Improving the characterization of spatio-temporal variable streambeds and river-aquifer exchange fluxes by assimilation of piezometric head and temperature data
A proper characterization of river-aquifer exchange fluxes is important for several reasons. It is for example important for groundwater management in order to determine the regional water balance (infiltrating/exfiltrating rivers strongly influence the amount of available water in an aquifer) and because infiltrating, contaminated stream water (e.g., bacteria, antibiotics, pesticides, industrial accidents) is a potential danger for groundwater quality. An accurate assessment of river-aquifer interactions can also be important for ecological corridors because exchange fluxes have a relatively large impact on the amount of river discharge under drought conditions. In order to assess whether a minimum river discharge, needed for the ecology close to the river, is maintained under drought conditions, it is essential to know the interactions between streams and groundwater.
The properties of streambeds together with the hydraulic gradients determine the fluxes of water, energy and solutes between streams and aquifers. Little permeable streambeds (e.g., streambeds which are clogged with a layer of very fine material) will result in smaller exchange fluxes. Highly permeable streambeds (e.g., stony streambeds in steeper streams with regular flooding events) exchange potentially more water, solutes and energy with the adjacent aquifer. A problem is that the material properties of streambeds are strongly heterogeneous in space and also in time. Flood events change streambeds and cause erosions in some river sections (resulting in more permeable riverbeds) and deposit large amounts of fine materials elsewhere (resulting in less permeable riverbeds). However, it is very difficult to characterize these erosion/sedimentation events deterministically.
Our approach is to investigate to what extend conventional and less conventional measurements (groundwater temperature data) in combination with modeling and data assimilation can help to characterize the spatio-temporal variability of riverbed properties. Main findings from previous and current studies are:
- For the river Limmat close to the city of Zurich (Switzerland) it was found that sudden flood events significantly change streambed permeabilities. It was also found that seasonal temperature variations have a significant influence on streambed permeabilities. Taking these variations into account with a coupled groundwater flow-heat transport modeling approach with temperature dependent hydraulic conductivities improves modeling results.
- A synthetic study revealed that with piezometric head measurements close to the stream it is possible to detect changes in riverbed permeability over time. This can be done with data assimilation methods that update model states and parameters in real-time. A disadvantage is that the model shows a delayed response with respect to sudden changes in the riverbed properties. This can be partly corrected by the use of inflation methods in combination with Ensemble Kalman Filtering.
- It was found that for the inverse estimation of the material properties of heterogeneous streambeds the best approach is a high-resolution representation of the streambed (stochastic field approach). It results in the best characterization of states and fluxes in a model for river-groundwater interaction. If the streambed properties are very coarsely represented with help of effective parameters, the net exchange fluxes over longer time periods along the stream are systematically underestimated (although the measured heads are reproduced well).
- Currently we are working on the incorporation of groundwater temperature data in a data assimilation scheme for a coupled flow-heat transport groundwater model. The joint assimilation of pressure and temperature data should allow a further improvement in the characterization of river-aquifer exchange fluxes and a better representation of groundwater temperatures. First results are promising.
W. Kurtz, H.J. Hendricks Franssen, P. Brunner and H. Vereecken. 20xx. The role of zonation of river bed conductivities on state-parameter updates with the Ensemble Kalman Filter. HESS. In preparation.
W. Kurtz, H.J. Hendricks Franssen and H. Vereecken. 20xx. Identification of time-variant river-bed properties with the Ensemble Kalman filter. Water Resources Research. Under major revision.
Irvine, D., P. Brunner, H.J. Hendricks Franssen and C.T. Simmons. 2012. Heterogeneous or homogeneous? Implications of simplifying heterogeneous streambeds in models of losing streams. Journal of Hydrology 424-425, 16-23. doi: 10.1016/j.jhydrol.2011.11.051. http://www.sciencedirect.com/science/article/pii/S0022169411008365
Engeler, I., H.J. Hendricks Franssen, R. Müller and F. Stauffer. 2011. The importance of coupled modelling of variable saturated groundwater flow-heat transport for assessing river-aquifer interactions. Journal of Hydrology 397, 295-305, doi:10.1016/j.hydrol.2010.12.007. http://www.sciencedirect.com/science/article/pii/S0022169410007626
Doppler T., H.J. Hendricks Franssen, H.P. Kaiser, U. Kuhlmann and F. Stauffer. 2007. Field evidence of a dynamic leakage coefficient for modelling river-aquifer interactions. Journal of Hydrology 347, 177-187..doi: 10.1016/j.jhydrol.2007.09.017. http://www.sciencedirect.com/science/article/pii/S0022169407005021
Link to project partners:
Forschungszentrum Jülich GmbH
Tel. +49 2461 61 2367
Alternative contact person:
Prof. Dr. Harrie-Jan Hendricks Franssen
Forschungszentrum Jülich GmbH
Tel. +49 02461 4462