# Planning MAR Schemes Using Physical Models: Comparison of Laboratory and Field Experiments
## 1 Introduction  
Managed aquifer recharge (MAR) is a water resources management approach used to mitigate the negative effects of overexploitation and climate change on groundwater resources and quality. It is further utilized to balance the temporal or local water demand and availability [1; 2]. It comprises the intended recharge of a groundwater body under controlled conditions [3]. MAR generally involves large-scale facilities, such as injection wells, infiltration basins, or recharge dams [4]. The construction of MAR facilities requires comprehensive planning to understand the local hydrogeological conditions, to achieve sustainable and controllable conditions, to reduce construction and maintenance costs, and to minimize the facility failure potential. Before building MAR facilities, planning is often accompanied by field and laboratory investigations. Pilot sites and preliminary studies are required by some MAR guidelines [5; 6]. Incorporating physical models helps to assess and define the requirements and constraints of applying MAR at a certain site. Further, the are also useful for optimizing the actual MAR site in terms of dimensions, monitoring, and operational parameters. Most commonly, surface infiltration experiments are used to understand the processes governing recharge into the unsaturated soil zone [7; 8; 9; 10; 11]. However,laboratory and field experiments for direct infiltration into the aquifer, e.g., through aquifer storage and recovery (ASR) wells, have been conducted as well [12; 13; 14]. The experimental design is conceptualized at different spatial scales. Usually laboratory experiments are preferred as field tests are time-consuming and costly [12] and are often impractical for detailed process assessment [15]. Another positive aspect of laboratory studies is that they can be conducted under adaptable and controllable boundary conditions [7; 13; 16; 17]. However, simplifications and scale-related limitations of small-scale experiments may lead to over- or underestimation of infiltration processes [8]. The extrapolation of results from controlled laboratory investigations to the field scale is highly uninvestigated in the context of MAR [7] and limitations of transferring results from laboratory experiments to the field are rarely discussed [9; 15]. These limitations can be linked to the shortcomings of laboratory experiments. Some issues, such as sidewall flow in laboratory columns, have been widely discussed [18; 19; 20; 21]. Another critical design issue, which is not restricted to column studies, is the occurrence of preferential flow paths through macropores or fingering [22; 23]. With regard to dimensionality, it has been stated that processes such as flow-bypassing can be not represented by 1D-systems [8]. The lower boundary of laboratory models is often operated as free drainage out of system, which may lead to the development of capillary fringes and the lower part of the system becoming saturated [20; 24]. Reproducing field climate conditions in the laboratory is restricted, even though seasonal conditions of temperature and radiation have a large impact on water flow and specifically clogging processes [25; 26]. To understand the restrictions and potential of different physical models for MAR assessment, we set up surface infiltration experiments in different scales (field, laboratory) and dimensions (1D, 3D) and with several soil types but with the same operational parameters (infiltration scenarios, water quality). The results from the different setups were compared regarding the reproduction of water flow dynamics and clogging processes during intermittent infiltration experiments. The results from the statistical comparison are discussed regarding the limitations and applicability of laboratory experiments.