## 3 Results  
### Sensitivity Analyses and Calibration Procedure  
Sensitivity analyses were performed in order to assess the most sensitive parameters for Tollense river catchment model calibration and were performed by changing different parameters within their allowable range. During the sensitivity analyses, only one parameter was adjusted at once, while others kept unchanged during each sensitivity test. Table 4 shows the considered variables and their definitions in the MIKE SHE and MIKE 11 model. Due to the close interaction of GW and SW, calibration of GW levels and SW discharges was conducted in parallel. Especially the calibration of GW level became challenging to the extreme soil heterogeneity, which is typical for glacial soils. Auto calibration was not applied in this study, as the intention was not to achieve an optimum fit between the observed and simulated data, as the model setup without auto calibration provides better process understanding and helps to identify the model deficiencies. Figure 8: Coupling scheme of a hydrological model MIKE SHE with a hydrodynamic model MIKE 11 based on literature [54]. Figure 7: **(left)** Weir gate openings from reference level at Tückhude and Osten. (**right**) The MIKE 11 model of Tollense river and its tributaries, channel cross sections (red rectangles), and control structures (black circles). Calibration is a process of model testing with known inputs and outputs and is used to estimate or adjust different parameters. The calibration process was initiated after the sensitivity analysis and following calibration techniques were applied for the different river flow calibration outcomes. In case 1--when the model was unable to simulate the peak flows, a careful review of the data was done for both the precipitation and river flow, as this happens normally due to the reason that either the rainfall station is not representative or due to the malfunctioning of the precipitation or flow gauges. In case 2--when the model continuously over predicted the flow, ET, soil water content, percolation, and GW recharge rates were adjusted. In case 3--where simulated flow followed the observed flow dynamics but lags the actual flow consistently, river bed leakage coefficient rates and Manning's roughness coefficient were adjusted. In case 4--when the model over predicted the peak flows but under predicted the average normal river flows, infiltration rate, interflow, and base flow recession parameters were adjusted. The coupled MIKE SHE and MIKE 11 model was calibrated by using the measured GW heads at seven selected GW observation wells shown in Figure 9, for the period ranging from November 2016 to April 2018. River flow calibration was based on flow measurements performed by StALU-MS and University of Rostock along the Tollense river at four different chainages in the river section from Klempenow to Demmin. Calibrated GW level and surface flow results are presented in Figures 10 and 11, respectively, for a period of nine years starting from 2010 to 2018. During the calibration process, hydraulic conductivity, BCs, roughness coefficient, and specific yield were adjusted to calibrate the coupled MIKE SHE and MIKE 11 model and their calibrated values are shown in Table 5.  
at the bank of Gemkow tributary, while other boreholes are located at banks of small tributaries present in different sub catchments of the Tollense river basin shown in Figure 9.  
The statistical performance of the coupled MIKE SHE and MIKE 11 model in terms of predicting the GW dynamics is shown in Table 6, whereas Figure 10 shows the comparison of simulated and observed GW levels at the selected seven borehole locations located in different sub catchments of the Tollense river basin. A good agreement was observed between simulated and observed GW levels. but simulated GW reacts more dynamically. A probable reason is the selection of too course spatial and temporal discretization to simulate strong soil heterogeneity of the catchment.