Value ($): 0.181 per tree (Source: http://cufr.ucdavis.edu/products/cufr_188_gtr186a.pdf)
Value ($): 0.12 per tree per year (Source: http://www.fs.fed.us/ne/syracuse/Data/State/data_IL.htm#statesum)
Value ($): 275 per tree (Source: http://www.fs.fed.us/ne/syracuse/Data/State/data_IL.htm#statesum)
Value High Range ($): 632 per tree (Source: Total value for trees in Chicago Region - we use $275 for initail value and add $9.65 per year for 37 years)
While we were able to estimate the total value of trees in the Chicago region, we were not able to value the hydrologic benefits of trees. It is commonly acknowledged that planting trees with a significant canopy on a site will reduce the amount of stormwater falling under the tree, at least initially, and reduce the runoff from the site. While there is little published data on the amount of the hydrologic benefits in the scientific literature, we did find one reference to such data in Stormwater Magazine.1
The article discusses a study done for Garland, Texas. The study “determined that increased tree cover could save even more. For example, a medium-size, 3.86-ac. residential site, with its 8 percent canopy cover, provided a 3 percent runoff reduction. If the site’s tree canopy were increased to 35 percent, runoff reduction would quadruple, to 12.8 percent; a canopy cover of 45 percent would bring that number to 16.1 percent. (American Forests, a national nonprofit organization, recommends that cities maintain a 40 percent tree cover.) The study, however, did not recommend how a site with such home density would accommodate the resultant roots that would accompany the increased tree canopy.
Using these figures, with the runoff reduction being roughly proportional to the canopy coverage, we can conclude that a 25 percent increase in cover (one of our green infrastructure options) would result in a reduction in annual runoff of 10.9 percent. This same reduction corresponds to a decrease in the green lot curve number of approximately 2.33. For a site to have a 10.9 percent runoff reduction, and for the trees to have only a 25 percent canopy, the curve number under the trees must be reduced 4 times 2.33, or by 9.32.
When the additional 25 percent tree coverage is selected, the calculator reduces the average site curve number by 2.33. It is assumed that this approximation applies regardless of the land cover under the trees—be it roof, pavement, lawn, or native vegetation.
CNT will be looking for more rigorous ways to model the hydrologic benefits of trees, and would appreciate user suggestions for improving the calculation of these benefits.
Green infrastructure directly and indirectly reduces commercial and residential energy use. Green roofs provide insulation and reduce the need for building air conditioning and heating. The City of Chicago estimates the energy savings benefits from its green roof at $0.18 per square foot (this value is included in the benefits calculator).2 In the Chicago area, the shade provided by trees also reduces direct energy use at a rate of 5 to 10 percent of heating and cooling costs for a 10 percent increase in tree cover.3 Finally, green infrastructure helps to mitigate the urban heat island effect—by which developed areas have a higher air temperature than less developed areas—and thus indirectly reduce the air conditioning needs of a community.4
Value ($): 86.42 per acre foot infiltrated (Source: http://www.sws.uiuc.edu/pubdoc/RI/ISWSRI-83.pdf)
Value Low Range ($): 39.89 per acre foot infiltrated (Source Low Range: http://www.sws.uiuc.edu/pubdoc/RI/ISWSRI-83.pdf)
Value High Range ($): 300.13 per acre foot infiltrated (Source: http://www.skokie.org/health/environfaqs.html)
In 1965, the Illinois State Water Survey (ISWS) estimated the recharge rates for 109 drainage basins for which sufficient data was available. The general conclusion was that “no simple relation exists between ground-water runoff and recharge rates.”5 They concluded that most recharge occurs during spring months when evapotranspiration is small and soil moisture is maintained at a high level by frequent rains. Recharge is low during summer and fall, except following heavy rains, and may be negligible during a cold winter. Only a small fraction of the annual precipitation percolates downward to the water table. The complexity of estimating recharge over large areas is indicated by the need for a 5-year investigation that is currently underway by the ISWS in Kane County.6
The recharge rates that were calculated in the 1965 report ranged from the equivalent of 0.027 inches of precipitation per year, in areas where there are relatively impermeable layers of rock over the aquifer, to the equivalent of 10 inches of precipitation, where a sand and gravel aquifer is overlain by permeable soils. In northeastern Illinois, the calculated recharge rates ranged from the equivalent of 1.28 to 4.5 inches of precipitation per year; the average for 9 basins in the Chicago region is 2.8 inches.
Estimating the benefits of green infrastructure related to groundwater recharge is, of course, also complex. A simple estimation method has been incorporated into the calculator to provide a very rough approximation of the potential benefits. In the Hydrologic Results, there is a comparison of the lot discharge (of surface water) for a 2-year storm between the green alternative and the conventional alternative. If that percent change is assumed to also relate to the change in annual discharge of surface water, the difference can be assumed to enter the ground instead of running off the surface. This increase in infiltration cannot, however, all be assumed to be recharged to groundwater.
The 1965 ISWS report contains, in Table 15, a comparison of water budget factors for three small basins in central Illinois. The average surface runoff in the basins is 19 percent of the average annual precipitation. The average groundwater recharge is 20 percent of the precipitation. Another 12 percent of the precipitation enters the ground, but is discharged into local streams rather than flowing down into the groundwater. About half of the precipitation is lost as evapotranspiration from the ground surface or vegetation.
So in order to approximate the annual recharge that can be obtained by employing green infrastructure, the following calculations are made:
Value ($): 0.18 Per Sq ft of roof garden per year (Source: http://tinyurl.com/7logo)
There are a wide variety of pollutants that enter waterways, lakes and ponds in the stormwater discharged from developed areas. Green infrastructure contains components that have the capacity to remove some of the pollutants. Two of the common indicators of stormwater quality are total suspended solids (the particles that give water its turbidity and can be removed with a filter) and total phosphorus (the combination of several compounds containing phosphorus that lead to excessive algal growth and oxygen depletion). Studies of these two pollutants have resulted in approximations of their concentrations in stormwater discharges from different types of urban surfaces.7,8
|Concentration of (mg/l)|
|Type of Surface||Total Suspended Solids||Total Phosphorus|
The calculator is designed to utilize the annual stormwater discharges from each of these types of surface for each alternative and calculate the pounds of each pollutant leaving each lot and the neighborhood.
Other research has been done to approximate the effectiveness of different components of the infrastructure in removing these and other pollutants.9,10
|Component||Total Suspended Solids||Total Phosphorus|
|Raingardens (Infiltration trench)||75%||60%|
|Filter Strips (Nat. Veg.)||65%||40%|
|Swales (Nat. Veg.)||65%||40%|
|Detention Basins (Wet Bottom)||60%||20%|
The pollution removal in each of the components of each alternative is calculated to yield a final concentration at the discharge, and an estimation of the annual reduction in quantity of each pollutant.
The removal of these pollutants from stormwater is rarely required, so that costs for their removal are not reported. However, sewage treatment facilities are responsible for removing most solids and, ever more frequently, phosphorus. The cost of removing a pound of suspended solids by filtration at the Washington, DC treatment plant is estimated to be $8.30.11 The cost of removing a pound of phosphorus from a large sewage treatment plant has been estimated as between $6 and $12. The calculator assumes a cost of $9.00. These costs are multiplied by the removals for each pollutant, and are shown as part of the benefits of the green infrastructure.
Value ($): 29.94 per acre feet of runoff reduced
(Source: http://www.mwrdgc.dst.il.us/finance/CAFR2004.pdf and http://www.mwrdgc.dst.il.us/ga/budget/2005%20Final/Section%20I%20-%20Foreward.pdf)
Uncontrolled stormwater runoff can cause erosion of topsoil and stream banks. Stormwater runoff control measures reduce erosion and limit the sediment that flows into waterways. Sediment can impact habitats, reduce navigability, and reduce the water storage capacity of lakes and ponds. Runoff from roads and construction sites led to the dredging of 12.6 million cubic yards of material from U.S. waters in 1995.12
Infrastructure improvements that control stormwater runoff reduce downstream flood damages as well as localized flooding of lawns and basements. Illinois had 7,054 claims of loss and damage due to flooding of households and businesses between 1983 and 2003. The average flooding loss insurance claim in Illinois was $15,450.13
The US Army Corps of Engineers has spent 15 years studying the Des Plaines River to develop strategies for reducing flood damages. In a 1996 report, they recommended 11 flood control structures that reduce damages by storing flood waters.14 Each of the structures was judged at that time to be cost-effective. The cost of all of the measures was estimated to be $73 million and the total storage to be constructed was 9,543 acre-feet. This leads to an average cost of some $10,000 per acre-foot.
The benefits of green infrastructure include reduction of outlet flows because of increased infiltration. These benefits are realized because of decreased needs for conveyance and detention basin capacity. Smaller detention basins could mean less storage would be available during major floods, counteracting the reductions in flows, so that the result would be no net reduction in flood damages downstream. However, the rainfall that causes flooding will continue for several days. Once filled, a detention basin will remain filled for several days, while some proportion of the infiltration capacity expanded by green infrastructure could remain during the duration of the flood. Thus, a modest benefit for flood reduction can probably be assumed for green infrastructure alternatives. For the calculator, this benefit is valued at $1,000 per acre-foot of reduced flow from the total site during the 100-year storm.
[On the detention spreadsheet, the maximum of (lp times td for conventional minus lp times td green).]
Green infrastructure options can provide additional habitat for wildlife and can also diversify existing habitats with native plant species. Green infrastructure may improve species habitat conditions by reducing pollutants such as phosphorous and sediment in streams. Some aquatic life is negatively impacted by the increased water temperature from stormwater flows, which green infrastructure is designed to help reduce.15
Roads have economic benefits, in that they contribute to the transport of goods and people. Roads also provide access to emergency services such as police and fire.16 The benefits of highways and transit systems in the U.S. to individuals and businesses are estimated at $790 billion per year.17 However, the benefits of new roads are much lower in areas that are already well connected than they are in areas without an interconnected transportation network.18 It’s been argued that the U.S. has already exceeded the economically optimal level of automobile travel, so that new roads have a net negative economic impact.19
Green infrastructure adds to the aesthetic value and even property value of a home. Studies have shown that trees increase the value of a home by 2 to 30 percent. After a certain point, however (one study suggests that 30 trees is the maximum) tree density begins to detract from the value of a home.20 The property value of trees is recognized in the calculator with a compensatory value21 of $632 per tree in Illinois.22 Compensatory value is often used in the insurance industry to estimate the cost of replacing a tree.23 Landscaping can add value to a home as well. Hedges and other landscaping can increase a home value by 3 to 12 percent.24
It is widely acknowledged that a home or commercial area near green space or water has a higher value than it would without those features. This increased value is usually measured in terms of sale or rental price. Studies have measured the premium of a house located near water to be 5-28 percent25 or more.26 Wet detention basins that are well designed are more likely to increase property values than dry ponds. Detention basins that are not well maintained may detract from property value.27
Reduced stormwater pollution has public health benefits in terms of reduced illness from eating contaminated seafood and swimming in contaminated waters.28
When a homeowner invests time and money, even modest amounts, in one or more raingardens, there is usually a significant benefit to the family. It may be that the garden is pleasing to the eye. It may satisfy a desire to help with environmental quality or to attract insects or birds. Or, most often, it is constructed to reduce the potential for damaging or messy flooding of a basement or a sensitive part of the landscape. These values may be difficult even for the family to define, and they are not likely to be documented by local agencies.
This leads to a dilemma. The calculator, without a benefit to balance the homeowner's investment, shows a negative life cycle cost for what might be the most cost-effective component of green infrastructure. In order to correct this anomaly, while being open about the subjectivity we employed, CNT developed the following rationale for quantifying the benefits of raingardens.
Reduced stormwater nuisances are estimated by assuming that, once a year, a resident avoids the necessity of cleaning up a mess caused by stormwater. This cleanup takes an average of 8 hours to clean up, and the resident’s time is worth $10 per hour. This labor value is specified in some programs where volunteer labor is used to match grant funds. Thus, the benefits of raingardens, in terms of reduced stormwater nuisances, are estimated to be $80 per year and are received by the homeowners. We assume this benefit regardless of the size and cost of the raingardens.
Green infrastructure provides recreation benefits from canoeing and fishing in local streams, lakes and ponds to bird watching as habitat increases and diversifies. Green infrastructure can also improve the water quality of regional waterways, making them safe for boating, swimming, and fishing.29
Reducing paved areas and using porous pavement can reduce the need for salt to de-ice sidewalks, driveways, and roads. Reduced salt use saves money—sidewalk salt can cost $0.05 per square foot treated;30 it also has habitat benefits, as salt can damage plants and waterways.31
The roof is one of the largest cost components in the green infrastructure model, but the benefits of having a warm, dry building outweigh the cost of installing and maintaining a roof.
The insulation that a green roof provides can have the added benefit of reducing noise in a home or business.32 Trees and shrubs may have sound absorptive properties as well.33