The USDA Root Zone Water Quality Model (RZWQM) was initiated in the middle of the 1980s to provide detailed mechanistic process simulation in the root zone based on lessons learned from other USDA models such as CREAMS (Chemicals, Runoff and Erosion from Agricultural Management Systems), GLEAMS (Groundwater Loading Effects of Agricultural Management Systems), NTRM (Nitrogen-Tillage-Residue Management), Opus, and PRZM (Pesticide Root Zone Model) (Ahuja et al., 2000a). The first version of RZWQM was released in 1992, and a Windows user interface was developed in 1998. Initially, the model contains a generic plant module (GPM) that may be parameterized for any annual crop (Hanson, 2000). Yet, as of today, this generic crop module has only been used for maize, soybean, and winter wheat. In the early 2000s, the DSSAT (Decision Support System for Agrotechnology Transfer) cropping system models (CSM) were incorporated into RZWQM and renamed as RZWQM2 (Ma et al., 2005, 2006). However, RZWQM and RZWQM2 are used interchangeably in the literature. The HERMES crop model was added into RZWQM2 as another option a decade later (Malone et al., 2017; Sima et al., 2020; Ma et al., 2021) (Fig. 1). To improve surface energy balance simulation, the SHAW model was incorporated into RZWQM2 (Ma et al., 2012a, Fang et al., 2014; Qi et al., 2016). Since the DSSAT and HERMES crop models are covered in other chapters (Boote et al., 2025; Nendel et al., 2025), we only detail the generic plant module in RZWQM2 along with some example applications.