Short name: HEC-ResSim
Long name: Hydrologic Engineering Center Reservoir System Simulation
Model type: Simulation of Reservoir Operations. Single events or continuous; from a single reservoir on a single stream to a multi-reservoir, highly developed, and interconnected system.
Usage: Short to medium range operation of multiple reservoirs; system operations for a common downstream control, including storage balancing. Applied for real-time decision support and planning studies: e.g., evaluating proposed reservoirs in a system, flood risk management, low flow augmentation, water supply, detailed reservoir regulation plan investigations, extreme storm events, etc.
Background: HEC-ResSim is the successor of HEC-5 developed to aid engineers and planners in predicting the behavior of reservoir systems in water management studies, decision-making process that reservoir operators must use to meet operating requirements for flood control, power generation, water supply, and environmental quality downstream of the reservoir. It represents the physical behavior of reservoir systems with a combination of hydraulic computations and hydrologic routing. The system is composed of junctions, routing reaches, diversions, and reservoirs to build a network capable of representing a single reservoir on a single stream or a large-scale, highly developed and interconnected multi-reservoir system. HEC-ResSim models can be integrated with other models to represent the full meteorologic, hydrologic, hydraulic, flood damage assessment for the real-time or planning operation application. The first version was released in 2003. The current version, 4.0, was released in 2025 and includes the major new feature for water quality calculations.
Developer: Hydrologic Engineering Center, U.S. Army Corps of Engineers
Modules: Watershed Setup, Reservoir Network, and Simulation
Channel routing: Lag & K, Modified Puls, Coefficient, Muskingum, Muskingum-Cunge 8-pt, Muskingum-Cunge prismatic, SSARR
Reservoir Routing: Accounting method, surcharge routing, emergency releases
Reservoir Operation: Rule-based, scripted, specified release, prescribed, storage-discharge curve, outlet structures, user overrides
Shortwave radiation: N/A
Longwave radiation: N/A
Precipitation: external variable, not computed by the model
Evapotranspiration: external variable, not computed by the model
Snowmelt: external variable, not computed by the model
Infiltration: Seepage and leakage: external variables, not computed by the model
Surface runoff: external variable, not computed by the model
Baseflow: N/A
Input data: Inflow time-series, stage, physical and operational characteristics of dam and reservoir: storage, elevation, area, volume, evaporation rate, leakage rate, operation location, index level, outlet capacity, control point data.
Integrated Suite of Software Tools: Reservoirs operations can be simulated using HEC-ResSim or as part of an integrated suite of software tools (CWMS – Corps Water Management System and HEC-WAT – Watershed Analysis Tool). Basic data required are physical and operational characteristics of dam and reservoir
Input format: HEC-DSS file format, programming API available. For maps: Shapefile, DEM, DXF, DLG, ASCII TIN, and raster images.
Input time interval: Options range from 1 minute to daily, with automatic linear interpolation to the simulation time interval.
Optimization or Calibration: No Optimization method in current version
Data Assimilation: –
Ensemble: Yes
Uncertainty: Ensemble and Monte Carlo compute types
Simulation Time Interval: 5-, 10-, 15-, 30-Minutes, 1-, 2-, 3, 4-, 6-, 12-hours, and 1-day
Model Output Time-Series: Reservoir storage, elevation, controlled release and uncontrolled spill parameters; Regulated, unregulated and cumulative local flow; Regulated, unregulated and cumulative local stages; Hydropower: generation efficiency, power head, hydraulic losses, energy generated, plant factor; Gate opening.
Time-Series format: HEC-DSS file format, programming API available
Model Output Statistics: Reservoir level, average, minimums, maximums, volumes necessary for showing mass balance, comparisons to observed data
Statistics Format: Summary Statistics/Extensible Markup Language (XML)
Inventory Platform: HEC-RTS, HEC-WAT (Watershed Analysis Tool)
Additional Platform: –
Installation: Easy – Automatic installation package
User Education: BSc
Degree of Difficulty: 3
GIS support: Can import shapefiles, including reservoirs, stream centerline, etc., but model is not georeferenced and and mapping is primarily for visual purposes.
Data Preparation: External with HEC-DSSVue
Parameter Estimation: None at this time
Model Calibration: Editors (to change parameter values)
Model Verification: Nash-Sutcliffe, percent bias, RMSE standard deviation
Hardware Requirements: Pentium III (or equivalent) processor or higher with more than 4GB of RAM is recommended to improve compute times and data visualization performance.
Operating System: Microsoft Windows 10+, Linux
Language of Core Code: Java
Open Source: No
Last Update and Version: September 2025 (Version 4.0)
Next Update and Version: planned Feb 2026 (Version 4.1)
Active Development Community: Yes
Platform Integration: optionally HEC-RTS, HEC-WAT (Watershed Analysis Tool)
Download URL: https://www.hec.usace.army.mil/software/hec-ressim/downloads.aspx
Free to Download and Use: Yes
Language of Software Interface: English
Online Support URL: https://www.hec.usace.army.mil/software/support_policy.aspx
Training Material URL (including example data sets): https://www.hec.usace.army.mil/confluence/ressimtr
Language of Trainings: English
Guidance Material URL (including case studies and benchmarking of performance/speed): https://www.hec.usace.army.mil/software/hec-ressim/documentation.aspx
Language of Guidance: English
References: Klipsch, D.J., Evans, T.A., 2009. Reservoir operations modeling with HEC-ResSim, Hydrologic Engineering Center. US Army Corps of Engineers Davis, CA 530–756.
Google Scholar currently lists over 900 journal articles, proceedings, reports, manuals, and other documents.
Owner, Developer:
US Army Corps of Engineers
https://www.hec.usace.army.mil
Mailing Address:
Department of The Army
Corps of Engineers
Institute for Water Resources
Hydrologic Engineering Center
609 Second Street
Davis, CA 95616-4687
HEC Phone: +1 530.756.1104
HEC Fax: +1 530.756.8250