USGS Says One Third of Harris County Now Impervious Cover

Did you know that one-third of the area in Harris County now has impervious cover? That Montgomery County had a 57.12% net increase of impervious surface area between 2001 and 2019? Or that 10% of land cover in the Lower 48 states changed during that same period? I discovered these and a multitude of other fascinating facts in a recently updated United States Geological Survey (USGS) website dedicated to monitoring changes in land cover, for example, from forested to developed.

When you live in an area for a long time, it’s easy to forget what happened two decades ago. And when you move to a new area, you just accept what is and don’t worry about what was.

But USGS gives you a quick and easy way to see and quantify changes in land use down to the county level. It’s useful in telling you where flood threats could develop over time and how fast they are developing.

About the USGS National Land Cover Database

USGS recently released updated land cover maps for the lower 48 United States. They show how the country’s landscapes have changed over an 18 year period in two- to three-year increments. It’s called the United States National Land Cover Database (NLCD). And it’s the fastest way to see how your county is changing.

Updates include 2001, 2004, 2006, 2008, 2011, 2013, 2016, and 2019.

Developed using Landsat imagery, NLCD classifies land cover into 16 groups with 30-meter resolution. The data includes both land-cover and urban imperviousness changes.

USGS claims 91 percent accuracy for the NLCD data. For more detail about how NLCD was developed see: Changes to the National Land Cover Database. More than nine billion pixels make up the land-cover dataset.

The USGS National Land Cover Database’s suite of GIS mapping products even includes a layer that defines the intensity of impervious surfaces across the United States. This information is used in runoff modeling, urban heat estimation, and a variety of other applications.

Mapping Land Cover Change in U.S. Over Time

Users can visualize land cover changes in the United States by accessing the the Enhanced Visualization and Analysis (EVA) tool. The online mapping tool was developed by USGS in partnership with the National Oceanic and Atmospheric Administration.

The tool allows users to select any county in the Lower-48 United States and generate a custom report on land cover change, developed areas, cropland change, and other factors.

Only one caution: the USGS site does not work with Apple’s Safari Browser. Mac users can use Firefox without problems, however. I have not tested other Mac browsers.

I ran two quick searches on Harris and Montgomery Counties by going to the EVA tool mentioned above. The findings astonished me.

Harris County Changes At a Glance

Between 2001 and 2019, in Harris County:

  • Almost one fifth of the land cover changed type (18.23%).
  • Developed portions of the county increased from 54.42% to 65.85% of the total acreage, a 20.99% percent net increase of developed area.
  • Forested parts of the county went from 10.64% to 6.29%, a percent net decrease of 40.92%.
  • The percent covered in wetlands went down from 8.28% to 7.02%, another percent net decrease of 15.24%.
  • The percentage of impervious surface increased from about a quarter to a third (26.28% to 33.39%), a percent net increase of 27.05%.
Screen showing development changes in Harris County with corresponding percentages of impervious cover. Green dots represent changes in land use. Clicking on icons in left column brings up different types of information.

MoCo Changes at a Glance

During the same period, in Montgomery County:

  • Even more land cover changed type (18.99%).
  • Developed portions of the county increased from 21.1% of the land area to 28.27%, a 33.97% net increase.
  • Impervious cover increased from 5.78% off the land area to 9.08%, a 57.12% increase.
  • Forested land decreased from 42.98% of the county to 38.96%, a 9.16% net decrease.
  • Wetlands decreased from 12.17% of the county to 11.35%, a 6.74% net decrease.
  • Agricultural land decreased from 12.28% to 10.31% of the county, a 16.04% net decrease.
Red areas represent areas in Montgomery County that changed land-cover type between 2001 and 2019.
Another screen showing areas in Montgomery County developed between 2001 and 2019.

Key Lesson

This database and GIS mapping system dramatize how quickly the region is growing and land use is changing.

Flood mitigation is or should be a two-pronged effort. We must fix problems that already exist downstream while hopefully preventing future problems from developing upstream. It’s not a just question of one county spending money to help prevent problems in another. It’s about surrounding counties protecting themselves. The outward expansion is relentless. People at the edge today will be downstream from someone else tomorrow.

There’s little anyone can do to change the FACT of development. But we can change the NATURE of development. If all new developments retained their own rain, no one would ever be doomed to the flood-mitigation treadmill of keeping up with ever-increasing amounts of upstream runoff.

Montgomery County already has a serious flooding problem of its own. Thousands of people flooded there during Harvey and Imelda.

Posted by Bob Rehak on 8/5/2021 based on USGS information

1437 Days since Hurricane Harvey

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.