Management Practices to Decrease Runoff in Urban Environments

By: Kristen Wickert, Water Resources Staff Forester, Texas A&M Forest Service - Houston

Figure 1: The Preserve on North Loop, Houston

Many posts throughout this blog emphasize the importance of the forest-water relationship in the rural forests of Texas. However, that does not mean that the forest-water relationship does not exist in the urban environment. Managing urban forests can be just as complex as managing rural forests. Specialized factors such as: increased population, impervious surfaces, and more elaborate pollution contributors, make managing urban forests a difficult task.

This is why the Texas A&M Forest Service is studying the effects of vegetation cover in urban environments on water quality and quantity. Our urban foresters in the Houston office performed a survey of an office complex that is known in the area for incorporating vegetation and larger developed trees in their building plan. This complex is called The Preserve. Built in 1971, The Preserve is located directly in the highly developed concrete jungle of Houston, on the 610 North Loop. Buildings were constructed around already established trees and replanting occurred in the open spaces between. Most of the trees are now well established and are larger than six inches in diameter.

Using a public domain software created by the Northern Research Station of the USDA, called i-tree Eco, the Texas A&M Forest Service was able to input sampled data from The Preserve property and generate economic values of the many benefits of the urban forest. A summary report of the benefits provided by the vegetation include: pollution removal, carbon storage, oxygen production, runoff reduction, building energy savings, avoided carbon emissions, and structural values.

Since this is a water resources blog, we will focus on the reduced runoff from having an urban forest incorporated in a building plan. There are three main factors that contribute to runoff reduction from vegetation: Canopy interception of rainfall, water infiltration promotion to the soil by the root system, and duff accumulation. The portion of the precipitation that reaches the ground and does not infiltrate into the soil becomes surface runoff, which costs tax money to clean in waste water treatment plants.

The i-tree Eco software calculates annual reduced surface runoff based on rainfall interception by vegetation, specifically focusing on the difference between annual runoff with or without vegetation. Although tree leaves, branches, and bark may intercept precipitation and thus mitigate surface runoff, only the precipitation intercepted by leaves is accounted for in the i-tree Eco analysis. The software bases the value of reduced runoff on the U.S. Forest Service's Community Tree Guide Series.

An excerpt from the report describes the amount of runoff reduced due to the projected 1,350 trees in the 12 acres of The Preserve at an estimated 47,200 cubic feet a year with an associated value of $3.14 thousand.” In actuality, this number is higher due to the fact that only leaf interception is considered, root infiltration is not considered, and the duff accumulation is absent from the report.

Figure 2: Protective duff covering the soil

The Preserve is a special office complex, because it is managed mainly without the use of lawn mowers once or twice a week; unlike most office complexes in urban environments. This enables the office complex grounds to actually mimic a functioning forest floor by accumulating duff. Duff is leaf litter and course woody debris that accumulates on the ground to form a protective shield against rain events and temperature changes. The duff layer of pine needles, leaves, and small course woody debris slows the infiltration rate of water into the soil, while still catching and holding the water, increasing absorption, and keeping moisture in the soil longer. This reduces the amount of water that passes over the saturated bare soils.  Therefore, avoiding runoff into the streets that could cause erosion and flooding.

The lessons we learn from our Forest-Water Best Management Practices have more applications than meets the eye. These valuable lessons are transferable to the urban environment in scales from small to large. The Texas A&M Forest Service is working hard to better the lives of rural and urban Texans by sharing this knowledge.

If you are interested in learning about more ways in which you can do your part in urban areas, please feel free to email me at kwickert@tfs.tamu.edu or to call me at 832-530-6468