Hydrological modeling plays a crucial role in assessing and predicting the behavior of natural and engineered waterways. In the United States, there are two widely used models for hydrological analysis are HEC-1 and HEC-HMS. Both models were developed by the United States Army Corps of Engineers Hydrologic Engineering Center (HEC). HEC stopped updating HEC-1 in the late 1990s and created HEC-HMS to replace HEC-1. In fact, it is not even possible to download HEC-1 from the Hydrologic Engineering Center’s website anymore. However, there are still engineers (and not just old ones) who prefer to use HEC-1 to build their hydrologic models. In the following blog post, I will discuss the differences between HEC-HMS and HEC-1. I will also discuss why I think some engineers still prefer HEC-1 to HEC-HMS.
About HEC-1
HEC-1 was created by the United States Army Corps of Engineers Hydrologic Engineering Center to simulate (numerically) a watershed’s response to precipitation. Its primary purpose was to serve as a tool for flood simulations. HEC-1 was created in the late 1960s using FORTRAN. Until 1984, it could only be run on a mainframe computer. Like many older engineering programs, HEC-1 files are a series of cards, which is reminiscent of the days when computers used punched cards.
About HEC-HMS
The Hydrologic Engineering System Hydrologic Modeling System (HEC-HMS) program also simulates rainfall-runoff response. Unlike HEC-1, it is currently being maintained and updated by the United States Army Corps of Engineers. HEC-HMS has been around for over 30 years. The first version of the program included most of the capabilities in HEC-1 with the advantage of including an unlimited number of hydrograph ordinates. The second version of the program was focused on including the capability for continuous simulation. In addition, a feature allowing for soil moisture accounting was added to the program as was the overtopping dam failure simulation option. The third version of the program included enhancements to the graphical user interface (GUI). Finally, the fourth version of HEC-HMS has focused on including additional computational features such as sediment transport and surface erosion.
HEC-HMS vs HEC-1
The following section discusses some differences between HEC-HMS and HEC-1.
Recession Baseflow
The recession baseflow method involves applying a recession constant to specify the rate at which recession flow decreases with time. In HEC-HMS, this parameter is defined as the ratio of the current recession flow rate to the recession flow rate one day earlier. In contrast, the HEC-1 parameter (RTIOR), is defined as the ratio of the current recession flow rate to the flow rate one hour later.
Clark Unit Hydrograph
While both HEC-HMS and HEC-1 include the Clark Unit Hydrograph, the programs differ slightly in how they handle this hydrograph. In both HEC-HMS and HEC-1, the tail of the unit hydrograph is truncated when the volume represented by the unit hydrograph exceeds 99.5% of the unit depth. However, in HEC-HMS, the ordinates are also adjusted to force a volume of exactly 100%. No such adjustments are made in HEC-1.
Kinematic Wave Routing
Testing of the kinematic wave routing method has revealed that an error exists in the way this method is implemented in HEC-1. These calculations have been corrected in HEC-HMS.
Green and Ampt Infiltration
HEC-1 did not switch between ponded (length of time required for the precipitation to saturate the soil such that surface runoff begins) and unponded situations correctly. This problem was corrected in HEC-HMS.
Why do I think some engineers prefer HEC-1 to HEC-HMS?
I have been using HEC-HMS since I started my first job out of school. However, over the years, I have come across several engineers (often very smart ones) who prefer HEC-1 even though it is outdated. I have always found this puzzling. However, after using the program myself, I have a better understanding of the thinking behind the preference for HEC-1.
Model complexity
In my opinion, HEC-1 is simpler to use in terms of model setup and input requirements. This means it is easier to learn and use for beginners in hydrological modeling. Although HEC-1 does not have a graphical user interface (GUI) like HEC-HMS, the input options are relatively straightforward as long as you have the HEC-1 User’s Manual to help you understand what information is required for each card. Overall, it takes less time, and is often easier to develop a HEC-1 model that runs. This is especially true if you are accustomed to HEC-1.
More control
I think many engineers also prefer HEC-1 because they feel like they have more control over the model. In contrast, HEC-HMS can feel like a bit of a “black box.”
Compliance
Some jurisdictions have requirements that make it difficult to develop an HEC-HMS model that complies with their requirements. For example, Maricopa County’s drainage manual seems to prefer HEC-1 over HEC-HMS. Maricopa County has even created a program called the Drainage Design Management System for Windows (DDMSW) to allow engineers and hydrologists to run an HEC-1 model on modern computers.
HEC-1 makes you feel like a programmer
Finally, I believe that many engineers prefer HEC-1 over HEC-HMS because it makes them feel like a programmer. It does seem like running batch files and looking at the text output files makes one feel smarter than looking at the cartoon-ish HEC-HMS screen. But it could just be me.
Final Thoughts
While I understand why people are fans of HEC-1, I do believe it is important to avoid using older programs as a crutch. Civil engineers are busy, and it can be difficult to find the time to learn new skills when it seems like there is an important deadline looming every Friday. However, I believe it is vital that everyone learn the newest programs in their field. While you may not be a “fan,” it is likely that a day will come when you are forced to “convert.” That process will be much more difficult if you must compete with other candidates with more current/relevant skills during a job search or learn a new program while trying to use it for a high-stakes project. Fortunately, HEC has many resources for those who are looking to learn more about the programs they offer. I recommend checking out their website and YouTube if you want to get started learning HEC-HMS.