Modeling dynamics in nitrate concentrations of stream discharge with the LGSI-WQ model: a case study for the Drentsche Aa
Supervisors: Paul Torfs (WUR), Joachim Rozemeijer (Deltares) and Ype van der Velde (UU)
In this research the Lowland Groundwater Surface water Interaction model for Water Quality (LGSI-WQ) was used to simulate the dynamical behavior of nitrate concentrations in the stream discharge of the Drentsche Aa (278 km2). It is assumed that the stream discharge concentration is a function of [1.] the nitrogen input (e.g. fertilizers, manure), [2.] the mass balance processes (i.e. regulator of availability) and [3.] the travel time distribution of discharge. Hence, the LGSI-WQ model was constructed by combining a lumped mass balance model [Van der Velde et al., 2010] and a model that derives dynamic travel time distributions [Van der Velde, 2012]. The mass balance model is used to describe the important chemical processes and subsurface interactions that take place in the root- and saturated zone (long-term/seasonal dynamics). The dynamic travel time distributions are responsible for the short-term variation in the nitrate concentrations, this is done by assuming a fixed relationship between the travel time and the (relative) nitrate concentration of a given water particle.
The results show that the LGSI-WQ model can adequately describe long-term trends in the nitrate concentration of the stream discharge (NS=0.68) and in the nitrogen storage of the root zone. This makes the LGSI-WQ particular interesting for performing scenario-studies. In this research we also investigated the model response to short-term (precipitation-) events; it can be seen that the LGSI-WQ does simulate short-term fluctuations but that the magnitude of these fluctuations is not as large as in the observed nitrate concentrations. We found that for simulation of short-term fluctuations the lack of high-frequency nitrate observations and uncertainties in the input data clearly limits the model performance. Furthermore, the daily time discretezation of the LGSI-WQ model is large compared to the time-scale of the fluctuations in the nitrate concentration (hours). The results of the sensitivity suggest that the parameters that determine the long-term processes are slightly dominant over the parameters that influence the short-term fluctuations. In general we can thus state that the LGSI-WQ in its current form is more suitable for simulating long-term trends than it is to simulate short-term fluctuations.
- Van der Velde, Y., Quantifying catchment-scale mixing and its effect on time-varying travel time distributions, Water Resources Research, 1,1,2012.
- Van der Velde, Y. G. de Rooij, J. Rozemeijer, F. Van Geer, and H.Broers, The nitrate response of a lowland catchment: on the relation between stream concentration and travel time dynamics, Water Resources Research, 46, W11, 534, 2010.