## 3 Results
### Water Balance  
A water balance estimation was performed following the calibration process as the interest of this study was to get a better insight into the interaction of the different subsystems and hydrological processes in highly GW influenced lowland river basins. The water balance was performed for the whole calibration period and additionally for each hydrologic year, which is defined between 1 st October to 30 th September of the next year in Germany. The total water input into the model was via precipitation and surface and subsurface inflow, which was further divided into ET, runoff and change in storage, and GW and SW interactions. Water balance error was calculated after balancing all the major hydrological components that includes precipitation, ET, runoff, surface and subsurface inflow, and change in storage. With a water balance error of less than 2% during the calibration period, the total estimated water balance is satisfyingly good.  
**Total water balance:** Overall water balance results shown in Figure 12a clearly show that a considerable portion of precipitations is going back to the atmosphere as an ET loss that represents approximate average of 60% of the total precipitation. Areas of small lakes, ponds, and forests contribute to the major part of ET loss. GW contribution to the SW flow is around 30%-35% via drainage, while SZ storage has decreased over the past nine simulated years, and this is evident in the simulated GW levels in the lowland catchment. Tollense river catchment has a very small cross boundary SZ inflow and GW storage is mainly dependent on local meteorological conditions, such as precipitation and ET.  
In the lowland catchment, the UZ is very shallow during the wet seasons. Due to this, infiltration and ET are vital processes that control the rate of recharge. Results illustrate that the SZ gains approximately 30%-40% of total precipitation, out of which a major portion is drained to the river via artificial drainage available in the study area.  
**Water balance in wet and dry hydrological years:** The water balance results for a wet hydrological year (2010-2011) and dry hydrological year (2017-2018) are shown in Figure 12b,c. During the wet hydrological year, ET is 56% of total P, while drainage contributes 34% of P into the river Tollense with a positive GW storage. During the dry hydrological year, ET raises up to 65% of total P with a raised drainage contribution of 46% in to the river flows, resulting in a negative GW storage. The water  
Figure 11: Observed (black squares) and simulated river (continuous black line) flows in the Tollense River catchment.  
balance results show that GW is a main contributor to the surface flows during low rainfall or partial drought periods and balance SW discharges in the Tollense river catchment.  
Figure 12: Water balance for the Tollense river catchment (400 km\({}^{2}\)) during wet hydrological year 2010-11 (**a**); dry hydrological year 2017–2018 (**b**); total water balance from 2010–2017 (**c**). All figures are in millimetres, where positive and negative storage change represents the ascending and descending change in water stored in SZ and UZ.  
Figure 13 below shows yearly water balance components for eight hydrological years starting from 2010 to 2017. GW recharge was estimated from the exchange between SZ and UZ. The water balance results need to be examined cautiously and water balance results in moderate climate lowlands, as it is controlled by climate variations and catchment characteristics. To ease the comparison between different components, all numbers in Figure 13 are in units of mm. Equation (1) can be applied to convert these units into volume units of m\({}^{3}\)/time.  
\[\text{N = \frac{\text{n}}{\text{1000}*A}}\text{t} \tag{1}\]  
where N = value in m\({}^{3}\)/year; n = value in mm; A = model area in m\({}^{2}\); t = simulation time period in years