Quoted from: https://www.cbrfc.noaa.gov/present/rdhm/RDHM_User_Manual.pdf

The features of HL-RDHM have evolved based on lessons learned from numerous scientific studies and practical applications since the mid 1990s. The HL-RDHM hydrologic system structure and some of its basic routing codes originated from the Nile Forecast System (Koren and Barrett, 1995). Several of the scientific subroutines accessible through HL-RDHM are identical to or slight modifications of National Weather Service River Forecast System (NWSRFS) operations. Koren et al., (2004) provide a description of the basic hydrologic system, a gridded implementation of the SAC-SMA model, the overland flow and channel routing algorithms, routing parameter estimation procedures, and representative results (Note that Koren et al., (2004) refer to the Hydrology Laboratory-Research Modeling System (HL-RMS), a precursor to HLRDHM 2.4.2). The HL-RDHM code base has been used for numerous other scientific studies and prototype operational applications (Cooper, 2006; Koren et al., 2005; Moreda et al., 2006; Reed, 2004; Reed et al., 2007; Reed et al., 2006; Schmidt et al., 2007; Shultz and Corby, 2006; Smith et al., 2004; Zhang et al., 2006). Cui et al. (2006) report on current software architecture features of HL-RDHM. Many more details on these features are provided in this User’s Manual and HL-RDHM Developer’s Manual. As HL-RDHM has evolved, so has crucial work into distributed parameter estimation for various hydrologic modeling techniques supported by HL-RDHM. Based on this work, default a-priori grids for many required parameters and connectivity files required for cell-to-cell routing have been derived for CONUS and are delivered to RFCs with HL-RDHM. Chapter 3 and Appendix A in this manual summarizes the modeling techniques available in HL-RDHM, the required parameters for each, and whether or not default, spatially variable, a-prioir parameter grids are provided for each parameter. Chapters 9 and 10 provide additional guidance on how to (1) estimate parameters for which no default values are provided, and (2) make adjustments to default parameter grids provided using manual and automatic calibration procedures.