Sunday, January 24, 2016

The Effect Urbanization Has On Watershed Runoff

A study conducted by Francisco Olivero and Buren B. DeFee in 2007 sought to understand how to most effectively urbanize a watershed area without dramatically increasing runoff and harming the local ecosystem. They wanted to study this so they could find a way to develop areas while being sustainable, and to see how much the watershed could take before runoff flooded the land. Therefore, the Harris County Appraisal District, creators of the study, turned their attentions to the Whiteoak Bayou Watershed in Texas, and studied its runoff level from 1949 to 2000.

To gather data, researchers first measured daily precipitation for the duration of the study, and averaged the daily amount of rainfall for all five decades it spanned. They found a statistically significant change in precipitation. They then measured development by gathering city and flooring plans for the 52-year period. They also visited the locations in person and took measurements of their areas to avoid error. The scientists then compared the precipitation data with the development plans, and synthesized both datasets into GIS applications (the Landscape Ecology Program) to create 52 individual maps of runoff data, which were then combined into a raster format.

This map shows development and "impervious" areas over the years (1949, 1972, 2000), areas which the researchers considered important to changing the amount of water runoff in the watershed. Using the maps as data, they used an algorithm to calculate the increase of runoff.

Ultimately, the researchers found that before 1970, urban development did not affect runoff levels as much as natural precipitation did. However, after 1970, the landscape was altered sufficiently that runoff depths and peak flows increased by 146% and 159%! This posed a flooding risk to the area around it. However, they also stressed that it was important not to blame urbanization for the increase entirely, as precipitation and global warming still had a hand in it.

Olivera, F., & DeFee, B. B. (2007). Urbanization and Its Effect On Runoff in the Whiteoak Bayou Watershed, Texas1.


  1. Do you think that after 1970, when the landscape was extremely different, that it's mostly because of urbanization and the increase in land use practices or because of increase in precipitation and global warming caused by pollutants?

    1. I would say that it's a mix of both. Climate change is an issue that is not dependent on only one variable, and the researchers also stated that while they believed urbanization caused up to 77% of the increased runoff, the other 23% was due to increased precipitation.

  2. Very interesting study. It looks like a large proportion of the water shed is urbanized (which has a high proportion of impervious land) which has caused runoff to increase. I wonder if they can prioritize land using GIS to best increase ground infiltration of water.