Effect of Urbanization on Runoff in the Whiteoak Bayou Watershed in Houston, Texas Francisco Olivera Texas A&M University Department of Civil Engineering Burren DeFee University of Arkansas at Monticello School of Forest Resources AWRA GIS in Water Resources Houston, May 8-10, 2006

Whiteoak Bayou Watershed The watershed is located in northwest Houston in Texas. Area = 223 km 2.

Whiteoak Bayou Watershed Precipitation and flow have increased significantly over time.

Whiteoak Bayou Watershed Cumulative annual runoff.

Whiteoak Bayou Watershed Peak 12-hour precipitation depths and peak flows have significantly increased over time.

Whiteoak Bayou Watershed Cumulative annual peak flow.

Whiteoak Bayou Watershed Parcel data was obtained from the Harris County Appraisal District. Data included: parcel digital map, parcel area, building square footage, number of floors and year of construction.

Whiteoak Bayou Watershed Developed parcels and roads.

Whiteoak Bayou Watershed One point per developed parcel

Whiteoak Bayou Watershed Percent developed area and percent impervious cover

Whiteoak Bayou Watershed Developing new parcels can change the spatial configuration of the developed areas.

Whiteoak Bayou Watershed Edge density and mean shape index.

Whiteoak Bayou Watershed Number and mean area of the developed patches.

Whiteoak Bayou Watershed Observed and predicted annual runoff values. Break point: 1972 – 1973.

Whiteoak Bayou Watershed Annual runoff assuming pre-1972 conditions.

Whiteoak Bayou Watershed Observed and predicted annual peak flows.

Whiteoak Bayou Watershed Peak flows assuming impervious remains lower than 10%.

Whiteoak Bayou Watershed Annual runoff depth for the period 1973 – 2000: Accounting for increasing urbanization level Average: 603 mm. Range: 310 mm – 1030 mm Assuming 1972 urbanization level (i.e., 10% impervious cover) Average: 340 mm. Range: 50 mm – 600 mm Annual runoff increment because of urbanization is 77%. Total annual runoff increment is 146%.

Whiteoak Bayou Watershed Peak flows for the period 1973 – 2000: Accounting for increasing urbanization level Average: 299 m 3 /s. Range: 108 m 3 /s – 711 m 3 /s Assuming 1972 urbanization level (i.e., 10% impervious cover) Average: 226 m 3 /s. Range: 57 m 3 /s – 434 m 3 /s Peak flow increment because of urbanization is 32%. Total peak flow increment is 159%

Whiteoak Bayou Watershed Peak flows tend to increase less than annual runoff depths because a significant part of the additional flow takes place during small events. It was observed that the 12-hour precipitation increased much more over time than the annual precipitation. Urbanization below a critical threshold value appears not to affect annual rainfall depth, after which it increases proportional to the developed area. That threshold value took place in in the Whiteoak Bayou watershed, when the watershed reached a 10% impervious cover. The variation in the number of developed patches can be interpreted as a measure of the connectivity of the impervious cover and of its capacity to convey flow. The number of patches reached a maximum value in 1972.

Whiteoak Bayou Watershed Future work: Apply same methodology to other cities in the world: Portland, Oregon, an example of a “planned” city as opposed to Houston, Texas. Madrid, in collaboration with the Castilla – La Mancha University in Spain. Consider non-linear regressions.