## 1 Introduction  
Hydrological processes in moderate climate lowland catchments can be complicated due to complex surface water (SW) and groundwater (GW) interactions, and due to this complexity, precise hydrological water balance information is a prerequisite to develop management practices for the sustainable use of water resources. Hydrological research and forecast has become increasingly important due to the growing problems with the available water resources, and due to this reason, the hydrological forecasts have been modernised, extending from simple flood predictions to detailed water management decisions and practices that require extensive hydrological processes information [1; 2; 3; 4]. Integrated hydrological models play an essential role by providing the necessary information regarding biological, physical, and chemical processes and their interactions within a catchment [5; 6]. In general, the temporal and spatial variations of GW and SW resources are imprecisely known which effects the proper management of water resources [7; 8; 9]. Physically-based dynamically-coupled SW and GW hydrological models are increasingly being used to examine the environmental interactions [10], transport of solute [11], flood modelling [12], and understanding of catchment hydrology [13; 14; 15; 16; 17; 18]. A good forecast of an integrated hydrological model does not only depend on the code but also on the ability of the modelling tool to adequately characterise the complex surface and subsurface hydrological processes and parameter distributions, which otherwise results in greater uncertainty and higher degree of errors [19; 20]. Lowlands are characterised mainly with high GW levels, low hydraulic gradients, and flat topography. This may lead to direct hydraulic interactions between the different subsystems which are not adequately described by pure hydrologic model approaches. Human interventions such as land use, artificial drainage, GW abstraction via pumps, river hydraulic structures, etc., also effect the natural water balance in lowlands. The complex SW and GW interactions can be reliably considered by bi-directionally-coupled hydrologic-hydraulic models of both systems. However, due to their complexity and numerical challenges, those models are rarely applied, so far. In this study a physically-based distributed hydrological model, MIKE SHE, coupled with a hydrodynamic model, MIKE 11, was set-up and calibrated for a typical lowland catchment in moderate climates, the Tollense River in the north-east of Germany. The Tollense river catchment was selected as study area in collaboration with the regional environmental protection agency (StALU-MS) due to its typical characteristics and related typical water management challenges (flood protection and maintenance of water infrastructures vs. near natural conditions, agricultural impact on water quality and balance) and the availability of the sufficient monitoring data. Dominant land use in the Tollense catchment is intensive agriculture. Large areas are provided with artificial drainage/GW abstraction pumps, while the Tollense river is equipped with two weirs named as Osten and Tuckhude to regulate the river flow. The artificial drainage is applied to lower the GW level for agricultural use, especially in the spring months, and accelerates flow and transport of nutrients in the catchment. Depending on the fertilizer application, plant growth state and the meteorological conditions fertilizer applied to the crops is partly washed off in GW and SW, leading to increased nutrient concentrations (namely nitrate) in both systems. Weirs serve opposite to the drainage by raising the SW levels on their upstream side (u/s), and results in SW contribution to the GW. The Tollense river also observes a back water effect at its confluence with the river Peene in Demmin, due to low hydraulic gradients influenced by the Baltic Sea.