## 3 Materials and Methods
### Process-Based or Conceptual Models
The coupled SWAT (version 2012/Rev664) and MODFLOW models [67; 68; 69] perform better during complex surface-groundwater interaction analysis and explicit modeling of the groundwater system and surface water [65], in comparison with field-scale cropping system models such as decision support system for agrotechnology transfer (DSSAT) [70], HYDRUS 1-D [71; 72], root zone water quality model (RZWQM) [73; 74] and leaching estimation and chemistry model (LEACHM) [75]. All the aforementioned cannot accept as a complementary component a hydrologic model so as to drive to better predictions. Modified croplands and watershed SWAT model coupled with the groundwater model and stream-aquifier interaction hydrologic MODFLOW compose the improved SWATMOD model [76], which demonstrates simulation capability of surface water, stream-aquifer interactions and groundwater [16]. The HYDRUS-1D model was employed and calibrated before use against nitrate concentration in soil water across all soil layers and the measured soil moisture. It was validated independently and finally started to simulate N-NO\({}_{3}\) leaching under certain conditions, e.g., controlled fertilization rate within prearranged irrigation depth or vice versa, i.e., regulated irrigation depth combined with a steady fertilization rate. Concurrently, it was assumed that there were different shifts in water table depth. Experimental site-specific soil physical and hydraulic properties were selected as well as data from an automated meteorological station, i.e., net radiation (Rn), barometric pressure (BP), temperature (T), wind speed (W), relative humidity (RH) and precipitation at 60 min intervals [72]. It is commonly employed in landfill hydrological evaluation [77]. The Penman-Monteith equation was utilized in conjunction with the meteo data obtained to determine the reference evapotranspiration [72]. HYDRUS-1D employs Richards' equation [78] to simulate the unsaturated water flow [79]. The mass balance of nitrification--first-order reaction rate constants--and denitrification along with N-NH\({}_{4}\)+ volatilization rate was taken into consideration. The soil-water characteristics curve was depicted by using the van Genuchten-Mualem equation [80] and the pedotransfer function model [49; 72] coefficients and parameters related to the hydraulic transport and transformation (see Figure 2). Solute transport in heterogeneous waste rocks could be simulated by HYDRUS-1D since it supports multiple porosity components [81]. Dynamic leaching flux and soil water storage including dissolved CO\({}_{2}\) and N\({}_{2}\)O conc. can be simulated with good results by implementing the HYDRUS-3D finite element model [82; 83]. It entails domain, finite element mesh, defined boundary conditions and the input of topsoil vertical layers based on Digital Elevation Model (DEM) surface and topsoil thickness information. It estimates soil and bedrock hydraulic properties including CO\({}_{2}\) and N\({}_{2}\)O storage, along with their leaching flow rate. Application of HYDRUS-3D over topsoil reveals the critical influencing, leaching factors (i.e., temperature, fertilizing, precipitations) of diluted CO\({}_{2}\) and N\({}_{2}\)O (the latter a strong GHG) in water and thus promotes the deeper understanding of the nitrogen cycle mechanism in terrestrial ecosystems [83]. Efforts were made to estimate (N) leaching quantification of irrigated maize by testing different fertilizers, regulating, through pumping and irrigation, the water table and adopting improved irrigation management practices. During these efforts, conventional fertilization practices were tested as a reference scenario and a field experimental trial was conducted for consecutive years. Subsoil zones were identified in which the leaching fluctuates within a hydrologic year [72]. Land use investigation combined with climate change was analyzed by implementing a global vegetation model (LPJ-GUESS) [84].