Effect of different river water quality model concepts used for river basin management decisions.

n this research the applicability of two different water quality concepts, a QUAL2E-based and a RWQM1-based water quality model is evaluated in terms of management decisions. The Dender river in Belgium serves as a case study for the application of the methodology. By using sensitivity analysis on both model concepts the important processes are revealed. Further, the differences between the predictions for a future scenario are analysed. The scenario chosen here is a reduction in fertiliser use of 90%, which reduces the diffuse pollution. This way, the advantages or disadvantages of using one concept against the other for this scenario are formulated. It was found that the QUAL-based models are more focussing on algae processes while the RWQM1 also takes into account processes in the sediment. Further the QUAL-based models are easier to calibrate, especially when only a small amount of data is available. Both concepts lead to more or less the same conclusions. However for some periods the differences become important and to reduce the uncertainty in those periods, more efforts should be spent in calibration and in better detection of parameters concerning sediment processes and diffusion.

Application of automated measurement stations for continuous water quality monitoring of the Dender river in Flanders, Belgium.

During the summer of 1999, two automated water quality measurement stations were installed along the Dender river in Belgium. The variables dissolved oxygen, temperature, conductivity, pH, rain-intensity, flow and solar radiation were measured continuously. In this paper these on-line measurement series are presented and interpreted using also additional measurements and ecological expert-knowledge. The purpose was to demonstrate the variability in time and space of the aquatic processes and the consequences of conducting and interpreting discrete measurements for river quality assessment and management. The large fluctuations of the data illustrated the importance of continuous measurements for the complete description and modelling of the biological processes in the river.

Propagation of uncertainty in diffuse pollution into water quality predictions: application to the River Dender in Flanders, Belgium.

The uncertainty of water quality predictions caused by uncertainty in the inputs related to emissions of diffuse pollution is analysed. An uncertainty analysis of the effects of diffuse pollution is essential to compare the cost and benefits of measures to lower those emissions. We focus on diffuse nitrate pollution due to fertiliser use. Using an efficient Monte Carlo method and Latin Hypercube sampling, the contribution to the overall uncertainty by each of the inputs is calculated. The modelling environment is ESWAT, an extension of SWAT, which allows for integral modelling of the water quantity and quality processes in river basins. The diffuse pollution sources are assessed by considering crop and soil processes. The crop simulations include growth, uptake of water and nutrients and several land management practices. The in-stream water quality model is based on QUAL2E. The spatial variability of the terrain strongly affects the non-point source pollution processes. The methodology is applied to the Dender basin in Belgium. Eight inputs have significant influence on the time that the nitrate content in the river is higher than 3 mg/l. The uncertainty analysis indicated wide uncertainty bounds (95% percentile bounds differ up to +/-50% from the average NO3 predictions).

Detection of the most optimal measuring points for water quality variables: application to the river water quality model of the River Dender in ESWAT.

This paper presents a methodology for the definition of an optimal set of sampling data for the calibration of a river water quality model. Starting with an extensive set of measurements, it is the aim to reduce those data to obtain just as much data as necessary for a calibration with an acceptable uncertainty in the parameters. The method requires a model for the river under examination and the availability of samples for a first calibration of the model. With the model, synthetic time series are generated, which can be used as virtual observations. In the next step, the method of D-optimal design is applied. The amount, frequency, period, place and kind of variables measured of the water samples that gives the most reliable estimates of the parameters of the model are considered to be the best observations that can be made for that river. Also, the percentage of improvement of the reliability can be defined, as a function of the observations. The method is applied to the river Dender.

Sensitivity analysis and calibration of the parameters of ESWAT: application to the River Dender.

The paper deals with the sensitivity analysis and parameter calibration of a complex river water quality model, implemented in ESWAT. The Extended SWAT includes a QUALIIE-based river quality simulator, in view of an integrated analysis of water quantity and quality management practises. The sensitivity analysis uses Latin Hypercube Sampling and criteria related to the duration of low concentrations of dissolved oxygen and the occurrence of high algae concentrations. The analysis on the river Dender shows that parameters related to the growth and die-off of the algae have the largest impact, while also the BOD decay constant and the benthic oxygen demand are important. A subsequent calibration of these most important parameters shows however that the optimal values of the parameters related to the activity of the algae are statistically not significant. This apparent contradiction is due to the poor information content of the measurements. It is concluded that the application illustrates the complementarity of the sensitivity analysis and the parameter calibration.

Optimisation of the discrete conductivity and dissolved oxygen monitoring using continuous data series obtained with automated measurement stations.

During the last five years, research on the relation between pollution loads and ecological river water quality has been done on the Dender river. In addition to biological sampling of macroinvertebrates and fish, automated measurement stations were used too to investigate the spatio-temporal variability of physical-chemical water pollution. This study on on-line water quality data collection is based on a measurement campaign during March-April 2000 with two automated measurement stations at two different sites: the flow control weirs at Geraardsbergen and Denderleeuw. These measurement stations contain sensors for temperature, turbidity, conductivity, pH, redoxpotential and dissolved oxygen. Short wave radiation as well as rainfall were monitored by means of pyranometers and rain gauges. A refrigerated sampler with 24 bottles allowed to take samples for additional laboratory analyses. In this study, continuous measurements of two physical-chemical parameters, conductivity and dissolved oxygen, were analysed to evaluate the adequacy of the current monitoring frequency in Flanders. Analysis showed that discrete conductivity measurements can be sufficient for trend detection, but the measuring frequency must be highly increased from one measurement per month to at least 8 measurements. Continuous measurements for conductivity are preferred because extreme values are obtained as well. For dissolved oxygen, a single measurement per month in not enough. The percentage of dissolved oxygen showed a strong diurnal variation with maxima in the late afternoon (photosynthesis) and minima at night (respiration). This parameter also differed significantly from day to day. Continuous measurements are therefore necessary for a reliable assessment of the dissolved oxygen budget of surface waters. When using discrete measurements for dissolved oxygen, a set time should be introduced to eliminate diurnal variation.