B. W. Webb

Estimating the discharge of contaminants to coastal waters by rivers: Some cautionary comments

Abstract Some of the problems associated with the estimation of contaminant loads transported by rivers using infrequent samples are described. The reliability of many load estimates, particularly those for substances where particulate-associated transport predominates, is questioned. A case study of the accuracy and precision of estimates of the suspended sediment load of the River Exe at Thorverton for the period 1978–1980 is used to demonstrate the potential reliability of loads calculated using a variety of estimation procedures. Underestimation and lack of precision are shown to be important problems.

Estimating the suspended sediment loads of rivers in the LOIS study area using infrequent samples

A central objective of the LOIS Community Research Programme is to provide estimates of the land–ocean flux of suspended sediment from the LOIS study area. While high frequency (15-minute interval) suspended sediment concentration data are recorded within the programme at six tidal limit sites, infrequent (weekly–monthly) sampling undertaken by the Harmonized Monitoring (HM) Programme provides additional concentration data for 23 tidal limit sites in the LOIS study area. In order that these infrequent data can be used to generate sediment flux estimates, the accuracy and precision of 22 load estimation procedures are assessed in the context of sampling frequency (weekly, fortnightly and monthly) and basin scales representative of the LOIS study area. Sampling frequency is demonstrated to exert a significant influence on the precision of the individual procedures, with precision being inversely related to sampling frequency. Accuracy is less clearly influenced by sampling frequency, although different procedures were identified as being the most accurate at the three sampling frequencies. Basin scale is shown to exert a significant influence upon accuracy and precision, with the performance of the load estimation procedures being inversely related to basin scale. For one interpolation procedure, an error correction procedure is developed, based on the strong relationship identified in this study between the error associated with individual flux estimates and the ratio of mean sampled discharge to the mean discharge obtained from the high frequency record. This relatively high resolution error correction method is applied to HM data from eight tidal limit sites in the LOIS area, and corrects the initial annual load estimates to provide values that are considered to be more representative of rivers from the LOIS study area. Copyright

SPATIAL AND SEASONAL VARIABILITY IN THE COMPONENTS OF THE RIVER HEAT BUDGET

Detailed hydrometeorological measurements have been used to establish the components of the river heat budget for 495 days covering 18 study periods and 11 study reaches in the Exe Basin, Devon, UK. Averaging the results across the whole data-set indicates that net radiation, friction, sensible heat transfer, condensation and bed conduction contributed 56.0, 22.2, 13.2, 5.8 and 2.8%, respectively, to the non-advective energy gains, whereas net radiation, evaporation, sensible heat exchange and bed conduction accounted for 48.6, 30.4, 10.6 and 10.4%, respectively, of the non-advective heat losses. Precipitation falling on the river channel had little impact on the river heat budgets, but energy advected in groundwater accounted for an average 5% of the heat storage in the river. The magnitude and importance of the river heat budget components were found to be variable in space and time. The influence of channel morphology, valley topography, riparian vegetation, substratum nature and hydrological conditions, especially the effects of river regulation, promoted inter-reach variability in the make up of the heat budget and caused significant differences in energy fluxes at a local scale. Heat budget components also exhibited considerable differences between seasons and varied from day to day for individual reaches.

Trends in stream and river temperature

Information on past and likely future trends in water temperature from different parts of the world is collated. The potential causes of trends in the thermal regimes of streams and rivers are many, but the existing database of water temperature information is inadequate to provide a global perspective on changes during the recent, let alone the more remote, past. Data from Europe suggest that warming of up to ca. 1°C in mean river temperatures has occurred during the 20th century, but that this trend has not been continuous, is distorted by extreme hydrological events, is not correlated with simple hydrometeorological factors and has been influenced by a variety of human activities. Predictive studies indicate that an accelerated rise in stream and river temperatures will occur during the next century as a consequence of global warming. However, forecasts must be tentative because future climatic conditions are uncertain and interactions between climate, hydrological and vegetation changes are complex.

Load estimation methodologies for British rivers and their relevance to the LOIS RACS(R) programme

Abstract Data collected from rivers within the LOIS study area have been analysed to test the reliability of commonly-used interpolation and extrapolation methods for estimating loads from the concentration and flow records that are typically available for rivers in Eastern England. Detailed time series of suspended sediment concentration and discharge, which were collected by means of turbidity monitoring and ultrasonic flow gauging on the River Wharfe at Tadcaster over a 6-month period, have been used to compute a reference load. These data were then artificially decimated to evaluate different procedures for calculating suspended sediment loads. None of the methods investigated produced very reliable load estimates when weekly suspended sediment concentration data were used, and replicate estimates ranged from 24.4 to 550.1% of the reference load. Extrapolation by means of a simple rating relationship was found to produce estimates of suspended sediment load with the highest level of accuracy and precision, but loads calculated by this procedure still varied from −57.2% to +29.1% of the true value at the 95% level of confidence. Reduction in sampling frequency from weekly to fortnightly or monthly intervals generally caused a greater underestimation and increased imprecision in the suspended sediment load estimates. The effect of applying different calculation procedures to estimate chemical fluxes was evaluated using information from the Harmonized Monitoring database for the River Derwent at Church Wilne. For 20 out of the 36 determinands investigated, the difference between the minimum and maximum load estimate, expressed as a percentage of the minimum value, exceeded 50%, and for five determinands, including arsenic in both dissolved and particulate form, the difference was 100% or more. The results of this preliminary analysis suggest that considerable caution should be exercised in calculating river fluxes to the LOIS study coastline, and underscore the need for new and improved procedures to be developed.

Long-term changes in river temperature and the influence of climatic and hydrological factors

Abstract River temperature is an important physical characteristic of water quality, and long-term monitoring in Austria has provided a unique perspective on river temperature changes during the 20th century. Significant rises in annual mean values were apparent and broadly followed changes in air temperature. However, especially in relation to seasonal mean values, trends were influenced by catchment characteristics and contrasts were evident between sites located in a headwater tributary, at the outlet of a catchment with a sizeable lake area, and on the mainstream of the Danube. There is evidence that inter-annual variations in water temperature were influenced by the climate pattern of the North Atlantic Oscillation, especially in the winter months.

LONG-TERM PERSPECTIVE ON THE NATURE OF THE AIR–WATER TEMPERATURE RELATIONSHIP: A CASE STUDY

Monthly mean data from a 90 year period relating to a small catchment (142.4 km2) in north-central Austria were used to provide a long-term perspective on the nature of the air–water temperature relationship. Annual mean values of air and water temperature were related in a relatively insensitive and scattered way (r2   95%, b > 0.65). A separate regression equation was needed to describe the behaviour of monthly mean water temperatures as the air temperatures fell below freezing. Analysis of air–water temperature regressions for individual months revealed a series of relations which were generally more scattered and less, but variously, sensitive than the ensemble relationship of monthly mean values. Monthly mean water temperatures could be predicted from the ensemble air–water temperature relationship and from the relations for individual months with root mean square errors of > 1.0 and < 0.8°C, respectively. Segmentation of air–water temperature regressions according to air temperatures above and below freezing did not significantly improve water temperature prediction. Hysteresis in, and the relatively low slope of, the air–water temperature relationships in the study catchment reflected the importance of snowmelt in the flow regime.

Water temperatures and heat budgets in Dorset chalk water courses

Field measurements have established the heat budgets for stations on two water courses which drain catchment areas dominated by the upper chalk in Dorset, UK. Information on the main advective and non-advective heat fluxes were collected during winter and summer periods in 1994 at a site near to a spring source on a tributary of the River Piddle, and at a station situated downstream on the River Bere. Inputs of heat energy were dominated by radiative fluxes, with net radiation receipt accounting on average for around 90% of the non-advective total in both summer and winter. Sensible heat (convective/conductive) transfer from the atmosphere was enhanced in the summer by the lower water temperature of these spring-fed streams, while relatively warm water temperatures during winter enhanced heat losses by evaporation, which was the dominant non-advective heat loss component in this season. Significant heat losses also occurred by sensible transfer and by back radiation. Summer measurements revealed that conduction of heat into the non-vegetated gravel bed of the River Piddle tributary was the dominant form of loss, but this output was strongly reduced in summer by weed cover on the bed of the River Bere. Considerable daily and diel variability was evident in non-advective heat flux components. Heat advected into the study reaches via precipitation was negligible, but groundwater inflows added to the heat storage of the water courses in both winter and summer. This effect was most marked at the headwater site and in the summer season because of lower streamflow discharge. Copyright

The composition of nutrient fluxes from contrasting UK river basins

Accurate estimates of N and P loads were obtained for four contrasting UK river basins over a complete annual cycle. The fractionation of these loads into dissolved and particulate, and inorganic and organic components allowed a detailed examination of the nutrient load composition and of the factors influencing both the relative and absolute magnitude of these components. The particulate phosphorus (TPP) loads account for 26–75% of the annual total phosphorus (TP) transport and are predominantly inorganic. The inorganic (PIP) and organic (POP) fractions of the TPP loads represent 20–47% and 6–28% of the annual TP transport, respectively. In contrast, the particulate nitrogen loads (TPN) represent 8% or less of the annual total nitrogen (TN) loads and are predominately organic. For dissolved P transport, the dissolved inorganic fraction (DIP) is more important, representing 15–70% of the TP loads, whereas the dissolved organic fraction (DOP) represents only 3–9% of the TP loads. The TN loads are dominated by the dissolved component and more particularly the total oxidized fraction (TON), which is composed of nitrate and nitrite and represents 76–82% of the annual TN transport. The remaining dissolved N species, ammonium (NH4-N) and organic N (DON) account for 0·3–1·2% and 13–16% of the annual TN transport, respectively. The TPN and TPP fluxes closely reflect the suspended sediment dynamics of the study basins, which are in turn controlled by basin size and morphology. The dissolved inorganic nutrient fluxes are influenced by point source inputs to the study basins, especially for P, although the TON flux is primarily influenced by diffuse source contributions and the hydrological connectivity between the river and its catchment area. The dissolved organic fractions are closely related to the dissolved organic carbon (DOC) dynamics, which are in turn influenced by land use and basin size. The magnitude of the NH4-N fraction was dependent on the proximity of the monitoring station to point source discharges, because of rapid nitrification within the water column. However, during storm events, desorption from suspended sediment may be temporarily important. Both the magnitude and relative contribution of the different nutrient fractions exhibit significant seasonal variability in response to the hydrological regime, sediment mobilization, the degree of dilution of point source inputs and biological processes.

A new approach to deriving 'best-estimate' chemical fluxes for rivers draining the LOIS study area.

A central objective of the LOIS Community Research Programme was to provide estimates of the land-ocean flux of chemical species from the LOIS study area. In the absence of detailed information on the concentration of most of the dissolved and particulate-associated chemical constituents, it has proved necessary to make use of the relatively infrequent samples collected in the UK Harmonised Monitoring Scheme, in order to estimate chemical fluxes for the LOIS rivers. In these circumstances, standard procedures may provide unreliable results, and a new approach has been developed to derive improved estimates of chemical flux from the LOIS study area. This approach is based on the generation of detailed synthetic time series of concentration, which are used to estimate the likely errors associated with different flux calculation procedures, especially those resulting from differences in the timing of chemographs and hydrographs during storm events. The approach was applied on a river by river and determinand by determinand basis to identify and to employ the best methods for computing fluxes of 45 chemical determinands from the LOIS study area during the period 1992-1996.