U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2015–5188

Prepared in cooperation with Douglas County Environmental Services and the Nebraska 

Environmental Trust

Water Balance Monitoring for Two Bioretention Gardens 

in Omaha, Nebraska, 2011–14
Front cover. Eastern Nebraska Office on Aging bioretention garden (photograph taken by Kent 

Holm, Douglas County Environmental Services, June 26, 2012. 

Back cover. Top, Douglas County Health Center bioretention garden 1 (photograph taken by David 

Rus, U.S. Geological Survey, June 17, 2011. Middle, Douglas County Health Center bioretention 

garden 1 (photograph taken by Kent Holm, Douglas County Environmental Services, August 3, 2011. 

Bottom, Eastern Nebraska Office on Aging bioretention garden (photograph taken by Kent Holm, 

Douglas County Environmental Services, June 17, 2011.
Water Balance Monitoring for Two 

Bioretention Gardens in Omaha, Nebraska, 

2011–14

By Kellan R. Strauch, David L. Rus, and Kent E. Holm

Prepared in cooperation with Douglas County Environmental Services and the 

Nebraska Environmental Trust

Scientific Investigations Report 2015–5188

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Director

U.S. Geological Survey, Reston, Virginia: 2016

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Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Strauch, K.R., Rus, D.L., Holm, K.E., 2016, Water balance monitoring for two bioretention gardens in Omaha, 

Nebraska, 2011–14, U.S. Geological Survey Scientific Investigation Report 2015–5188, 19 p., http://dx.doi.

org/10.3133/sir20155188.

ISSN 2328-0328 (online)
iii

Acknowledgments

The authors also thank the Douglas County Health Center and the Eastern Nebraska Office 

on Aging for granting access to their properties for this study.

v

Contents

Acknowledgments........................................................................................................................................iii

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Purpose and Scope ..............................................................................................................................2

Study Area..............................................................................................................................................2

Douglas County Health Center Bioretention Garden.............................................................2

Eastern Nebraska Office on Aging Bioretention Garden......................................................2

Methods..................................................................................................................................................6

Precipitation..................................................................................................................................6

Stormwater Inflow Volume.........................................................................................................9

Overflow ........................................................................................................................................9

Evapotranspiration ....................................................................................................................11

Subsurface Storage ..................................................................................................................11

Rainfall Event Duration Determination...................................................................................12

Water Balance Monitoring.........................................................................................................................13

Douglas County Health Center Site .................................................................................................13

Performance on an Event Basis..............................................................................................13

Monitoring Period Summary....................................................................................................15

Eastern Nebraska Office on Aging Site ..........................................................................................15

Summary........................................................................................................................................................17

References Cited..........................................................................................................................................18

Figures

1. Map showing location of bioretention garden study sites in Omaha, Nebraska ..............3

2. Bioretention garden at the Douglas County Health Center site, Omaha, Nebraska .........4

3. Bioretention garden at the Eastern Nebraska Office on Aging site, Omaha, 

Nebraska........................................................................................................................................7

4. A slotted grate affects overflow estimates at the Douglas County Health Center

bioretention garden....................................................................................................................10

5. Graphs showing performance measures at the bioretention garden at the Douglas

County Health Center site..........................................................................................................14

6. Graph showing relation between peak inflow rate and peak outflow rate for

measured events at the Douglas County Health Center bioretention garden..................15

7. Graph showing annual water balance components for measured events during

2011–14, normalized to the bioretention garden area at the Douglas County Health

Center site....................................................................................................................................15

8. Graph showing annual water balance components for all events during 2011–14,

normalized to the bioretention garden area at the Eastern Nebraska Office on

Aging site .....................................................................................................................................16
vi

Tables

1. Summary of plant species and calculated landscape coefficient for the

bioretention gardens at the Douglas County Health Center and Eastern Nebraska

Office on Aging sites ..................................................................................................................12

2. Monitoring period summary of water balance components for measured events

during 2011–14 for the bioretention garden at the Douglas County Health Center 

site ................................................................................................................................................16

3. Monitoring period summary of water balance components for all events during

2011–14 for the bioretention garden for the Eastern Nebraska Office on Aging site......17

Conversion Factors

Inch/Pound to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2)

square foot (ft2) 0.09290 square meter (m2)

Volume

cubic foot (ft3) 28.32 cubic decimeter (dm3) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

Flow rate

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

inch per hour (in/h) 0.0254 meter per hour (m/h)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as °F = (1.8 × 

°C) + 32.

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as °C = (°F – 

32) / 1.8.

Datum

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).
Water Balance Monitoring for Two Bioretention Gardens 

in Omaha, Nebraska, 2011–14

By Kellan R. Strauch,1 David L. Rus,1 and Kent E. Holm2

1

U.S. Geological Survey

2

Douglas County Environmental Services

Abstract

Bioretention gardens are used to help mitigate stormwater 

runoff in urban settings in an attempt to restore the hydrologic 

response of the developed land to a natural predevelopment 

response in which more water is infiltrated rather than routed 

directly to urban drainage networks. To better understand the 

performance of bioretention gardens in facilitating infiltration of 

stormwater in eastern Nebraska, the U.S. Geological Survey, in 

cooperation with the Douglas County Environmental Services 

and the Nebraska Environmental Trust, assessed the water balance of two bioretention gardens located in Omaha, Nebraska 

by monitoring the amount of stormwater entering and leaving the gardens. One garden is on the Douglas County Health 

Center campus, and the other garden is on the property of the 

Eastern Nebraska Office on Aging.

For the Douglas County Health Center, bioretention garden 

performance was evaluated on the basis of volume reduction 

by comparing total inflow volume to total outflow volume. The 

bioretention garden reduced inflow volumes from a minimum of 

33 percent to 100 percent (a complete reduction in inflow volume) depending on the size of the event. Although variable, the 

percent reduction of the inflow volume tended to decrease with 

increasing total event rainfall. To assess how well the garden 

reduces stormwater peak inflow rates, peak inflows were plotted 

against peak outflows measured at the bioretention garden. Only 

39 of the 255 events had any overflow, indicating 100 percent 

peak reduction in the other events. Of those 39 events having 

overflow, the mean peak reduction was 63 percent.

No overflow events were recorded at the bioretention 

garden at the Eastern Nebraska Office on Aging; therefore, data 

were not available for an event-based overflow analysis. Monitoring period summary of the water balance at both bioretention 

gardens indicates that most of the stormwater in the bioretention 

gardens is stored in the subsurface. 

Evapotranspiration was attributed to a small percentage 

of the outputs on an annual basis (3 percent at Douglas County 

Health Center site and 5 percent at Eastern Nebraska Office on 

Aging site), which indicates that vegetative water uptake is not a 

primary factor in the water budget.

Introduction

Urbanization can radically alter the flow regime of a 

stream system by creating impermeable areas that favor 

stormwater runoff instead of infiltration and by constructing 

drainage networks that quickly transport the stormwater out 

of the urban area. When coupled with the pollutants typically 

associated with urbanization, this disturbance to the flow 

regime typically degrades the quality of the urban stream 

(National Research Council, 2009). Areas that are drained by 

combined sewer systems—where stormwater sewers (drainage networks that drain surface water from impervious land) 

and sanitary sewers (drainage networks in which sewage from 

houses and commercial buildings are collected and transported to be treated) are combined in one pipe and can be 

released untreated into streams during rainfall events—present a greater challenge to stormwater management because of 

the additional human-health threats associated with sanitary 

sewage from combined sewer systems. In response, stormwater best management practices are used to reduce stormwater 

effects. A stormwater best management practice is a “technique, process, activity, or structure to reduce the pollutant 

content of stormwater discharge, and could be implemented 

singly or in tandem to maximize effectiveness” (Fletcher and 

others, 2015).

The bioretention garden is one type of stormwater best 

management practice. In a typical bioretention garden design, 

stormwater is directed from impervious surfaces into a vegetated basin with an underdrain (Water Environment Federation, 2012). The soils are usually amended to promote plant 

growth and enhance evapotranspiration processes and nutrient 

uptake. The underdrain is lined by a variety of porous media 

and generally functions as a filter for the stormwater. The 

purpose of the bioretention gardens is to attenuate and delay 

peak flows and to reduce the total volume of stormwater runoff into the combined storm sewer system. Natural processes 

are used to infiltrate, filter, store, evaporate, and detain runoff 

similar to how the water would have been processed in a more 

natural setting. In the case of combined sewer systems such as 

in Omaha, Nebraska, bioretention gardens have the potential 

to decrease the frequency and magnitude of instances when 

untreated sewage is released into receiving waters.
2 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

Although bioretention gardens have been used for several 

years in the United States, questions exist regarding their 

effectiveness locally. To better understand the performance of 

bioretention gardens in eastern Nebraska, the U.S. Geological 

Survey, in cooperation with Douglas County Environmental 

Services and the Nebraska Environmental Trust, assessed the 

water balance of two bioretention gardens by monitoring the 

amount of stormwater entering and leaving the gardens during 

2011‒14.

Purpose and Scope

The purpose of this report is to present the results of 

water balance monitoring for two bioretention gardens in 

Omaha, Nebraska. Water balance data were collected at the 

two bioretention gardens from May through October in 2011 

and from April through October in 2012, 2013, and 2014.

Study Area

In Omaha, Nebraska, two bioretention gardens at two different locations were constructed to receive stormwater from 

adjacent buildings and parking lots (fig. 1). The two bioretention gardens are in the central part of the city, approximately 

0.5 mile apart. One garden is on the east side of the Douglas 

County Health Center (DCHC) campus approximately two 

blocks south of the intersection between 40th and Pacific 

Streets, and the other garden is on the Eastern Nebraska 

Office on Aging (ENOA) property at the southwest corner 

of the intersection between 42nd and Center Streets (fig. 1). 

The mean annual temperature in Omaha during 1981‒2010 

was 51.1 degrees Fahrenheit with seasonal means ranging 

from 85.2 degrees Fahrenheit in the summer to 16.1 degrees 

Fahrenheit in the winter (National Oceanic and Atmospheric 

Administration, 2015a). The mean annual precipitation during 

1981‒2010 was 30.6 inches (in.) with most of the precipitation 

falling in the spring, summer, and autumn.

Both bioretention gardens were constructed in soils 

consisting of firm to stiff clay fill material that overlie a firm 

clay of low plasticity (Peorian Loess; Edward M. Schmackenberg, Olsson Associates, written commun., 2010). Each garden 

has an underdrain in the bottom of the bioretention garden 

and drains into the combined sewer system. The underdrain 

consists of a 4-in. diameter perforated pipe that is surrounded 

by coarse gravel and wrapped in a geotextile filter fabric (Paul 

Woodward, Olsson Associates, written commun., 2010). The 

gravel-wrapped pipe rests in a 2- by 2-foot (ft) infiltration 

trench that is filled with an even mixture of fine sand and 

compost. The remainder of the bioretention cell was covered 

with 6 in. of compost that was tilled into the native soil. The 

rain gardens were constructed in July 2010 and planting was 

finished in August 2010.

A feature of the underdrain is the inclusion of an adjustable valve that allows for the controlled drainage of the 

bioretention garden. The valve is adjusted to maximize the 

detention capacity of the bioretention garden and still preserve 

the ability to drain the garden in 24‒48 hours (City of Omaha, 

2014). To simplify the measurements required for the water 

balance equation, the underdrain valves remained closed 

for the duration of the monitoring period. Each bioretention 

garden has an overflow standpipe that is connected to the 

storm sewer and is set at a height to ensure that the bioretention garden’s berms are not overtopped, which corresponds to 

0.39 and 1.66 ft above the land surface of the DCHC and the 

ENOA bioretention gardens, respectively.

Douglas County Health Center Bioretention 

Garden

The DCHC bioretention garden consists of two gardens 

joined together in series (fig. 2A, 2B, 2C). Stormwater runoff 

is directed into an inflow channel and then enters garden 1. 

If the ponding depth in garden 1 exceeds the height of an 

intermediate pipe connecting the two gardens (corresponding to a height of 0.39 ft above the surface of the bioretention 

garden 1), the excess water flows into garden 2. If the ponding 

depth in garden 2 exceeds the height of the overflow standpipe, the excess water flows to the combined sewer. Unfortunately, an undocumented drainpipe at the bottom of garden 2 

was uncovered near the end of the monitoring period (fig. 2C). 

The drain was originally covered by soil but became exposed 

during the course of the study; therefore, an unmeasured volume of water drained from garden 2 to the storm drain. As a 

result, the water balance was computed on flows into and outputs from bioretention garden 1 only; garden 2 was excluded 

from all computations.

Bioretention garden 1 occupies a surface area of about 

2,100 square feet (ft2) and has a surface storage volume of 

700 cubic feet (ft3), corresponding to the pool storage before 

the garden overflows. The DCHC bioretention garden receives 

stormwater from 1.3 acres (56,600 ft2); therefore, garden 1 

provides 538 ft3 of storage per acre of drainage area. The 

drainage area is almost completely impervious and consists 

of the adjacent building rooftop and parking lot (fig. 2A). The 

runoff from the rooftop is collected into downspouts that direct 

the flow to the parking lot to the east and is then redistributed 

into sheet flow on the parking lot. The stormwater continues 

flowing east on the parking lot to the curb that separates the 

bioretention garden from the parking lot (fig. 2B). Once the 

flow reaches the curb, the flow is channeled into the bioretention garden at the inlet and routed through the flume into the 

bioretention garden.

Eastern Nebraska Office on Aging Bioretention 

Garden

The ENOA bioretention garden initially had a drainage 

area of about 0.48 acre; however, after the initial 2 years of 

monitoring (2011–12), modifications were made at the bioretention garden that increased the drainage area to 0.78 acre 
Introduction 

3

EXPLANATION

& Monitoring site locations 0 1 2 0.5 Miles

0 1 2 0.5 Kilometers

&

&

-95°58' -96°

41°15'41°14'

Douglas County Health Center Bioretention Garden

Eastern Nebraska Office on Aging Bioretention Garden

0 1 2 MILES 0.5

0 1 2 KILOMETERS 0.5

EXPLANATION

& Study site location

Base from Esri digital data, 2014

Figure 1. Location of bioretention garden study sites in Omaha, Nebraska.
4 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

Base from Esri, 2014

#

#

#

|

EXPLANATION

# Transducer

| Weather station

UnderdrainDrainage area boundary for  bioretention garden 1

0 50 100 FEET

0 50 100 METERS

N

A

Inflow 

channel 

and flume

Garden 2

Garden 1

Figure 2. Bioretention garden at the Douglas County Health Center site, Omaha, Nebraska. A, drainage area and bioretention garden 1 layout; 

B, photograph showing 

bioretention garden 1 and monitoring equipment as viewed from the southwest corner of the bioretention garden on September 1, 2014; and 

C, cross-sectional sketch of 

bioretention gardens 1 and 2.
Introduction 5

Parshall flume 

and transducer

Weather

station

Transducer

B

Vertical scale greatly exaggerated

Tranducers

Mulch

Native soil

Inflow

Bioretention

garden 1

Bioretention

garden 2

Infiltration trench (perforated pipe 

wrapped in coarse gravel and a 

geotextile membrane laid in a mixture of 

equal parts of fine sand and compost) 

Infiltration trench (perforated pipe 

wrapped in coarse gravel and a 

geotextile membrane laid in a mixture of 

equal parts of fine sand and compost) 

0.5-foot topsoil amendment

Overflow standpipe

Intermediate pipe that 

connects bioretention 

garden 1 and 2

Undocumented

drainpipe

Combined

sewer

Valve

0.39 foot 0.7 foot

1.2 feet

1.5–2.3 feet

C

Figure 2. Bioretention garden at the Douglas County Health Center site, Omaha, Nebraska. A, drainage area and 

bioretention garden 1 layout; B, photograph showing bioretention garden 1 and monitoring equipment as viewed from the 

southwest corner of the bioretention garden on September 1, 2014; and C, cross-sectional sketch of bioretention gardens 1 

and 2.—Continued
6 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

(fig. 3A, 3B, 3C) and increased the size of the parking lot 

drainage grates. The drainage area is almost completely 

impervious and consists of a part of the ENOA building’s 

rooftop, surrounding parking lot, and an area of greenspace 

to the southeast of the building (fig. 3A). During construction 

of the bioretention garden, the rooftop downspouts associated 

with the western part of the ENOA building were disconnected 

from the storm sewer and routed to the bioretention garden’s 

inflow channel. Unfortunately, the storm sewer connection to 

the downspouts associated with the eastern part of the roof 

could not be located. As a result, stormwater from the eastern part of the rooftop continues to drain to the storm sewer, 

and the corresponding area of the roof is not considered to 

contribute flow to the bioretention garden as shown as the 

noncontributing drainage area in figure 3A. The runoff flow for 

the parking lot is collected into three lateral parking lot drain 

grates (fig. 3A) and is then routed by pipe to the bioretention 

garden inflow channel. The ENOA bioretention garden has a 

surface area of about 2,900 ft2 and a surface storage volume 

of 3,500 ft3, which corresponds to the pool storage before the 

garden overflows; overflows are routed through a valved pipe 

to the combined sewer (fig. 3C). The ENOA site bioretention garden initially provided 7,300 ft3

 of storage per acre and 

4,500 ft3 of storage per acre after modifications were made at 

the site.

Methods

The general monitoring design corresponded to a water 

balance approach to characterize the inputs, outputs, and 

change in storage within the bioretention gardens. The water 

balance equation that was used in the monitoring period analysis can be represented numerically as follows:

V V P S + = W O V V F U + + F E V V T S + +VSS (1)

where VP is the volume of precipitation that falls 

directly into the bioretention garden, VSW is the volume of stormwater inflow to the 

bioretention garden, VOF is the volume of outflow from the bioretention 

garden by way of the overflow standpipe, VUF is the volume of outflow from the bioretention 

garden by way of the underdrain, VET is the volume of evapotranspiration from the 

bioretention garden, VS is the change in volume of storage of ponded 

water within the bioretention garden, and VSS is the change in volume of storage in the 

subsurface within the bioretention garden 

(this includes any losses to infiltration, 

such as water that leaves the bottom of the 

rain garden and moves to the water table or 

out of the system horizontally).

The components of the water balance that were measured 

or calculated were VP, VSW, VOF, and VET. Component VUF was 

assumed to be zero because the valves on the underdrains 

were closed at both bioretention gardens for the duration of 

the monitoring period. Although component VS could be estimated from water-level measurements within the bioretention 

gardens, VS was neglected from the water balance analyses 

because the depth of ponding at the end of the monitoring 

period was zero; therefore, change in storage of ponded water 

was always zero. The remaining component in equation 1, VSS, 

was estimated using equation 1. Therefore, for the analysis in 

this report, the water balance inputs are VP and VSW, and the 

outputs are VOF, VET , and VSS.

For the event-based analysis, only the components VSW, 

VP, and VOF were used. Although component VS was able to be 

estimated from water-level measurements within the bioretention gardens, VS

 was neglected from the event-based analyses 

because time periods were selected to represent rainfall events 

such that the change in storage of ponded water was always 

zero. Components VET and VSS were not used in the event-based 

analysis because this analysis was used to characterize the 

reduction in stormwater volume and peak discharges to the 

storm sewer system primarily during time periods of overflow, 

and these components take place over longer time periods.

All of the components of the water balance were measured every 15 minutes during nonflow periods; however, 

VSW, VOF, and VS were measured every minute when flow was 

measureable or when water depths were higher than 0.03 ft 

in each sensor (ponding). Collected data were stored using 

electronic data loggers and transmitted in near real time to the 

U.S. Geological Survey National Water Information System 

Web site (U.S. Geological Survey, 2015). All data collected 

for this study are available on the National Water Information 

System Web site (http://waterdata.usgs.gov/nwis/sw) using site 

number 411450095582201 for the DCHC site and site number 

411429095583801 for the ENOA site. The monitoring methods associated with each of the water balance components are 

described in greater detail in the following sections.

Precipitation

Precipitation data were collected and summed in 15-minute intervals using a tipping bucket rain gage during the monitoring periods at both locations. The data from the rain gages 

were cross-checked with other nearby precipitation data to 

verify accuracy (National Oceanic and Atmospheric Administration, 2015b). Rainfall intensity information presented in the 

report was calculated using the 15-minute rainfall totals and 

scaling them to hourly rates of inches per hour. Precipitation 

data are presented in units of volume (for example, cubic feet) 

by multiplying the lengths (summed rainfall totals in inches 

converted to feet) by the surface area of the bioretention garden (in square feet).
Introduction 

7

Base from Esri, 2014

#

|

#

EXPLANATION

#Transducer

| Weather station

Drainage area boundary added 2013Noncontributing drainage area boundaryDrainage area boundary 2012

Inflow pipeLateral drain gratesUnderdrain

0 20 40 FEET

0 20 40 METERS

Inflow channel and flume

A

N

Figure 3. Bioretention garden at the Eastern Nebraska Office on Aging site, Omaha, Nebraska. A, drainage area and bioretention garden layout; B, photo showing bioretention garden and monitoring equipment as viewed from east of the bioretention garden on September 1, 2014; and C, cross-sectional sketch of the bioretention garden.
8 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

Transducer

Weather

station

Parshall flume

and transducer

B

Native soil

Vertical scale greatly exaggerated

Valved pipe

0.5-foot topsoil 

amendment

1.66 feet

2.1 feet

1.5–2.3 feet

Tranducer

Mulch

Inflow

Overflow 

standpipe

Combined

sewer

Infiltration trench (perforated pipe 

wrapped in coarse gravel and a geotextile 

membrane laid in a mixture of equal parts 

of fine sand and compost) 

C

Figure 3. Bioretention garden at the Eastern Nebraska Office on Aging site, Omaha, Nebraska. A, drainage area and 

bioretention garden layout; B, photo showing bioretention garden and monitoring equipment as viewed from east of the 

bioretention garden on September 1, 2014; and C, cross-sectional sketch of the bioretention garden.—Continued
Introduction 9

Stormwater Inflow Volume

Stormwater inflow volume to the bioretention gardens 

was measured using a Parshall flume with an attached stilling 

well. For both bioretention gardens, the stormwater runoff 

was directed into a channel and routed through a Parshall 

flume with a 9-in. contraction before entering the bioretention 

gardens. Vented submersible pressure transducers were used 

to measure the water level in the stilling well using methods 

described in Sauer and Turnipseed (2010). The water level was 

used in the Parshall flume equation to determine stormwater 

inflow (Bureau of Reclamation, 2001): 

Q Ch SW a

n = (2)

where

QSW is the stormwater inflow, in cubic feet per 

second;

C is a size-specific coefficient of discharge, or 

3.07 for the 9-inch flume contraction;

ha is the measured water level in the stilling 

well, in feet; and

n is a size-specific exponent, or 1.53 for the 

9-inch flume contraction.

A stormwater inflow volume, VSW, was obtained by multiplying each QSW measurement by the corresponding measurement interval and summing those individual volumes within a 

given rainfall event. To display the data in units of length (for 

example, feet), the volumes (for example, cubic feet) were 

divided by the surface area (square feet) of the bioretention 

garden. The result of this calculation was multiplied by 12 to 

display the data in inches. 

The design of the DCHC bioretention garden provided 

little free board between the bottom of the inflow channel 

and the maximum ponding depth in garden 1. Ponding depths 

greater than 0.7 ft above the bottom of garden 1 inundated 

the Parshall flume (fig. 2C) and invalidated the flow relation above those depths. Consequently, the larger magnitude 

rainfall events that produced ponding depths exceeding 0.7 ft 

had to be omitted from the analyses of the DCHC bioretention 

garden.

Overflow

Each of the bioretention gardens had overflow standpipes 

that allowed for outflow to the storm sewer system when 

the water level exceeded the height of the standpipes. At the 

DCHC bioretention garden, the overflow height corresponded 

to 0.39 ft above the land surface of garden 1 (fig. 2C). At the 

ENOA bioretention garden, the overflow height corresponded 

to 1.66 ft above the land surface (fig. 3C). Water depths in 

each of the bioretention gardens were measured using vented 

submersible pressure transducers (Sauer and Turnipseed, 

2010) installed in the bottom of the gardens. During the monitoring period, water depths did not exceed 1.66 ft at the ENOA 

bioretention garden. In contrast, water depths at the DCHC 

bioretention garden frequently exceeded 0.39 ft. Slotted grates 

placed on top of the standpipes (fig. 4A) prevented debris from 

entering the storm sewer but also complicated the estimate of 

overflow through the standpipes for the DCHC bioretention 

garden. Johnson and Chang (1984) characterized flow through 

grates using a combination of the weir spillway equation 

(equation 3) and the orifice flow equation (equation 4) as 

follows: 

Q Ph OF,weir

. = 3 0. 1 5

(3)

where

QOF,weir is the overflow estimated from the weir 

spillway equation, in cubic feet per second; 

3.0 is the coefficient of discharge;

P is the outside perimeter of the slotted area 

section of the grate, in feet (3.3 feet 

for the Douglas County Health Center 

bioretention garden); and 

h is the water depth above the overflow grate, in 

feet. 

Q A gh OF,orifice = 0 6. 7 2 (4)

where

QOF,orfice is the overflow estimated from equation 3 

(orifice flow), in cubic feet per second;

A is the total slot area of the grate (0.32 square 

foot for the Douglas County Health Center 

bioretention garden);

g is the gravitational constant (32.2 feet per 

second squared); and

h is the water depth above the overflow grate, in 

feet.

A relation between overflow (QOF) and depth above the 

overflow standpipe was developed using equations 3 and 4 

(fig. 4B). At low values of h, overflow is governed by the 

hydraulic conditions of weir flow. As the water depth increases 

into intermediate values of h, overflow transitions to orifice 

flow. In this intermediate range of h, a vortex appears over the 

grate, and both of the equations overestimate flow; however, 

accounting for this transitional overestimation is not well 

defined in the literature. Discussions with the manufacturer of 

the grates led to the following approach: (1) the intermediate 

range was defined as those values of h where the weir and orifice equations (equations 3 and 4) produced overflow estimates 

within 10 percent of one another, and (2) the overflow within 

the intermediate range was considered to be 90 percent of the 

mean values calculated from equations 3 and 4. Based on the 

geometry of the DCHC grate, this intermediate range was at h

values between 0.16 and 0.19 ft. At h values greater than the 

intermediate range, overflow is governed by orifice flow. 
10 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

A

Photograph by David Rus, U.S. Geological Survey, 2011

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Overflow discharge, in cubic feet per second

Depth of water over standpipe grate, h, in feet

Overflow estimated from weir equation, QOF, weir

Overflow estimated from orifice equation, QOF, orfice

Overflow estimated using combined equations, QOF

EXPLANATION

B

Figure 4. A slotted grate 

affects overflow estimates at 

the Douglas County Health 

Center bioretention garden. 

A, photograph of a slotted 

grate covering the overflow 

standpipe and B, the relation 

of overflow to water depth 

above the standpipe at the 

garden.
Introduction 11

An assumption for this approach to estimating overflow 

was that the grate over the standpipe was never clogged (partially or entirely) by debris; however, in reality, this assumption was not always met. Debris was periodically cleaned from 

the grate, but the level of clogging and frequency of cleaning 

were not documented; therefore, the computation of overflow 

may overestimate its true value. Similar to stormwater inflow, 

the outflow volume by way of the overflow standpipe, VOF, 

was obtained by multiplying each QOF measurement by the 

corresponding measurement interval and by summing those 

individual volumes within a given rainfall event. To display 

the data in units of length (for example, feet), the volumes (for 

example, cubic feet) were divided by the surface area (square 

feet) of the bioretention garden. The result of this calculation 

was multiplied by 12 to display the data in inches.

Evapotranspiration 

To determine the amount of water that was lost to the 

atmosphere, evapotranspiration (ET) was calculated at each 

bioretention garden by scaling the reference evapotranspiration (ET0

) by a landscape coefficient. Measurements of net 

radiation, air temperature, relative humidity, and wind speed 

were used to calculate daily ET0

 using the Penman-Monteith 

equation (Monteith and Unsworth, 1990; Allen and others, 

1998). Reference evapotranspiration represents ET, in inches, 

over a well-watered grass of uniform height that completely 

shades the ground (Allen and others, 1998). To adjust the 

ET0

 to represent the actual ET of the bioretention gardens, a 

weighted landscape coefficient (KL) was used (Costello and 

others, 2000). The landscape coefficient modifies the commonly used crop coefficient approach of Allen and others 

(1998) by accounting for species type, density, and microclimate using the following equation: 

K K L s = × K K d m × c (5)

where

KL is the landscape coefficient,

Ks is the species factor,

Kd is the density factor, and

Kmc is the microclimate factor.

Costello and others (2000) developed the landscape coefficient method for California landscapes and provide ranges 

of appropriate values for species type, density, and microclimate. The climatic and soil differences between California 

and Nebraska introduce the potential for error that is primarily 

manifested in the species factor. The water-use characteristics 

of a particular species may differ between the two geographical regions. However, the landscape coeffiecient method was 

considered superior to the standard crop coefficient approach 

because crop coefficients do not exist for the plants within the 

bioretention garden. In addition, KL values have been used to 

estimate ET of rain gardens in Wisconsin (Selbig and Balster, 

2010). For the two Omaha bioretention gardens, the median 

values for the species type factor were assigned to each plant 

species present in the bioretention gardens. Because the 

bioretention gardens were only constructed a year before the 

monitoring period, the plantings were not fully established, 

and an increase in plant density during the monitoring period 

was not indicated. As a result, a low plant density factor of 

0.7 was assigned to each bioretention garden. A microclimate 

factor value of 1.0 was applied to both monitoring locations 

because the reference evapotranspiration stations for both 

sites were located in the gardens and microclimate adjustments would be accounted for in the calculated ET0

. For each 

species, a landscape coefficient was calculated using equation 

5 and then weighted based on the percentage of the area each 

plant species occupied that was determined by planting areas 

in the design plans (Paul Woodward, Olsson Associates, written commun., 2010; table 1). The weighted KL values for the 

DCHC and ENOA bioretention gardens were 0.25 and 0.30, 

respectively. The landscape coefficient was then multiplied by 

ET0

 to determine the actual ET at each bioretention garden. 

To display the data in in units of volume (for example, cubic 

feet), the actual ET (inches) was divided by 12 to convert to 

feet and then multiplied by the surface area of the bioretention 

garden (square feet).

Subsurface Storage

Subsurface storage, VSS, was calculated using the water 

balance equation. Because the other components were either 

measured or were zero, subsurface storage was estimated as 

the residual by rearranging equation 1 as follows:

V V SS = + P S V V W O − − F E V T (6)

where

VSS is the volume of subsurface storage in the soil 

system from the bioretention garden, VP is the volume of precipitation that falls 

directly into the bioretention garden, VSW is the volume of stormwater inflow to the 

bioretention garden, VOF is the volume of outflow from the bioretention 

garden by way of the overflow standpipe, 

and

VET is the volume of evapotranspiration from the 

bioretention garden.

To display the data in units of length (for example, feet), the 

volumes (for example, cubic feet) were divided by the surface 

area (square feet) of the bioretention garden. The result of this 

calculation was multiplied by 12 to display the data in inches.

The estimation of subsurface storage as the residual 

of the remaining components of the water balance neglects 

the effects of measurement error. The precision of tipping 

bucket rain gages varied with rainfall intensity; however, the 

precision generally varied by 2.9 percent when measured at 

a 15-minute interval (Ciach, 2002). Parshall flumes produce 
12 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

Table 1. Summary of plant species and calculated landscape coefficient for the bioretention gardens at the Douglas County 

Health Center and Eastern Nebraska Office on Aging sites.

[Ks, species factor; Kd, density factor; Kmc, microclimate factor; KL, landscape coefficient; --, no value]

Plant species Percentage of 

area

Ks Kd Kmc KL

Douglas County Health Center

Sorghastrum nutans 11.1 0.5 0.7 1 0.35

Sedum spp. (some are Hylotelephium) 8.0 0.2 0.7 1 0.14

Schizachyrium scoparium 22.8 0.2 0.7 1 0.14

Rudbeckia spp. 4.1 0.5 0.7 1 0.35

Liatris spicata 14.9 0.5 0.7 1 0.35

Calamagrostis x acutiflora 6.8 0.5 0.7 1 0.35

Bouteloua gracilis 7.7 0.2 0.7 1 0.14

Asclepias incarnata 17.1 0.5 0.7 1 0.35

Andropogon gerardii 7.3 0.2 0.7 1 0.14

Weighted KL -- -- -- -- 0.25

Eastern Nebraska Office on Aging

Sedum spp. (some are Hylotelephium) 6.4 0.2 0.7 1 0.14

Schizachyrium scoparium 6.4 0.2 0.7 1 0.14

Rudbeckia spp. 6.6 0.5 0.7 1 0.35

Monarda didyma 5.5 0.5 0.7 1 0.35

Miscanthus sinensis 6.4 0.5 0.7 1 0.35

Liatris spicata 6.8 0.5 0.7 1 0.35

Hemerocallis spp. 7.7 0.5 0.7 1 0.35

Helenium autumnale 10.6 0.5 0.7 1 0.35

Eupatorium spp. 6.2 0.5 0.7 1 0.35

Cornus sericea 4.3 0.8 0.7 1 0.56

Calamagrostis x acutiflora 11.2 0.5 0.7 1 0.35

Bouteloua gracilis 8.2 0.2 0.7 1 0.14

Asclepias incarnata 7.2 0.5 0.7 1 0.35

Andropogon gerardii 6.8 0.2 0.7 1 0.14

Weighted KL -- -- -- -- 0.30

flow estimates within 5 percent of actual values (Bureau of 

Reclamation, 2001; Heiner and others, 2010). The other two 

components, VOF and VET,

 however, are much more poorly 

constrained in their precision. The values of VOF are assumed 

to be within 25 percent of actual values, and because of the 

grate clogging, the flow volume has a tendency to be overestimated. Because of the dependence on several empirical factors 

used in the estimation of VET, this component is assumed to 

vary by 50 percent on a daily basis but is likely to be more 

accurate at a monthly or greater time step. Because of the 

errors associated with the VSS component being calculated 

as the residual, VSS is tabulated in the report as a range to 

account for the uncertainty in the actual value. This range 

was calculated by finding the maximum and minimum values 

using the accuracy of the measurements for each component 

that were discussed above.

Rainfall Event Duration Determination

The performance of the bioretention gardens was evaluated for a series of rainfall events that will be referred to as 

“events” throughout report. The start of an event was determined when rainfall began at the site. The end of an event 

was determined when the water level was equal to zero in the 

stilling well of the inflow flume or when the water level in the 

bioretention garden was zero, whichever occurred last. 
Water Balance Monitoring 13

Water Balance Monitoring

Water balance monitoring at the DCHC bioretention 

garden was analyzed on a rainfall event basis and also summarized for the monitoring period. No overflow events were 

recorded at the ENOA bioretention garden; therefore, only a 

monitoring period analysis of the water balance was tabulated 

for that site. The lack of overflows could be because of the 

larger available storage volume per acre (4,500 ft3 per acre) at 

the ENOA site compared to the 538 ft3 per acre at the DCHC 

site, indicating that the ENOA bioretention garden could be 

oversized for the drainage area. 

Douglas County Health Center Site

At the DCHC bioretention garden, water balance components of precipitation, stormwater inflow volume, outflow 

volume by way of the overflow standpipe, evapotranspiration, 

and subsurface storage were tabulated by event and on annual 

(excluding winter) time steps for the monitoring period. As 

stated in the “Stormwater Inflow Volume” section, events that 

produced ponding depths higher than 0.7 ft were excluded 

from the analyses. Although the high volume events represented only 26 out of 281 events, they represented 43 in. or 

48 percent of the total rainfall of 89 in. recorded during the 

monitoring period. As a result, monthly or annual performance 

metrics and totals are affected because these large rainfall 

events would have more outflow volume than the rainfall 

events that were assessed. 

Performance on an Event Basis

Events for the bioretention garden at the DCHC site 

were separated using the criteria described previously in the 

“Methods” section. At the DCHC bioretention garden, 255 

of the 281 events were included in the analysis, and 39 of the 

255 events led to overflow of the garden. The analyzed events 

had a minimum rainfall of 0.01 in. and a maximum rainfall of 

1.4 in., with a minimum rainfall intensity of 0.04 inch per hour 

and a maximum intensity of 2.1 inches per hour. The storms 

captured by the monitoring characterize small to medium size 

events. 

Performance on a volume-reduction basis was evaluated 

by comparing total event rainfall to percent reduction of inflow 

volume (fig. 5A) and by comparing total inflow volume to total 

outflow volume (fig. 5B). The bioretention garden reduced 

inflow volumes from a minimum of 33 percent to 100 percent (a complete reduction in inflow volume) depending on 

the size of the event. Although variable, the percent reduction of the inflow volume tended to decrease with increasing 

total event rainfall. The line of equality on figure 5B shows 

the stormwater response that would have taken place had the 

garden not been installed. As expected, all of the outflows 

from the garden were to the right of the line of equality, which 

indicates that the garden reduced outflow volume, and all 

event total inflows less than the garden volume (700 ft3) were 

100 percent captured. Of the 67 events that produced inflow 

volumes greater than 700 ft3 but less than 3,000 ft3, 38 events 

were completely captured. A linear regression line was fit to 

events where the inflow volume exceeded 1,000 ft3 and was 

less than 5,400 ft3. Some of the events at the site that had 

inflow volumes less than 5,400 ft3 may have been excluded 

in this analysis because an event that had an inflow less than 

5,400 ft3 could exceed the 0.7-ft ponding depth limitation and, 

therefore, would have been excluded. The linear regression 

line had a slope of 0.59, indicating that when the garden was 

overflowing, only 59 percent of the inflow was outflow (or 

a volume reduction of 41 percent). This slope of 0.59 at the 

DCHC bioretention garden also is similar to the slope of 0.72 

at a bioretention garden in Maryland (Davis and others, 2012). 

The variability in the relation shown in figure 5 indicated by 

the coefficient of determination of 0.61, where unity would 

be a perfect correspondence between inflow and a proportional volume reduction as outflow, also indicates that rainfall 

intensity could be one of the factors affecting the reduction 

in volume. For example, the 18-hour event on September 10, 

2013, had a total rainfall of 0.72 in., a maximum intensity of 

0.32 inch per hour, with a 5-day time period since the previous event and produced 3,000 ft3

 of inflow that was contained 

by the garden. In contrast, the 6-hour event on October 22, 

2014, had a total rainfall of 0.76 in., a maximum intensity of 

0.88 inch per hour, with a 9-day time period since the previous 

event and produced 3,100 ft3 of inflow and 807 ft3 of outflow. 

Other factors that may contribute to the variability in volume 

reduction shown in figures 5A–5B could be antecedent moisture conditions of the bioretention garden and the timing and 

duration of the high intensity rainfall in the event.

In addition to volume reduction, another benefit of bioretention gardens is the ability to reduce peak inflow rates. To 

assess how well the garden reduces stormwater peak inflow 

rates, peak inflows were plotted against peak outflows measured at the bioretention garden (fig. 6). The line of equality 

shown in figure 6 indicates the stormwater peak that would 

have recorded in the absence of the bioretention garden. As 

expected, all of the peak outflows plot to the right of the line 

of equality. Only 39 of the 255 analyzed events caused outflow 

through the overflow standpipe, indicating complete capture 

of stormwater for all other events. Of those 39 events having 

overflow, the mean reduction in peak inflows was 63 percent. 

Reductions in the peak inflows varied between 17 and 100 percent. Similar to the volume reduction, a threshold seemed to 

exist above which peak outflows were observed; however, 

variability in the peak reduction response was greater than in 

the volume-reduction response. Consequently, a linear relation 

between peak inflow and peak outflow was not developed. The 

variation in the peak inflow reduction possibly can be attributed to rainfall intensity and to the timing and duration of the 

high intensity rainfall of the event. 
14 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Percent reduction of inflow volume

Total event rainfall, in inches

Data point

EXPLANATION A

September 10, 2013

October 22, 2014

y = 0.59x–710.20

Coefficient of determination (R2) = 0.61

0

1,000

2,000

3,000

4,000

5,000

6,000

0 1,000 2,000 3,000 4,000 5,000 6,000

Outflow volume (y), in cubic feet

Inflow volume (x), in cubic feet

Line of equality

Garden volume

Linear regression line for inflow 

volumes greater than 1,000 cubic 

feet and less than 5,400 cubic feet

Data point

EXPLANATION

B

October 22, 2014

September 10, 2013

Figure 5. Performance measures at the bioretention garden at the Douglas County Health Center site. A, relation between total event 

rainfall and percent reduction of inflow volume; B, relation between inflow volume and outflow volume for measured events.
Water Balance Monitoring 15

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Peak outlow, in cubic feet per second

Peak inflow, in cubic feet per second

Line of equality

Data point

EXPLANATION

Figure 6. Relation between 

peak inflow rate and peak 

outflow rate for measured 

events at the Douglas County 

Health Center bioretention 

garden.

Monitoring Period Summary

Water balance components for measured events at the 

DCHC bioretention garden were tabulated for each monitoring period; totals and the amounts of individual components 

are shown in figure 7 and listed in table 2, which also includes 

percentages of total outputs for the three output components 

(subsurface storage, outflow volume, and evapotranspiration). Subsurface storage composed the largest percentage of 

output from the bioretention garden ranging from a minimum 

of 64 percent to a maximum of 98 percent, and the total for 

all 4 years was 73‒93 percent. Outflow was the second largest 

percentage for outputs ranging from a minimum of 7 percent 

to a maximum of 20 percent, and the total was 14 percent 

for the monitoring period. Evapotranspiration generally was 

constant, accounting for about 2 percent of the output, except 

for the exceptionally hot and dry year of 2012 when the 

percentage lost to evapotranspiration was double (4 percent) 

than that of the other 3 years. This higher evapotranspiration 

value in 2012 was due to a drought that affected the region 

from November 2011 through March 2013 (National Drought 

Mitigation Center, 2015). 

Eastern Nebraska Office on Aging Site

The ENOA bioretention garden did not record any overflow events and had a total reduction in flow; therefore, data 

were not available for an event-based overflow analysis at the 

0

50

100

150

200

250

300

350

2011

inflow

2011

outflow

2012

inflow

2012

outflow

2013

inflow

2013

outflow

2014

inflow

2014

outflow

Monitoring period water balance component, in inches

EXPLANATION

Evapotranspiration

Outflow volume

Subsurface storage

Inflow volume

Precipitation

Range of values of subsurface storage

Figure 7. Annual water balance components for measured 

events during 2011–14, normalized to the bioretention garden area 

at the Douglas County Health Center site.
16 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14 Table 2. Monitoring period summary of water balance components for measured events during 2011–14 for the bioretention garden at the Douglas County Health Center site. [Values in inches were normalized by the bioretention garden area] Year Inputs Outputs Percentage of total outputs Precipitation1 Inflow volume1 Outflow volume1 Evapotranspiration Subsurface storage Outflow volume Evapotranspiration Subsurface storage Inches2 Cubic feet Inches2 Cubic feet Inches2 Cubic feet Inches2 Cubic feet Inches2 Cubic feet May 2011 through October 2011 11 1,900 190 33,300 14 2,500 5 900 166‒197 29,000‒34,500 7 2 82‒98 April 2012 through October 2012

9 1,600 145 25,400 30 5,300 7 1,200 99‒136 17,300‒23,800 19 4 64‒88

April 2013 through 

October 2013

14 2,500 281 49,200 33 5,800 6 1,100 231‒282 40,400‒49,400 11 2 78‒96

April 2014 through 

October 2014

14 2,500 274 48,000 58 10,200 6 1,100 193‒256 33,700‒44,800 20 2 67‒89

Total 48 8,400 891 155,900 135 23,600 24 4,200 688‒871 120,400‒152,400 14 3 73‒93

1Only storms that were able to be measured at the site are included in summary; thus, the summary does not provide a complete total for the monitoring period because of monitoring limitations.2Values in inches were obtained by dividing the volume in cubic feet by the surface area of the garden (2,100 square feet) and multiplying by 12 to convert to inches.

bioretention garden. Water balance components were summarized on a monitoring period basis during 2011–14. 

Water balance components for all events at the ENOA 

bioretention garden were tabulated and summarized for each 

monitoring period; totals and the amounts of individual components are shown in figure 8 and listed in table 3, which also 

includes percentages of total output for the three output components (subsurface storage, evapotranspiration, and outflow 

volume). The large increase in the amount of inflow volume 

measured during 2011–12 compared to inflow volume measured during 2013–14 is shown in figure 8. The reason for this 

increase in inflow is because of modifications to the parking 

lot drain inlets at the ENOA site and because of the increased 

drainage area at the site as described previously in the “Study 

Area” section. Subsurface storage composed the largest percentage of output from the bioretention garden ranging from 

a minimum of 48 percent to a maximum of 100 percent, and 

the total for all 4 years was 87–99 percent. Evapotranspiration 

ranged from a minimum of 3 percent in 2014 to a maximum 

of 32 percent during the drought in 2012. For the monitoring 

period, evapotranspiration composed 5 percent of the total 

outputs from the bioretention garden. Because the ENOA 

bioretention garden did not record any overflow events, the 

outflow was zero, and the bioretention garden reduced 100 

percent, or 115,700 ft3, of the amount of stormwater flow that 

would have been output to the combined sewer system during 

the 2011–14 monitoring period.

0

50

100

150

200

250

Monitoring period water balance component, in inches

EXPLANATION

2011

inflow

2011

outflow

2012

inflow

2012

outflow

2013

inflow

2013

outflow

2014

inflow

2014

outflow

Evapotranspiration

Subsurface storage

Inflow volume

Precipitation

Range of values of subsurface storage

Figure 8. Annual water balance components for all events 

during 2011–14, normalized to the bioretention garden area at the 

Eastern Nebraska Office on Aging site.
Summary 17

Table 3. Monitoring period summary of water balance components for all events during 2011–14 for the bioretention garden for the Eastern Nebraska Office on Aging site.[Values in inches were normalized by the bioretention garden area]

Year

Inputs Outputs Percentage of total outputs

Precipitation Inflow volume Outflow volume Evapotranspiration Subsurface storage Outflow 

volume Evapotranspiration Subsurface storage Inches1 Cubic feet Inches1 Cubic feet Inches1 Cubic feet Inches1 Cubic feet Inches1 Cubic feet

May 2011 through 

October 2011

17 4,100 35 8,400 0 0 5 1,200 42‒52 10,200‒12,400 0 9 81‒100

April 2012 through 

October 2012

10 2,500 13 3,000 0 0 7 1,800 11‒20 2,700‒4,900 0 32 48‒88

April 2013 through 

October 20132

21 5,100 154 37,100 0 0 7 1,600 157‒180 37,900‒42,300 0 4 90‒100

April 2014 through 

October 20142

37 9,000 192 46,300 0 0 7 1,700 208‒236 50,200‒55,300 0 3 91‒100

Total 86 20,900 392 94,800 0 0 26 6,300 418‒488 100,900‒114,900 0 5 87‒99

1Values in inches were obtained by dividing the volume in cubic feet by the surface area of the garden (2,900 square feet) and multiplying by 12 to convert to inches.2Drainage area changed after 2012.

Considering the water balance monitoring at both biore-

tention gardens, a small percentage of the outputs was attrib-

uted to evapotranspiration on an annual basis (3 percent at the 

DCHC site [table 2] and 5 percent at the ENOA site [table 3]), 

which indicates that vegetative water uptake is not a primary 

factor in the water budget. The evapotranspiration values 

(5–7 in.) are less than published annual mean estimates of 

21–27 in. for the study region (Szilagyi, 2015); however, this 

deviation is mainly attributed to the low plant species factors 

in the landscape coefficient method that was used to scale the 

reference evapotranspiration (ET0) as described in the “Evapo-

transpiration” section. Vegetation, however, is important in the 

function of the bioretention garden because vegetation aids in 

maintaining infiltration rates through combating soil compac-

tion in the gardens (Hatt and others, 2009).

Summary

Urbanization can radically alter the flow regime of a 

stream system by creating impermeable areas that favor 

stormwater runoff instead of infiltration and by constructing 

drainage networks that quickly transport the stormwater out of 

the urban area. Areas that are drained by combined sewer sys-

tems—where stormwater and sanitary sewage are combined 

and can be released untreated into streams during rainfall 

events—present a greater challenge to stormwater manage-

ment as a result of the additional human-health threats associ-

ated with stormwater runoff. In response, stormwater best 

management practices are used to reduce stormwater effects. 

A bioretention garden is one type of stormwater best manage-

ment practice. In the case of combined sewer systems such as 

in Omaha, Nebraska, bioretention gardens have the potential 

to decrease the frequency and magnitude of instances when 

untreated sewage is released into receiving waters. To better 

understand the performance of bioretention gardens in eastern 

Nebraska, the U.S. Geological Survey, in cooperation with 

Douglas County Environmental Services and the Nebraska 

Environmental Trust, assessed the water balance of two biore-

tention gardens by monitoring the amount of stormwater enter-

ing and leaving the gardens. The gardens are on the Douglas 

County Health Center campus approximately two blocks south 

of 40th and Pacific Streets and on the Eastern Nebraska Office 

on Aging property at the southwest corner of 42nd and Center 

Streets. Water balance components of precipitation, stormwa-

ter inflow, outflow, reference evapotranspiration, and subsur-

face storage were either measured directly or computed. Water 

balance data were collected at the two bioretention gardens 

from May through October in 2011 and from April through 

October in 2012, 2013, and 2014.

At the Douglas County Health Center site, bioreten-

tion garden performance on a volume-reduction basis was 

evaluated by comparing total inflow volume to total outflow 

volume. The bioretention garden reduced inflow volumes 

from a minimum of 33 percent to 100 percent (a complete 
18 Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14

reduction in inflow volume) depending on the size of the 

event. Although variable, the percent reduction of inflow 

volume tended to decrease with increasing total event rainfall. To assess how well the garden reduces stormwater peak 

inflow rates, peak inflows were plotted against peak outflows 

measured at the bioretention garden. Only 39 of the 255 events 

had any overflow, indicating 100 percent peak reduction in the 

other events. Of those 39 events having overflow, the mean 

peak reduction was 63 percent.

No overflow events were recorded at the bioretention 

garden at the Eastern Nebraska Office on Aging site; therefore, 

data were not available for an event-based overflow analysis. Monitoring period summary of the water balance at both 

bioretention gardens indicates that most of the stormwater in 

the bioretention gardens is stored in the subsurface. A small 

percentage of the outputs was attributed to evapotranspiration 

on an annual basis (3 percent at Douglas County Health Center 

site and 5 percent at Eastern Nebraska Office on Aging site), 

which indicates that vegetative water uptake is not a primary 

factor in the water budget.

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Strauch and others—Water Balance Monitoring for Two Bioretention Gardens in Omaha, Nebraska, 2011–14—Scientific Investigations Report 2015–5188

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20155188