MOHID Land

MOHID Land is the hydrology module within the three-dimensional MOHID Water Modelling System. MOHID is an integrated system for water flow, consisting of three main modules: Water for three-dimensional fluid dynamics, Land for hydrology and Soil for groundwater flow. The MOHID Land module is a watershed mathematical model — or hydrological transport model — designed to simulate the flow of water in a drainage basin and aquifer. The processes which are simulated include two-dimensional overland runoff, infiltration into the ground, one-dimensional drainage network flow through rivers and canals, as well as (saturated and unsaturated) porous medium transport. The interactions between the different processes (like water exchange between aquifer and river) are calculated dynamically by the model, using the hydraulic gradients. The different processes occurring in a basin are programmed in different modules. The user can choose which modules to activate, allowing simulation of the desired ones only.

MOHID Land was developed within the framework of three EU-funded projects: EcoRiver,^[1] TempQsim^[2] and ICReW^[3] for the simulation of water flow in watersheds with pathways for river and groundwater flow. The porous media module was developed in close collaboration with soil scientists from EAN-INIA (Portuguese National Agronomic Station). Recently the ESA finaced Aquapath-Soil^[4] project to apply mohid land to support irrigation using Satellite LAI as input to the model, obtaining maps of Actual Evapotranspiration. Example of map obtained can be seen in Google Maps (see map).

Main processes solved

• Drainage network (kinematic wave equation);
• 2D overland flow (kinematic wave equation);
• Infiltration calculated by Richards equation or Green–Ampt approach;
• Soil redistribution by Richards equation;
• Aquifer flow by Darcy's law;
• Evapotranspiration is calculated with the 'FAO crop reference evapotranspiration', a standardized Penman–Monteith equation.^[5]

Dynamical time step adaptation

MOHID Land uses an adaptive time-stepping method in its main hydrodynamic cycle. Within an iterative cycle, if the water volume — of reach or overland flow or porous media — varies more than a user defined percentage during two consecutive time steps, the model automatically decreases the time step. Thereafter the model recalculates the current solution with a smaller time step for the affected process (reach or overland flow or porous media). This process is repeated until the volume variation is less than the user defined value mentioned above. The time step dynamically increases again when the model verifies that flow is “stable”. For example within the module "Drainage Network" the time step may be reduced to very short intervals during flush events. This procedure avoids the occurrence of negative volumes and optimizes the time it takes to make a certain simulation, without compromising model stability. Time steps of the processes — computed in the different sub-models — can be chosen differently, adding more to the optimization of the computational cost.

GIS Model Interfaces

You can download interfaces to prepare inputs and to analyse model results. Presently available two options:

• a completely free interface from Mohid Website (www.mohid.com).
• a beta version of a new interface can be downloaded from Action Modulers.

Source code

The latest source code of the model can be downloaded from Codeplex (Mohid Land in CodePlex). Just click on the download link, no need to register. For advanced users you can download the entire solution ready to compile.

Other users can just browse around the code of each module to check out equations. In the links below you^[who?] can go directly to the version from 4 Feb 2011 and see code in html:

• Mohid Land Is the Main Program of Mohid Land
• Module Basin Is the top level of RunOff, Infiltration, River Flow and Vegetation
• Module Porous Media Simulates Water Flow in variable saturated soils
• Module Porous Media Properties Deals with all property transport in Porous Media.
• Module Sediment Quality Zero-dimensional model for primary production, nitrogen and carbon cycle in the Porous Media Soil and Aquifer)
• Module PhreeqC Zero-dimensional model for chemistry equilibrium of solution, pure phases, gas phase, solid phase, exchangers and surfaces in Porous Media (Soil and Aquifer)
• Module Run Off Module which calculates the Surface RunOff
• Module Run Off Properties Deals with all property transport in Runoff
• Module Vegetation Module to simulate plant development
• Module Drainage Network Module which simulates a 1D Drainage Network System
• Mohid River Network Program that allows to run river using SWAT-Mohid discharges
• Module CEQUALW2 U.S. Army Corps of Engineers zero-dimensional model for primary production that can be run to simulate water quality in the river
• Module Triangulation This is one of the modules for interpolation. This is useful to obtain distributed rain in space based on point time series.
• etc

Notes

1. ^ EU EcoRiver project
2. ^ EU TempQsim project
3. ^ EU ICREW project
4. ^ Aquapath-Soil project
5. ^ FAO crop reference evapotranspiration

References

• Ramiro Neves et al. (2008), Sustainable Use and Development of Watersheds, http://www.springerlink.com/content/t8178785515h7247/, retrieved 2008-10-02 
• Pedro Bagulho Galvão; Ramiro Neves; Adelio Silva; Pedro Chambel Leitão; Frank Braunchweig (2003), "Integrated watershed modeling", ESA Meris Workshop, Frascati, Italy, http://earth.esrin.esa.it/pub/ESA_DOC/meris_workshop_2003/papers/51_bagulho.pdf, retrieved 2008-10-30 
• Rosa Trancoso; Frank Braunchweig; Pedro Chambel Leitão; Mathias Obermann; Ramiro Neves (2009), "An advanced modelling tool for simulating complex river systems", Science of The Total Environment, http://linkinghub.elsevier.com/retrieve/pii/S0048969709000217, retrieved 2009-03-20