A Brief Explanation of "Urbanization and its effect on runoff in the Whiteoak Bayou Watershed, Texas"

Urbanization and its effect on runoff in the Whiteoak Bayou Watershed, Texas.

Francisco Olivera and Buren B. DeFee, 2007

A study was conducted just north of Houston, TX on the Whiteoak Bayou Watershed to determine “the capacity for a watershed to urbanize without losing its hydrologic response and the relationship between that response to the spatial configuration of the developed areas studied.” Wow that’s a mouthful of phrases that Olivera and DeFee try to convey in this paper. Luckily, readers will be privileged to an explanation of Olivera and DeFee’s objectives in this study…

This study included data on the hydrologic responses to urbanization of the watershed from 1949 to 2000. First off, a watershed is a geological area where water from rain, melting snow or ice converges into this basin and feeds into bodies of water such as rivers, streams, lakes, oceans. So, hydrologic response (units) refers to areas within a watershed that react similarly to given inputs to the watershed. In this case, the input is run off from urban and rural areas around the watershed pertinent to this study (see Title). Hydrologic response is also a good analytical tool for determining spatial heterogeneity, uneven terrain distributions, within the watershed.

   

An important factor in Olivera and DeFee’s interrogation of the watershed pertains to land use effects on run off. This is very important since different types of land uses can determine the amount of run off into the watershed. Land development occurs consecutively from natural or wild areas to more urban or farm use areas which highly determines the way water flows across these different land scapes. Naturally, urbanization has its varying consequences to the drainage of a watershed some of which include the increase in runoff, “the increase in event peak flows and a decrease in base flows from subsurface waters.” Again, a mouthful of niche geological watershed phrases…

If you’re a novice watershed analyst, it would behoove you to finish this paragraph. If you’re a savy, well versed watershed geological enthusiast, feel free to skip to the next section or read this section and critique my basic understanding of the terms. First term refers to “event peak flows.” They refer to the annual increases in water flow with large amounts of precipitation.  Second term refers to “base flows from subsurface waters.” This is just the lowest water flows from waters below the surface also referred to as ground water. 

Due to the consequences of urbanization on the drainage of watersheds, Olivera and DeFee acknowledge that sustainable development and land-use planning can be viable ways to alleviate symptoms of urbanization. The use of geographical information systems (GIS) is an exceptional method in determining spatial variability in land use planning. Olivera and DeFee use spatial metrics in a GIS to “describe the broad-scale patterns of the human-developed portion of the landscape.” This spatial configuration in a GIS is very useful because it illuminates the broad changes in landscape over the half-century studied.  *The term, "impervious" refers to structures covered by asphalt, cement, and stone such as: pavement, roads, roofs, etc.

Figure 4. From Top to Bottom, Development State in 1949, 1972,

and 2000. Black refers to developed patches and gray to undeveloped

patches. On average, developed patches are 40% impervious

Following a substantial amount of data analysis and regression models, Olivera and DeFee came to several conclusions pertaining to the effects of urbanization on runoff of the Whiteoak Bayou Watershed that drains into the rivers and streams and subsurface waters of the greater Houston area, from 1949-2000.

1       Annual runoff depths and annual peak flow depths depended on the annual precipitation depth. (See Figure 3, below).

2      Olivera and DeFee found that since 1970, annual runoff increased by 146% and 159%. Urbanization is responsible for 77% and 32% of the increase and precipitation is responsible for the remaining 39% and 96%.

3       Prior to 1970, the watershed did not significantly feel the effects of urbanization and therefore had no modification of its hydrologic response. Afterwards, the effects of runoff started kicking in once the watershed reached it’s capacity, which according to the EPA is when the watershed reaches an impervious area of 10%. Olivera and DeFee infer that this is due to land development changes that increased the watershed’s conveyance capacity and therefore the change in its hydrologic response.

4       Olivera and DeFee also conclude that the information provided by their study should not be generalized and more research needs to be published on the subject.