 |
| Titel |
A supply-based concentration rating curve to predict total phosphorus concentrations in the Rhine River |
| VerfasserIn |
M. Van der Perk, M. F. A. Vogels |
| Konferenz |
EGU General Assembly 2012
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069878
|
|
|
|
|
|
| Zusammenfassung |
| Concentration rating curves are useful for the analysis of the response of sediment or solute
concentrations to changes in stream discharge or for the interpolation of infrequent
concentration measurements in time with discharge as auxiliary variable, for example to
estimate annual sediment or solute loads. A known limitation of rating curves is that their
performance is generally poor, which can be partly attributed to the fact that rating curve
methods neglect the hysteresis effects in the concentration response to changes in discharge.
To enhance the performance of rating curve models, they should account for these hysteresis
effects.
Here, we present a supply-based concentration rating curve for total phosphorus
concentrations in the Rhine River, the Netherlands, which does account for the above
hysteresis effects. The supply-based concentration rating curve has four components:
1) The traditional power law rating curve of the form C = a Qb where C is the
phosphorus concentration [M L-3], Qis the river discharge [L T-1], and a and b are
constants [-];
2) A long-term linear trend;
3) A seasonal trend of the form C(t) = Acos [2Ï(t – Tk)/T] where A is the concentration
amplitude [M L-3], t is the time (T), Tk is the phase shift (T), and T is the period [T]
(365.25 d).
4) A discharge dependent supply or loss term of the form C = -ΔS/(QΔt), where S is the
phosphorus stock [M]. The phosphorus stock was assumed to increase linearly during periods
of deposition, i.e. the discharge is below a critical discharge. If the discharge is greater than
the critical discharge during a sufficiently long period (> 16 days), the decrease in
phosphorus stock was assumed to be proportional to the excess discharge above the critical
discharge.
For model parameterization and calibration, we used the daily Aqualarm data of total
phosphorus concentrations and the Waterbase data of water discharge measured daily
by Rijkswaterstaat (Dutch Ministry of Infrastructure and the Environment) at the
Lobith-Bimmen monitoring station at the German-Dutch border between 1 April 2004 and 20
July 2010. The model parameters were calibrated by a step-wise procedure which involved in
some steps visual calibration (e.g. concentration amplitude, critical discharge for
erosion/deposition) and in other steps regression analysis (e.g. long-term linear trend, power
law rating curve).
The total phosphorus concentrations show a long-term linear decrease of 1.0 Ã 10-5 mg
l-1 d-1. The amplitude of the seasonal fluctuation in phosphorus concentration was
estimated to be 0.03 mg l-1. The critical discharge for erosion and deposition was estimated
to be 1900 m s-1, the increase in phosphorus stock during deposition periods 9300 kg d-1,
and the supply from the phosphorus stock to the river water 32 kg d-1per m3 s excess
discharge.
The squared Pearson’s correlation coefficient between the observed and predicted total
phosphorus increased from 0.16 for the traditional power-law concentration rating curve
(Nash’s efficiency coefficient = 0.13) to 0.36 for the supply-based rating curve (Nash’s
efficiency coefficient = 0.34). This implies that inclusion of the long-term and seasonal trends
and a discharge dependent supply and loss term considerably enhances the performance and
predictive power of the concentration rating curve model. As the response to changes in
discharge is different for dissolved and particulate total phosphorus, a further improvement
of model performance can likely be achieved by deriving separate concentration
rating curves for dissolved total phosphorus and sediment-associated phosphorus. |
|
|
|
|
|
|