   

13.
The variations in the density of the ionospheric F2 layer maximum (NmF2) under the action of the zonal plasma drift perpendicularly to the magnetic (B) and electric (E) fields in the direction geomagnetic west-geomagnetic east have been studied using the three-dimensional nonstationary theoretical model of electron and ion densities (N e and N i ) and temperatures (T e and T i ) in the low-latitude and midlatitude ionospheric F region and plasmasphere. The method of numerical calculations of N e , N i , T e , and T i , including the advantages of the Lagrangian and Eulerian methods, is used in the model. A dipole approximation of the geomagnetic field (B), taking into account the non-coincidence of the geographic and geomagnetic poles and differences between the positions of the Earth’s and geomagnetic dipole centers, is accepted in the calculations. The calculated NmF2 and altitudes of the F2 layer maximum (hmF2) have been compared with these quantities measured at 16 low-latitude ionospheric sounding stations during the geomagnetically quiet period October 11–12, 1958. This comparison made it possible to correct the input model parameters: the NRLMSISE-00 model [O], the meridional component of the neutral wind velocity according to the HWW90 model, and the meridional component of the equatorial plasma drift due to the electric field specified by the empirical model. It has been indicated that the effect of the zonal E × B plasma drift on NmF2 can be neglected under daytime conditions and changes in NmF2 and hmF2 under the action of this drift are insignificant under nighttime conditions north of 25° and south of ?26° geomagnetic latitude. The effect of the zonal E × B plasma drift on NmF2 and hmF2 is most substantial in the nightside ionosphere approximately from ?20° to 20° geomagnetic latitude, and the neglect of this drift results in an up to 2.4-fold underestimation of NmF2. The found dependence of the effect of the zonal E × B plasma drift on NmF2 and hmF2 on geomagnetic latitude is related to the longitudinal asymmetry of B, asymmetry of the neutral wind about the geomagnetic equator, and changes in the meridional E × B plasma drift at a change in geomagnetic longitude.  相似文献   

14.
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15.
Intense quasimonchromatic geomagnetic pulsations with a period of ～15 min, observed on the Earth’s surface in the near-noon sector at the beginning of the recovery phase of a very strong (Dst min = ?260 nT) magnetic storm of May 15, 2005, are analyzed. The variations were registered at auroral latitudes only in the X field component, and wave activity shifted into the postnoon sector of the polar cap an hour later; in this case pulsations were observed in the X and Y field components. Within the magnetosphere the source of magnetic pulsations could be the surface waves on the magnetopause caused by the pulse of the solar wind magnetic pressure. Geomagnetic pulsations in the polar cap, observed in phase at different latitudes, could apparently reflect quasiperiodic variations in the NBZ system of field-aligned currents. Such variations can originate due to the series of pulsed reconnections in the postnoon outer cusp at large (～20 nT) positive B z values and large (about ?40 nT) negative values of IMF B x .  相似文献   

16.
The North Anatolian Fault (NAF) extends for about 1500 km from Karliova to the east, to the Egean Sea in the west. The Marmara region, located near the western end of the NAF, is a tectonically active zone characterized by the transition between a strike slip stress regime and an extensional one in the Aegean Sea. Microseismic studies performed around the Marmara Sea in 1995 [Tectonophysics 316, 2000, 1], and just before the 1999 Izmit Earthquake Bull. Seism. Soc. Am. 92, 2002a, 361;J. Seismol. 6, 2002b, 287) permitted the analysis of the evolution of seismicity connected to this destructive earthquake and its sequels. Several observations indicate that the aftershock distribution fits well the EW orientation of the NAF, but the ruptures are not simple and linear as a first glance would suggest. Instead they are segmented in at least five pieces as shown by the slip variation and aftershock clusters, showing complexity at different scales (Bull. Seism. Soc. Am. 92, 2002a, 361). There is still a gap, across the northern border of the Marmara Sea that has not ruptured, and this is the only sector that did not break on the NAF since the 1939 great Erzincan earthquake. Will it rupture as a whole with a large magnitude earthquake, or by segments with smaller magnitude events? The Hurst analysis of the overall behaviour of the seismicity in the Marmara region since historical times shows that if a large earthquake occurs in the near future, it might break the complete gap. The Hurst character of the time variation of seismicity is persistent with H= 0.82. The aftershocks of the 1999 Izmit earthquake can be analyzed by using the Hurst method, showing an exceptionally high persistent memory.  相似文献   

17.
It has been two decades since the last comprehensive standard model of ambient earth noise was published Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The PETERSON model was updated by analyzing the absolute quietest conditions for stations within the GSN (Berger et al. in J Geophys Res 109, 2005; Mcnamara and Buland in Bull Seism Soc Am 94:1517–1527, 2004; Ringler et al. in Seismol Res Lett 81(4) doi:10.1785/gssrl.81.4.605, 2010). Unfortunately, both the original model and the updated models did not include any deployed station in North Africa and Middle East, which reflects the noise levels within the desert environment of those regions. In this study, a survey was conducted to create a new seismic noise model from very broadband stations which recently deployed in North Africa. For this purpose, 1 year of continuous recording of seismic noise data of the Egyptian National Seismic Network (ENSN) was analyzed in order to create a new noise model. Seasonal and diurnal variations in noise spectra were recorded at each station. Moreover, we constructed a new noise model for each individual station. Finally, we obtained a new cumulative noise model for all the stations. We compared the new high-noise model (EHNM) and new low-noise model (ELNM) with both the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The obtained noise levels are considerably lower than low-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993) at ultra long period band (ULP band), but they are still below the high-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The results of this study could be considered as a first step to create permanent seismic noise models for North Africa and Middle East regions.  相似文献   

18.
In this research, the simulation of Urmia Lake water level fluctuation by means of two models was applied. For this, Support Vector Machines (SVM), and Neural Wavelet Network (NWN) models that conjugated both the wavelet function and ANN, developed for simulating the Urmia Lake water level fluctuation. The yearly data of rainfall, temperature and discharge to the Urmia Lake and water level fluctuation were used. Urmia Lake is the biggest and the hyper saline lake in Iran. The outcome of the SVM based models are compared with the NWN. The results of SVM model performs better than NWN and offered a practical solution to the problem of water level fluctuation predictions. Analysis results showed that the optimal situation occurred with use of precipitation, temperature and discharge for all station and water level fluctuations at the lag time of one year (RMSEs) of 0.23, 0.41 m obtained by SVM, NWN, respectively, and SSEs of 0.43, 1.33 and R 2 of 0.97, 0 obtained by SVM, NWN, respectively. The results of SVM model show better accuracy in comparison with the NWN model.  相似文献   

19.
Using model simulations, the morphological picture (revealed earlier) of the disturbances in the F 2 region of the equatorial ionosphere under quiet geomagnetic conditions (Q-disturbances) is interpreted. It is shown that the observed variations in the velocity of the vertical E × B plasma drift, related to the zonal E y component of the electric field, are responsible for the formation of Q-disturbances. The plasma recombination at altitudes of the lower part of the F 2 region and the dependence of the rate of this process on heliogeophysical conditions compose the mechanism of Q-disturbance formation at night. The daytime positive Q-disturbances are caused exclusively by a decrease in the upward E × B drift, and this type of disturbances could be related to the known phenomenon of counter electrojet. Possible causes of formation of the daytime negative Q-disturbances are discussed.  相似文献   

20.
The use of submersed macrophyte tissue δ 15N to quantify the level of WWTP effluent use in a highly urbanized and agricultural river was evaluated using several methods. Macrophytes, NH4 + and NO3 ? were collected by canoe along two 10 km reaches of river, upstream and downstream of two major municipal WWTPs over 3 years. NH4 + decreased in concentration while δ 15N–NH4 + increased as a function of distance downstream of both WWTPs, changing in one survey from 13 to 31 ‰ over 1 km. This increase is attributed to the combined effects of volatilization, nitrification and uptake. While NO 3 ? concentrations increased downstream of the WWTP over one of the survey reaches, δ 15N–NO 3 ? showed no prominent trend with distance at either. Macrophyte tissue δ 15N increased with distance downstream of both WWTPs, with a slope not significantly different from that of δ 15N–NH4 + suggesting that macrophytes incorporate effluent NH4 + as their main N source in those areas. However, mixing models suggest that towards the end of the reach, where source separation is distinct, macrophytes may utilize background NO 3 ? . Our study indicates the difficulty of deriving precise estimates of effluent use by macrophytes in a system where the δ 15N of the effluent changes rapidly. It also illustrates the utility of macrophytes in describing those changes where the effluent is too attenuated to allow for direct isotopic analysis.  相似文献   

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