## 2 Materials and Methods
### Modeling Framework  
The GSFLOW model coupled the Precipitation Runoff Modeling System (PRMS) with the 2005 version of the Modular Groundwater Flow Model (i.e., MODFLOW-2005). The GSFLOW model can run either PRMS-only or MODFLOW-2005-only or both. There are three conceptual regions involved in the GSFLOW model (Figure 2). The first region is between the plant canopy on the top and the soil zone on the bottom. The second region consists of streams and lakes. The third region is beneath the soil zone. In the GSFLOW model, the responses of the hydrologic processes in the first region are simulated by the PRMS model. However, the processes in two other regions are simulated with the MODFLOW-2005 model. Because the GSFLOW model integrates many simulation modules, the formats, contents, and module communications must be defined in the input files for both PRMS and MODFLOW models. All the formats of input files in the GSFLOW model had been organized based on modular modeling system files [23] and followed the standard MODFLOW-2005 input requirements [14]. The PRMS model can be used to simulate and assess the effects of the various combinations of climate, and land-use on the watershed response based on changes in water balance relations, streamflow regimes, soil-water relations, and groundwater recharges. The PRMS model uses a series of reservoirs to simulate the hydrologic processes of the watershed. A watershed is an area or ridge of land that separates water that flows to different rivers, basins, or sea. The streamflow and detention reservoir segments to the drainage network are simulated by surface runoff, interflow, and groundwater discharge. In the PRMS model, the calculations of evaporation, transpiration, surface runoff, and infiltration require different types of climate data. These data include precipitation, air temperature, and solar radiation [15]. When the precipitation reaches the land surface, the water may be stored in the sealed zone, infiltrated into the soil zone, evaporated, or become surface runoff. In the PRMS model, the subsurface is represented by three conceptual reservoirs, namely the soil zone, unsaturated zone, and groundwater reservoir. The soil and unsaturated zones account for the different fractions of volumetric water flowing downslope, which may be further removed by plants or combined with groundwater. The water in the soil zones may become surface runoff, interflow, and recharge. It can still be lost because of the evaporation and transpiration based on plant type, solar radiation, and air temperature. The water stored in the unsaturated zones can be available for the gravity drainage to the groundwater reservoirs or for the interflow that flows to the streams or lakes. The water stored in the groundwater reservoirs can become available for the groundwater discharge to streams or be lost to subsurface sinkholes. In the study, the watersheds in the GSFLOW model were discretized into many specified hydrologic response units (HRUs). The discretization and determination of the HRUs are based on site-specific hydrologic and physical characteristics, including drainage boundary, land-surface altitude, land-surface slope, land-surface aspect, plant type, plant cover, land-use, precipitation distribution, temperature distribution, solar radiation distribution, soil morphology, geology, or flow direction [11]. Each HRU is assumed to be homogeneous to represent the hydrologic and physical characteristics. The flow paths and fluxes in each HRU and the adjacent HRUs are computed based on the concept of mass conservation and user-defined input data and model parameters in the GSFLOW model. Note that the simulations of watershed responses are discretized into a daily time-step model to handle the daily observations of precipitation, air temperature, and solar radiation. In the study, the groundwater flow processes, such as areal recharge, leakage from streamflow to aquifers, groundwater discharge to streams, and groundwater pumping, were simulated by using the MODFLOW-2005 model.