# Assessing Groundwater Level with a Unified Seasonal Outlook and Hydrological Modeling Projection
## 2 Methodology
### Watershed Models  
Rainfall-induced surface runoff was obtained from the HEC-HMS model in this study. Three major modules are included in the HEC-HMS: the basin, meteorological, and control specification modules [29; 30]. The Soil Conservation Service (SCS) curve number (CN) method is used to calculate the sub-basin loss in the basin module. The parameters of the initial loss, CN value, and impervious percentage are evaluated on the basis of land-use types in the study area. The SCS unit hydrograph method is applied to calculate sub-basin routings, using data from historical records to calculate the discharge and lag time. For the meteorological module, HEC-HMS provides multiple options to generate regional rainfall distribution. The rainfall station weighting method is then used to determine the areal weights for various sub-watersheds using the Thiessen polygon method. Therefore, HEC-HMS was applied to simulate the runoff, and its results were adopted as inputs for subsequent watershed model simulations (see the third stage in Figure 1).  
The WASH123D numerical model, developed by [PERSON] et al. [18; 27; 28], which conceptualizes the watershed system as a 1D (one-dimensional) stream-river network, 2D (two-dimensional) overland regime, and 3D (three-dimensional) subsurface media, was applied in this study. The model was selected as the most appropriate protocol because it simulates all flows that comprise a watershed. Moreover, it is capable of simulating problems related to various spatial and temporal scales as long as the continuum assumptions are valid. The theory built into WASH123D is based on the conservation laws of fluid and momentum, associated with constitutive relationships among the fluxes, state variables, and appropriately formulated equations for source/sink terms. The governing equations for the one-dimensional river flow are derived from the momentum equation, which is based on the results of the law of conservation of water mass in the continuity equation and linear momentum conservation. For the two-dimensional overland flow, the above assumptions hold except for the conservation principle of linear momentum along with the \(x\)-direction and \(y\)-direction results in the two momentum equations. The governing equations for subsurface flow through saturated-unsaturated porous media are derived on the basis of the water mass conservation law. The cross-section-averaged 1D diffusive wave equation, depth-averaged 2D diffusive wave equation, and Richard's 3D equation are applied in the simulations. For surface flow problems, the semi-Lagrangian method (backward particle tracking) is used to solve governing equations. Moreover, the governing equations of subsurface flow are discretized with the Galerkin finite element method [18]. WASH123D has been applied to many regional projects and was chosen by the US Army Corps as the core computational code for coastal and watershed studies [18]. A revamped version of the WASH123D model has been applied to many flood mitigation and groundwater resource problems [7; 35; 36; 37].