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A model for simulating water-based transport processes in and on the surface soil.

Author: Roger E. Smith

Platform: PC

Documentation: PDF

User Manual: PDF

Download:  OPUS Modeling Software

Brief Description:

Opus (not an acronym) simulates the vertical movement and transport of water in the rootzone soil profile, plus the surface water movement and transport of material during rainfall runoff on a small catchment. The catchment spatial limits are set by a) the areal extent for which a single soil profile is representative, b) the areal extent of a single vegetation type of complex, and c) the uniformity of rainfall, that is, the area for which a single rainfall record is applicable. The watershed treated can include second order streams or drainage paths, but complex runoff stream networks are not supported.

Opus is a continuous simulation model with internally nested hierarchical time scales, so that time steps are chosen to match the process dynamics. Time steps during a storm, for example, are much smaller than those for periods of no rain. Simulated soil processes include unsaturated soil water flow, plant growth and transpiration, soil evaporation, soil water transport and decomposition of adsorbed chemicals, soil C, N and P cycling and residue decomposition, using the Century model (Parton et al., 1988). Surface water flow and soil erosion and transport are simulated during runoff events. Subsurface drains may be treated as well.

The changes in soil and plant cover due to agricultural management are simulated based on user input of the types of management actions (tillage, fertilization, spraying, etc.) and the schedule of such actions. Plants are grown after planting using a mechanistic model in which the plant responds to nitrogen, water, and temperature stresses. Chemicals applied are traced through the soil and are subject to adsorption kinetics as well as environmental degradation. Manure and fertilizers may also be applied. A rotation period of up to 5 years can be treated. The overall simulation period may be as long as is necessary for the purpose of the user, such as comparison of the effects of alternate management methods.


Model Input:

Parameters must be input to describe the soil hydraulics, the basic soil organic matter, the field slope and size, the soil erodibility characteristics, depth and spacing of any drains, vegetation growth parameters, and other basic information. The user may choose either English or metric input units. Two ASCII files, in template format, describe field parameters and weather for the simulation period. A third optional file allows Opus to use more detailed weather and hydrologic records if available.

Currently under development is a graphic user interface that will lead the user through the development of a parameter input file, or allow editing of an existing file. It will include help references and guidelines for parameter value limits.

Opus can accept a variety of input rain record types, including detailed rainfall rate records or daily rainfall totals, or can generate an artificial record using statistical parameters for the particular location of interest, from furnished parameter maps (Richardson and Wright, 1984). Opus needs some information on the daily maximum and minimum temperature and solar radiation, but it can decompose monthly mean records for these variables into a viable daily sequence, or (for the continental US) generate a record based on local statistical parameters, or use a record of local daily recorded values. It can also use monthly records of average daily pan evaporation in place of solar radiation to estimate a daily record of potential ET.


Model Output:

The model produces a variety of output files, in part depending on the simulation options chosen. The simulation of residue and soil nitrogen cycles, pesticide transport, and erosion processes are optional. There is a main output file giving summaries of water balance on a daily, monthly, and annual basis, plus optional output files for water and chemical distributions in the soil profile, for hydrologic detail, and for daily plant status. The user may specify English or metric input and output units.


System Requirements:

The currently supplied executable runs on a 32bit Windows system, but if needed a DOS based version can be supplied. The model simulations take from about 20 seconds to 2 minutes per year of simulation, on a 90mhz pentium PC, depending on the complexity of simulation.


Availability and Documentation:


The model is furnished to users with a Documentation volume describing the theory and equations used, and a User Manual outlining the input parameters needed. The User Manual in many cases offers guidelines for parameter selection. For potential users, other publications describing the model and example applications are available (Smith, 1995; Santos et al. 1996). The model is available through this website. Downloadable documentation is not available, unfortunately, and we are currently unable to provide the technical documentation as it is being revised. We hope to provide a downloadable version soon.. Contact the author for further details and any questions.



Parton, W.J., J.W.B. Stewart, and C.V. Cole, 1988. Dynamics of C, N, P, and S in grassland soils: A model. Biogeochemistry, 5:109-131.

Santos, D.V., R.E. Smith, P.L. Sousa, and L.S. Pereira, 1996. Calibration and validation of model Opus for water and nitrate simulation. In: R. Ragab, D.E. El-Quosy, B. van den Broek, and L.S. Pereira, editors, "Crop-Water-Environment Models" , ICID Cairo International Conference, pp. 17-28

Smith, R.E., 1992. Opus: An Integrated Simulation Model for Transport of Nonpoint-source pollutants and the Field Scale. Vol. 1, Documentation. USDA-ARS, ARS-98, 120 pp.

Ferreira, V.A., and R.E. Smith, 1992. -------. Vol. II, User Manual, 100 pp+appendices.

Smith, R.E. 1995. Opus simulation of a wheat/sugarbeet plot near Neuenkirchen, Germany.Ecological Modelling, 81:121-132.