The USDA-ARS Root Zone Water Quality Model (RZWQM)

Authors: Zhang, Huihui; Ma, Liwang; QI, ZHIMING - McGill University - Canada; Hanson, Jonathan

Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: 8/5/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: The USDA Root Zone Water Quality Model (RZWQM), developed in the 1990s, simulates agricultural management effects on crop production and environmental quality. It includes modules for water movement, soil nutrient cycles, plant growth, chemical transport, and energy balance. The first released version contains a generic plant growth module that could be parameterized for any annual crops and was used extensively in the corn belt to simulate corn and soybean production under various water and nitrogen management in the MSEA (Management Systems Evaluation Areas) projects. It is also applied to winter wheat production in the Great Plains. Internationally, the generic plant growth module was used to simulate winter wheat and corn in China and corn in Portugal. Since the 2000s, RZWQM added the DSSAT (Decision Support System for Agrotechnology Transfer) crop model and the HERMES model as options for users. Many users are using the DSSAT option due to its easy to calibrate and support from DSSAT developers. The model is still maintained by USDA-ARS and used worldwide for simulating various agricultural management practices (irrigation,fertilization, tillage, manure application, pesticide fates, and soil carbon sequestration) under the changing environment.

Technical Abstract: The USDA Root Zone Water Quality Model (RZWQM) was developed to simulate detailed mechanistic processes in the root zone, building on insights from earlier USDA models such as CREAMS, GLEAMS, and NTRM. Released in 1992, RZWQM initially featured a generic crop module adaptable to any annual crop. However, this module has primarily been applied to crops like corn, soybean, and winter wheat. In the early 2000s, RZWQM was integrated with the DSSAT crop model, leading to the development of RZWQM2, which allowed users to simulate plant growth using a more crop-specific modeling system. Later, a even simpler generic crop module, HERMES, was added to RZWQM2 at request of users. The plant growth modules are interrelated with other abiotic modules, including water balance, soil carbon and nitrogen cycles, surface energy balance, and solute transport, among others. The model operates on a daily time step but calculates water balance on a sub-hourly basis, solving the Richards’ equation with plant water uptake and soil evaporation as key factors. The model also simulates potential evapotranspiration using the Shuttleworth-Wallace equations, incorporating surface residues and partial canopy effects. Soil organic matter is divided into three pools with unique degradation rates, and microbial populations mediate soil carbon and nitrogen cycling. For chemical transport, solutes move with water fluxes and undergo sorption-desorption reactions in the soil. Additionally, agricultural management practices such as irrigation, fertilization, tillage, planting, and pesticide applications are simulated with a management module. In this book chapter, we focused on the generic plant growth module and its primary applications for irrigation and nitrogen management. Comparison with the DSSAT module in RZWQM2 is also analyzed for corn, soybean and winter wheat simulations.