Since the late 1960s, hydrologic modeling focused more and more on interactions between surface and groundwater flow systems, leading to the development of integrated modeling approaches simulating flow in these two systems and the simulation of interactions between them. These interactions are of variable character as well as spatial and temporal heterogeneity. The primary objective of this study is the application and investigation of a loosely coupled modeling approach, combining two well-known models of the United States Geological Survey, the Precipitation Runoff Modeling System (PRMS) and the Modular Finite-Difference Groundwater Flow Model (MODFLOW), in order to simulate complex hydrological processes in a mesoscale watershed. Within the scope of the hydrologic modeling groundwater recharge, the parameter unidirectionally linking the surface water and groundwater system is used to couple these two models loosely. Considering factors that have a great influence on groundwater recharge (e. g. climate, topography, soil, and land use), the potential of modeling the hydrologic system with a distributed surface water model is explored. In addition, the option of constraining the PRMS parameter determining groundwater recharge, and therefore the loosely coupled surface-groundwater model, with observed head levels only, is investigated. This modeling approach is applied to the Esperstedter Ried basin, an ungauged, mesoscale, groundwater-dominated catchment in central Germany. The study represents a good example of problems frequently occurring in hydrologic modeling. Due to the lack of observed streamflow and surface runoff data, the calibration and validation of any hydrologic model for this watershed proves to be difficult. Furthermore, the basin features a strong surface-groundwater interaction of unknown degree. This research paper describes the concept of an inexpensive and simple method of combining the advantages of two established hydrologic modeling approaches by using output information of the surface water model as input for the groundwater model. The basic assumption is made that groundwater recharge rates provided by PRMS are reasonable because the estimation procedure benefits from the fairly high resolution of meteorological, land use and soil parameters of the surface water model. At present, the approach presented in this paper is limited to a unidirectional coupling of the surface and groundwater model, which is only accounting for groundwater estimates provided by PRMS as input for MODFLOW; whereas possible upward movement of water, i.e. groundwater seepage to the surface is neglected. Performance and efficiency of this approach are assessed in the scope of applying the model in steady-state as well as transient mode to the Esperstedter Ried basin. The results of this case study, which produced a number of reasonable as well as some unsatisfying simulations, demonstrate the potential of coupling a surface water model and a groundwater model to obtain more complex and accurate analyses and simulations of hydrologic systems. However, the study also shows the need for further research in regard to constraining the parameter linking both models, i.e. groundwater recharge. Furthermore, the necessity for a more dynamic and complex method of integrating surface and subsurface interactions is revealed, which accounts for downward as well as upward movement of water in the subsurface system.
