EXTENDED – The American Institute of Hydrology (AIH) is issuing this Request for Proposals (RFP)

Update: We’ve extended the deadline from April 12 to April 19th! 

The American Institute of Hydrology (AIH) is issuing this Request for Proposals (RFP) to invite entities with specialized hydrology experience in hydrology and qualifications related to development of and training to support hydrology-related examination preparation. Responses are due April 19, 2022.

AIH intends to enter a multi-year contract with a single entity to provide certification exam support services. Note that the single contracted entity may enter into agreements with other entities or individuals to propose on services in responses to the RFP and for performance of services.

The American Institute of Hydrology (AIH) was founded in 1981 as a non-profit scientific and educational organization dedicated to the certification and registration of professionals in all fields of hydrology. AIH is the only nationwide organization that offers certification to qualified hydrologic professionals. AIH’s goal is to promote hydrology as a science and profession and to help protect public interest from non-professional practices.

View the Revised RFP

AIH – The Recertification Process

AIH – The Recertification Process

American Institute of Hydrology certified members provide expert knowledge, specialized skills, and adhere to the highest standards and ethics in the field of hydrology. Recertification of  members assures that they have continued to keep updated with current research, standards and practices. This ongoing educational requirement helps protect public interests and the profession from non-professional, sub-standard or unethical practices in a field involving complex water issues. Because of the continually changing nature of the field of hydrology and societal challenges, certified members need to continue their professional development throughout their careers and keep current in their specialty fields by engaging in professional practice, participating in continuing education courses, keeping up with technical literature, and attending professional meetings and seminars. Professional Development Hour (PDH), or Professional Development Credit (PDC), is defined as one contact hour of instruction, presentation, or study towards the goal of staying current in the field of practice. Specific competency requirements are determined by the Executive Committee on a periodic basis.


Certified members are responsible for keeping records of the number of PDH/PDC they earn along with associated documentation that can be validated by AIH. Documentation is due by January 15 of every 5 years, computed from the anniversary year of the individual member, in order to remain in compliance with certification. A minimum of 60 PDH/PDCs is required over 5 years to be eligible for recertification.



Dates & Deadlines

Certemy has a notification system implemented for those in the recertification phase which notifies those who are approaching their application deadline 3 months ahead of time and again one month ahead. Given the recertification process spans over a 5-year period and requires professionals to provide proof of continuing education over this period of time, we understand that the notification process we currently have in place may not have been an appropriate amount of time to obtain the prerequisites for recertification – specifically the professional development credits component. As a result of this, we have restructured our notification process so that, moving forward, our members will receive notifications beginning 6 months ahead of time rather than 3 months. This will then be followed up by a 3-month notification with an additional final notification 1 month ahead of application due dates. 

AIH is revising the recertification process to make it easier for members to upload their recertification data. Implementation of this process is underway but not yet completed. This will be completed within the next 90 days. With that said, the Executive Committee recently voted on extending the recertification deadline an additional 6 months for any of our AIH professionals that are currently in the recertification phase. It is our hope that this will allow time to obtain the proper material needed to recertify. 

Certified members whose recertification is due in 2022 now have until July 15, 2022 to complete the process. 


Membership Dues

Annual membership dues are required every year, regardless of if you are in a recertification phase or not, in order to continue practicing under your earned credential.


Recertification Fees

Every 5 years, certified members must provide proof of eligibility to recertify, along with recertification fees. These fees are separate from your annual membership dues. 

The points above regarding dates & deadlines, membership dues, and recertification fees are what we hope is captured as a main takeaway from our members reading this message. We greatly value each of our members and we thank you for your continued support of AIH!

Awards Nominations

The American Institute of Hydrology (AIH) recognizes individuals for outstanding accomplishments in the fields of groundwater, surface water, water quality, and institute development. These awards are named after prominent scientists and engineers, who have made numerous lasting contributions to the hydrologic sciences. AIH Awards include:

  • Charles V. Theis Award for Groundwater
  • Ray K. Linsley Award for Surface Water
  • Robert G. Wetzel Award for Water Quality
  • Founders Award for Institute Development
Nominees Do Not Need to Be a Member of AIH!

The nomination packet for each award should contain the following:

  1. A formal nomination letter by the nominator, outlining the education, career, and a record of performance detailing accomplishments and noteworthy impact achieved by the potential candidate.
  2. A current resume of the nominee.
  3. A minimum of two and maximum of four supporting letters with a limit of three pages each.

The supporting letter writers should state briefly how they know the nominee and describe why the nominee is deserving of the award. Please submit your nomination to admin@aihydrology.org by Friday, March 25, 2022. Award winners will be notified and the awards will be presented at an upcoming conference to be determined later in the year.

The Charles Vernon (C.V.) Theis Award was established in 1986, to recognize individuals who have made outstanding contributions in groundwater hydrology. Charles V. Theis graduated with a PhD from the University of Cincinnati in June 1929 and made numerous contributions to the field of groundwater science throughout his life.

The Ray K. Linsley Award was established in 1986, to recognize individuals who have made outstanding contributions in surface water hydrology. Dr. Linsley made numerous contributions to surface water hydrology and was the principal author of the textbook Applied Hydrology, published by McGraw-Hill in 1949. He directed the PhD dissertations of 35 students at Stanford University covering research on mechanics of overland flow, rainfall synthesis, stochastic hydrology, and modeling of the hydrologic cycle. These efforts led to the development of the Stanford Watershed Model, a state-of-the-art tool of hydrologists worldwide.

The Robert G. Wetzel Award recognizes individuals who have made outstanding contributions in the field of water quality. This award was dedicated in 2006, to the family of the late Robert G. Wetzel. Dr. Wetzel was a true leader in the field of freshwater science. His thoughts and brilliant synthesis of all aspects of lakes and streams are included in more than 30 books and 400+ publications including the definitive college textbook of this field, Limnology.

The AIH Founders Award was established in 1990, to recognize individuals who have provided outstanding, long and dedicated service to the Institute. It was established to honor the AIH founders Sandor Csallany, Alex Zaporozec, and Roman Kanivetsky. The award is given at the discretion of the AIH Executive Board to a member in good standing.

President’s Message

AIHPresident Jamil Ibrahim

Greetings! The end of the calendar year is a common period for self-reflection by individuals and organizations. As I look back while concluding the first year of my two-year term as your American Institute of Hydrology (AIH) President, I’m delighted about the accomplishments of your AIH leadership team during 2021. Our achievements this year, however, are not just a function of what we did during 2021 – they consider the steps taken by AIH over the past few years to address challenges and introduce new perspectives, along with the great work of your leadership team.

We did great things in 2021! Thank you to all who contributed. Here’s a snapshot of our accomplishments:

  • Boosted AIH’s membership by 22% compared to 2020 and received 35% more new member applications during 2021 compared to the previous year.
  • Held the Institute’s first virtual meet-and-greet event with AIH members.
  • Connected our AIH community through in-person Water New Year celebrations in Minneapolis, Minnesota and Sacramento, California. One of the three co-founders of AIH, Roman Kanivetsky, joined in the Minneapolis event!
  • Welcomed our first Hydrologic Technician (HT) to serve as a member of AIH’s leadership team on our Executive Committee (EC).
  • Completed a widely popular and informative webinar series on the topic of Forecast Informed Reservoir Operations (FIRO). Our FIRO series, provided in collaboration with the American Water Resources Association (AWRA), featured our nation’s top experts on FIRO, an innovative strategy for water resources management that leverages use of advanced hydrological and meteorological data.
  • Convened a new Diversity, Equity, and Inclusion (DEI) Task Force composed of AIH members and members from the EC. Stay tuned – you will have an opportunity to learn more and support the important activities our DEI Task Force will be initiating within the next few months.
  • Revamped our membership application and database system. Despite the glitches our administration team encountered after the rollout, we’re confident our modernization changes will improve ease of application development by prospective members, application review and processing by our Board of Registration, and provide enhanced member benefits through access of information and content.
  • Presented to audiences at professional conferences about AIH and the importance of Professional Hydrologist (PH) certification for hydrologists and HT certification for hydrologic technicians, including: Groundwater Resources Association of California’s (GRAC) Future of Water Conference; Consortium of Universities for the Advancement of Hydrologic Science’s (CUAHSI) Biennial Colloquium; AWRA’s 2021 Annual conference; and the University of Minnesota’s 2021 Minnesota Water Resources Conference.
  • Improved social media presence and member connectivity.
  • Coordinated with researchers for the U.S. Department of Labor on development of information on hydrologist and hydrologic technician occupations.

Also, we are nearly complete with revisions and updates to AIH’s Bylaws and a contracting solicitation for AIH examination support services–both scheduled for distribution early in 2022.

Again – Thank you to all who have stepped forward to take on roles to help advance the mission of AIH. Member participation is vital to AIH’s success and we are eager to engage more members in AIH activities. Even if not interested in taking on a leadership role for AIH or getting involved in various subcommittees or groups, we request all our members to be ambassadors for AIH and its certified members. Please contact me or others on our leadership team to learn more about how to get involved.


Jamil S. Ibrahim PH, PMP, ENV SP

AIH President, 2021-2022

Former AIH President Delivers a Keynote Address at the Latin American Congress of Civil Engineering Students

Former AIH President Delivers a Keynote Address at the Latin American Congress of Civil Engineering Students

From November 14-20, 2021, over 120 civil engineering students from nine Latin American countries attended the COLEIC (Latin American Congress of Civil Engineering Students) 2021 event in Tarija, Bolivia.

Dr. Miguel A. Medina, Jr., PH (AIH President, 2009-2010) delivered a keynote presentation on November 17th titled “Water Resources of Bolivia, Integration of Hydrologic Cycle Processes, Climate Change Challenges, Adaptation and Professional Opportunities.” That same day he was invited to the Municipality of Tarija and presented with an ordinance declaring him a Distinguished Visitor. While at the Tarija City Hall, he was interviewed by a local journalist regarding his findings of potential climate change in Tarija.


The following morning a national Bolivian TV (UNITEL) news crew interviewed him live at the Hotel Vendimia lobby on the topic of Bolivian water resources and management, as well as potential climate change implications. The broadcast was also presented again that evening on a national news segment.

On November 18th, Medina also participated in a forum discussion on climate change implications for the civil engineering and hydrology professions.

For his presentation, Medina examined the annual maxima series of precipitation at three recording stations in the region: Tarija (1944-2020), Juntas (11976-2020) and Cañas (1977-2020).

Nonstationary time series analysis with the Generalized Extreme Value Distribution (GEV) was applied to the three stations.

Both Tarija and Juntas exhibited a slight decrease in precipitation. However, at a higher elevation over a mountain ridge, Cañas exhibited a substantial increase in precipitation.

Both deterministic and statistical methods were discussed, as well as surface-subsurface interactions across the hydrologic cycle. The presentation ended with recommendations for infrastructure adaptation to climate change, increased monitoring needs, and a review of current standards. Specific recommendations followed for future training of civil engineers and hydrologists to enhance their professional opportunities.

Institute Development Recap

Institute Development Recap

This past fall, the American Institute of Hydrology participated in two hydrology related conferences: 2021 Minnesota Water Resources Conference (MWRC) on October 19, 2021 and the American Water Resources Association (AWRA) on November 10, 2021.

Minnesota Water Resources Conference
Minnesota Water Resources Conference

At the MWRC conference, Salam Murtada (VP of Institute Development), Brennon Schaefer (VP of Communication) and John Nieber (past president of AIH) co-authored a presentation titled “American Institute of Hydrology: Certifying the Practice of Hydrology”. The presentation provided an overview of AIH, history, current membership composition, certification process and on-going activities. It was attended by over 100 people. The conference, which was held virtually this year, was attended by over 800 water resources professionals representing students, university teachers, consultants and government officials at the federal, state and local levels. The MWRC is held annually around the middle of October in St. Paul, Minnesota.

At the AWRA conference, AIH participated at the gold level sponsorship. During the reception, Salam Murtada gave a brief presentation about AIH followed by award announcements for three outstanding hydrologists: Vicki Kretsinger Grabert who was awarded the Charles V. Theis Award for contributions to groundwater, Dr. Phil Bedient who was awarded the Ray K. Linsley award for contributions to surface water and Dr. Ying Ouyang who was awarded the Robert G. Wetzel award for contributions to water quality. The AWRA conference was also held virtually this year due to the pandemic.

Vicki Kretsinger Grabert, PH     Dr. Phil BedientDr. Ying Ouyang, PH
Charles V. Theis Award for Contributions to GroundwaterRay K. Linsley Award for Contributions to Surface WaterRobert G. Wetzel Award for Contributions to Water Quality

We hope to continue collaborating with MWRC, AWRA and other organizations such as the Consortium of Universities for the Advancement of Hydrologic Science, Inc (CUAHSI), Groundwater Resources Association (GRA) and the American Geoscience Institute (AGI) among many others.

Membership Renewals

Membership Renewals

Please be reminded that your membership with AIH will expire on January 15, 2022. We encourage you to renew your dues now. Renewal of dues should be completed through the Certemy platform.

We hope you will take the time to renew your membership and remain part of our community. AIH’s dedication to the organization and commitment to the hydrology industry is unprecedented. You want to continue to be a part!

We value all contributions to AIH, as we recognize that without committed and involved members, AIH would not be where it is today. Your involvement is important and very much appreciated.

AIH offers numerous reasons to continue to stay involved:

  • Continuous membership is required to maintain current status as a certified member.
  • AIH offers access to its online membership directory in a member portal environment.
  • Receive peer, employer, industry, and client recognition of technical expertise in hydrology.
  • Hydrologist certification satisfies requirements for jobs in many states.
  • Access to National Registry of Certified Professional Hydrologists network.
  • Discount on AIH publications and events.

We would like to thank the entire American Institute of Hydrology (AIH) organization for its involvement and support this past year. We accomplished a lot!

If your certification type has changed since you last renewed, or you find yourself having issues accessing your Certemy profile, please contact the AIH office at admin@aihydrology.org or call (916) 231-2149.

Spotlight on Recent AIH Exams and on New Members

Spotlight on Recent AIH Exams and on New Members

Congratulations to those who recently passed their exams during November 2021 and all new members welcomed during the second half of 2021!


  • Dillon Vogt, HIT

Professional Hydrologist

  • Megan Arpino, PH (surface water)
  • William Chatron, PH (surface water)
  • Justin Coffman, PH (surface water)
  • Rheannon Hart, PH (surface water)
  • Matt Sparacino, PH (surface water)
  • Max Strickler, PH (surface water)
  • Kaveh Zamani, PH (surface water)

Book Review by Donald Anderson, PH, Cresta Hydrologics, LLC

By David Owen, Riverhead Books, 2017

The Colorado River is often referred to as “America’s hardest-working river.” As it winds through some of the driest land in the country, it supplies water for more than 40 million people and 5.7 million acres of irrigated lands in seven western states, plus a small slice of Mexico. It eventually dribbles into the dry soils of Baja California, Mexico before reaching its natural historic outlet – the Gulf of California.

The aforementioned population and irrigated acreage values should be considered rather provisional, given the ongoing drought that is diminishing the basin’s water supplies. The Bureau of Reclamation cites a 34% probability that water levels in the massive Lake Powell Reservoir, which stores runoff from the 108,000-square-mile “upper basin” and regulates that water for “lower basin” use, will drop too low to generate hydropower in 2023. This has never happened before Thus, Reclamation implemented emergency measures this year to move 181,000 acre-feet of water down to Lake Powell from its higher-elevation reservoirs in an unprecedented scramble to prop up reservoir storage.

How did America’s hardest-working river come to this sorry state? Where the Water Goes, by David Owen, takes the reader on a journey to understand the convoluted story of how and why the Colorado River can no longer provide what we’ve asked of it in the past.  Consider that  when Owen’s book was published just four years ago, Lake Powell still held 7.4 million more acre-feet of water than today!

Owen’s account is aimed at a broad audience. His writing is light on numbers but heavy on stories illuminating the people, places, and activities associated with the river and its myriad users. He winds his way down the river system in a rental car, from the Colorado headwaters through Utah, Arizona, Nevada and California. We meet families irrigating vineyards in Colorado’s Grand Valley. We meet water managers in Denver and Las Vegas. We meet environmentalists seeking to re-hydrate desiccated wetlands in Mexico’s Colorado River Delta. We tag along with farmers in California’s Imperial Valley growing lettuce for America’s dinner tables, and forage for a global cattle market.

Through these stories, Owen (a staff writer for New Yorker) effectively describes the various hopes, expectations, regulations and stresses that have shaped management of Colorado River flows since the first 14-mile canal was hand-dug in 1890 to move water out of the basin’s headwaters to thirsty agricultural lands on the far side of the Rocky Mountains. His journey is a travelogue as much as a treatise on the Colorado River. This may annoy readers who might prefer more hydrology and fewer human-interest digressions. Nevertheless, through intriguing discourse, Owen skillfully portrays  the convoluted history of the Colorado River – a waterway that has produced, over time, the maddeningly complex body of laws and regulations governing its management.

Importantly, Owen also illustrates how this byzantine body of regulations frequently stands in the way of implementing effective long-term solutions to the problem of over-allocation. As Owen puts it, “If you picked just about any high school civics class in the country and gave its students a year to gather information and think, they could almost certainly come up with an approach to western water use that would be more rational than the arcane patchwork we have currently.”

But, as Owen notes, that’s not going to happen. Instead, those responsible for managing the Colorado will need to collectively determine a path forward through this tangled administrative puzzle by applying creativity, collaboration, and — inevitably — large sums of cash needed to overhaul the status quo.

Owen helpfully reviews various possible paths forward in his final chapter “What is to be Done?” Various proposals — ranging from cloud-seeding to reducing water allocations to importing water from the Great Lakes — are individually examined, highlighting each of their weaknesses. The clear take-home message is that there are no simple solutions. But, also, there is little time to waste. The river is grossly over-allocated, climate change will almost certainly aggravate supply shortages, and managers simply don’t have all the tools they need today to resolve the challenges they’ve inherited.

With Lake Powell now depleted to its lowest level since first filling in the late Sixties, and with the first mandatory water-use cutbacks already impacting water users in Arizona (thereare more to come), we’ve essentially run out of time. Where the Water Goes will help readers retrace the steps that brought us to this eleventh-hour dilemma.

About the Reviewer

Prior to his recent retirement from federal service, Donald Anderson served as the Instream Flow Coordinator for the Upper Colorado River Endangered Fish Recovery Program, a public-private partnership that recovers endangered native fish populations while water development continues in the upper Colorado River basin. Mr. Anderson now provides his part-time consulting services through Cresta Hydrologics, LLC, based in Denver, Colorado.

Comparing the Results of HEC-RAS and DHM for Two Dimensional Diffusion Wave

By Theodore .V. Hromadka II (1) and Prasada Rao (2)

(1) Professor, Department of Engineering-Mathematics, United States Military Academy, West Point, NY.

(2) Professor, Department of Civil and Environmental Engineering, California State University, Fullerton, CA.


For simulating flows where inertial forces dominate over frictional forces, as in many types of floods, the solution of the two dimensional diffusion wave equation will suffice (compared to a one dimensional simulation). The associated numerical models are reliable and computationally efficient, thus not warranting the amount of resources that are required for solving the full momentum equations. In this work, the reliability of the USGS Diffusion Hydrodynamic Model (DHM) is cross checked by comparing its results with the more widely used HEC-RAS model. The close agreement between the two sets of computational results underscores the reliability of the legacy DHM.

Key words: Two dimensional models, Diffusive wave, HEC-RAS, DHM


Numerical models developed in the 1980’s and earlier, during which period, computer memory and speed were significant modeling constraints, are increasingly called “legacy” models. The mathematical underpinnings in these models coupled with their reliability has made their solution to serve as a benchmark solution for the modern software. That is, providing another computational opinion to the problem under study using the prior technology. While some legacy models have been altered to accommodate more complex fluid dynamics (by adding modules to simulate flow turbulence, simplifying the data input requirements, and/or enhancing the model output/visualization modules), other legacy models have focused on access to their core algorithms, laying a foundation to newer models. DHM [1] is an example of such a legacy, first generation, hydraulics model developed for the USGS in the mid 1980’s time period, which led to publication of a USGS Technical Report in 1987. The model was written in Fortran 77, and has been extensively applied to different overland, coupled channel, and overbank flow scenarios. The DHM also served as a foundation for other finite-difference algorithms [2] resulting in additional computational programs for solving a variety of transport problems.

DHM solves the two-dimensional overland flow equations coupled with one-dimensional open channel flow equations and includes interfaces between these two flow regimes using source and sink term approximations. It is one of the first general purpose computational solutions to a two dimensional formulation of the Navier-Stokes equations. The model is capable of approximating such hydraulic effects as backwater, drawdown, channel overflow, storage, and ponding, among other hydraulic topics of interest.

The application of legacy hydraulic models, including DHM, for large-scale applications on modern computers is constrained by multiple factors. One is related to recompiling the code to take into account new processors and architecture. Porting the Fortran 77 codes to the new computing platforms and recompiling them can be a challenge and may warrant rewriting some coding statements that are not supported by the new compilers. Changes to the legacy codes may require adding new functionalities. In this case, the DHM code was re-compiled using the Intel visual Fortran compiler. The end windows environment executable file is robust, and the time required to run the model, because of the various optimization modules in the compiler, has been significantly reduced. Another factor is that DHM was written when memory requirements in computers were limited. This limitation translated to using a smaller size of the arrays, and hence its application over large-scale computational domains was not feasible. To address this, the legacy codes can be modified to include the array standards that were introduced to Fortran in the 2000’s timeframe. The size of the dimension array in the DHM was increased from 250 to 9999. Larger arrays are possible as well. Similarly, for codes in which the system of equations is assembled into the matrix form Ax=B, the solver phase can incur a large cost. Matrices are often so large that the standard numerical methods become unsatisfactory and cannot be implemented on even high-performance computers. In a typical code, the largest portion of CPU time is spent in solving systems of linear algebraic equations [4]. It is noted that for an applied researcher, a solution module merely represents a means to an end of solving the flow equations, while for a “solver” developer, the application is a source of sparse equations to be solved. In such cases, using an appropriate solver coupled with a pre-conditioner can make the legacy codes more computationally efficient.

In this work, the results from DHM are compared with those of HEC-RAS (ver. 5.07) for a flow scenario accompanied by a flood. Our primary goal is to underscore the reliability of DHM.

Overview of the Models

DHM was developed for the USGS in the late 1970’s and is one of the first computational hydraulics programs. It solves the simplified Navier-Stokes equations written in diffusion form. For the uniform grid elements, the integrated finite difference version of the nodal domain integration method is used for solving the equations. Characteristic features of the DHM include (a) computational domain is made of square cells; (b) flow can enter or leave the cell through any of the four interfaces (c) minimum and maximum time step size that can be used in the computation needs to be specified, and (d) flow variables are calculated at the center of the cell. The required input conditions at the center of each cell are bottom elevation, initial water depth, and roughness value; The model uses an integrated finite difference numeric scheme for solving the flow equations. The model’s companion website www.diffusionhydrodynamicmodel.com has the relevant source codes, executable files, and complete documentation along with the results from various case studies of applications.

HEC-RAS allows the user to perform one-dimensional steady flow, one and two-dimensional unsteady flow calculations, sediment transport, and water quality modeling [4]. It gives the option of either solving the two dimensional diffusion wave (default mode) or the full dynamic equations. HEC-RAS and DHM share many similarities in that both models calculate the flow variables at the center of the computational cell with the given input details at the center of the cell (elevation, roughness value, initial depth). HEC-RAS uses an implicit finite volume solution of the flow equations. The model provides the flexibility of generating either uniform square, hexagonal, or adaptive mesh, based on the terrain characteristics. It also offers many other features for solving one and two dimensional steady and unsteady river flow simulations like coupling one and two dimensional models, computational mesh development and refinement tools, geometric and hydraulic input and output tools, multiple boundary options, moving sediment and water quality analysis tools, and other options as discussed in the current version of the HEC-RAS user manual [4].

Case Study Test Problem and Flow Details

In the case under study, a rectangular channel is specified that is 41 miles long and 800ft wide with a longitudinal slope of 2.48%. The computational domain is discretized using uniform square cells of dimensions 160ft x 160ft. There are a total of 9471 cells defined in the test model domain. The two channel “walls” defined by the DHM were modeled by cells with high bottom elevations. The simulation was performed over 73 hours. An initial water depth of 0ft in all the cells was specified. The inflow hydrograph (Figure 1) was specified at the center cell of the upstream end. The inflow has a peak value of 79141 cfs at time = 36 hours and a flow of 16500 cfs at time = 72 hours. A critical depth boundary condition was specified along the downstream boundary nodes. Of interest is the cumulative outflow at the downstream boundary.

Computational Results

Figure 1 plots the inflow hydrograph and the two outflow hydrographs for a channel bottom Mannings roughness (n) value of 0.035. The peak outflow and its time of arrival, which are of primary interest in the analysis, are similar for both the configured models. The program’s default values (like tolerance variables, time step, threshold values, etc.) were used, and no effort was made to optimize them.

Figure 1. Inflow and Outflow hydrographs for n = 0.035
Figure 1. Inflow and Outflow hydrographs for n = 0.035

Figure 2 plots the corresponding outflow hydrographs for a channel bottom roughness (n) value of 0.06.

Figure 2. Inflow and Outflow hydrographs for n = 0.06
Figure 2. Inflow and Outflow hydrographs for n = 0.06

The demonstration problems show that the considered two computational models can produce very similar outcomes. This is an important result because the two models differ in their underpinnings. The respective partial differential equations being modeled differ, and the computational efforts also differ, causing a difference between the two modeling outcomes. However, the computational effort needed to use the HEC-RAS model is substantially more than the DHM formulation, which calls into question the use of the HEC-RAS in similar practical problems. It is noted that a recent advance in the DHM is a code that runs on a hand-held calculator.


In this effort, the performance characteristics of two well-known hydraulic models, HEC-RAS and DHM are analyzed. The focus was on comparing their respective solution of the two dimensional diffusion flow equation. Given that the Diffusive Wave formulation of the flow equations is the HEC-RAS default option, and the Diffusive Wave formulation is also the default flow routing option in the legacy program DHM, a unique opportunity exists to test both computational programs and compare outcomes. The reliability of DHM, being one of the first computational models for solving the simplified overland flow equations, was reinforced by comparing its output with the HEC-RAS output. It is concluded that other computational outcomes from legacy models like DHM can act as a baseline analysis for new computational models and paradigms that continue to evolve.

Author Details

Hromadka & Associates’ Principal and Founder, Theodore Hromadka II, Ph.D., Ph.D., Ph.D., PH, PE, has extensive scientific, engineering, expert witness, and litigation support experience. His frequently referenced scientific contributions to the hydrologic, earth, and atmospheric sciences have been widely published in peer-reviewed scientific literature, including 30 books and more than 500 scientific papers, book chapters, and government reports. His professional engineering experience includes supervision and development of over 1500 engineering studies. He is currently a faculty member at the United States Military Academy at West Point, New York.

Prasada Rao is a Professor in the Civil and Environmental Engineering Department at California State University, Fullerton.



1. Hromadka II TV, Yen CC. A diffusion hydrodynamic model. Water resources investigations report. U.S. geological survey; 1987: 87– 4137, https://pubs.er.usgs.gov/publication/wri874137

2. O’Brien JS, Julien PY, Fullerton WT. Two‐dimensional water flood and mudflow simulation. J Hydraulic Eng ASCE. 1993; 119: 244–261.

3. Eriksson K, Estep D, Hansbo P, and Johnson C. Computational Differential Equations. Cambridge University Press, New York, 1996.

4. Brunner, GW., CEIWR-HEC, US Army Corps of Engineers, HEC-RAS River Analysis System, 2D Modeling User’s Manual, Version 5.07, March 2021. https://www.hec.usace.army.mil/software/hec-ras/