Description

Root Zone Water Quality Model (RZWQM) simulates major physical, chemical, and biological processes in an agricultural crop production system. RZWQM is a one-dimensional (vertical in the soil profile) process-based model that simulates the growth of the plant and the movement of water, nutrients and agro-chemicals over, within and below the crop root zone of a unit area of an agricultural cropping system under a range of common management practices. The model includes simulation of a tile drainage system.

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Water Quality Applications

The primary use of RZWQM will be as a tool for assessing the environmental impact of alternative agricultural management strategies on the subsurface environment. These alternatives may include: conservation plans on field-by- filed basis; tillage and residue practices; crop rotations; planting date and density; and irrigation- , fertilizer- , and pesticide-scheduling (method of application, amounts and timing). The model predicts the effects of these management practices on the movement of nitrate and pesticides to runoff and deep percolation below the root zone. That is, the model predicts the potential for pollutant loadings to the groundwater thus allowing an assessment of nonpoint-source pollutant impacts on surface and ground water quality.

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Features

RZWQM consists of six major scientific submodules or processes that define the simulation program, a Numerical Grid Generator, and an Output Report Generator. Interaction between these programs is achieved through the use of seven input datafiles and three generated output files. The user can create and modify input files using a commercial editor. The model generators three general output files with twenty-five optional debugging output files that provide detailed results generated by the model. The Output Report Generator uses model results to create summary tables and publication quality graphical output in 2- and 3-dimensional formats. The most recent version of the model will have a Windows 95 user interface.

1. Physical processes include a large number of hydrologic processes; infiltration; chemical transport during infiltration; chemical transport to runoff during rainfall, water and chemical flow through soil matrix and macropores (i.e., root and worm channels), soil heat flow; fluctuating water table; tile drain, bare and residue-covered soil evaporation; crop transpiration; and soil water and chemical redistribution between rainfall and irrigation events. Snow accumulation and melt are also considered.
   
2. Plant growth processes predict the relative response of plants (corn, soybean, wheat) to changes in environment. Environmental changes can be manifest either as normal variations in climatic variables or by differences in management practices. The model simulates carbon dioxide assimilation, carbon allocation, dark respiration, periodic tissue loss, plat mortality, root growth, water and nutrient (currently only N) uptake.
   
3. Soil chemical processes consist of the soil inorganic environment in support of nutrient processes, chemical transport, and pesticide processes. The chemical state of the soil is characterized by soil pH, solution concentrations of the major ions, and adsorbed cations on the exchange complex. The model is capable of handling soil solution chemistry across a wide range of soil pH.
   
4. Nutrient processes define carbon and nitrogen transformation within the soil profile. Given initial levels of soil humus, crop residues, other organics, and nitrate and ammonium concentrations, the model simulates mineralization, nitrification, immobilization, denitrification, and volatilization of appropriate nitrogen.
   
5. Pesticide processes include the transformations and degradation of pesticides on plant surfaces, plant residue, the soil surface, and in soil profile. Given the plant, crop residue, soil and pesticide characteristics, coupled with environmental conditions, the model simulates the fate of pesticides above and within the soil. Adsorption coefficients are updated daily to account for variations in organic matter decomposition and bulk density changes. Degradation algorithms allow for 1st order, 2 compartment/ 1st order, specific pathway, and daughter product dissipation.
   
6. Management processes consist of description of management activities influencing the state of the root zone. It includes tillage practices and the impacts on surface roughness, soil bulk density, and macroporosity; fertilizer, pesticide, and manure applications; crop planting; irrigation scheduling for flood furrow, sprinkler, and drip systems; and BMP algorithms for dynamic nitrogen-rate determination. Soil surface reconsolidation as a function of time, rainfall, and tillage. Decomposition and bioincorporation of surface residues as affected by water content and temperature, to describe ridge-tilled and no-tilled systems.

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Limitations

• The crops parameterized are limited to corn, soybean and wheat.
• Both input and output are in metric units.
• The complexity of the processes and the need to interpret model results favor the technical staff of most agencies as model users.
• Frozen soil dynamics are not considered.
• Rainfall is entered as break point increments
• A fairly detailed description of the soil profile and initial state has to be known to give good simulation response for the system.

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Support

The documentation and the model are currently being prepared for commercialization through a Cooperative Research and Development Agreement with the Water Resources Publications, Englewood, CO. In the meantime, information on the model is available upon request from the Agricultural Systems Research Unit by contacting the Research Leader.

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