Workpackage 4

Workpackage 4

WP4: Model potential consequences of drought for water quality

Questions and hypotheses:

What are the potential consequences of full and partial intermittency for the cumulative self-purification capacity of the respective streams?

  • H5: Reaches with full intermittency will yield a lower cumulative self-purification capacity than those with partial intermittency and permanent reaches.

What is the relationship between stream discharge and self-purification capacity? Is there a limiting discharge for sustaining the natural self-purification capacity of the stream?

  • H6: There exists a non-linear relationship between stream discharge and self-purification capacity which in turn is limited by a minimum discharge value.

Method: Fluvial hydrodynamic modelling

In order to determine the self-purification capacity of an entire reach depending on the discharge, we employ a modelling framework based on our in-house two-dimensional hydrodynamic model (RSim-2D, Tritthart et al. 2011). The required digital elevation models for the investigated reaches will be provided by the water management authorities partnering with us in the project; missing topographic data as well as calibration data for the model (i.e. water surface elevations and flow velocities at various instances in time) will be determined along with the aforementioned field sampling campaign. The 2D hydrodynamic models will be established for three river reaches and, following the objective of the study, calibrated (using a roughness-calibration approach) for low to medium discharges. Once calibrated, we will run the models for the hydrograph of the vegetation period (March to October) of the first project year, as well as a documented wet year and a respective dry year. This will yield unsteady and spatially detailed areas of (i) wetted stream bed surfaces and (ii) wetted interstitial zones. By integrating over time the respective surfaces with the self-purification rates in terms of DOC and nutrient exchange per area and time (obtained from WP2 and WP3), a cumulative self-purification capacity of the river reach throughout the vegetation period is obtained. This result will be validated for parts of the vegetation period of the first project year using the actual water quality parameters determined upstream and downstream of the reach. The comparison of reaches with full, partial and near-zero intermittency as well as wet and dry years will finally yield the potential consequences of drought for water quality. Moreover, the established modelling framework will be used to determine a relationship between stream discharge and purification capacity of the stream. The application of the model for very low discharges will show whether sustaining the stream’s self-purification capacity is linked to a minimum discharge.