What is the primary purpose of USGS Hydrological Modeling Software?

The primary purpose of USGS Hydrological Modeling Software is to simulate and predict the movement, distribution, and quantity of water across watersheds, linking surface water and groundwater systems. These tools are essential for water resource management, planning, and assessing the impacts of climate change and human activities on water availability and quality.

What does the acronym MODFLOW-OWHM stand for, and what is its main function?

MODFLOW-OWHM stands for the One-Water Hydrologic Flow Model. Its main function is to integrate and simulate all components of the human and natural water cycle, including supply-and-demand processes, managed water use, and the interaction between surface water, groundwater, and imported water. This makes it crucial for complex conjunctive-use water management issues.

How does the Basin Characterization Model (BCM) help in assessing climate change impacts?

The BCM is a regional water balance model that assesses climate change impacts by calculating key water budget components like runoff, recharge, soil moisture, and evapotranspiration based on climate scenarios (precipitation and temperature) and landscape attributes. It provides a spatially explicit view of how water resources are affected by changing climate patterns across a watershed.

What is the function of the Cascade Routing Tool (CRT) in hydrological modeling?

The Cascade Routing Tool (CRT) is used to define and compute the cascading surface and shallow subsurface flow paths within a watershed simulation. It calculates how water moves among Hydrologic Response Units (HRUs) and streams, providing essential data on runoff and interflow directions before water enters the stream network. This is critical for accurate surface water modeling.

What unique process is simulated by the INFIL model?

The INFIL model simulates the temporal and spatial distribution of daily net infiltration of water across the lower boundary of the root zone. This is a critical process, as the calculated net infiltration is assumed to be equivalent to the net recharge entering the underlying unconfined water-table aquifer. It helps bridge the gap between surface water processes and groundwater recharge.

How does MODPATH Observation Process (MODPATH-OBS) relate to groundwater contamination?

MODPATH-OBS is a post-processing tool that utilizes particle tracking (usually performed by MODPATH) to calculate simulated equivalents to transport observations. In the context of contamination, it simulates the movement of contaminant particles to help compare model results with field data, such as the observed location and extent of contaminant plumes.

Is MODFLOW only used for groundwater modeling?

While MODFLOW (Modular Groundwater Flow Model) started as the industry standard for three-dimensional groundwater flow, its descendants have expanded significantly. Modern versions, particularly the MODFLOW-OWHM framework, are designed to be integrated, simulating both groundwater flow and surface water processes, thus covering the entire water cycle.

What are some examples of hydrological modeling software not developed by the USGS?

The article mentions several other widely used hydrological modeling software programs, including SWAT (Soil and Water Assessment Tool), VIC (Variable Infiltration Capacity), TOPKAPI, HBV-Light, and PREVAH. These models, alongside USGS tools, are used globally to address various water-related issues like floods, droughts, and climate change impacts.

What is an HRU, and why is it important in watershed modeling?

An HRU stands for Hydrologic Response Unit. It is a spatial unit used in watershed modeling, defined as an area with similar soil, land cover, slope, and hydrological characteristics. Using HRUs (as done by the Cascade Routing Tool) simplifies the watershed by grouping areas with similar responses to precipitation, making hydrological computations feasible and efficient across large regions.

How is the One-Water concept integrated into the MODFLOW-OWHM?

The One-Water concept is integrated by treating the human and natural components of the water cycle as a single, connected system. MODFLOW-OWHM achieves this by coupling traditional hydrological processes (rainfall, runoff, infiltration) with human infrastructure (wells, canals, reservoirs) within a single simulation framework, allowing for true integrated water resource analysis.

Can these models be used for flood forecasting and drought assessment?

Yes. While the USGS models (like MODFLOW-OWHM) are generally designed for long-term resource management and assessment, their fundamental output (simulated runoff, streamflow, and water storage) is crucial for both applications. Additionally, other models mentioned (like SWAT and VIC) are commonly applied to real-time flood forecasting and long-term drought monitoring and assessment globally.

Do the USGS models account for human water use, such as irrigation and pumping?

Yes, especially the advanced integrated model, MODFLOW-OWHM. This model explicitly includes components for simulating human actions like agricultural demands (irrigation), municipal water use, groundwater pumping from wells, and return flows from wastewater treatment. It places these processes into a comprehensive supply-and-demand framework.

5 USGS Water Modeling Tools: An In-Depth Look

By Alex

20 minutes read

Are you interested in learning how USGS hydrological modeling software is transforming so we can better manage water resources? It is estimated that the global hydrological modeling market is expected to grow from $12.13 Billion in 2015 to $22.17 Billion by 2026, at a CAGR of 5.7% (MarketsandMarkets, 2020). This growth rate indicates a significant increase in demand for hydrological modeling software, highlighting its importance in water resources management.

Conceptual visual of 5 key USGS Water Modeling Tools used for groundwater and surface water analysis. — **Fig. 1.** The five key USGS Hydrological Modeling Software programs, including BCM and MODFLOW, are utilized for water resource management.

Table. 1. 5 USGS Hydrological Modeling Software

Model	Application
Basin Characterization Model (BCM)	Water resources planning and management
MODFLOW-OWHM	Assessment of impacts of management scenarios on groundwater resources
MODPATH Observation Process	Understanding the potential impacts of contamination on water resources
Cascade Routing Tool	Assessment of impacts of management scenarios (e.g., dam operations) on downstream water resources
INFIL3.0	Understanding how much water is entering the groundwater system in a given area

Download Full Table of5 USGS Hydrological Modeling Software

Hydrological modeling is critical for comprehending and managing water resources. It involves simulating the movement of water through various components of the hydrological cycle, such as precipitation, evaporation, runoff, and groundwater flow. Hydrological modeling software finds extensive application in the field of water resources management, flood forecasting, and environmental impact assessments. It enables hydrologists, engineers, and researchers to simulate and predict the behavior of water systems under different scenarios, facilitating the decision-making processes (Horton et al., 2021; Baydaroğlu et al., 2022).

The output format of hydrologic modeling software typically includes visual representations, such as maps, graphs, and tables, which provide insights into water quantities, flow patterns, and other hydrological parameters (Sahu and Prasad, 2018; Fatichi et al., 2017). This article provides a comprehensive overview of hydrological modeling software, primarily USGS products, along with their key features and some notable examples.

Compilation of hydrologic modeling software maps for predicting water flow and aquifer behavior. — **Fig. 2.** Comprehensive visualization of water systems generated by Hydrologic Modeling Software.

1. The USGS and its Contribution to Hydrological Modeling Software Market

The U.S. Geological Survey (USGS) has been at the forefront of hydrological modeling software development since the 1960s. Their models are widely used to predict the responses of hydrological systems to changing stresses and assess the movement of solutes and contaminants in water (USGS, 2018). Many approaches to hydrologic modeling have been developed. In the following, five key software programs for hydrologic modeling are introduced.

1.1. Basin Characterization Model (BCM)

The Basin Characterization Model (BCM) is a regional water balance model that has been applied to numerous watersheds at a fine scale of 270 m grid cells to assess the impacts of climate change on both water availability and ecosystems. The BCM calculates water balance components, including runoff, recharge, evapotranspiration, soil moisture, and stream flow, based on climate, topography, soils and underlying geology, and the solar-driven energy balance. This map‐based, mechanistic model focuses on the interactions between climate (rainfall and temperature) and landscape attributes (topography, soils, and underlying geology) to calculate the water balance of a given watershed (USGS, 2018).

The BCM enables the generation of scenarios at different time steps, allowing for a detailed analysis of water resources. The results of this model testing permit a better interpretation of where hydrologic simulations perform better or worse due to the influences of landscape variables. The BCM can be used to identify locations and climatic conditions that generate excess water by quantifying the amount of water available either as runoff or as in-place recharge monthly (Flint et al. 2015; Thorne et al. 2012; Micheli et al., 2012).

Fig. 3. Understanding water balance processes within the Basin Characterization Model (BCM).

1.2. MODFLOW-OWHM: One-Water Hydrologic Flow Model

MODFLOW-OWHM is an Integrated Hydrologic Model (IHW) that simulates and analyzes water movement within a physically based supply-and-demand framework. The MODFLOW One-Water Hydrologic Flow Model combines multiple MODFLOW variants in one cohesive simulation software. It is the most complete version of the MODFLOW family of hydrologic simulators that is designed to address various conjunctive-use issues, water management, water-food security, and climate-crop-water scenarios, facilitating the management of human and natural water movement. Conjunctive use is the combined use of groundwater and surface water (Hanson et al. 2014).

MF-OWHM includes new features since MODFLOW-2005, such as the Surface-water Routing Process (SWR), Seawater Intrusion (SWI), and Riparian Evapotranspiration (RIP-ET), and new solvers such as Newton-Raphson (NWT) and nonlinear preconditioned conjugate gradient (PCGN). MF-OWHM represents a complete operational hydrologic model that fully links the movement and use of groundwater, surface water, and imported water for consumption by irrigated agriculture, as well as water used in urban areas and by natural vegetation. From large- to small-scale settings, MF-OWHM has unique capabilities to simulate and analyze historical, present, and future conjunctive-use conditions (Boyce et al. 2020).

Flow diagram showing MODFLOW as the base, leading to descendants like MODFLOW-SWR and MODFLOW-OWHM. — Fig. 4. **Components of the MODFLOW family, a key USGS Hydrological Modeling Software, contributing to MODFLOW-OWHM.**

1.3. MODPATH Observation Process

The MODPATH Observation Process (MODPATH-OBS) is a program developed to calculate simulated equivalents to transport observations derived from particle tracking. It helps analyze and interpret hydrological settings and can be used for transient and steady-state flow simulations. The simulated equivalents supported by MODPATH-OBS are (1) distance from a source location at a defined time or proximity to an observed location; (2) time of travel from an initial location to defined locations, areas, or volumes of the simulated system; (3) concentrations used to simulate groundwater age; and (4) percentages of water derived from contributing source areas. This program can also account for simple decay or production, but it cannot account for diffusion (Hanson et al. 2013).

Schematic of particle tracking in MODPATH-OBS, showing observed contaminant plumes on the model grid — **Fig. 5**. MODPATH-OBS, a particle tracking model and key USGS Hydrological Modeling Software.

1.4. Cascade Routing Tool (CRT)

The U.S. Geological Survey Cascade Routing Tool (CRT) is a powerful computer program used to compute flow paths among Hydrologic Response Units (HRUs) and Groundwater Reservoirs (GWRs), defined for watershed simulation that includes the Precipitation-Runoff Modeling System (PRMS). The CRT provides a valuable output that defines cascading surface and shallow subsurface flow paths for grid-based model domains. CRT also includes an option to condition the grid-scale Digital Elevation Model (DEM) to fill unintended swales and to provide continuous down-sloping HRUs that follow streams.

CRT requires a land-surface elevation for each HRU of the model grid. Additionally, a list of the HRUs is required, which must include streams, swales, lakes, and other cascade termination features, along with indices that uniquely define each of these features. Cascades are predefined flow paths that can be used for routing surface and shallow subsurface lateral flow in hydrologic models. Using a cascade module can allow for a more realistic representation of the effects of topography and topology on watershed drainage relative to a simulation without cascades. CRT is used externally to the watershed model so that the computed cascades can be evaluated before a simulation to make sure that water is routed appropriately (Henson et al. 2013).

Diagram of cascading flow: water movement through HRUs and streams in both topography and grid models. — Fig. 6. Grid-based and topography-based HRUs for defining cascading flow paths in hydrologic models.

1.5. INFIL

INFIL is a grid-based, distributed-parameter, deterministic water-balance model that calculates the temporal and spatial distribution of daily net infiltration of water across the lower boundary of the root zone. The primary climatic inputs to the model are daily precipitation and maximum and minimum air temperature. It considers various components of the water balance, such as precipitation; snowfall accumulation, sublimation, and snowmelt; evapotranspiration; drainage and water-content redistribution; surface-water runoff; etc.

Diagram of the hydrologic cycle: snow, precipitation, infiltration, evaporation, and net infiltration. — **Fig. 7.** Diagram of water-balance processes controlling net infiltration across the root zone, as simulated by the INFIL model.

INFIL gives an overview of the water movement across the root zone and can be valuable for understanding water-table aquifers. The INFIL model is based on a conceptual model of the physical processes that control net infiltration across the lower boundary of the root zone. The conceptual model was developed to represent the major components of the water balance for arid to semiarid environments but may also be applicable to more humid regions (FILL, 2008).

2. Other Notable Hydrological Modeling Software

Apart from the USGS-developed models, several other notable hydrological modeling software tools are available in the market. Other hydrologic models that help with water resources comprehension, forecasting, and management include HEC-HMS (flash flood risk assessment) (Mandal and Chakrabarty, 2016); SCS-CN (Soil Conservation Service Curve Number) (Sahu et al., 2023); SWAT (Soil and Water Assessment Tool) (Arnold et al., 2012); MODSIM (a Decision Support System for Integrated River Basin Management) (Berhe et al., 2013); GERM (Glacier Evolution Runoff Mode); HYPE (Hydrological Predictions for the Environment); PREVAH (Precipitation–Runoff–Evapotranspiration HRU Model); and VIC (Variable Infiltration Capacity model) (Horton et al., 2021; Wi et al., 2017).

Hydrological modeling software (SWAT, VIC, GERM) for assessing climate, flood, and drought impacts. — **Fig. 8.** Using global hydrological modeling software to assess climate change, floods, droughts, and cryosphere impacts.

Conclusion

Hydrological modeling software is an essential tool for understanding and managing water resources. With recent advances, these software tools have revolutionized our ability to predict and manage water-related sustainability issues. The USGS has been a significant contributor to the development of hydrological models, including the Basin Characterization Model (BCM) and MODFLOW-OWHM. Hydrological modeling software finds extensive application in various fields related to water resources management, flood forecasting, and environmental impact assessments. While these tools offer numerous benefits, it is important to consider their limitations and validate and calibrate models using observed data. As technology advances, hydrological modeling software is expected to evolve further, incorporating artificial intelligence, machine learning, and socio-economic factors for better accuracy and user-friendly interfaces.

5 USGS Hydrological Modeling Software