GHOST (Generic Hydrologic Overland-Subsurface Toolkit) MM-pihm is a distributed coupled surface-subsurface hydrologic open source code commonly used to predict the hydrological response of medium-size watersheds over period of months (Qu et al., 2007; Yu et al., 2013). GHOST is a reformulation of MM-pihm to perform multi-year simulations in large basins. Modifications and modules specifically developed in GHOST was directed by: Increasing performance Coupling fluxes capable of reproducing long-term continuous basin behavior Capturing physical processes in Iowa watersheds Robust algorithms

Improving computational efficiency OF the code Code consistent with new version of Sundials Parallelized with OpenMP Variable time step (depending on Sundials convergence efficiency) Restart option Improved interpolation schemes Improving computational efficiency OF the code

Development of mathematical expressions for lateral fluxes Robust scheme for lateral fluxes in 2D surface elements New formulation for lateral fluxes between 2D surface elements and channel

Development of mathematical expressions for vertical Fluxes New formulation for infiltration, exfiltration and recharge models New formulation to include temporary streams. Development of channel infiltration and exfiltration models

Development of specific models for IOWA watersheds Hello Development of specific models for IOWA watersheds Lake module Retention basin module

Lake defined in overland grid with inactive cells in space occupied by the lake. Fluxes to lake include overland flow, precipitation, stream flow, groundwater flow and evaporation Bridge head losses between lakes included Rating curves to calculate outflow Lake storage-depth relationship to compute lake depth Lake module

Retention basin module Implemented in river segments Contribution from overland and stream flows Groundwater contribution Outlet rating curves Project storage-depth relationship to compute lake depth Evaporation and infiltration included

ET model Root depth, vegetation height and crop coefficient as a function of time Optional use of potential evapotranspiration from NLDAS New model for winter processes considering soil temperature and reduction of infiltration in frozen soils Implementation of Panday and Huyakorn (2004) model

Outputs No. File Purpose 1 Simulation_name.plt Visualization of hydrological response in 2D elements 2 Simulation_name_R.plt Visualization of hydrological response in river segments 3 Simulation_name_WaterBalance.plt Integrated variables over the watershed area to compute water budget 4 Simulation_name_RiverFile.plt Evolution of flowrate for selected river segments 5 Simulation_name_GWFile.plt Evolution of groundwater for chosen elements 6 Simulation_name_Continuity.plt Continuity file to check mass conservation 7 Simulation_name_Grid.plt Actual mesh after internal modification 8 Simulation_name_CVODE.log Output information of Sundials solver 9 Simulation_name_Performance.txt Numerical performance of the simulation 10 Simulation_name_P_ProjectFile.plt Evolution of project/wetland variables 11 Simulation_name_P_ProjectStats.plt % time project is above and below given elevations 12 Simulation_name_L_LakeFile.plt Evolution of lake variables