## 1 Introduction
In the last decade, the region has faced strongly varying meteorological conditions, with very wet years and partial flooding of agricultural areas (e.g., in 2011 and 2017) and very dry years with partial drought (e.g., in 2018). This illustrates that the development of adaptive water management is challenging and requires a deterministic model to forecast probable climate and land use scenarios and to assess the effect of water management options. Before deciding for the chosen model combination, four different process models were reviewed, which integrate GW and SW processes: SWAT (soil and water assessment tool), HSPF (hydrological simulation program--FORTRAN), SWIM (soil and water integrated model), and a bi-directional coupling of MIKE SHE and MIKE 11. By focusing mainly on moderate climate lowland catchments with intensive agriculture, high GW tables and decreased retention times, these models were compared on the basis of their data requirement, desired complexity level, and their ability to simulate relevant hydrologic and hydraulic processes. Both SWAT and MIKE SHE are able to simulate the desired hydrological processes shown in Figure 1, but in case of simulating the hydraulic structures and their operations, a coupled MIKE SHE and MIKE 11 model stands out as a suitable simulation tool, as the SWAT model does not take into account hydraulic structures and their operational and control strategies. Both SWAT and the MIKE SHE model require more input data in comparison to HSPF and SWIM. SWIM and SWAT are based on similar governing equations and input data requirements, but SWIM does not simulate ponds, lakes, wetlands, and drainage systems. Both HSPF and SWIM do not provide a satisfying solution to include pump flow and drainage. HSPF is more suitable to study the fate and transport of nitrogen in unsaturated, saturated, and surface flow zones. In order to simulate the backwater effect and control structures and their operational strategies, all these models need to be coupled with hydrodynamic models. Here, MIKE SHE is advantageous, since it provides a ready coupling interface to the hydrodynamic model MIKE 11, which can simulate backwater effects, hydraulic structures (e.g., weirs, gates, etc. ), and their operations [21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32]. MIKE SHE is a physically-based distributed hydrological model that allows the integrated modelling of hydrological processes and can simulate the SW and GW interactions within a catchment [4; 5]. MIKE SHE has the ability to simulate the catchment areas ranging from less than 1 km\({}^{2}\)[33; 34; 35; 36] to thousands of km\({}^{2}\)[37; 38; 39]. MIKE SHE water movement module consists of six components that describe the hydrological cycle, including precipitation/interception, evapotranspiration (ET), saturated zone (SZ), unsaturated zone (UZ), overland (OL) flow, and exchange between SW and GW [36]. As the above described hydrologic, hydraulic, and infrastructural conditions apply large areas in the North and West of Europe, North Western, North America, and (with different meteorological conditions) also in Asia, the chosen model combination should be interesting for these areas too. The research main questions of the present study include:  
1. Possibility to satisfyingly incorporate all relevant processes and boundary conditions and to holistically calibrate the model. 2. Quantification of the meteorological effects on water balance components and the exchange of GW and SW during dry and wet hydrological years. 3. Effects of weather variations on seasonal dynamics of GW levels and river discharges in lowlands. 4. Use of physically-based distributed coupled models and their ability to simulate lowlands with moderate climatic conditions.