Essential Guide to WMS Rain-on-Grid Modeling 

In the realm of water resources engineering, Rain-on-Grid (RoG) modeling has rapidly transitioned from a computationally prohibitive research method to an industry standard. Moving away from traditional lumped-parameter unit hydrographs in favor of applying spatially distributed precipitation directly to a 2D computational mesh, a frequent question arises: Can Aquaveo’s Watershed Modeling System (WMS) effectively handle true rain-on-grid workflows?

The short answer is yes, but the technical reality is highly dependent on the computational engine you select within the WMS interface. Here is a practical, engineering-focused breakdown of how to leverage WMS for RoG, and how it compares to standard hydrodynamic tools like HEC-RAS 2D.

Understanding WMS: A Modeling Interface, Not a Single Model

WMS, developed by Aquaveo, is best understood as a GIS-based interface that integrates multiple hydrologic and hydraulic models, rather than a standalone solver. This flexibility allows users to select from engines such as GSSHA and HEC-HMS, each with different capabilities for handling rainfall and runoff processes.

Rain-on-Grid Capability in WMS

WMS does support spatially distributed (gridded) precipitation inputs, including raster and netCDF datasets. These inputs can be mapped onto computational grids, forming the basis for rain-on-grid style simulations. However, true rain-on-grid modeling—where rainfall is directly applied to a 2D hydraulic surface and routed dynamically—depends on the selected engine.

Best Option in WMS: GSSHA

Among the available models, GSSHA is the most suitable for rain-on-grid applications. It is a fully distributed, physics-based model that simulates:

• Overland flow on a grid

• Infiltration and soil moisture

• Groundwater-surface water interaction

Because GSSHA routes water across a 2D grid, it aligns closely with the conceptual framework of rain-on-grid modeling. According to the U.S. Army Corps of Engineers, GSSHA was specifically designed for distributed watershed simulation, making it well-suited for spatial rainfall analysis (Downer & Ogden, 2004).

Limitations Compared to HEC-RAS Rain-on-Grid

Despite these capabilities, WMS does not natively replicate the workflow of HEC-RAS 2D rain-on-grid modeling. HEC-RAS applies rainfall directly to a hydraulic mesh and solves flow using diffusion wave or full momentum equations, producing detailed depth and velocity fields.

In contrast:

• GSSHA models emphasizes hydrologic processes first, with simplified hydraulic routing

• HEC-HMS models remains lumped or semi-distributed, not fully grid-based hydraulics

Conclusion: When to Use WMS for Rain-on-Grid

WMS is an exceptionally powerful platform for rain-on-grid modeling, provided you utilize the GSSHA engine. However, practitioners must apply the right tool to the right boundary conditions:

1. Use WMS (GSSHA) when the project scope demands a highly detailed, spatially distributed hydrologic model. If your primary challenge is accurately quantifying runoff generation over complex terrain, variable soil profiles, and dynamic weather systems, GSSHA excels.

2. Use HEC-RAS 2D (or SMS) when the project is primarily a hydraulic challenge, requiring detailed water surface elevations, shear stress calculations, and complex flow paths around urban infrastructure.

By understanding the distinction between hydrologic routing and hydrodynamic routing, you can confidently deploy WMS for robust rain-on-grid analysis without pushing the software beyond its intended physical constraints. Download WMS now to begin rain-on-grid modeling.