What’s a HUC? Understanding hydrologic units

Story County is taking the forward-looking step of assessing all its HUC12 watersheds so it has solid information for managing its water resources.  Prairie Rivers of Iowa is excited to be part of the project: our team is busy mapping potential conservation practices using the Agricultural Conservation Planning Framework.  That’s great, you might say, but what’s a HUC?

HUC12 watersheds in Story County.  Can you locate your watershed?

We’re used to thinking of nested levels when it comes to our home addresses.  You live in a city or township within a county within a state within a nation.

Jackson Township, Boone County, Iowa, USA

In the same way, you have a watershed address.  Your land probably drains to a creek that drains to a river that drains to a larger river.  The further downstream you go, the larger the watershed.

Onion Creek watershed, Ioway Creek watershed, South Skunk River watershed, Mississippi River basin

The US Geologic Survey’s Watershed Boundary Dataset is a good way of representing this watershed address.  The entire United States is divided up into a nested six-level hierarchy of drainage areas.  At each level, two digits are tacked on to the Hydrologic Unit Code (HUC).  When we talk about a “huck twelve” (HUC12), we mean a small hydrologic unit with a twelve-digit code, on the order of 10-40 thousand acres.

Having a unique code and uniform coverage is essential for computer-based work—so there’s no confusion about which “Clear Creek” we are talking about—so you’ll see hydrologic units used as an organizing framework for a number of exciting mapping tools.  These include the Agricultural Conservation Planning Framework (which maps good sites for practices like grassed waterways and saturated buffers), the Iowa BMP Mapping Project (which maps some existing conservation practices), the CarbonScapes Atlas (which shows the climate benefits of healthy soils), and the Iowa Daily Erosion Project (used to estimate topsoil loss for any day of the year).

However, when using this framework, there’s caveat you need to keep in mind: not all hydrologic units are watersheds.  James Omernik et al. pointed out the issue this month in a new paper in the Journal of Environmental Management:

The problem is that it is impossible to map watersheds, basins, or catchments of relatively equal size and cover the whole country. The hydrologic unit framework is in fact composed mostly of watersheds and pieces of watersheds. The pieces include units that drain to segments of streams, remnant areas, noncontributing areas, and coastal or frontal units that can include multiple watersheds draining to an ocean or large lake. Hence, half or more of the hydrologic units are not watersheds as the name of the framework “Watershed Boundary Dataset” implies. Nonetheless, hydrologic units and watersheds are commonly treated as synonymous, and this misapplication and misunderstanding can have some serious scientific and management consequences.

We encountered this issue with our current watershed planning project. About 210,622 acres drain into the South Skunk above Ames.  However, that watershed is too big for a HUC10, so the section that includes Keigley Branch was separated from two headwater areas near Ellsworth and Jewell.  33,385 acres drain to Keigley Branch, but that watershed is too big for a HUC12 so it was split between the headwaters and the lower section.  Rain that falls on Little Wall Lake and its tiny watershed probably stays in the lake until it evaporates, but those acres were lumped into a HUC12 with land that drains to the South Skunk above Story City, the “Miller Creek-South Skunk River Watershed”.

*Not a true watershed

After successfully working with the Ioway Creek Watershed Management Authority to write a management plan and get funding for education and conservation practices in the Ioway Creek Watershed, tackling another HUC10 was a logical next step.  We hope to add additional watershed projects in the future to further improve water quality in the South Skunk River.  However, the hyphenated names make it confusing to talk about.  Bear with us.

So what’s a HUC, again?  HUC stands for the Hydrologic Unit Code of a watershed (or part of a watershed) in the national Watershed Boundary Dataset.  The number of digits in the code indicates the scale of the hydrologic unit.  A ten-digit hydrologic unit (HUC10) is a bit smaller than a county, a twelve-digit hydrologic unit (HUC12) is a bit smaller than a township.  Add digits to specify a smaller watershed within a larger river basin.  Think of it as the zip code for your watershed address.

-Dan Haug, Watershed Educator, July 19