U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5008 

In cooperation with the Wisconsin Department of Natural Resources

A Comparison of Runoff Quantity and Quality from Two 

Small Basins Undergoing Implementation of Conventionaland Low-Impact-Development (LID) Strategies: Cross Plains, 

Wisconsin, Water Years 1999–2005 

A Comparison of Runoff Quantity and 

Quality from Two Small Basins Undergoing Implementation of Conventionaland Low-Impact-Development (LID) 

Strategies: Cross Plains, Wisconsin, 

Water Years 1999–2005

In cooperation with the Wisconsin Department of Natural Resources

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2008–5008
U.S. Department of the Interior

DIRK KEMPTHORNE, Secretary

U.S. Geological Survey

Mark D. Myers, Director

U.S. Geological Survey, Reston, Virginia: 2008

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Suggested citation:

Selbig, W.R., and Bannerman, R.T., 2008, A comparison of runoff quantity and quality from two small basins 

undergoing implementation of conventional- and low-impact-development (LID) strategies: Cross Plains, Wisconsin, water years 1999–2005: U.S. Geological Survey Scientific Investigations Report 2008–5008, 57 p.
iii

Contents

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

Introduction............................................................................................................................................ 2

Purpose and Scope ..................................................................................................................... 3

Description of Study Area .......................................................................................................... 3

Low-Impact-Development Practices in Study Area .............................................................. 6

Methods of Data Collection ................................................................................................................ 7

Basin-Outlet Flow Measurement and Calibration.................................................................. 7

Flow Measurement............................................................................................................. 7

Calibration of Flows............................................................................................................ 8

Precipitation.................................................................................................................................. 9

Water Temperature ..................................................................................................................... 9

Runoff-Sample Collection................................................................................................ 10

Comparison of Runoff Quantity from the Basins ........................................................................... 10

Precipitation................................................................................................................................ 11

Runoff Volumes .......................................................................................................................... 12

Temporal Changes in Runoff Quantity in the Low-Impact-Development 

(LID) Basin.......................................................................................................................... 15

Performance of Infiltration Basin................................................................................... 19

Comparison of Runoff Water Quality from the Basins.................................................................. 27

Storm Loads................................................................................................................................ 27

Water Temperature ................................................................................................................... 33

Limitations of This Study and Suggestions for Future Work........................................................ 36

Summary and Conclusions................................................................................................................ 37

Acknowledgements............................................................................................................................ 38

References........................................................................................................................................... 38

Appendixes .......................................................................................................................................... 41

Appendix 1. Quality Assurance and Quality Control Data................................................... 42

Appendix 2. Detailed Event Characteristics and Stormwater-Quality Data..................... 43
iv

Figures

1–4. Photographs showing:

1. Location of study basins, Cross Plains, Wis........................................................... 4

2. Land use in the low-impact-development and conventional basins 

and location of water-quality monitoring stations, Cross Plains, Wis............... 5

3. Example of stormwater-runoff controls using low-impact-development practices ............................................................................................................ 7

4. Parshall flume and H-flume used to measure discharge at the lowimpact-development (LID) basin outlet................................................................... 8

5–6. Graphs showing:

5. Relation between actual discharge determined using a rhodamine 

dye tracer and measured discharge, computed by water-level and 

velocity data, during free-flow conditions at the conventionalbasin outlet .................................................................................................................. 9

6. Precipitation totals for April through October at the low-impactdevelopment (LID) and conventional basins during each study year.............. 11

7. Histograms showing the frequency of rainfall events resulting in discharge 

from the study area as a function of increasing precipitation depth in 

the low-impact-development (LID) and conventional basins .................................... 12

8–10. Graphs showing:

8. Relation of cumulative-discharge volume, as a percentage of total 

discharge volume measured between 1999–2005, to increasing 

precipitation depth in the conventional and low-impact-development 

(LID) study basins ..................................................................................................... 14

9. Relation of cumulative-discharge volume to precipitation depth for 

residential land use in Madison, Wis., based on model predictions ............... 15

10. Hydrologic response of low-impact-development (LID) and conventional basins to two consecutive precipitation events, Cross Plains, Wis. .... 16

11–14. Photographs showing:

11. Yearly changes in construction activity and housing density in the 

low-impact-development (LID) basin, 1999–2004................................................ 18

12. Approximate locations of double-ring infiltrometer tests in the infiltration basin in the low-impact-development (LID) basin.................................. 25

13. Layer of silt that has formed from excessive ponding of runoff near 

the west-inlet and main-outlet culverts in the infiltration basin, in the 

low-impact-development (LID) basin .................................................................... 26

14. Concentrated sediment in snow piles in the conventional-development basin.................................................................................................................. 31

15. Temperature of runoff, air temperature, cumulative precipitation, and 

water levels measured in the conventional-development basin............................... 34

16. Temperature of runoff, air temperature, cumulative precipitation, and 

water levels measured in the low-impact-development (LID) basin........................ 35
v

Tables

1. Breakdown of land use in the developed portion of the low-impact-development (LID) and conventional-development basins ........................................................ 6

2. Summary statistics for measured discharge events at the low-impactdevelopment (LID) and conventional-development basin monitoring stations, 

water years 1999–2005...................................................................................................... 13

3. Comparison of precipitation and runoff characteristics in the low-impactdevelopment (LID) and conventional-development basins over increasing 

percentages of basin development................................................................................ 17

4. Reduction of runoff volume in the low-impact-development (LID) basin by 

the infiltration basin, 2002–2005 ...................................................................................... 20

5. Runoff volume reduction by the infiltration basin as a function of increasing 

precipitation intensity ....................................................................................................... 24

6. Infiltration rates measured at seven locations in the infiltration basin, 

2000–04 ................................................................................................................................ 26

7. Regression results for storm loads in the low-impact-development (LID) 

and conventional-development basins.......................................................................... 27

8. Storm-load summary statistics at the low-impact-development (LID) and 

conventional basin outfalls, water years 1999–2005 ................................................... 28

9. Estimate of annual loads in the low-impact-development (LID) and 

conventional-development basins, water years 1999–2005....................................... 30

10. Contributions of constituent load as a percentage of annual load for 

select precipitation events in the low-impact-development (LID) and 

conventional-development basins.................................................................................. 32

11. Estimate of annual yields of total solids, total suspended solids, and total 

phosphorus in the low-impact-development (LID) and conventionaldevelopment basins, water years 1999–2005................................................................ 33
vi

Conversion Factors, Horizontal Datum, and Abbreviated 

Units of Measurement

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 0.4047 hectare (ha)

square foot (ft2) 0.09290 square meter (m2)

Volume

gallon (gal) 3.785 liter (L) 

cubic foot (ft3) 0.02832 cubic meter (m3) 

liter (L) 0.2642 gallon (gal)

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)

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

ton, short (2,000 lb) 0.9072 megagram (Mg)

ton per day per square mile 

[(ton/d)/mi2]

0.3503 megagram per day per square 

kilometer [(Mg/d)/km2]

Application Rate

pounds per acre per year 

[(lbs/acre)/yr]

1.121 kilograms per hectare per year 

[(kg/ha)/yr]

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

follows:

°F=(1.8×°C)+32

Horizontal coordinate information is referenced to the 1991 adjustment of the North 

American Datum of 1983 (NAD 83/91).

Concentrations of chemical constituents in water are given either in milligrams per liter 

(mg/L) or micrograms per liter (μg/L).

Water year is defined as the 12-month period October 1 through September 30. The water 

year is designated by the calendar year in which it ends.
vii

Other abbreviations

BMPs best-management practices

IBI index of biotic integrity

LID low-impact development

NOAA National Oceanic and Atmospheric Administration

RPD relative percent difference

WDNR Wisconsin Department of Natural Resources

WSLH Wisconsin State Laboratory of Hygiene

A Comparison of Runoff Quantity and Quality from 

Two Small Basins Undergoing Implementation of 

Conventional- and Low-Impact-Development (LID) 

Strategies: Cross Plains, Wisconsin, Water Years 

1999–2005

By William R. Selbig1 and Roger T. Bannerman2

Abstract

Environmental managers are often faced with the task 

of designing strategies to accommodate development while 

minimizing adverse environmental impacts. Low-impact 

development (LID) is one such strategy that attempts to 

mitigate environmental degradation commonly associated 

with impervious surfaces. The U.S. Geological Survey, 

in cooperation with the Wisconsin Department of Natural 

Resources, studied two residential basins in Cross Plains, 

Wis., during water years 1999–2005. A paired-basin study 

design was used to compare runoff quantity and quality 

from the two basins, one of which was developed in a conventional way and the other was developed with LID. The 

conventional-developed basin (herein called “conventional 

basin”) consisted of curb and gutter, 40-foot street widths, 

and a fully connected stormwater-conveyance system. The 

LID basin consisted of grassed swales, reduced impervious 

area (32-foot street widths), street inlets draining to grass 

swales, a detention pond, and an infiltration basin. Data 

collected in the LID basin represented predevelopment 

through near-complete build-out conditions. 

Smaller, more frequent precipitation events that 

produced stormwater discharge from the conventional 

basin were retained in the LID basin. Only six events 

with precipitation depths less than or equal to 0.4 inch 

produced measurable discharge from the LID basin. Of 

these six events, five occurred during winter months when 

underlying soils are commonly frozen, and one was likely 

a result of saturated soil from a preceding storm. In the 

conventional basin, the number of discharge events, using 

1

 U.S. Geological Survey, Middleton, Wisconsin

2

 Wisconsin Department of Natural Resources, Madison, Wisconsin

the same threshold of precipitation depth, was 180, with 

nearly one-half of those resulting from precipitation depths 

less than 0.2 inch. Precipitation events capable of producing appreciable discharge in the LID basin were typically 

those of high intensity or precipitation depth or those that 

occurred after soils were already saturated. Total annual 

discharge volume measured from the conventional basin 

ranged from 1.3 to 9.2 times that from the LID basin.

Development of the LID basin did not appreciably 

alter the hydrologic response to precipitation characterized 

during predevelopment conditions. Ninety-five percent or 

more of precipitation in the LID basin was retained during 

each year of construction from predevelopment through 

near-complete build-out, surpassing the 90-percent benchmark established for new development by the Wisconsin 

Department of Natural Resources. The amount of precipitation retained in the conventional basin did not exceed 

94 percent and fell below the 90-percent standard 2 of the 

6 years monitored.

Much of the runoff in the LID basin was retained by 

an infiltration basin, the largest control structure used to 

mitigate storm-runoff quantity and quality. The infiltration basin also was the last best-management practice 

(BMP) used to treat runoff before it left the LID basin as 

discharge. From May 25, 2002, to September 30, 2005, 

only 24 of 155 precipitation events exceeded the retention/infiltrative capacity of the infiltration basin. The 

overall reduction in runoff volume from these few events 

was 51 percent. The effectiveness of the infiltration basin 

decreased as precipitation intensities exceeded 0.5 inch per 

hour.
Annual loads were estimated to characterize the 

overall effectiveness of low-impact design practices for 

mitigating delivery of total solids, total suspended solids, and total phosphorus. Annual loads of these three 

constituents were greater in the LID basin than in the 

conventional basin in 2000 and 2004. Seventy percent or 

more of all constituent annual loads were associated with 

two discharge events in 2000, and a single discharge event 

produced 50 percent or more of constituent annual loads in 

2004. Each of these discharge events was associated with 

considerable precipitation depths and (or) intensities, 

ranging from 4.89 to 6.21 inches and from 1.13 to 

1.2 inches per hour, respectively. These same storms did 

not contribute as much of the annual load in the conventional basin. With large storms and saturated soils, the ability of low-impact design techniques to reduce runoff, and 

thus constituent loads, can be greatly diminished. 

For both the LID and conventional basins, the temperature of runoff was largely affected by ambient air temperatures. However, the temperature of discharge from the 

LID basin increased upon runoff cessation. This increase 

is likely due to solar heating of water that is temporarily 

stored in the detention pond and infiltration basin. 

Introduction

Conversion of rural and agricultural lands to developed urban areas is a leading contributor of nonpointsource pollution. Contamination from nonpoint sources 

is now the largest cause of water-quality impairment in 

the United States because of past and current successes 

in controlling point sources (U.S. Environmental Protection Agency, 2001). Converting the natural landscape into 

residential or commercial land use can drastically alter its 

hydrologic characteristics. Land that was once covered 

with native vegetation is often replaced with impervious 

surfaces such as driveways, rooftops, streets, and parking 

lots. Precipitation that previously infiltrated into surrounding soils is now collected and conveyed by concrete 

structures into nearby streams, rivers, or lakes to prevent 

localized flooding. Collectively, impervious areas introduced by traditional urban landscapes lead to more diverse 

pollutants, reduced pollutant removal during overland 

flow, reduced infiltration, and increased peak flows which, 

in turn, can aggravate stream erosion (Davis, 2005). Previous studies have demonstrated that only a small percentage 

of connected impervious area in a watershed can degrade 

stream ecology. Wang and others (2001) noted that as the 

impervious fraction in a watershed exceeded 12 percent, 

decreases in fish density, species richness, diversity, index 

of biotic integrity (IBI) score, base flow, and increased 

bank erosion were measurable. 

Although water quality across the country has 

improved appreciably since passage of the Clean Water 

Act in 1972, challenges still remain. In 2000, water-quality 

assessments by states indicated that 39 percent of assessed 

stream miles, 45 percent of assessed acres of lakes, and 

51 percent of assessed estuary areas failed to meet criteria for one or more designated uses (U.S. Environmental 

Protection Agency, 2002). The top causes of impairment 

in assessed stream miles were siltation, nutrients, bacteria, 

metals (primarily mercury), and oxygen-depleting substances. Pollution from urban and agricultural land that is 

transported by precipitation and runoff was found to be the 

leading source of impairment (U.S. Environmental Protection Agency, 2002).

Environmental managers are often faced with the task 

of designing strategies to accommodate expanding development while minimizing adverse environmental impacts. 

Low-impact development (LID) is one such strategy that 

attempts to mitigate environmental degradation commonly associated with traditional residential or commercial construction practices. LID attempts to maintain or 

replicate the predevelopment hydrologic regime through 

use of design techniques that create a functionally equivalent hydrologic landscape (U.S. Environmental Protection 

Agency, 2000). LID principles are based on controlling 

stormwater at the source by incorporating the hydrologic 

functions of storage and infiltration into small-scale, 

distributed, structural and nonstructural controls. Some 

LID practices include, but are not limited to, decreasing 

impervious surfaces by narrowing street widths, creating micro-scale stormwater retention and detention areas, 

increasing flow paths, preserving highly permeable soils, 

incorporating vegetated swales and permeable paving 

into the building plan, and preventing soil compaction by 

discouraging the use of heavy equipment (Coffman, 2000; 

Liaw and others, 2000). 

Although individually many of these stormwater 

controls have been proven to reduce stormwater-runoff 

volumes and (or) improve water quality, there are very few 

large-scale studies that evaluate the LID concept. Previous studies used computer-simulated models to attempt to 

estimate the hydrologic benefits of LID. Results of these 

studies indicate that it is possible to reduce hydrologic 

impacts using a LID design instead of traditional, fully 

connected stormwater systems (Liaw and others, 2000; 

2 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Holman-Dodds and others, 2003; Brander and others, 

2005). However, environmental limitations, zoning and 

regulatory statutes, safety and public-health concerns, and 

economic viability have limited opportunities to apply 

the LID philosophy (Davis, 2005). For these reasons, the 

U.S. Geological Survey (USGS), in cooperation with the 

Wisconsin Department of Natural Resources (WDNR), 

undertook a study in Cross Plains, Wis. (fig. 1), to evaluate 

runoff quantity and quality from a LID and a conventional 

basin. The purpose of the investigation was to determine 

whether implementing several LID practices in a residential basin could reduce the quantity and (or) improve the 

quality of stormwater runoff measured at the basin outlet 

when compared to a nearby residential subdivision that 

was constructed using more conventional “curb and gutter” 

techniques. 

The WDNR has promulgated a series of performance 

standards and prohibitions with regard to nonpoint-stormwater sources. These standards are intended to be minimum benchmarks of performance necessary to achieve 

water-quality goals. For proposed development of new 

residential areas, the site must be designed to infiltrate at 

least 90 percent of the predevelopment infiltration volume, 

based on an average annual rainfall (Wisconsin Administrative Code, 2002). Incorporating low-impact practices as 

part of residential design may be one way to achieve this 

State-mandated standard.

Purpose and Scope

This report describes the methods used in and the 

results from a study comparing the runoff quantity and 

quality of a conventionally designed residential basin 

to one in which LID strategies were implemented. Two 

basins were selected to represent different construction 

philosophies. The “conventional” basin, developed in the 

late 1980s and early 1990s, used traditional engineering 

practices such as curb and gutter, larger street widths, and 

a fully connected stormwater-conveyance system. The LID 

basin, developed in the late 1990s and early 2000s, utilized 

low-impact design concepts distributed throughout the 

drainage basin. Automated, intensive stormwater sampling 

was done during storm-runoff periods from predevelopment through near-complete build-out of the LID basin 

(October 1998 to September 2005). The objectives of the 

study were to determine whether using LID techniques in a 

residential subdivision would reduce the volume as well as 

improve the quality of stormwater runoff when compared 

to a residential subdivision built using a more conventional, fully connected stormwater-conveyance system. 

Concentrations of total solids, total suspended solids, 

and total phosphorus in stormwater-runoff samples were 

used to compute storm loads for each contaminant at the 

outlets of the two basins. Comparison of constituent loads 

was made between the conventional and LID basins to 

evaluate the hydrologic benefits of a low-impact design. 

Water temperature was measured at the basin outlets to 

characterize the temperature of stormwater stemming from 

the concrete storm-sewer conveyance system in the conventional basin and compare it to temperatures from the 

more natural, vegetated system used in the LID basin. This 

study supports an ongoing effort by the USGS and WDNR 

to identify existing and new methods to reduce nonpointsource pollution from urban areas.

Description of Study Area

The study area is near the southwest corner of the 

village of Cross Plains, Wis. (fig. 1). The climate is typical 

of interior North America, with a large annual temperature 

range and frequent, short-period temperature changes. 

Nearly 60 percent of the annual precipitation falls in May 

through September, with annual precipitation averaging 

31.73 in. (National Oceanic and Atmospheric Administration, 1999–2005). The conventional and LID study basins 

were within 1⁄4 mi of each other to minimize climatic variability between basins. Each basin can be described as a 

valley with forested upland hills surrounding a valley floor 

that was previously used for agriculture. The valley floor is 

composed primarily of silt loam. The valley walls are soilcovered sandstone, limestone, and dolomite bedrock. Each 

basin drains to nearby Black Earth Creek, a cold-water 

trout stream that drains the Black Earth Creek watershed, 

which is generally hilly and has steep-sided valleys. 

The conventional basin has a drainage area of 137 

acres; 32 acres are developed on the valley floor (fig. 1). 

The remaining 105 acres are forested uplands. Construction of the conventional basin began in 1988 and was 

completed by 1991. Land use in the developed portion of 

the conventional and LID basin is shown in figure 2 and 

table 1. Land use is mixed, with residential and commercial areas making up approximately 75 and 25 percent of 

the developed area, respectively. Fifty percent of the basin 

is pervious; grassed lawns make up the largest percentage (48 percent) of a single land-use classification (table 

1). The stormwater-conveyance system is composed of a 

directly connected network of curb and gutters draining to 

a large-diameter, concrete storm-sewer pipe that eventually 

leads to a small detention pond outside the conventional 

basin. 

Introduction 3
WISCONSIN

Dane County

Base from Dane County Land Information Office Orthophotography, 1:10,000, 2005, Wisconsin Transverse Mercator projection, NAD 83/91.

Orthophoto from U.S. Geological Survey, 1:12,000, 2000, Wisconsin Transverse Mercator projection, NAD 83/91.

Madison

Cross Plains

Low-impactdevelopment basin

Conventional basin

89°39'W 89°40'W

43°6'30"N 43°6'N

0 0.2 0.1 MILES0 0.2 0.1 KILOMETERS

EXPLANATION

Drainage basin boundaryDevelopment basin boundary

Figure 1. Location of study basins, Cross Plains, Wis.

4 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Land cover

EXPLANATION

Infiltration basinStreetParking lotRoofDrivewaySidewalkPlaygroundPondLawnForest

Monitoring station

0 500 250 FEET

0 125 62.5 METERS

89°40'0"W 89°39'10"W

43°6'40"N 43°6'20"N

Base from Dane County Land Information Office Orthophotography, 1:10,000, 2005, Wisconsin Transverse Mercator projection, NAD 83/91.

Orthophoto from U.S. Geological Survey, 1:12,000, 2000, Wisconsin Transverse Mercator projection, NAD 83/91.

Figure 2. Land use in the low-impact-development (left) and conventional-development (right) basins and location of water-quality monitoring stations, Cross Plains, Wis.

Introduction 5
Table 1. Breakdown of land use in the developed portion of the low-impact-development (LID) and conventional-development 

basins.

[Values in parentheses represent percent of developed area rounded to nearest whole number; %, percent; --, land use not present in basin; <, less than]

Characteristic

Study basin

LID Conventional

Residential Residential Commercial

Drainage area (acres) 37.7 32.0b

Land use (acres)

Driveway 1.6 (4%) 0.8 (3%) 0.3 (1%)

Lawns 24.4a (64%) 13.5 (42%) 1.8 (6%)

Roofs 4.7 (12%) 3.7 (11%) 1.1 (4%)

Sidewalks 0.6 (2%) 0.7 (2%) --

Streets 3.8 (10%) 2.9 (9%) 0.4 (1%)

Parking lot -- 1.8 (6%) 4.2 (13%)

Forest 0.5 (1%) 0.8 (3%) --

Park 0.1 (<1%) -- --

Other 2.0 (5%) 0.03 (<1%) --

a

 10 percent of this land-use category was zoned as commercial but was maintained as lawn during study period.

b

 Total drainage area represents a combination of residential and commercial land use.

The LID basin drainage area is 192 acres; 37.7 acres 

are developed on the valley floor (fig. 1). The remaining 154.3 acres are forested uplands. Construction of the 

basin began in May 1999 and was nearly complete at the 

conclusion of this study (2005). Prior to development, 

the area had been used for agriculture but was left fallow 

for approximately 1 year before the study commenced. 

Land use is dominated by single-family residential lots; 

however, a small percentage of area zoned for commercial 

development remained undeveloped during the course of 

the study and was therefore classified as lawns (table 1). 

Over two-thirds of the land in the LID basin is considered 

pervious, a reflection of the low-impact concept. Lawns 

represent the majority of pervious surfaces (64 percent of 

the basin area). The LID basin also includes smaller areas 

that do not fit into one of the major source-area categories. 

A community park occupies a small percentage of the LID 

basin (less than 1 percent). This area was not included in 

the “lawn” category because the physical maintenance 

and use associated with a public park may be different 

from that of a typical residential lawn. Similarly, the LID 

basin includes a large infiltration basin planted with native 

prairie species. This area makes up nearly 5 percent of the 

basin area. A detention pond near the center of the basin 

covers approximately 1 percent of the basin area. Each of 

these have been lumped into a single “other” source-area 

category (table 1). Roofs and streets make up the majority 

of impervious surfaces, with 12 and 10 percent of the basin 

area, respectively. 

Low-Impact-Development Practices in 

Study Area

A variety of techniques for erosion control and stormwater management were employed in the LID basin. The 

erosion-control practices included sediment basins, silt 

fences, use of erosion fabric on street swales, and timely 

seeding of disturbed areas. Runoff from upland areas 

was reduced with a combination of earthen berms and 

rock trenches. The LID basin was designed to maximize 

infiltration of runoff by locating infiltration practices based 

on predevelopment soil and permeability rates. Some of 

the techniques used to manage stormwater in the LID 

basin include reduction of street widths from 40 to 32 ft 

to minimize impervious cover, use of grass swales instead 

of storm sewers, routing of stormwater from street inlets 

to grass swales, protection of existing woodlands, use of a 

detention pond to reduce solids loading to the infiltration 

basin, use of an infiltration trench, and construction of a 

6 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Figure 3. Example of stormwater-runoff controls using low-impact-development practices.

Infiltration basin with prairie vegetation

Grassed swales

handled by electronic dataloggers. Data were automatically retrieved twice daily by way of telephone modems. 

Precipitation data were collected at both the conventional 

and LID basins by means of a tipping-bucket rain gage 

calibrated to 0.01 in. 

Flow Measurement

An automated monitoring station was used to measure flow and collect samples from a 4.50-ft-diameter 

circular storm sewer at the basin outlet in the conventional 

basin (fig. 2). Water levels were measured with a bubblegage system and pressure transducer. Measurements were 

made at two locations near the pipe floor approximately 20 

ft apart. Measuring water level at two locations provided 

redundancy and minimized the potential for missing data if 

one of the sensors became inoperable. Ultrasonic velocity sensors were used to measure instantaneous velocity 

near the locations of the upstream and downstream bubble 

line. A fifth-order polynomial was used to determine the 

cross-sectional area of the pipe as a function of water level. 

Instantaneous discharge was then calculated by multiplying the cross-sectional area of the pipe by the associated 

mean velocity. Whenever possible, the downstream water 

level and velocity measurements were used to compute 

discharge in the conventional basin. Storm-runoff volumes 

were computed by summing the 1-minute-interval instantaneous discharge over the runoff duration. When water 

depth at the sensor was less than approximately 1 in., 

flow calculations were not considered reliable because the 

velocity sensor and bubble line were not fully submersed. 

Given the large diameter of the drainage pipe in the conventional basin, all flows occurring at less than 1 in. depth 

were considered insignificant to the overall event volume. 

large infiltration basin planted with deep-rooted prairie 

species native to the area. Some of the practices used in the 

LID basin are illustrated in figure 3. 

Surface runoff was diverted from impervious surfaces to one or more structures for temporary storage and 

infiltration. This “treatment-train” approach increased the 

opportunity for stormwater-runoff infiltration and waterquality improvement. For example, runoff originating from 

the street surface was directed to a grassed swale, where it 

would then drain into either an infiltration basin or a detention pond for storage during smaller storm events. For 

larger storm events, a combination of V-notch and broadcrested weirs at the outlet of the detention pond discharged 

excess runoff into the infiltration basin before the runoff 

migrated toward the basin outlet. 

Methods of Data Collection

During this study, water-quantity and -quality data 

derived from the two residential basins were collected, 

characterized, and interpreted. Water-quality concentrations and subsequent loads were collected from the basin 

outlets (locations shown in fig. 2).

Basin-Outlet Flow Measurement and 

Calibration

Storm runoff was measured and sampled at the basin 

outlets in the conventional and LID basins. Locations of 

the basin-outlet monitoring stations are shown in figure 

2. Each monitoring station was equipped with automated 

water-quality samplers and instruments to measure runoff 

discharge. Measurement, control, and storage of data were 

Methods of Data Collection 7
Figure 4. Parshall flume (left) used to measure discharge at the low-impact-development (LID) basin outlet. The Parshall flume 

was later replaced by an H-flume (right) to accommodate larger discharge events.

Stormwater runoff leaving the LID basin was routed 

to a Parshall flume with a 0.25-ft throat width attached to 

a wooden retaining wall (fig. 4). The Parshall flume was 

replaced with a 2-ft H-flume on July 14, 2000 (fig. 4), to 

increase the amount of runoff that could be passed without 

overtopping the wooden retaining wall. Water levels in 

both the Parshall and H-flume were measured with a 

bubble-gage system and pressure transducer. Conversion 

of water levels to discharge was based on standard rating 

curves for Parshall and H-flumes of the indicated size. A 

Flo-Dar device was mounted to the top of the H-flume in 

April 2001 to record water levels with an ultrasonic-based 

pulse-echo measurement. Data from the Flo-Dar were 

used to provide an independent check on the bubble-gagesystem and pressure-transducer values, as well as to fill 

in periods of missing data. Storm-runoff volumes were 

computed by summing the 1-minute-interval instantaneous 

discharge over the runoff duration.

Instrumentation was installed in the LID basin to 

measure both detention-pond and grass-swale runoff 

volumes prior to drainage into the infiltration basin. 

Water levels in both the detention pond and grass swales 

were measured using a bubble-gage system and pressure 

transducer. Conversion of water levels to discharge was 

based on rating curves from as-built engineering specifications for the V-notch weir at the detention-pond outlet and 

published ratings for the trapezoidal flumes placed in the 

center of each grass swale. Installation of each monitoring 

station for the detention pond and grass swales was completed in August 2001 and May 2002, respectively.

Calibration of Flows

Water levels were periodically calibrated at the conventional and LID monitoring stations. Water levels were 

adjusted by applying corrections that represented the difference between water levels measured manually and those 

measured electronically by the pressure transducer. Corrections were made if the difference between the manual 

and electronic water-level measurements exceeded 0.02 

ft. Because a rated flume was used at the LID basin outlet, 

water level was the only parameter requiring calibration. 

Calibration of the ultrasonic velocity sensor at the 

conventional-basin outlet was done with a dye tracer 

and laboratory fluorometer. A known concentration of 

rhodamine dye was continuously injected at a constant 

rate sufficiently upstream from the monitoring station 

to allow for complete mixing during stormwater-runoff 

events. Samples of the dye were acquired with a dedicated 

automated sampler. Discrete samples were collected at 

predetermined, equal increments of rising or falling water 

levels. Samples were collected and returned to the Wisconsin Water Science Center in Middleton, Wis., for analysis. 

A detailed description of the methods used to conduct the 

dye-tracer calibration can be found in Wilson and others 

(1986). Resulting concentrations of rhodamine dye in each 

discrete sample were converted into an instantaneous discharge and then compared to the corresponding discharge 

measured at the basin outlet with the ultrasonic velocity 

sensor. Figure 5 illustrates the linear relation between the 

two methods of discharge computation. Of the 38 dye 

samples collected during the velocity-sensor calibration 

8 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
R2 = 0.89

0

10

20

30

40

50

60

70

0 10 20 30 40 50 60 70

DISCHARGE FROM WATER-LEVEL AND VELOCITY DATA,

IN CUBIC FEET PER SECOND

DISCHARGE FROM DYE-TRACER METHOD,

IN CUBIC FEET PER SECOND

Line of equal discharge 

Figure 5. Relation between actual discharge determined using a rhodamine dye tracer and measured discharge, computed by 

water-level and velocity data, during free-flow conditions at the conventional-development-basin outlet.

period, 35 were associated with water levels less than 

1.0 ft. Therefore, determination of discharges by use of a 

dye tracer for water levels greater than 1.0 ft can only be 

estimated by extrapolation. However, over 90 percent of all 

runoff events measured at the conventional basin outlet had 

maximum water levels less than or equal to 1.0 ft. Figure 5 

illustrates that the velocity of runoff measured in the pipe 

was greatly underestimated, resulting in discharges that 

were approximately one-third those computed from the 

dye-tracer measurements. Linear regression was used to 

derive an adjustment factor for those discharges that were 

computed by use of the velocity sensor. This adjustment 

factor was applied to all discharges recorded at the conventional-basin outlet for the entire study period. Church and 

others (1999) evaluated the bias and variability of several 

methods of flow measurement, including the sensors used 

in this study. Velocity sensors using acoustic or electromagnetic detection consistently underestimated actual 

values. Therefore, errors associated with velocity measurements at the conventional-basin outfall are not unique and 

highlight the importance of calibrating instrumentation 

used for water measurements. 

Precipitation

Continuous precipitation data were collected by 

means of tipping-bucket rain gages in the LID and conventional study basins. Each rain gage measured precipitation 

in 0.01-in. increments. These rain gages were not designed 

to measure snowfall; however, there were several runoff 

events during winter months where precipitation was in 

the form of rain instead of snow. Precipitation data were 

compiled, and statistical summaries were computed for 

both rain-gage locations. 

Water Temperature

Continuous water temperature was measured with an 

insulated thermocouple wire during periods of runoff. The 

LID thermocouple was inside the culvert approximately 

5 ft upstream from the H-flume approach section (fig. 4). 

The conventional-basin thermocouple was in the concrete 

pipe near the sample-intake orifice. 

Methods of Data Collection 9
Runoff-Sample Collection

Sample collection was activated by a rise in water 

level in the storm-sewer pipe or flume during a storm 

event. Once the water-level threshold was exceeded, typically a depth of 0.10 to 0.15 ft from the pipe/flume floor, 

the volume of water passing the station was measured and 

accumulated at 1-minute increments until a volumetric 

threshold was reached. At that point, the sampler collected a discrete water sample, and the volumetric counter 

was reset. The process was repeated until the water level 

receded below the water-level threshold ensuring adequate 

coverage of the storm hydrograph.

For the majority of sampled runoff events, the discrete 

flow-weighted samples were collected and combined into 

a single water sample, then split and processed for analysis. A Teflon-coated, stainless-steel churn splitter was used 

to composite and split samples. Processed samples were 

refrigerated until delivered to the laboratory, usually within 

48 hours after runoff cessation, for determination of concentrations of total solids, total suspended solids, and total 

phosphorus. Mass of a particular constituent, expressed as 

storm load, was computed by multiplying the event mean 

concentration by the total storm volume. In some cases, 

discrete samples were processed individually to determine 

instantaneous concentrations over the storm hydrograph. 

For these runoff events, the integration method was used to 

determine a storm load (Porterfield, 1972).

Field and sample-processing equipment blanks were 

collected at the control and test basin monitoring stations 

to evaluate the integrity of the water-quality sampling 

process, identify whether sample contamination existed 

and, if so, to identify possible sources. Blank samples were 

obtained by drawing deionized water through the suction line and sampler into a collection bottle. The Teflon 

sample line and automatic sampler were not cleaned 

before obtaining blank samples. Blank water collected in 

the 1-liter plastic sample bottle(s) was then split through a 

Teflon-lined churn splitter into plastic laboratory-prepared 

sample bottles. Samples were placed on ice and delivered 

to the Wisconsin State Laboratory of Hygiene (WSLH) for 

analysis. Deionized blank water also was used to isolate 

individual elements of the sampling process from source to 

delivery. These samples were not delivered to the WSLH 

unless erroneous concentrations were found in the original blank sample. Blank-sample results are detailed in 

appendix table 1-1. Replicate samples also were collected 

to evaluate the inherent variability in the sampling analyses 

and methods.

The bias and variability identified by analysis of 

blanks and replicates were within acceptable limits for 

total solids, total suspended solids, and total phosphorus. 

Two of the blank samples collected at the LID-basin outlet 

had concentrations above the detectable limit; one for 

total suspended solids and the other for total phosphorus. 

Because the concentrations of these two constituents were 

much lower than the majority of those measured in waterquality samples, they were considered insignificant to the 

overall integrity of the water-quality sampling process. 

Only one blank sample collected at the conventional basin 

showed constituent concentrations slightly above the 

detectable limit. Although the concentrations were within 

the range of those measured in water-quality samples at 

this site, a review of field notes indicated the possibility of 

contamination during sample acquisition. Therefore, the 

results from this sample were excluded from analysis. 

Replicate samples were submitted to verify reproducibility in the sample acquisition and splitting process as 

well as analytical methods conducted in the laboratory. 

Replicate samples were checked for precision on the basis 

of an absolute relative percent difference (RPD). Replicate samples are listed in appendix table 1-2. The RPD 

values for replicate samples collected at both the LID and 

conventional basin outlets were within an acceptable range 

of error (25 percent). 

Comparison of Runoff Quantity from 

the Basins

One of the underlying principles of LID is to maintain or replicate predevelopment hydrologic conditions as 

closely as possible (Davis, 2005). Advances in structural 

and nonstructural runoff controls, such as green roofs and 

reduced impervious area, could provide a way to mitigate 

storm runoff that might otherwise go untreated. To quantify potential hydrologic benefits of LID, the LID basin, in 

which a variety of best-management practices (BMPs) was 

used, was monitored to detect changes in water quantity 

and quality from predevelopment phase through final 

build-out. Data from the LID basin were compared to the 

conventional basin, an existing, fully developed basin built 

with conventional construction techniques, with a fully 

connected stormwater-conveyance system.

10 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
0

5

10

15

20

25

30

35

40

1999 2000 2001 2002 2003 2004 2005

YEAR

(APRIL–OCTOBER)

PRECIPITATION AT LID, CONVENTIONAL, AND DANE COUNTY REGIONAL AIRPORT 

RAIN GAGES, IN INCHES 

Low-impact-development basin

Conventional basin

Dane County Regional Airport

30-year normal 

Figure 6. Precipitation totals for April through October at the low-impact-development (LID) and conventional-development 

basins during each study year. Precipitation measured at the Dane County Regional Airport in Madison, Wis., (approximately 

15 miles from the study area) represents a 30-year normal (1976–2005) for the same 5-month span and is represented by the 

horizontal dotted line.

Precipitation

Differences in rainfall patterns between study basins 

from one year to the next could potentially affect results 

from data analyses. An examination of rainfall was done 

to identify potential sources of bias. Analyses of rainfall 

patterns were limited to April through October of each 

water year because rain gages in the study area were not 

equipped to measure the water equivalency of snowfall. In 

addition, nearly 75 percent of annual precipitation in the 

Cross Plains area typically occurs during April through 

October (National Oceanic and Atmospheric Administration, 1999–2005). The 30-year normal (1976–2005) 

precipitation for April through October was 23.47 in. at 

the National Oceanic and Atmospheric Administration 

(NOAA) weather station at the Dane County Regional 

Airport in Madison, Wis. (National Oceanic and Atmospheric Administration, 1999–2005). Data from the rain 

gages in each study basin show that rainfall was above the 

30-year normal for 4 of 7 years monitored in the LID study 

basin (1999, 2000, 2001, and 2004) and below the 30-year 

normal for 3 years (2002, 2003, and 2005) (fig. 6). Rainfall 

in the conventional basin was above the 30-year normal for 

4 of the 7 years (1999, 2000, 2001, and 2004) and below 

normal for the remaining 3 years (2002, 2003, and 2005). 

Rainfall recorded at the Dane County Regional Airport 

exhibited the same overall pattern (fig. 6).

A Kendall test for trend (Helsel and Hirsch, 1992) 

was performed on 7 years of precipitation data (1999–

2005), covering April through October, collected at the 

conventional and LID study basins. No significant trend 

was identified at the 5-percent significance level. A similar 

test was performed for the precipitation data collected at 

the Dane County Regional Airport. Again, no significant 

trend was identified. 

Comparison of Runoff Quantity from the Basins 11
0

10

20

30

40

50

60

70

80

90

100

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0

STORM PRECIPITATION, IN INCHES

NUMBER OF EVENTS

Low-impact-development basin

Conventional-development basin

Figure 7. Histograms showing the frequency of rainfall events resulting in discharge from the study area as a function of 

increasing precipitation depth in the low-impact-development (LID) and conventional-development basins.

Event rainfall also was evaluated for differences 

between study basins. A storm event was defined as any 

precipitation event greater than 0.1 in. in depth preceded 

by 6 or more hours of no precipitation. The Wilcoxon 

rank-sum test (Helsel and Hirsch, 1992), used to test 

whether one group tends to produce larger observations 

than a second group, revealed no significant differences (at 

the 5-percent significance level) in event rainfall between 

the study basins for each of the 7 years monitored. Climatic characteristics for sampled precipitation events in 

each basin are detailed in tables 2-1 and 2-2 in appendix 2.

Runoff Volumes

Summary statistics for event volumes measured in 

the conventional and LID basins are presented in table 

2. Hereafter in this report, a “discharge event” is defined 

as rainfall or snowmelt that results in discharge from the 

study area. Differences in the number of discharge events, 

annual discharge volumes, and peak-flow rates were 

identified between the two study basins. Mean and median 

annual discharge volumes in the LID basin were less than 

those in the conventional basin during each study year. 

Because these values represent runoff from all precipitation events, they reflect the numerous times when there 

was no discharge from the LID basin; however, those 

precipitation events that did not generate runoff in the 

LID basin were excluded from the total number of runoff 

events described in table 2. The number of precipitation 

events generating a measurable amount of discharge from 

the conventional basin far exceeded those in the LID basin 

(table 2). Smaller, more frequent precipitation events that 

produced discharge from the conventional basin were 

retained and infiltrated in the LID basin. Histograms are 

presented in figure 7 for precipitation events generating a 

measurable volume of discharge from the conventional and 

LID basins, respectively. Only six events with precipitation depths less than or equal to 0.4 in. produced discharge 

from the LID basin. Of these six events, five occurred during winter months when soils are commonly frozen, and 

one was likely a result of saturated-soil conditions from a 

preceding storm. In the conventional basin, the number of 

discharge events, using the same threshold of precipitation 

depth, was 180, with nearly one-half of those resulting 

from precipitation depths less than 0.2 in. (fig. 7). The total 

volume of discharge generated from these 180 events was 

12 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Table 2. Summary statistics for measured discharge events at the low-impact-development (LID) and conventional-development 

basins monitoring stations, water years 1999–2005.

[<, less than; ft3/s, cubic feet per second]

Statistic

Water year a

1999 2000 2001 2002 2003 2004 2005

Low-impact-development basin

Volume (inches)b

Mean <0.01 0.03 0.03 <0.01 0.01 0.03 0.01

Median .00 .00 .00 .00 .00 .00 .00

Maximum .05 .42 1.11 .11 .12 .67 .30

Minimum .00 .00 .00 .00 .00 .00 .00

Sum .16 1.51 1.72 .34 .49 1.81 .74

Number of events c 13 17 16 11 11 18 12

Peak flow (ft3/s)c

Mean .37 4.05 2.62 .66 1.18 1.99 .59

Median .20 .36 .77 .35 1.05 .73 .23

Maximum 2.19 21.84 22.28 1.93 4.11 10.10 2.20

Minimum .02 .07 .05 .10 .02 .11 .05

Conventional-development basin

Volume (inches)b

Mean .03 .04 .05 .03 .03 .04 .03

Median .02 .01 .02 .02 .02 .02 .01

Maximum .12 .33 .83 .13 .24 .30 .37

Minimum <.01 <.01 <.01 <.01 <.01 <.01 <.01

Sum 1.50 2.22 2.74 1.61 1.40 2.44 1.92

Number of eventsc 49 57 50 55 50 63 58

Peak flow (ft3/s)c

Mean 9.01 10.16 8.46 7.14 7.73 10.27 5.20

Median 3.87 3.10 3.57 4.97 4.17 5.07 2.74

Maximum 79.30 76.30 78.10 27.20 79.40 96.00 25.80

Minimum .28 .10 .36 .15 .19 .12 .10

a

 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 

2004 water year occurred October 1, 2003, through September 30, 2004).

b

 Represents all precipitation and snowmelt events, including those that did not produce discharge.

c

 Represents only those precipitation and snowmelt events that produced discharge.

nearly 10 times the total volume generated by the 6 events 

in the LID basin. This difference is likely due to the larger 

percentage of impervious surface and the fully connected 

stormwater-conveyance system used in this basin. Stormwater is quickly routed from impervious surfaces into 

a concrete collection system, with little opportunity for 

retention and infiltration. 

For small, frequent storm events, the hydrologic 

impact associated with traditional, fully connected systems 

appears to have been reduced in the LID basin by incorporating structural and nonstructural controls designed 

to reduce stormwater runoff. Holman-Dodds and others 

(2003) came to similar conclusions based on model results 

comparing a fully developed landscape using traditional 

Comparison of Runoff Quantity from the Basins 13
0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

PRECIPITATION DEPTH, IN INCHES

CUMULATIVE DISCHARGE VOLUME, IN PERCENT

Low-impact-development basin

Conventional-development basin

Figure 8. Relation of cumulative-discharge volume, as a percentage of total discharge volume measured between 1999–2005, 

to increasing precipitation depth in the low-impact-development (LID) and conventional-development basins.

stormwater management as opposed to one that uses infiltration-based, low-impact design. The cumulative effect on 

discharge volume from these small, frequent precipitation 

events becomes apparent in each study basin. The annual 

sum of all discharge events measured in the conventional basin exceeded those measured in the LID basin, 

in some cases by an order of magnitude (table 2). Total 

annual discharge volume measured from the conventional 

basin ranged from 1.3 to 9.2 times that for the LID basin. 

Cumulative discharge volume measured at the LID and 

conventional basins as a percentage of total volume with 

increasing precipitation depths from 1999 through 2005 

is shown in figure 8. Fifty percent of discharge volume in 

the conventional basin was associated with precipitation 

depths less than 1 in. (fig. 8). This agrees with previous 

work by Pitt and others (1999) describing the quantity 

of runoff in Madison, Wis., that can be expected with 

increasing rainfall depths from a typical medium-density 

residential development (fig. 9). Compared to the conventional basin, less than 1 in. of precipitation in the LID 

basin was associated with only 16 percent of cumulative 

discharge. One-half of total discharge volume was associated with precipitation depths greater than 3 in. compared 

to 18 percent in the conventional basin (fig. 8). Nearly 20 

percent of the total discharge volume from the LID basin 

came from a single, large event in August 2001. More than 

9.5 in. of rain fell within 24 hours, exceeding the 100-year, 

24-hour precipitation depth by more than 3 in. (Huff and 

Angel, 1992). Although precipitation events like this are 

atypical, they serve as a reminder that the influence of lowimpact design practices on runoff becomes less noticeable 

for large, intense precipitation events or storm events that 

occur in rapid succession when soils are saturated. Many 

BMPs are not designed to adequately control the volume 

of runoff associated with these events. 

Similar to discharge volumes, peak-flow rates in the 

conventional basin were higher than in the LID basin. 

Precipitation events capable of producing appreciable 

discharge from the LID basin were typically associated 

with those of high intensity, substantial depth, or occur14 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
100

80

60

PERCENT ASSOCIATED WITH RAIN, OR LESS

40

20

0

0.01 0.1

RAIN (INCHES)

1 10

Cumulative rain count

Cumulative commercialrunoff quantity

Cumulative residentialrunoff quantity

Figure 9. Relation of cumulative-discharge volume to precipitation depth for residential land use in Madison, Wis., based on 

model predictions (modified from Pitt,1999).

rence after soils were already saturated. The LID basin, 

when compared to the conventional basin, was effective 

at reducing discharge volume and peak flows for the first 

event but was relatively ineffective during the second event 

(fig. 10). The time of concentration decreases from 24 to 

11 minutes during the second event as storage capacity is 

reduced and soils become saturated. Despite the reduction 

in the ability to effectively manage discharge during the 

second event detailed in figure 10, the time of concentration in the LID basin is still greater than the conventional 

basin where, by comparison, very little time lapses from 

the onset of rainfall to measurable discharge in the storm 

sewer. The conventional basin demonstrated a similar 

hydrologic response during both events, as illustrated in 

figure 10. Hydrologic characteristics for sampled precipitation events in each basin are detailed in tables 2-1 and 

2-2 in appendix 2.

Temporal Changes in Runoff Quantity in the 

Low-Impact-Development (LID) Basin

Changes in hydrologic response in the LID basin 

were characterized over time from predevelopment to near 

complete build-out. Predevelopment conditions were monitored from June 24, 1998, through May 14, 1999. Development of the LID basin began on May 14, 1999, with 

deployment of earth-moving machinery. Because measurements of water quantity and quality during the predevelopment phase of this study did not span a full year, comparison to subsequent years cannot be made. Therefore, 

an examination of precipitation characteristics, including 

depth, intensity, and antecedent dry days was done on data 

collected at a nearby U.S. Geological Survey monitoring 

station (05406470), located approximately 1.25 miles from 

Comparison of Runoff Quantity from the Basins 15
0

5

10

15

20

25

30

35

05/21/04

06:00

05/21/04

08:24

05/21/04

10:48

05/21/04

13:12

05/21/04

15:36

05/21/04

18:00

05/21/04

20:24

DATE AND TIME

DISCHARGE, IN CUBIC FEET PER SECOND

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

CUMULATIVE PRECIPITATION, IN INCHES

Conventional-development basin discharge

Low-impact-development basin discharge

Cumulative precipitation

Tc lag = 24 minutes Tc lag = 11 minutes

Figure 10. Hydrologic response of low-impact-development (LID) and conventional-development basins to two consecutive 

precipitation events, Cross Plains, Wis. [Tc , time of concentration]

the LID basin, to estimate hydrologic conditions in the 

LID basin during May and June 1998. The largest storm 

event during this period produced 2.27 in. of precipitation 

in a 24-hour period (June 18, 1998). Two storms with similar characteristics to this storm occurred after the waterquantity and -quality monitoring station was installed in 

the LID basin. The first storm event occurred on June 27 

and the second on September 14, 1998, producing 2.13 

and 2.43 in. of rain, respectively, in a 24-hour period. Neither storm produced measurable discharge. Therefore, it 

was assumed that no discharge occurred during May–June 

1998, which was not monitored during the predevelopment 

phase. For consistency, characterization of hydrologic 

response to precipitation events in the LID basin during the 

predevelopment phase, as well as all subsequent phases of 

build-out, was based on a 12-month period starting in May 

and ending in April. The hydrologic response of the LID 

basin during the predevelopment and active construction 

phases of the study is shown in table 3. For comparison, 

similar hydrologic characteristics of the conventional basin 

also are detailed in table 3. However, because monitoring 

equipment was not installed in the conventional basin until 

August 1998, data collected during the predevelopment 

year was incomplete. Unlike the LID basin, nearly every 

precipitation event in the conventional basin produced 

measurable discharge, making it difficult to estimate the 

volume of discharge for storms prior to site installation. 

Each year of active construction was differentiated 

by the level of activity recorded in the basin. Construction activity and housing density in the LID basin during 

each study year are shown in figure 11. The first year 

of construction included not only construction of a few 

single-family homes but also land disturbances typically 

associated with site grading and installation of infrastructure such as roads, storm sewers, and stormwater-control 

structures. Estimates of housing density, represented by a 

percentage of complete build-out of the area illustrated in 

figure 2 are detailed in table 3. Ninety-five percent or more 

of precipitation falling in the LID basin was retained during each construction year. The conventional basin consis16 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Table 3. Comparison of precipitation and runoff characteristics in the low-impact-development (LID) and conventional-development basins over increasing percentages of 

basin development. [--, insufficient data]

Year of active development (May through April)

Predevelopment 1999–2000 2000–2001 2001–2002 2002–2003 2003–2004 2004-2005

LID Conven- tional LID Conven- tional LID Conven- tional LID Conven- tional LID Conven- tional LID Conven- tional LID Conven- tional

Number of precipitation eventsa 63c -- 55 51 61 48 53 58 38 40 48 48 54 57Total precipitation volume (inches)a 35.4 -- 26.7 28.5 36.9 33.8 37.6 37.2 19.8 21.8 32.5 33.2 30.5 32.2Number of discharge eventsb 7 -- 10 55 19 51 18 61 7 46 18 60 19 68Total discharge volume (inches)b .11 -- .19 1.7 1.47 3.4 1.75 4.1 .40 1.9 1.26 2.9 1.61 3.9Precipitation retained (percent) 100 -- 99 94 96 90 95 89 98 91 96 91 95 88Basin development (percent) 0 -- 20 100 35 100 50 100 75 100 85 100 95 100

a Does not include snowfall.b Includes snowmelt events.c Includes precipitation events recorded by nearby rain gage for May through June 1998.

tently retained less, ranging from 88 to 94 percent of total 

precipitation. The percent of precipitation retained in the 

conventional basin is similar to that measured from other 

residential basins constructed in a similar style in Madison, 

Wis. (Selbig and Bannerman, 2007). These figures are 

slightly underestimated because they include the volume 

of discharge due to snowmelt but do not account for added 

precipitation volumes from the water equivalency of 

snow during winter months. An examination of climatic 

records collected at the Dane County Regional Airport 

for December through March of each study year showed 

a range of 1.3 (2002–2003) to 3.1 (2004–2005) additional 

inches of precipitation falling on each basin from snow 

(National Oceanic and Atmospheric Administration, 

1999–2005). Adding the estimated water equivalency of 

snow to the total precipitation amounts detailed in table 3 

had little effect on the percentage of precipitation retained 

during each study year. The largest percentage of total 

precipitation retained in the LID basin occurred during 

the predevelopment period. Of the 63 precipitation events 

measured during this period, only 7 produced any measur-

able discharge. Three of the seven runoff events occurred 

during February 1999 and were a direct result of snow-

melt. The first year of construction (1999–2000) had the 

second highest percentage of runoff retained. During this 

period, the majority of construction activity was related 

to site grading and infrastructure, and a limited amount of 

newly created impervious surfaces contributed to discharge 

volumes. As more houses were built in the basin during 

each successive construction year, the percentage of runoff 

retained tended to decrease. One notable exception was the 

2002–2003 construction year. Annual rainfall during this 

period was well below normal. The number of discharge 

events recorded during the 2002–2003 construction year 

was similar to that measured during the predevelopment 

phase. Although the percentage of precipitation retained 

in 2002–2003 was greater than in previous construction 

years, it was still less than in the predevelopment period; 

that is, the natural landscape appears to retain a greater 

percentage of rainfall than the LID basin does. 

Although the LID basin was able to retain a larger 

percentage of rainfall during each year of construction 

than the fully developed conventional basin, comparisons 

are best made during 2004–2005 when the LID basin was 

nearly 100 percent developed. During this year, more than 

3 times as many discharge events were measured at the 

outfall for the conventional basin. After normalizing by 

basin area, the volume of discharge leaving the conven-

tional basin was nearly 4 in. compared to 1.6 in. in the LID 

basin (table 3). 

Comparison of Runoff Quantity from the Basins 17
September 1999 September 2000

September 2001 September 2002

September 2003 September 2004

Figure 11. Yearly changes in construction activity and housing density in the low-impact-development (LID) basin, 1999–2004.

18 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
In Wisconsin, proposed development of new residential areas must be designed to infiltrate at least 90 percent 

of the predevelopment infiltration volume (Wisconsin 

Administrative Code, 2002). The quantity of discharge 

from the LID basin during predevelopment and the 2004–

2005 study periods was compared to estimate whether 

State infiltration performance standards for new residential 

development would likely be achieved. Differences in climatic conditions between these two periods could produce 

large differences in the reduction of runoff volumes. The 

Wilcoxon rank-sum test (Helsel and Hirsch, 1992) was 

used to determine whether climatic conditions in the LID 

basin during the predevelopment phase were different from 

those measured during 2004–2005, when the basin was 

95 percent developed. The test was done on storm-event 

precipitation depth and 60-minute intensities measured in 

the LID basin. The null hypothesis states that there is no 

difference in the distribution of depth or intensity between 

study periods. The alternative hypothesis is that there is a 

significant difference, which can be inferred to be a result 

of climatic variation. Results of the test indicate no difference in precipitation depth or 60-minute precipitation 

intensities between the predevelopment and 2004–2005 

study phases at the 5-percent significance level; therefore, 

because the LID basin retained 95 percent of rainfall during the period at which it was most developed, the infiltration standard of 90 percent of predevelopment infiltration 

volume was satisfied (table 3).

Performance of Infiltration Basin

Stormwater in the LID basin was routed to one or 

more retention structures. Stormwater in excess of retention capacity of lawns and grassed swales was directed to a 

1.75-acre infiltration basin along the northern boundary of 

the development (fig. 2). This was the largest control structure used to mitigate storm-runoff quantity and quality. It 

was also the last BMP used to treat stormwater before the 

stormwater discharged from the LID basin. 

In May 2002, instrumentation was installed to monitor the quantity of runoff into the infiltration basin. The 

total volume of water entering the infiltration basin was 

the sum of runoff volumes contributed from the detention 

pond, grass swales, and direct precipitation. The reduction 

in runoff volume attributed to the infiltration basin was 

determined by comparing total runoff into the infiltration 

basin to total discharge from the study area. From May 

25, 2002, to September 30, 2005, 179 rain and snowmelt 

events were measured in the LID basin. Of these events, 

24 were either snowmelt or a combination of rainfall 

and snowmelt. The percentage volume reduction by the 

infiltration basin for those 24 events could not be determined because the volume of water entering (and within) 

the infiltration basin as snowmelt could not be quantified. 

Precipitation characteristics and the volume of runoff 

entering and leaving the infiltration basin for the remaining 

155 precipitation events are detailed in table 4. The overall 

volume of runoff into the infiltration basin was reduced by 

51 percent before leaving the study area as discharge. The 

majority of precipitation events produced a small amount 

of runoff that was completely retained by the infiltration 

basin. Much of the potential runoff volume was intercepted 

prior to entering the infiltration basin. A combination of 

storage, retention, and infiltration by forested hillslopes, 

lawns, grassed swales, and a detention pond within the 

LID basin provided an overall decrease in the potential 

volume of runoff of 96 percent prior to reaching the 

infiltration basin (table 4). In general, precipitation from 

low-intensity, long-duration storm events was absorbed by 

the surrounding landscape, whereas precipitation generated 

from high-intensity storm events typically exceeded infiltration rates of underlying soils, resulting in larger volumes 

of runoff leaving the LID basin as discharge. 

A stepwise multivariate linear-regression analysis 

incorporating precipitation depth, 15-, 30-, and 60-minute 

precipitation intensities, and antecedent dry days was done 

to test whether a relation existed between percent runoffvolume reduction in the infiltration basin and climatic 

variables. Results of the test indicated that 60-minute 

precipitation intensities were able to explain 29 percent of 

the variation in the percentage of runoff volume reduced 

in the infiltration basin at the 5-percent significance level. 

There was a negative relation between 60-minute precipitation intensities and the percentage reduction between 

influent and effluent runoff volumes in the infiltration 

basin; therefore, the effectiveness of runoff-volume reduction in the infiltration basin, and perhaps the LID basin in 

general, decreased with increasing precipitation intensity. 

The mean and median runoff-volume reductions by the 

infiltration basin when grouped by increasing ranges of 

60-minute precipitation intensities are shown in table 5. 

The effectiveness of the infiltration basin decreased as 

precipitation intensities exceeded 0.5 in. per hour. Only 

the 24 events that produced enough stormwater to exceed 

the retention and infiltrative capacity of the infiltration 

basin (table 4) were used to provide data for table 5. The 

inability of the LID basin to retain precipitation events 

of appreciable depth and (or) intensity might be a result 

of one or a combination of several factors, which may 

include the rate of precipitation exceeding the maximum 

Comparison of Runoff Quantity from the Basins 19
Table 4. Reduction of runoff volume in the low-impact-development (LID) basin by the infiltration basin, 2002–2005.

[mm/dd/yyyy, month, day and year; min., minute; ft3, cubic foot; --, no data]

Storm date

(mm/dd/yyyy)

Storm 

duration 

(hours)

Precipitation 

depth 

(inches)

Precipitation intensity

(inches per hour)

Antecedent 

dry time

(days)

Total 

volume IN

(ft3

)

Total 

volume OUT

(ft3

)

Volume reduction

(percent)

15-min. 30-min. 60-min. Infiltration basin Rest of LID basin

05/25/2002 6.4 0.69 0.32 0.26 0.22 12.1 6,028 0 100 005/28/2002 6.2 .15 .24 .18 .11 3.4 958 0 100 006/02/2002 2.0 .19 .24 .18 .15 4.6 1,214 0 100 006/03/2002 58.3 2.38 1.52 .90 .53 .3 132,703 74,036 44 9306/10/2002 1.6 .28 .68 .38 .23 5.3 2,430 0 100 006/13/2002 4.7 .32 .60 .42 .27 2.8 3,621 0 100 006/14/2002 8.0 .11 .32 .16 .08 .6 703 0 100 006/26/2002 4.2 .54 1.08 .66 .35 11.4 3,450 0 100 007/20/2002 .4 .37 1.23 -- -- 24.3 2,370 0 100 007/22/2002 6.8 .84 1.54 .90 .50 1.4 6,690 0 100 007/27/2002 4.7 .17 .14 .09 .06 4.9 1,092 0 100 008/04/2002 8.2 .60 1.42 .83 .44 7.7 4,142 0 100 008/11/2002 1.1 .63 .98 .65 .55 7.3 5,825 0 100 008/12/2002 9.7 .32 .18 .18 .14 1.1 2,161 0 100 008/17/2002 .7 .16 .48 .29 -- 4.0 1,291 0 100 008/21/2002 3.1 .25 .67 .46 .24 4.4 11,648 0 100 008/22/2002 6.0 .31 .14 .13 .11 .3 19,132 0 100 009/02/2002 4.6 .56 .25 .23 .22 10.7 5,252 0 100 009/10/2002 1.1 .15 .25 .20 .14 8.2 977 0 100 009/19/2002 4.0 .30 .32 .22 .14 8.5 1,897 0 100 009/19/2002 1.2 .14 .36 .22 .13 .3 862 0 100 009/20/2002 14.0 .18 .29 .16 .08 .6 1,150 0 100 009/25/2002 .0 .33 -- -- -- 4.9 2,121 0 100 009/28/2002 7.9 .40 .18 .18 .11 3.3 3,036 0 100 010/01/2002 35.5 .53 .24 .16 .13 2.5 6,936 0 100 010/03/2002 32.2 1.11 1.28 .88 .64 .8 56,117 23,777 58 9410/24/2002 26.5 .47 .16 .14 .10 19.7 3,003 0 100 0

20 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Storm date

(mm/dd/yyyy)

Storm 

duration 

(hours)

Precipitation 

depth 

(inches)

Precipitation intensity

(inches per hour)

Antecedent 

dry time

(days)

Total 

volume IN

(ft3)

Total 

volume OUT

(ft3)

Volume reduction

(percent)

15-min. 30-min. 60-min. Infiltration basin Rest of LID basin

11/10/2002 9.1 .49 .20 .20 .16 15.9 3,131 0 100 011/18/2002 5.9 .25 .16 .12 .08 7.3 1,597 0 100 012/17/2002 15.0 .57 .32 .16 .14 29.1 3,642 0 100 001/31/2003 4.9 .18 .08 .06 .06 43.8 1,150 0 100 003/28/2003 16.7 .37 .08 .06 .05 8.2 3,486 0 100 004/19/2003 18.0 .22 .08 .06 .05 12.9 1,406 0 100 004/30/2003 52.1 2.12 1.32 1.22 .70 9.8 98,363 61,949 37 9405/04/2003 13.4 .46 .24 .18 .17 2.4 7,003 0 100 005/07/2003 22.6 .42 .44 .34 .20 1.8 11,446 492 96 9705/09/2003 28.4 .51 .32 .28 .24 .9 22,676 10,143 55 9405/10/2003 60.9 1.48 2.28 1.26 .98 .7 96,171 81,268 15 9205/14/2003 23.9 .61 .20 .18 .15 .8 15,858 4,614 71 9705/28/2003 5.6 .22 .40 .20 .20 13.1 1,406 0 100 005/30/2003 12.9 0.38 0.68 0.56 0.30 2.1 2,681 0 100 006/06/2003 7.7 .26 .28 .20 .14 6.2 1,661 0 100 006/08/2003 1.8 .25 .60 .34 .18 1.3 1,630 0 100 006/08/2003 .6 .37 .72 .70 -- .6 5,298 0 100 006/18/2003 1.5 .24 .48 .34 .21 10.1 1,533 0 100 006/22/2003 .2 .11 .44 -- -- 3.9 703 0 100 006/24/2003 1.7 .30 .56 .36 .22 1.4 1,917 0 100 006/24/2003 .2 .11 -- -- -- .4 703 0 100 006/25/2003 7.7 .39 .84 .54 .29 .9 2,572 0 100 006/28/2003 11.1 1.01 .80 .54 .39 2.1 14,386 0 100 007/03/2003 5.2 2.21 3.96 3.90 2.12 4.8 14,119 0 100 007/04/2003 1.2 .22 .36 .22 .18 .7 1,406 0 100 007/05/2003 2.0 .30 .24 .24 .20 .9 2,670 0 100 007/06/2003 55.8 1.82 3.88 2.04 1.04 1.3 73,453 41,562 43 95

Table 4. Reduction of runoff volume in the low-impact-development (LID) basin by the infiltration basin, 2002–2005—Continued.[mm/dd/yyyy, month, day and year; min., minute; ft3, cubic foot; --, no data]

Comparison of Runoff Quantity from the Basins 21
Storm date

(mm/dd/yyyy)

Storm 

duration 

(hours)

Precipitation 

depth 

(inches)

Precipitation intensity

(inches per hour)

Antecedent 

dry time

(days)

Total 

volume IN

(ft3

)

Total 

volume OUT

(ft3

)

Volume reduction

(percent)

15-min. 30-min. 60-min. Infiltration basin Rest of LID basin

07/09/2003 7.0 .11 .20 .10 .06 1.1 4,480 0 100 007/15/2003 2.9 .94 1.76 1.70 .93 4.8 47,182 0 100 007/21/2003 28.2 .97 1.68 1.02 .73 5.8 48,248 31,467 35 9408/02/2003 .2 .14 -- -- -- 11.4 894 0 100 008/16/2003 .3 .18 .68 -- -- 14.2 1,150 0 100 008/20/2003 1.7 .11 .12 .10 .08 4.0 703 0 100 008/28/2003 .9 .46 1.44 .88 -- 8.0 3,168 0 100 009/12/2003 61.5 3.56 1.00 .68 .48 14.5 135,824 57,344 58 9509/22/2003 2.6 .11 .28 .20 .10 7.8 703 0 100 010/11/2003 1.9 .25 .36 .22 .17 18.9 1,608 0 100 010/13/2003 11.2 .66 .28 .26 .21 2.0 4,631 0 100 010/24/2003 4.8 .51 .36 .32 .30 10.3 3,535 0 100 011/01/2003 21.7 1.35 .44 .30 .20 8.0 8,625 0 100 011/03/2003 70.8 3.64 1.28 1.00 .76 .5 518,093 306,858 41 8011/17/2003 13.5 .24 .08 .04 .04 11.6 1,533 0 100 011/22/2003 33.5 2.05 1.48 .88 .63 4.5 280,524 123,068 56 8112/04/2003 13.0 .14 .04 .04 .03 10.7 894 0 100 012/09/2003 23.2 1.17 .20 .18 .16 4.0 160,625 19,362 88 8112/27/2003 5.2 .18 .16 .10 .07 11.6 1,150 0 100 003/10/2004 4.3 .11 .12 .06 .05 4.3 703 0 100 003/13/2004 5.0 .23 .16 .10 .08 2.9 1,469 0 100 003/24/2004 4.5 .32 .24 .16 .11 6.5 2,044 0 100 003/25/2004 39.1 1.07 .76 .68 .51 1.4 55,745 35,087 37 9303/28/2004 8.5 .33 .24 .18 .14 1.2 2,108 0 100 004/16/2004 .1 .12 -- -- -- 19.2 1,222 0 100 004/20/2004 5.6 0.47 0.20 0.18 0.17 3.6 4,442 0 100 0

Table 4. Reduction of runoff volume in the low-impact-development (LID) basin by the infiltration basin, 2002–2005—Continued.[mm/dd/yyyy, month, day and year; min., minute; ft3, cubic foot; --, no data]

22 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Storm date

(mm/dd/yyyy)

Storm 

duration 

(hours)

Precipitation 

depth 

(inches)

Precipitation intensity

(inches per hour)

Antecedent 

dry time

(days)

Total 

volume IN

(ft3)

Total 

volume OUT

(ft3)

Volume reduction

(percent)

15-min. 30-min. 60-min. Infiltration basin Rest of LID basin

04/24/2004 17.6 .29 .24 .14 .07 4.0 1,853 0 100 005/08/2004 3.4 .11 .28 .14 .07 12.8 703 0 100 005/08/2004 4.6 .42 .64 .38 .22 .4 2,922 0 100 005/10/2004 12.8 .45 .48 .34 .22 1.1 5,693 0 100 005/13/2004 5.5 .48 .76 .50 .27 2.8 12,223 0 100 005/14/2004 7.6 .27 .12 .08 .07 .5 1,725 0 100 005/17/2004 6.9 .75 2.24 1.36 .71 3.3 24,734 4,985 80 9605/21/2004 74.7 6.21 2.40 1.68 1.13 3.2 503,562 469,610 7 8905/25/2004 1.5 .18 .44 .24 .17 .6 1,150 0 100 005/29/2004 65.3 1.67 1.24 .80 .54 4.2 66,551 27,994 58 9506/09/2004 6.0 .12 .32 .18 .09 8.9 767 0 100 006/10/2004 20.6 .94 .56 .44 .29 .3 12,619 0 100 006/12/2004 2.1 .42 .84 .72 .39 .7 13,606 2,411 82 9606/16/2004 10.0 .48 .36 .30 .24 4.8 3,299 0 100 006/21/2004 3.2 .21 .20 .14 .09 4.1 1,342 0 100 007/03/2004 21.0 1.08 1.20 .98 .61 12.1 32,841 6,532 80 9707/09/2004 1.2 .17 .36 .24 .16 5.0 1,258 0 100 007/16/2004 24.0 1.05 1.72 1.20 .62 6.9 98,939 59,435 40 8707/21/2004 2.6 .39 .56 .34 .19 3.9 2,598 0 100 007/29/2004 7.4 .28 .32 .18 .12 8.0 1,789 0 100 007/30/2004 1.6 .14 .32 .20 .11 .7 894 0 100 008/01/2004 1.2 .35 .72 .40 .33 1.7 2,417 0 100 008/02/2004 2.0 .38 .72 .52 .28 1.1 2,890 0 100 008/03/2004 20.2 1.20 3.84 2.02 1.09 1.2 68,053 36,253 47 9308/18/2004 .9 .33 .76 .58 -- 14.1 2,672 0 100 008/24/2004 .4 .12 .44 -- -- 6.1 767 0 100 0

Table 4. Reduction of runoff volume in the low-impact-development (LID) basin by the infiltration basin, 2002–2005—Continued.[mm/dd/yyyy, month, day and year; min., minute; ft3, cubic foot; --, no data]

Comparison of Runoff Quantity from the Basins 23
Table 5. Runoff volume reduction by the infiltration basin as a function of increasing precipitation intensity.

[>, greater than]

Statistic

Percent reduction in potential runoff for indicated 

precipitation intensity, 60-minute

(inches per hour)

0–0.5 0.5–1.0 >1.0

Mean (percent reduction) 69 43 32

Median (percent reduction) 71 44 43

Number of events 8 13 3

rate of infiltration, contribution of additional stormwater 

from forested uplands that normally have little to no runoff 

from smaller precipitation events, saturated soil from prior 

storms, or frozen soil. 

Infiltration rates in the infiltration basin were measured at various locations during the summers of 2000–

2004 (except 2001). Infiltration tests also were done in 

2005 but were considered unreliable and were not used. A 

double-ring infiltrometer was used to quantify the steady 

rate of infiltration, which is often equated to the saturated 

hydraulic conductivity of underlying soils (Dierks, 2007). 

The approximate location of each double-ring infiltrometer test is shown in figure 12. The estimated infiltration 

rates at each location are provided in table 6. Infiltration 

rates varied within locations from one year to the next and 

between locations in any given year. Variation in infiltration rates could be a function of changing vegetation and 

root growth, clogging of pores and openings as a result of 

sedimentation during periods of episodic inundation, or 

simply inconsistent location of the double-ring infiltrometer from prior tests. Attempts were made to minimize 

errors associated with reproducing the test location by 

identifying each site with latitude and longitude coordinates acquired by a handheld global positioning system. 

These devices were accurate to within 20 ft, thereby 

allowing for slight inaccuracies in placement of the 

infiltrometer. Infiltration rates are much lower along the 

western edge of the infiltration basin as was determined at 

points 2 and 3 (fig. 12 and table 6). The reduced infiltration rates are likely a result of the sloping topography of 

the infiltration basin. Ideally, runoff entering the infiltration basin from the detention pond would spread evenly 

across the width of the infiltration basin before leaving the 

site as discharge through the main culvert on the northern edge; however, because the infiltration basin was not 

level, runoff would move preferentially along the western boundary of the infiltration basin and pond near the 

northwest corner. Field surveys were done in September 

2001 to confirm the overall slope of the infiltration basin. 

The contours of the infiltration basin and locations of the 

detention-pond inlet, grassed-swale inlets, and main-culvert outlet are shown in figure 12. As suspected, the lowest 

elevation was in a small area near the main-culvert outlet. 

Over time, fine silts and clays suspended in runoff near 

this area settled out and clogged the upper layers of soil, 

effectively reducing the infiltrative capacity of underlying 

soils. A layer of silt that has developed over time near the 

western grassed-swale-culvert inlet and main-culvert outlet 

is shown in figure 13. Had runoff into the infiltration basin 

been spread across a uniform, level surface, it is likely 

some of the precipitation events that produced stormwater 

in exceedance of the retention capacity of the infiltration 

basin may have instead been completely retained. The 

volume of runoff capable of being stored in the infiltration 

basin was estimated based on survey data and the depth 

of water required before reaching the H-flume invert. The 

infiltration basin should have been able to receive approximately 39,000 ft3

 of runoff (assuming a level surface and 

no infiltration). Based on data from table 4, 8 of the 24 

precipitation events that produced enough runoff to leave 

the basin as discharge may have been fully retained. 

24 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Figure 12. Approximate locations of double-ring infiltrometer tests in the infiltration basin in the low-impact-development (LID) 

basin. Note how color-shaded contours change from yellow to brown, revealing a preferential pathway for water to move from 

the detention-pond inlet toward the main outlet. Elevation data obtained by a total-station survey.

2

7

6

1

5

4

3

East inlet

culvert

West inlet

culvert

Detention

pond

Main outlet

culvert

Water-quality monitoring station

2

7

6

1

5

4

3

East inlet

culvert

West inlet

culvert

Detention

pond

Main outlet

culvert

Water-quality monitoring station

Detention-pond

inlet culvert

89°39'54" 89°39'50"

43°6'42" 43°6'40" 43°6'38"

Base from Dane County Land Information Office Orthophotography, 1:10,000, 

2005, Wisconsin Transverse Mercator projection, NAD 83/91.

EXPLANATION

High (101)

Double-ring

infiltrometer test 

location and test 

number

Elevation of surface of 

infiltration basin, in feet

above arbitrary datum

Sites

Low (94)

Culvert

4

0 50 100 FEET

0 12.5 25 METERS

Comparison of Runoff Quantity from the Basins 25
Figure 13. Layer of silt that has formed from excessive ponding of runoff near the west-inlet and main-outlet culverts in the 

infiltration basin in the low-impact-development basin (April, 2001).

Detention-pond inlet culverts

West-inlet culvert

Table 6. Infiltration rates measured at seven locations in the infiltration basin, 2000–04. Locations of tests shown in figure 12. 

[--, test not performed]

Infiltration 

test location

Infiltration rate by year (inches per hour)

2000 2002 2003 2004 Average

1 -- 0.62 1.47 2.12 1.40

2 0.19 .26 .16 .37 .25

3 .49 .18 1.2 .26 .53

4 .86 22.5 6.3 5.81 8.87

5 -- 1.06 -- 4.11 2.59

6 1.22 1.37 .34 .41 .84

7 .91 1.36 -- 1.02 1.10

26 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Table 7. Regression results for storm loads in the low-impact-development (LID) and conventional-development basins. 

[P, precipitation depth; Peak Q, peak discharge; In, n-minute maximum precipitation intensity; APIn, antecedent rainfall n days before the storm; T, starting serial date of runoff period; cos, trigonometric cosine function; sin, trigonometric sine function; R2

, fraction of variation explained by the regression]

Dependent variable Independent variable Sample size Adjusted R2 Standard error

Conventional-development basin

Total solids Peak Q, cos(T) 28 0.767 0.79

Total suspended solids I

15, Peak Q, cos(T) 28 .778 .89

Total phosphorus I

15, Peak Q, sin(T) 25 .883 .5

LID basin

Total solids P, I15, Peak Q, sin(T) 66 .614 .77

Total suspended solids P, I15, Peak Q, sin(T), P+API5 66 .689 .71

Total phosphorus P, I15, Peak Q, sin(T), P+API3 62 .694 .56

Comparison of Runoff Water Quality 

from the Basins

Storm Loads

In addition to water quantity, changes in the quality of stormwater were measured to determine whether 

low-impact design practices could reduce sediment and 

sediment-associated constituent loads and yields at the 

basin outfall compared to those measured in the conventional basin. Loads were computed by multiplying the 

event mean concentrations by storm-runoff volumes. A 

complete list of sampled event-mean concentrations and 

loads can be found in tables 2-3 through 2-6, respectively, 

in appendix 2. 

In many cases, the variability in storm loads is large 

enough to mask potential differences in conventional and 

low-impact design practices. In addition, because the 

development and implementation of BMPs used in the 

LID basin spanned several years, differences in storm 

loads from one year to the next could result from differences in hydrologic conditions rather than from the BMPs. 

Furthermore, because not every runoff event was sampled, 

comparison of constituent loads between the conventional 

and LID basins using only those runoff events that were 

sampled could be misleading. One solution is regression 

analysis relating storm loads to variables representing climatologic and seasonal conditions. If the explanatory variables represent the climatologic and seasonal conditions, 

then the variability remaining in the regression residuals 

represents the combination of lack-of-fit for the regression model and changes induced by the low-impact design 

practices (Graczyk and others, 2003). Using methods 

described in Graczyk and others (2003), stepwise regressions were done for each basin and constituent to estimate 

loads for storm events that were not sampled. The final 

regressions were based on log transformations of measured 

storm loads and climatologic and seasonal variables. The 

outcome of the regression analysis is shown in table 7. 

Descriptive statistics for solids and phosphorus loads 

measured and estimated at the conventional and LID-basin 

outfalls during each study year are summarized in table 8. 

Mean total and total suspended solids loads were greater 

in the LID basin than in the conventional basin two of 

the seven study years. Mean total phosphorus loads were 

greater in the LID basin for four of the seven study years. 

Examination of the LID- and conventional-basin loads 

revealed a highly skewed distribution. Large precipitation 

events can skew the distribution of solids loads. One 

such event occurred in August 2001 when more than 

9.5 in. of rain was recorded in the study area within 

48 hours. Similarly, precipitation events that did not 

produce any discharge from the study sites were included 

in the dataset used to develop table 8. The sediment and 

phosphorus loads associated with these events were zero. 

The effect of including these events is represented by 

the median value in table 8. Because of the numerous 

precipitation events that did not produce runoff from the 

LID basin, median values for total solids, total suspended 

solids, and total phosphorus loads were less than those for 

the conventional basin for all seven years of the study. 

Comparison of Runoff Water Quality from the Basins 27
Table 8. Storm-load summary statistics at the low-impact-development (LID) and conventional-development-basin outfalls, 

water years 1999–2005.

[<, less than]

Statistic Water year a

1999 2000 2001 2002 2003 2004 2005

Low-impact-development basin

Total solids (tons)b

Number of observations c 9 17 15 11 9 12 6

Minimum .00 .00 .00 .00 .00 .00 .00

Maximum .55 5.25 11.63 .52 .85 6.86 .55

Median .00 .00 .00 .00 .00 .00 .00

Mean .02 .27 .28 .03 .06 .22 .09

Standard deviation .08 .92 1.60 .10 .17 .99 .09

Total suspended solids (tons) b

Number of observations c 9 17 15 11 9 12 6

Minimum .00 .00 .00 .00 .00 .00 .00

Maximum .39 2.95 10.33 .20 .41 4.00 .02

Median .00 .00 .00 .00 .00 .00 .00

Mean .01 .13 .22 .01 .03 .11 <.01

Standard deviation .06 .49 1.42 .03 .08 .58 <.01

Total phosphorus (pounds) b

Number of observations c 9 17 15 11 9 12 6

Minimum .00 .00 .00 .00 .00 .00 .00

Maximum .94 54.13 31.56 1.19 2.46 19.76 .67

Median .00 .00 .00 .00 .00 .00 .00

Mean .04 1.42 .81 .08 .21 .80 .03

Standard deviation .14 7.23 4.35 .22 .52 3.01 .13

a

 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 

2004 water year occurred October 1, 2003 through September 30, 2004).

b

 Represents only those precipitation events, excluding snowmelt, that produced discharge.

c

 Represents all precipitation events, excluding snowmelt, including those that did not produce discharge.

28 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Statistic Water yeara

1999 2000 2001 2002 2003 2004 2005

Conventional-development basin

Total solids (tons)b

Number of observations c 46 43 43 52 42 47 42

Minimum <.01 <.01 .01 <.01 <.01 <.01 <.01

Maximum 3.84 2.46 20.85 8.89 .70 1.76 1.03

Median .07 .03 .13 .07 .06 .06 .04

Mean .19 .21 .77 .28 .12 .18 .12

Standard deviation .58 .51 3.16 1.21 .15 .35 .21

Total suspended solids (tons)b

Number of observations c 46 42 43 52 42 51 48

Minimum <.01 <.01 <.01 <.01 <.01 <.01 <.01

Maximum .69 2.04 1.53 .28 .30 .52 .20

Median .03 .02 .04 .03 .03 .02 .02

Mean .06 .15 .10 .05 .05 .07 .04

Standard deviation .11 .43 .25 .05 .07 .11 .05

Total phosphorus (pounds) b

Number of observations c 46 42 43 52 42 51 48

Minimum .01 .01 .02 .01 <.01 <.01 <.01

Maximum 1.76 5.36 6.25 .75 .78 1.49 1.29

Median .14 .08 .15 .16 .12 .13 .12

Mean .22 .45 .43 .19 .20 .27 .23

Standard deviation .28 1.08 1.00 .16 .19 .33 .31

a

 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 

2004 water year occurred October 1, 2003 through September 30, 2004).

b

 Represents only those precipitation events, excluding snowmelt, that produced discharge.

c

 Represents all precipitation event, excluding snowmelt, including those that did not produce discharge.

Table 8. Storm-load summary statistics at the low-impact-development (LID) and conventional-development-basin outfalls, 

water years 1999–2005—Continued.

[<, less than]

Comparison of Runoff Water Quality from the Basins 29
Table 9. Estimate of annual loads in the low-impact-development (LID) and conventional-development basins, water years 

1999–2005.

Study basin Water year a

1999 2000 2001 2002 2003 2004 2005 1999–2005

Total solids (tons)

LID 1.2 15.5 14.7 1.7 2.4 10.3 1.0 49.8

Conventional 8.9 9.0 33.2 14.3 5.0 8.4 5.1 84.1

Total suspended solids (tons)

LID .8 7.4 11.5 .4 1.0 5.3 .0 29.5

Conventional 2.8 6.3 4.5 2.5 2.2 3.4 1.7 23.4

Total phosphorus (pounds)

LID 2.0 82.6 42.0 4.1 8.2 37.7 1.7 181.3

Conventional 9.9 18.9 18.5 10.0 8.3 13.8 10.8 90.2

a

 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 

2004 water year occurred October 1, 2003 through September 30, 2004).

Annual loads were computed to better characterize 

the overall effectiveness of low-impact design practices 

for mitigating delivery of total and suspended solids and 

total phosphorus loads. Annual total suspended solids and 

total phosphorus loads were greater in the conventional 

basin than the LID basin for all years except 2000, 2001, 

and 2004 (table 9). Because of the strong association of 

total phosphorus with sediment, total phosphorus loads 

tended to increase or decrease with total suspended solids 

loads (Sartor and Boyd, 1972; Waschbusch and others, 

1999; Kronvang and others, 1999). The annual total solids 

load was greater in the LID basin for water years 2000 

and 2004 but was approximately one-half that measured in 

the conventional basin in 2001. During this year, a single 

event sampled in the conventional basin had an unusually 

large total solids concentration of 10,000 mg/L, an order 

of magnitude higher than that for any other sampled runoff 

event. An examination of field notes during this combination snowmelt and rainfall event indicated a heavy layer 

of sediment in the sample containers. Photographs taken a 

few days later document appreciable street sediment and 

debris collected in snow piles along the street edge (fig. 

14). As snow melted, these concentrated sediments were 

likely entrained in runoff and washed into nearby storm 

drains. This event produced more than one-half of the 

annual total solids load in the conventional basin in 2001. 

Similarly, closer examination of individual storm loads 

in the LID basin for water years 2000, 2001, and 2004 

showed that the majority of annual constituent loads were 

attributed to one or two large runoff events. Select runoff 

events in the LID and conventional basins for water years 

2000, 2001, and 2004 are shown in table 10. Two runoff 

events were responsible for contributing 70 percent or 

more of all total solids, total suspended solids, and total 

phosphorus loads in 2000, and a single runoff event produced the majority of constituent loads in 2001 and 2004 

(table 10). Each of these events was associated with considerable precipitation depths and (or) intensities. These 

same storms did not contribute as much of the annual load 

in the conventional basin. With large storms and saturated 

soils, the ability of low-impact design techniques to reduce 

runoff, and thus constituent loads, can, at least in this case, 

be greatly diminished. 

Annual yields were computed by dividing the annual 

load by contributing drainage area. The annual yields of 

total solids, total suspended solids, and total phosphorus 

are shown in table 11. Because both the LID and conventional basins had nearly the same contributing area, comparison of annual yields between the two basins revealed 

similar conclusions as annual loads. An analysis of annual 

yields using a drainage boundary defined by developed 

area only, not including the surrounding undeveloped 

hillslopes and forested uplands, resulted in similar conclusions. These areas that were not included typically contribute little to no runoff (Montgomery and others, 1997); 

however, it is likely these areas contributed some volume 

30 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Figure 14. Concentrated sediment in snow piles in the conventional-development basin. As the snow melted, the sediment 

became entrained in runoff, resulting in unusually high concentrations of solids.

U.S. Geological Survey water-quality

monitoring station

Comparison of Runoff Water Quality from the Basins 31
Table 10. Contributions of constituent load as a percentage of annual load for select precipitation events in the low-impact-development (LID) and 

conventional-development basins.[mm/dd/yyyy, month, day, and year; in/h, inch per hour]

Water 

year1

Date

(mm/dd/yyyy)

Precipitation 

depth

(inches)

Storm 

intensity

(in/h)

Total solids Total suspended solids Total phosphorus

Event load

(tons)

Percent of 

annual load

Event load

(tons)

Percent of 

annual load

Event load

(tons)

Percent of 

annual load

Low-impact-development basin

2000 05/17/2000 5.09 1.2 5.3 32 3.0 35 54.1 65

2000 05/30/2000 4.89 1.14 6.2 38 3.0 35 18.5 222001 08/01/2001 9.68 3.15 11.6 74 10.3 83 31.6 732004 05/21/2004 6.21 1.13 6.9 61 4.0 63 19.8 51

Conventional-development basin

2000 05/17/2000 4.93 1.3 2.1 23 1.9 30 5.4 282000 05/30/2000 5.14 .88 2.5 27 2.0 32 4.1 222001 08/01/2001 8.82 2.6 2.0 6 1.5 34 6.3 342004 05/21/2004 6.71 1.22 3.4 40 1.1 32 3.1 22

1 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 2004 water year occurred 

October 1, 2003 through September 30, 2004).

32 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Table 11. Estimate of annual yields of total solids, total suspended solids, and total phosphorus in the low-impact-development 

(LID) and conventional-development basins, water years 1999–2005.

[lbs/acre, pounds per acre; <, less than]

Study basin Water yeara

1999 2000 2001 2002 2003 2004 2005

Total solids (lbs/acre)

LID 13 161 154 18 25 108 11

Conventional 129 132 484 209 72 123 75

Total suspended solids (lbs/acre)

LID 8 77 120 4 11 55 <1

Conventional 40 92 66 36 32 49 25

Total phosphorus (lbs/acre)

LID .01 .43 .22 .02 .04 .20 .01

Conventional .07 .14 .13 .07 .06 .10 .08

a

 Water year is a 12-month period from October 1 through September 30, and is designated by the calendar year in which it ends (for example, the 

2004 water year occurred October 1, 2003 through September 30, 2004).

of runoff to the drainage network of the developed basins 

during one or more events with substantial precipitation 

depths and (or) intensities. Because these events were 

responsible for the majority of constituent loading, computations based only on the developed area might result in an 

overestimation of annual yields in each basin.

Water Temperature

The temperature of urban streams is often affected 

directly by urban runoff. Previous studies indicate that as 

rainfall passes over impervious surfaces such as rooftops, 

driveways, roads, and parking lots, it absorbs some of the 

energy stored as heat in the surface (Roa-Espinosa and 

others, 2003). Galli (1990) demonstrated an increase in 

base-flow water temperature of 0.14°C for every 1-percent 

increase in watershed imperviousness. Although the LID 

and conventional basins constitute only a small percentage 

of the Black Earth Creek watershed area (0.3 and 0.2 percent, respectively), the percentage of impervious surface 

within each basin is quite different. Runoff leaving each 

basin eventually drains into nearby Black Earth Creek, 

a Class I trout stream. The difference in the temperature 

of runoff between a basin constructed using low-impact 

techniques as opposed to a more conventional basin could 

potentially be used to predict impacts to receiving waters 

given future watershed build-out scenarios. Certain species 

of fish, such as trout, require low daily mean temperatures 

of less than 22°C for survival (Lyons and others, 1996) and 

are particularly sensitive to temperature fluctuations. 

A typical response in water temperature to rainfall 

measured at the LID- and conventional-basin outlets is 

illustrated in figures 15 and 16, respectively. Water levels 

in each basin also are shown in figures 15 and 16 to 

show changes in temperature before, during, and after a 

rainfall event. The temperature of runoff measured at the 

conventional-basin outlet is not largely affected by heating of impervious surfaces prior to rain events because of 

its location. As rainfall first moved into the study basin 

on May 21, 2004, the temperature of runoff was close to 

ambient air temperatures (fig. 15). This is likely because 

rain fell in the early morning hours; impervious surfaces 

cooled overnight and were not subject to additional heating prior to rainfall. After runoff cessation, temperatures 

inside the storm-sewer pipe returned to pre-event levels. 

A second pulse of rainfall moved through the study area 

Comparison of Runoff Water Quality from the Basins 33
Figure 15. Temperature of runoff, air temperature, cumulative precipitation, and water levels measured in the conventionaldevelopment basin.

0

5

10

15

20

25

30

35

TEMPERATURE, IN DEGREES CELSIUS

0

1

2

3

4

5

6

7

W

ATER LEVEL, IN FEET, AND CUMULATIVE PRECIPITATION, IN INCHES

DATE/TIME

05/20/04

00:00

05/20/04

12:00

05/21/04

00:00

05/21/04

12:00

05/22/04

00:00

05/22/04

12:00

05/23/04

00:00

05/23/04

12:00

05/24/04

00:00

05/24/04

12:00

05/25/04

00:00

Water temperature

Air temperature

Water level

Cumulative precipitation

(fig. 16). Additional rainfall and cooler air temperatures 

during the afternoon hours on May 21 resulted in gradually decreasing runoff temperatures; however, unlike the 

conventional basin, the temperature of runoff in the LID 

basin increased more dramatically after precipitation ended 

and air temperatures increased. This warming effect can be 

seen during the afternoon hours on May 22 and again on 

May 23 when air temperatures were increasing and there 

was no additional rainfall (fig. 16). The temperature of 

runoff follows the same overall trend as air temperature but 

appears to lag behind by a few hours. This is likely due to 

the absence of tree canopy for shading and the presence of 

the wet detention basin in the LID basin. Water in the wet 

detention basin stores heat during dry periods until runoff 

from a rainfall event moves the warmer water into the 

infiltration basin and replaces it with cooler runoff, which 

is stored in turn. Runoff entering the infiltration basin from 

the wet detention pond and other drainage areas is spread 

uniformly over a large surface area to promote infiltration. 

in the afternoon, just after the air temperature reached 

its highest point of the day. The temperature of runoff 

was again similar to air temperatures initially but quickly 

cooled as rain persisted in the basin. Heating of impervious surfaces during the afternoon hours may have elevated 

the temperature of runoff somewhat prior to the second 

pulse of rainfall on May 21, but a drop in air temperature, 

additional rainfall, and cooler ambient temperatures in 

the storm sewer produced a cooling effect. Fluctuations in 

runoff temperatures beyond this point did not exceed more 

than a couple of degrees despite periods of atmospheric 

heating with no additional rainfall. 

The temperature of runoff in the LID basin is 

largely controlled by ambient air temperatures, just as in 

the conventional basin. Runoff temperatures in the LID 

basin were similar to ambient air temperatures during 

the cooler morning hours of May 21, 2004. As rainfall 

subsided, the temperature of runoff measured at the LID 

outlet increased at nearly the same rate as air temperature 

34 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Figure 16. Temperature of runoff, air temperature, cumulative precipitation, and water levels measured in the low-impactdevelopment (LID) basin.

0

5

10

15

20

25

30

35

TEMPERATURE, IN DEGREES CELSIUS

0

1

2

3

4

5

6

7

W

ATER LEVEL, IN FEET, AND CUMULATIVE PRECIPITATION, IN INCHES

DATE/TIME

05/20/04

00:00

05/20/04

12:00

05/21/04

00:00

05/21/04

12:00

05/22/04

00:00

05/22/04

12:00

05/23/04

00:00

05/23/04

12:00

05/24/04

00:00

05/24/04

12:00

05/25/04

00:00

Water temperature

Air temperature

Water level

Cumulative precipitation

This water is typically shallow and exposed to direct solar 

heating, allowing for cooler runoff to warm rapidly during 

dry periods. 

The paired t-test (Helsel and Hirsch, 1992) was used 

to determine whether daily mean temperatures of runoff 

in the LID basin were greater or less than those in the conventional basin. The test was done on runoff-temperature 

data measured in the conventional and LID basins for 

April through November to eliminate periods of snowmelt. 

The null hypothesis states that there is no difference in the 

means of temperature between the LID and conventional 

basins. The alternative hypothesis is that there is a significant difference that can be inferred to be a result of basin 

characteristics. Results of the test indicate that the temperature of runoff in the LID basin is greater than that in the 

conventional basin at the 5-percent significance level. 

Water temperature may be considered one of the most 

important factors in determining the geographic distribution, growth rate, and survival of fish and other aquatic 

organisms (Bartholow, 1989; Regier and others, 1990; 

Armour, 1991; LeBlanc and others, 1997). Some of the 

variables having the most effect on stream temperature 

include, but are not limited to, riparian vegetation, groundwater discharge, and channel morphology. These variables 

also are highly influenced by land use. Low-impact design 

practices appear to mitigate temperature associated with 

runoff by retaining and infiltrating the majority of stormwater runoff into underlying aquifers, thereby helping 

to sustain base flow (Pitt and others, 1994). Maintaining 

a steady supply of ground-water discharge to a stream 

is important because critical temperature thresholds for 

aquatic survival are often associated with low-flow periods 

during summer (LeBlanc and others, 1997). 

Comparison of Runoff Water Quality from the Basins 35
Limitations of This Study and 

Suggestions for Future Work

This study compared the runoff quantity and quality of a conventionally developed residential basin to an 

adjacent basin in which LID strategies were implemented. 

Limitations of this study and suggestions for future work 

are discussed below.

•	 Previous studies determined through numerical 

simulations that developments adopting a low-impact approach, with a large portion of the land kept 

in a natural condition, produced less runoff than 

conventional developments (Brander and others, 

2005). Unlike in a conventionally developed basin, 

LID attempts to maintain or replicate the predevelopment hydrologic regime through use of design 

techniques that create a functionally equivalent 

hydrologic landscape (U.S. Environmental Protection Agency, 2000). This distinction between construction philosophies became evident in this study 

when comparing the percentage of impervious 

surface in the conventional basin to that in the LID 

basin. The conventional basin had nearly twice the 

area of mapped imperviousness than the LID basin, 

largely due to commercial parking lots. The large 

difference in this factor between these two types 

of basins presented challenges in this study when 

comparing runoff quantity and quality. A greater 

percentage of impervious surfaces in a basin could 

result in greater runoff volumes. 

•	 During the study period, development activities 

were continuing in the LID basin; hence, the results 

from that basin do not necessarily represent fully 

developed conditions.

•	 Although the study basins were located relatively 

close to one another, there were differences in 

observed rainfall. Slight differences in precipitation 

totals may lead to differences in stormwater loads 

and yields. 

•	 Replacing natural landscapes with urban areas can 

introduce a wide variety of anthropogenic contaminants such as total and dissolved metals, oil 

and grease, pesticides, and polycyclic aromatic 

hydrocarbons (Davis, 2005; Bannerman and others, 

1996). Because constituents analyzed as part of this 

study were limited to total solids, total suspended 

solids, and total phosphorus, little can be inferred 

about the potential benefits of low-impact design 

on reducing concentrations of other contaminants. 

Future work could include comparison of the concentrations and loads of additional contaminants 

between the LID and conventional basins.

•	 Daily mean temperatures of runoff in the LID 

basin were higher than in the conventional basin. 

Although the frequency of runoff events in the LID 

basin was less than that in the conventional basin, 

their duration was considerably longer. The length 

and magnitude of water temperature excursions 

above or below a threshold may have an effect on 

the survival of fish species (Mohseni and others, 

1998). For example, Bear and others (2007) measured a drop in survival rates of rainbow trout—

which is a common species found in Black Earth 

Creek—in Montana streams after only a few days 

when stream temperatures were at or above 26°C. 

The temperature of runoff leaving the LID basin 

occasionally exceeded this threshold. Although 

detailed runoff temperature data from both study 

basins were collected as part of the current study, 

additional data would need to be collected to allow 

prediction of the short- and long-term thermal 

response in Black Earth Creek during and after 

storm-runoff events.

•	 The ability to detect long-term trends in sediment 

and phosphorus yields was limited given the short 

period of observation in the continually developing LID basin. Additional data would need to 

be collected once the LID basin was completely 

developed in order to fully characterize the waterquantity and -quality benefits of a LID design. 

36 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Summary and Conclusions

As development continues to push further into the 

natural landscape, controlling nonpoint sources of contamination has become a major focus for the regulatory community. During the course of development, land that was 

once covered with vegetation is sometimes replaced with 

impervious surfaces that are associated with more diverse 

pollutants, reduced infiltration, and increased peak stream 

flows. Environmental managers are often faced with 

designing strategies to accommodate development while 

minimizing adverse environmental impacts. Low-impact 

development (LID) is one such strategy that attempts to 

mitigate environmental degradation commonly associated 

with impervious surfaces. By incorporating the hydrologic 

functions of storage, infiltration, and volume and frequency of discharges into small-scale, distributed, structural, and nonstructural controls, LID principles attempt to 

control stormwater at the source. Many of the stormwater 

controls used as part of low-impact design have been 

successfully evaluated individually. Although hydrologic 

models have demonstrated the potential environmental 

benefit of incorporating multiple stormwater controls, 

there is a paucity of large-scale field studies evaluating the 

aggregate effect of these practices implicit in a low-impact 

design.

To that end, the U.S. Geological Survey, in cooperation with the Wisconsin Department of Natural Resources, 

did a study to compare conventional and LID practices 

with respect to the quantity and quality of stormwater 

runoff. Specifically, this study examined the hydrologic 

response of a residential basin from predevelopment 

through near-complete build-out. The basin (referred to as 

the “LID” basin) was developed using several stormwater 

controls that are often implemented as part of LID, such as 

reduced street widths to minimize impervious cover, grass 

swales, street inlets that discharge to grass swales, protection of existing woodlands, a detention pond, an infiltration trench, and a large infiltration basin planted with 

deep-rooted prairie species native to the area. Water-quantity and -quality data from the LID basin were compared 

to data from a fully developed residential basin (referred 

to as the “conventional basin”) that was constructed using 

a more conventional, fully connected stormwater-conveyance system, such as curbs and gutters and concrete storm 

sewers. 

Results of the study show that the quantity of stormwater discharge leaving a residential basin can be reduced 

by incorporating LID practices. Precipitation from smaller, 

more frequent events that produced stormwater discharge 

from the conventional basin was retained within the drainage boundaries of the LID basin. The cumulative effect of 

these small, frequent precipitation events became apparent 

when examining cumulative discharge volumes from each 

study basin. Fifty percent of total runoff volume leaving 

the LID basin as discharge was associated with precipitation depths greater than 3 inches compared to 18 percent 

in the conventional basin. For each study year, annual 

discharge volume measured from the conventional basin 

exceeded that from the LID basin, in some cases by an 

order of magnitude. During the 2004–2005 study year, 

when the LID basin was near complete build-out, 

95 percent of the annual precipitation was retained onsite. 

Much of the runoff reduction was attributed to a combination of low-impact practices such as lawns, grassed swales, 

a detention pond, and forested hillslopes. 

Runoff in the LID basin was received by a 1.75-acre 

infiltration basin, the largest control structure used to 

mitigate runoff quantity and quality. It also was the last 

structure used to treat runoff before it left the LID basin 

as discharge. A volumetric mass balance was computed 

for all stormwater-runoff volumes entering and leaving 

the infiltration basin from May 2002 through September 

2005. During this period, 51 percent of all influent was 

retained in the infiltration basin. Examination of individual 

precipitation events showed a relation between the amount 

of runoff reduced by the infiltration basin and precipitation intensity. Reductions in stormwater runoff in the LID 

basin diminished as precipitation intensities increased. As 

precipitation intensities exceeded 0.5 inches per hour, the 

average percentage reduction in runoff volumes dropped 

to 43 percent. The failure of the LID basin to fully retain 

precipitation events of appreciable depth and (or) intensity might be a result of one or a combination of several 

factors, including the rate of precipitation exceeding the 

maximum rate of infiltration, additional volume of runoff 

from forested uplands that normally have little to no runoff 

from smaller precipitation events, saturated soil from prior 

storms, or frozen soil. 

A comparison of annual loads characterized the 

overall effectiveness in this study of low-impact design 

practices at mitigating delivery of total solids, total 

suspended solids, and total phosphorus loads. Annual constituent loads were greater in the conventional basin than 

in the LID basin for all but 3 study years, during which 

more than 50 percent of annual load in the LID basin was 

associated with one or two discharge events. Although 

low-impact design practices are able to retain and reduce 

Summary and Conclusions 37
stormwater runoff, and thus constituent loads, larger, less 

frequent precipitation events can greatly diminish this 

capability. 

The temperature of runoff in both the LID and 

conventional basins was largely controlled by ambient air 

temperatures. A greater percentage of impervious surface 

area in the conventional basin resulted in slight heating of 

runoff initially, but cooler air temperatures and precipitation rapidly reduced the temperature of runoff leaving 

the basin as discharge. Once in the enclosed storm-sewer 

conduit, the temperature of discharge in the conventional 

basin remained constant. The absence of storm sewers 

in the LID basin allowed stormwater runoff retained in 

the infiltration basin and detention pond to be exposed to 

direct solar heating. This resulted in elevated discharge 

temperatures several days after a storm event. 

Acknowledgements

The authors thank Eric Booth, Justin Haasch, and 

Kristian Corby of the U.S. Geological Survey for their tireless efforts in the field; Jerry Gray of the Village of Cross 

Plains for providing valuable engineering information; and 

Joyce Powers of Prairie Ridge Nursery for planting and 

maintaining the infiltration basin with a diverse assortment 

of native prairie species. Without their cooperation and 

effort, this study would not have been possible.

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Wisconsin, 1994–95, U.S. Geological Survey WaterResources Investigations Report 99–4021, 47 p.

Wilson, J.F., Cobb, E.D., and Kilpatrick, F.A., 1986, Fluorometric procedures for dye tracing: U.S. Geological 

Survey Techniques of Water-Resources Investigations, 

book 3, chap. A12, 34 p.

Wisconsin Administrative Code, 2002, Wisconsin Department of Natural Resources—Runoff management: 

Chap. NR 151 [variously paginated].

References 39

Appendixes

Appendixes 41
Appendix table 1-1. Results of blank-sample analyses in the low-impact-development (LID) and conventional-development 

basins.

[mm/dd/yyyy, month/day/year; mg/L, milligram per liter; <, less than; --, no data]

Sample name Date

(mm/dd/yyyy)

Phosphorus, 

total recoverable

(mg/L)

Total suspended solids

(mg/L)

Total solids

(mg/L)

LID-3 04/17/2001 <0.005 <2 --

LID-3 07/09/2002 -- 3 <50

LID-3 06/10/2004 .008 <2 <50

LID-2 06/01/2005 <.001 <6 <6

Conventional-3 04/19/2001 .015 9 --

Conventional-3 07/09/2002 -- <2 <50

Conventional-3 07/02/2004 <.005 <2 <50

Conventional-2 06/01/2005 <.001 <6 <6

Appendix table 1-2. Results of replicate-sample analyses in the low-impact-development (LID) and conventional-development 

basins.

[mm/dd/yyyy, month/day/year; mg/L, milligram per liter; RPD, relative percent difference; LID, low-impact-development; --, no data; n/a, no value available]

Sample name Date

(mm/dd/yyyy)

Phosphorus, 

total recoverable

(mg/L)

Absolute 

RPD

(percent)

Total 

suspended solids 

(mg/L)

Absolute 

RPD

(percent)

Total solids

(mg/L)

Absolute 

RPD

(percent)

LID-32 09/08/2001 -- n/a 61 17 196 0

LID-32R1 09/08/2001 -- 52 196

Conventional-35 09/07/2001 -- n/a 59 13 98 21

Conventional-35R1 09/07/2001 -- 68 124

Conventional-85 05/11/2005 0.142 7 -- n/a -- n/a

Conventional-85R1 05/11/2005 .152 -- --

1

 Replicate sample.

Appendix 1. Quality-Assurance and Quality-Control Data

42 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix 2

Appendix 2-1. Hydrologic and precipitation characteristics of sampled discharge events in the 

low-impact-development (LID) basin............................................................................................................... 44

Appendix 2-2. Hydrologic and precipitation characteristics of sampled discharge events in the 

conventional-development basin ..................................................................................................................... 46

Appendix 2-3. Event mean concentrations for sampled discharge events in the low-impact-development 

(LID) basin............................................................................................................................................................. 50

Appendix 2-4. Event mean concentrations for sampled discharge events in the conventional-development 

basin ...................................................................................................................................................................... 51

Appendix 2-5. Constituent loads computed for sampled discharge events in the low-impact-development 

(LID) basin............................................................................................................................................................. 54

Appendix 2-6. Constituent loads computed for sampled discharge events in the conventional-development 

basin ...................................................................................................................................................................... 55

Appendix 2. Detailed Event Characteristics and Stormwater-Quality Data

Appendix 2 43
Appendix table 2-1. Hydrologic and precipitation characteristics of sampled discharge events in the low-impact-development (LID) basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation 

depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3

)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

2 10/05/1998 17:19 10/05/1998 20:07 2.8 1.07 1.88 1.68 1.06 423 <1 0.20 0.53 02/11/1999 13:27 02/11/1999 16:30 3.0 .63 1.48 .98 .58 1,486 <1 .57 1.84 04/22/1999 19:21 04/23/1999 09:52 14.5 1.95 .48 .44 .38 28,080 2 2.19 .25 05/16/1999 21:56 05/17/1999 19:31 16.2 1.92 4.28 2.52 1.33 933 <1 .04 .26 06/30/1999 23:38 07/01/1999 09:52 5.2 1.05 1.16 .74 .48 1,391 <1 .06 2.17 07/17/1999 02:46 07/17/1999 15:10 3.8 1.25 1.48 1.24 .91 484 <1 .04 .28 07/20/1999 13:48 07/24/1999 18:40 100.9 1.59 2.48 1.96 1.40 29,315 3 .36 1.99 08/23/1999 08:36 08/25/1999 10:35 50.0 1.07 1.60 1.12 .65 7,318 1 .10 4.5

10 02/23/2000 09:00 02/24/2000 22:00 37.0 .29b .12 .10 .08 46,837 23 1.22 44.011 02/25/2000 18:14 02/26/2000 12:49 18.6 .72 .80 .48 .30 19,872 4 .59 .8

13, 14, 15a 04/20/2000 00:16 04/22/2000 07:42 55.4 1.02 .60 .40 .35 29,039 1 .36 .5

16 05/17/2000 12:49 05/19/2000 21:53 57.1 5.09 4.20 2.32 1.20 294,926 8 21.84 5.417 05/30/2000 06:23 06/01/2000 15:00 56.6 3.67 2.48 1.46 .84 264,142 10 11.65 .818 06/13/2000 15:36 06/15/2000 02:54 35.3 2.79 2.87 1.60 .93 180,680 9 15.47 1.119 07/02/2000 19:34 07/03/2000 17:07 21.5 .90 2.60 1.52 .81 6,428 1 .17 4.020 07/10/2000 03:40 07/11/2000 02:25 22.7 .71 .64 .42 .29 4,182 1 .14 .721 08/05/2000 11:09 08/06/2000 03:54 16.7 1.84 3.00 2.20 1.46 21,980 2 1.35 4.022 08/17/2000 02:23 08/18/2000 01:50 23.4 .92 .88 .68 .50 5,210 1 .17 3.723 09/11/2000 07:37 09/12/2000 14:03 30.4 1.08 1.48 .88 .81 8,873 1 .24 7.924 09/22/2000 09:16 09/24/2000 03:07 41.9 .55 .28 .22 .17 13,435 4 .28 2.425 04/09/2001 23:18 04/10/2001 03:10 27.2 .82 .32 .32 .27 9,634 2 .62 3.126 04/11/2001 06:01 04/12/2001 13:48 31.8 .76 .72 .60 .40 36,253 7 1.55 1.127 04/21/2001 00:11 04/21/2001 23:55 4.7 .42 .56 .36 .23 8,312 3 .34 .728 05/20/2001 23:50 05/22/2001 05:59 30.1 1.71 1.32 1.02 .71 38,431 3 2.10 9.829 05/23/2001 01:34 05/23/2001 21:03 12.1 .38 .24 .16 .11 2,195 1 .11 .830 06/05/2001 01:29 06/06/2001 06:40 29.2 .66 .56 .42 .33 17,194 4 .69 3.231 08/01/2001 18:00 08/03/2001 13:30 43.5 9.68 6.08 4.54 3.15 774,516 11 22.28 7.732 09/07/2001 19:05 09/09/2001 00:02 28.9 1.26 2.44 1.52 .81 20,736 2 .84 .333 09/18/2001 23:28 09/20/2001 10:48 35.3 1.02 .36 .28 .25 25,350 4 .85 1.5

44 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix table 2-1. Hydrologic and precipitation characteristics of sampled discharge events in the low-impact-development (LID) basin—Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation 

depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

34 09/22/2001 22:34 09/24/2001 15:23 40.8 1.66 0.92 0.78 0.62 88,344 8 3.90 2.135 10/22/2001 14:36 10/23/2001 22:14 31.6 1.21 .84 .66 .44 6,592 1 .25 8.736 11/23/2001 22:17 11/25/2001 13:20 39.0 .97 .80 .48 .30 9,798 1 .35 4.837 02/18/2002 22:10 02/21/2002 20:19 70.2 1.54 .24 .18 .16 52,842 5 1.07 8.438 04/07/2002 15:14 04/09/2002 16:25 49.2 .99 .16 .12 .10 14,826 2 .44 .439 03/04/2004 18:04 03/06/2004 12:24 42.3 1.13 .48 .30 .22 78,883 10 1.97 2.040 03/25/2004 14:20 03/27/2004 05:25 39.1 1.07 .76 .68 .51 35,087 5 1.43 1.441 05/21/2004 07:16 05/24/2004 10:00 74.7 6.21 2.40 1.68 1.13 469,610 11 9.48 3.242 05/29/2004 06:41 06/01/2004 00:00 65.3 1.67 1.24 .80 .54 27,994 2 1.18 4.243 07/03/2004 15:03 07/04/2004 12:00 21.0 1.08 1.20 .98 .61 6,532 1 .31 12.144 07/16/2004 10:01 07/17/2004 10:00 24.0 1.05 1.72 1.20 .62 59,435 8 2.36 6.945 08/03/2004 17:51 08/04/2004 14:00 20.2 1.20 3.84 2.02 1.09 36,253 4 1.77 1.246 01/12/2005 13:04 01/13/2005 15:38 26.6 .33b .16 .12 .11 27,233 12 1.09 10.047 03/21/2005 14:00 03/24/2005 19:10 82.0 snowmelt -- -- -- 21,910 <1 .28 .448 03/24/2005 10:00 03/25/2005 21:45 35.8 snowmelt -- -- -- 5,358 <1 .16 .049 03/30/2005 13:33 03/31/2005 17:14 27.7 .51 .72 .46 .25 4,095 1 .13 1.1

a Single discharge event separated into three sampled periods.b Combination rainfall and snowmelt event.

Appendix 2 45
Appendix table 2-2. Hydrologic and precipitation characteristics of sampled discharge events in the conventional-development basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed; n/a, data not available]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation

 depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3

)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

1 10/05/1998 18:01 10/05/1998 19:55 1.9 1.10 2.00 1.68 1.09 35,420 6 30.8 n/a3 04/21/1999 20:58 04/26/1999 03:50 102.9 3.42 4.28 2.22 1.17 81,005 5 11.4 1.24 05/16/1999 16:25 05/17/1999 22:45 30.3 2.39 4.56 2.62 1.38 81,361 7 79.3 10.05 06/06/1999 14:24 06/07/1999 04:24 14.0 .89 1.68 1.30 .77 33,741 8 18.2 2.17 07/16/1999 20:34 07/18/1999 02:00 29.4 1.86 1.36 1.30 1.01 69,510 8 17.0 7.88 07/20/1999 13:07 07/21/1999 20:07 31.0 2.00 3.00 2.42 1.69 63,393 6 29.6 1.39 08/23/1999 08:35 08/24/1999 14:45 30.2 1.27 1.72 1.32 .76 44,820 7 18.3 4.5

12 04/19/2000 07:45 04/19/2000 12:40 4.9 .52 .28 .26 .20 9,125 4 3.3 23.6

12a 04/20/2000 00:18 04/21/2000 10:00 32.5 1.02 .60 .40 .35 46,039 9 10.8 .5

13 05/17/2000 12:47 05/21/2000 15:15 98.5 4.93 4.54 2.57 1.34 232,123 9 76.3 4.814 05/26/2000 22:14 06/01/2000 18:13 122.4 5.14 2.76 1.58 .88 209,190 8 45.7 5.315 06/12/2000 05:50 06/15/2000 22:15 88.4 3.18 2.87 1.60 .93 199,351 13 70.6 7.016 07/02/2000 19:36 07/04/2000 21:07 37.4 .91 2.60 1.54 .83 53,295 12 43.2 3.917 07/10/2000 03:41 07/11/2000 16:00 36.3 .67 .63 .42 .28 46,576 14 9.3 .718 08/05/2000 11:10 08/07/2000 17:00 53.8 2.02 3.28 2.38 1.59 80,440 8 38.7 4.119 08/17/2000 02:23 08/19/2000 06:00 51.6 .91 .94 .67 .47 44,139 10 11.6 3.720 09/11/2000 07:38 09/13/2000 17:00 57.4 1.10 1.43 .86 .79 52,536 10 25.1 7.921 09/22/2000 09:36 09/24/2000 00:30 38.9 .61 .29 .22 .17 28,450 9 4.9 2.522 01/29/2001 11:08 01/31/2001 03:00 39.9 .83 .29 .22 .19 66,804 16 3.7 61.323 02/08/2001 14:47 02/10/2001 16:00 49.2 1.18 .32 .29 .26 82,153 14 4.4 8.524 02/24/2001 09:21 02/25/2001 11:50 26.5 .24 .11 .09 .07 31,240 26 2.7 13.725 04/10/2001 22:49 04/14/2001 00:20 73.5 .78 .67 .51 .35 29,384 8 14.2 .026 04/21/2001 00:10 04/22/2001 15:00 38.8 .47 .63 .40 .25 20,409 9 10.4 .727 05/10/2001 19:40 05/12/2001 21:50 50.2 .65 1.28 .77 .46 41,351 13 22.1 .728 05/20/2001 23:58 05/23/2001 01:27 49.5 1.62 .96 .78 .52 70,074 9 10.6 8.129 05/23/2001 01:28 05/24/2001 22:00 44.5 .37 .20 .16 .11 23,065 13 3.5 .030 06/05/2001 01:29 06/07/2001 12:00 58.5 .70 .68 .50 .38 63,906 18 7.9 3.231 06/11/2001 22:04 06/13/2001 21:10 47.1 2.33 2.24 2.06 1.51 172,661 15 38.9 1.932 07/18/2001 07:21 07/20/2001 13:00 53.6 .90 2.80 1.64 .88 38,983 9 27.4 .9

46 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix table 2-2. Hydrologic and precipitation characteristics of sampled discharge events in the conventional-development basin—Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed; n/a, data not available]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation

 depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

33 08/01/2001 18:00 08/04/2001 23:00 77.0 8.82 4.87 3.67 2.64 581,107 13 78.1 7.734 08/24/2001 19:05 08/27/2001 04:00 56.9 1.11 .94 .86 .72 49,664 9 12.7 2.235 09/07/2001 12:19 09/11/2001 20:00 103.7 2.35 2.54 1.60 .83 116,172 10 35.3 .936 09/18/2001 23:44 09/22/2001 17:00 89.3 1.22 .32 .29 .25 91,392 15 9.7 1.537 09/22/2001 22:34 09/25/2001 06:00 55.4 1.61 1.01 .80 .61 64,039 8 26.0 .238 10/22/2001 14:38 10/24/2001 08:09 41.5 1.10 .86 .61 .41 34,534 6 8.6 8.839 11/23/2001 22:17 11/25/2001 19:00 44.7 .80 .70 .41 .25 32,721 8 8.9 4.840 02/18/2002 22:13 02/23/2002 21:00 118.8 1.44 .22 .18 .16 93,388 13 2.5 8.441 03/08/2002 16:53 03/12/2002 19:00 98.1 .25 .36 .25 .14 21,812 17 5.3 .542 04/07/2002 03:39 04/07/2002 06:48 3.1 .26 .25 .22 .14 20,005 15 2.5 4.343 08/04/2002 02:25 08/04/2002 10:33 8.1 .66 1.60 .92 .48 23,214 7 18.0 7.744 08/11/2002 16:34 08/11/2002 17:44 1.2 .98 2.04 1.14 .91 38,108 8 27.2 7.345 08/21/2002 18:01 08/22/2002 13:00 19.0 1.01 1.88 1.02 .51 30,426 6 17.1 4.546 09/02/2002 03:46 09/02/2002 11:01 7.2 .99 1.04 .80 .52 43,075 9 12.3 10.647 09/28/2002 21:44 09/29/2002 05:29 7.8 .67 .96 .66 .34 25,552 8 14.5 8.248 03/04/2004 17:40 03/05/2004 11:28 17.8 1.20 .52 .34 .24 74,929 13 7.7 2.949 03/25/2004 20:09 03/26/2004 03:17 7.1 1.10 .68 .66 .51 48,617 9 9.1 1.650 05/08/2004 06:31 05/09/2004 00:53 18.4 .61 .84 .48 .28 22,379 7 14.5 12.751 05/10/2004 02:24 05/10/2004 15:05 12.7 .51 .56 .40 .25 17,935 7 6.9 1.152 05/17/2004 20:01 05/18/2004 03:01 7.0 .77 2.28 1.36 .72 23,862 6 22.8 3.353 05/21/2004 07:16 05/21/2004 08:28 1.2 .80 1.92 1.02 .75 32,735 8 25.7 3.254 05/29/2004 06:54 05/29/2004 12:05 5.2 .64 .28 .24 .21 26,030 8 3.2 4.255 05/30/2004 07:58 05/30/2004 10:42 2.7 .71 1.28 .84 .58 27,061 8 13.5 .856 05/31/2004 06:54 05/31/2004 21:00 14.1 .19 .16 .14 .10 15,345 16 8.0 .857 06/09/2004 21:42 06/10/2004 12:15 14.5 .46 -- -- -- 16,194 7 7.7 9.058 06/10/2004 14:15 06/11/2004 05:00 14.8 .69 -- -- -- 26,353 8 6.8 .159 06/12/2004 00:06 06/12/2004 02:04 2.0 .45 .80 .76 .42 16,232 7 7.5 .860 06/16/2004 20:26 06/17/2004 03:46 7.3 .52 .36 .32 .25 20,274 8 5.6 4.861 06/24/2004 06:04 06/24/2004 11:21 5.3 .39 .32 .22 .16 14,529 7 4.7 2.7

Appendix 2 47
Appendix table 2-2. Hydrologic and precipitation characteristics of sampled discharge events in the conventional-development basin—Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed; n/a, data not available]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation

 depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3

)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

62 07/03/2004 15:06 07/04/2004 00:04 9.0 1.40 1.72 1.24 0.76 53,015 8 31.6 5.863 07/09/2004 11:33 07/09/2004 12:43 1.2 .35 .76 .58 .33 11,141 6 7.1 5.564 07/16/2004 10:01 07/16/2004 16:41 6.7 1.94 5.08 2.96 1.50 83,778 9 96.0 4.865 07/21/2004 07:49 07/21/2004 10:36 2.8 .39 .44 .28 .16 15,284 8 5.3 4.666 07/29/2004 13:07 07/29/2004 17:31 4.4 .26 .32 .16 .12 8,334 6 4.5 8.167 08/01/2004 05:59 08/01/2004 07:08 1.2 .32 .68 .40 .30 12,909 8 8.2 1.768 08/02/2004 10:00 08/02/2004 11:43 1.7 .45 .88 .62 .35 16,702 7 8.4 1.169 08/03/2004 17:51 08/04/2004 01:25 7.6 1.20 3.84 2.02 1.09 45,887 8 60.9 1.370 08/18/2004 17:14 08/18/2004 18:40 1.4 .47 1.20 .86 .46 17,594 8 12.3 14.771 09/01/2004 03:48 09/01/2004 05:39 1.9 .50 .96 .56 .40 19,549 8 11.8 5.172 09/15/2004 09:29 09/15/2004 15:32 6.0 .56 1.08 .56 .28 21,598 8 18.4 14.273 10/07/2004 23:24 10/08/2004 06:29 7.1 .87 .64 .58 .48 39,630 9 10.8 6.174 10/22/2004 17:14 10/23/2004 09:23 16.2 .76 .24 .20 .19 33,845 9 3.9 3.575 10/29/2004 22:38 10/29/2004 23:07 .5 .49 1.64 -- -- 17,514 7 25.8 3.176 11/26/2004 22:08 11/27/2004 16:24 18.3 .48 .12 .08 .07 8,736 4 1.1 6.977 12/05/2004 21:12 12/06/2004 07:29 10.3 .34 .20 .16 .14 11,047 7 1.7 8.278 12/07/2004 00:44 12/07/2004 11:18 10.6 .57 .16 .16 .14 26,418 9 2.8 .779 01/01/2005 19:30 01/02/2005 18:00 22.5 snowmelt -- -- -- 54,887 <1 10.9 22.080 01/12/2005 06:46 01/13/2005 02:00 19.2 1.54b .80 .70 .61 47,572 6 3.2 9.581 03/22/2005 12:00 03/23/2005 19:00 31.0 snowmelt -- -- -- 47,696 <1 2.7 .682 03/24/2005 12:00 03/24/2005 19:00 7.0 snowmelt -- -- -- 23,262 <1 2.1 .783 03/30/2005 13:31 03/30/2005 18:18 4.8 .53 .84 .52 .27 16,617 6 10.5 1.084 04/06/2005 18:54 04/06/2005 20:38 1.7 .42 .76 .56 .37 15,664 7 7.4 7.085 05/11/2005 03:50 05/11/2005 10:04 6.2 .77 .32 .30 .28 40,592 11 6.4 1.986 05/13/2005 00:49 05/13/2005 06:34 5.7 .4 .56 .40 .31 18,933 10 8.8 1.687 05/18/2005 21:36 05/19/2005 03:14 5.6 .39 .36 .30 .20 15,701 8 5.4 5.688 06/05/2005 01:06 06/05/2005 03:44 2.6 .34 .48 .38 .27 384 <1 .3 8.689 06/24/2005 18:38 06/25/2005 04:44 10.1 .49 .28 .18 .12 1,336 1 1.2 9.990 06/30/2005 01:41 06/30/2005 01:45 .1 .11 -- -- -- 1,686 3 .2 4.0

48 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Duration

(hours)

Precipitation

 depth

(inches)

15-minute 

precipitation 

intensity

(in/h)

30-minute 

precipitation 

intensity

(in/h)

60-minute 

precipitation 

intensity

(in/h)

Total 

event 

volume

(ft3)

Runoff

(%)

Peak 

flow

(ft3/s)

Antecedent 

dry time

(days)

91 07/21/2005 08:45 07/21/2005 13:28 4.7 1.42 3.28 2.30 1.25 37,675 5 21.1 0.992 07/25/2005 00:47 07/26/2005 02:20 25.5 1.69 2.48 1.32 .78 49,441 6 23.3 1.593 08/11/2005 07:12 08/11/2005 23:41 16.5 .42 .32 .20 .18 11,118 5 3.5 16.294 08/26/2005 19:29 08/26/2005 20:30 1.0 .24 .40 .38 .23 10,045 8 7.0 8.495 09/13/2005 16:17 09/13/2005 16:35 .3 .23 .88 -- -- 7,750 7 16.7 17.896 09/19/2005 05:07 09/19/2005 07:18 2.2 .56 1.28 .80 .43 16,137 6 13.4 5.597 09/22/2005 02:49 09/22/2005 12:29 9.7 .4 .56 .36 .26 15,362 8 9.2 2.898 09/25/2005 09:46 09/25/2005 21:08 11.4 1.06 1.64 1.02 .61 37,292 7 21.8 2.9

a Single discharge event separated into two sampled periods.b Combination rainfall and snowmelt event.

Appendix table 2-2. Hydrologic and precipitation characteristics of sampled discharge events in the conventional-development basin—Continued.[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; in/h, inch per hour; ft3, cubic foot; %, percent; ft3/s, cubic foot per second; <, less than; --, not computed; n/a, data not available]

Appendix 2 49
Appendix table 2-3. Event mean concentrations for sampled discharge events in the low-impact-development (LID) basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; mg/L, milligram per liter; --, not computed; n/a, data not available]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(mg/L)

Total suspended 

solids (mg/L)

Total phosphorus

(mg/L)

2 10/05/1998 17:19 10/05/1998 20:07 152 82 --

3 02/11/1999 13:27 02/11/1999 16:30 3,110 2,550 --

4 04/22/1999 19:21 04/23/1999 09:52 n/a n/a n/a

5 05/16/1999 21:56 05/17/1999 19:31 926 300 1.77

6 06/30/1999 23:38 07/01/1999 09:52 1,270 108 --

7 07/17/1999 02:46 07/17/1999 15:10 880 974 --

8 07/20/1999 13:48 07/24/1999 18:40 n/a n/a --

9 08/23/1999 08:36 08/25/1999 10:35 n/a n/a n/a

10 02/23/2000 09:00 02/24/2000 22:00 n/a n/a n/a

11 02/25/2000 18:14 02/26/2000 12:49 n/a n/a n/a

13, 14, 15a 04/20/2000 00:16 04/22/2000 07:42 n/a n/a n/a

16 05/17/2000 12:49 05/19/2000 21:53 570 320 2.94

17 05/30/2000 06:23 06/01/2000 15:00 356 91 .53

18 06/13/2000 15:36 06/15/2000 02:54 434 132 .59

19 07/02/2000 19:34 07/03/2000 17:07 352 212 .57

20 07/10/2000 03:40 07/11/2000 02:25 216 84 .28

21 08/05/2000 11:09 08/06/2000 03:54 804 600 1.03

22 08/17/2000 02:23 08/18/2000 01:50 134 44 .27

23 09/11/2000 07:37 09/12/2000 14:03 350 252 .71

24 09/22/2000 09:16 09/24/2000 03:07 80 22 .27

25 04/09/2001 23:18 04/10/2001 03:10 228 35 .30

26 04/11/2001 06:01 04/12/2001 13:48 260 82 .36

27 04/21/2001 00:11 04/21/2001 23:55 182 40 .28

28 05/20/2001 23:50 05/22/2001 05:59 148 72 .25

29 05/23/2001 01:34 05/23/2001 21:03 108 12 .11

30 06/05/2001 01:29 06/06/2001 06:40 108 22 .16

31 08/01/2001 18:00 08/03/2001 13:30 n/a n/a n/a

32 09/07/2001 19:05 09/09/2001 00:02 196 61 --

33 09/18/2001 23:28 09/20/2001 10:48 116 8 .30

34 09/22/2001 22:34 09/24/2001 15:23 176 54 .39

35 10/22/2001 14:36 10/23/2001 22:14 116 16 .56

36 11/23/2001 22:17 11/25/2001 13:20 114 13 .35

37 02/18/2002 22:10 02/21/2002 20:19 222 21 .24

38 04/07/2002 15:14 04/09/2002 16:25 182 14 .22

39 03/04/2004 18:04 03/06/2004 12:24 252 77 .31

40 03/25/2004 14:20 03/27/2004 05:25 288 82 .44

41 05/21/2004 07:16 05/24/2004 10:00 468 273 .67

42 05/29/2004 06:41 06/01/2004 00:00 206 80 .32

43 07/03/2004 15:03 07/04/2004 12:00 112 31 .27

44 07/16/2004 10:01 07/17/2004 10:00 382 224 .66

45 08/03/2004 17:51 08/04/2004 14:00 348 175 .84

46 01/12/2005 13:04 01/13/2005 15:38 646 22 .37

47 03/21/2005 14:00 03/24/2005 19:10 136 23 .44

48 03/24/2005 10:00 03/25/2005 21:45 164 15 .31

49 03/30/2005 13:33 03/31/2005 17:14 156 25 .24

a

 Single discharge event separated into three sampled periods.

50 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix table 2-4. Event mean concentrations for sampled discharge events in the conventional-development basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; mg/L, milligram per liter; --, not computed; n/a, data not available; <, less than]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(mg/L)

Total suspended 

solids (mg/L)

Total phosphorus

(mg/L)

1 10/05/1998 18:01 10/05/1998 19:55 138 125 --

3 04/21/1999 20:58 04/26/1999 03:50 n/a n/a n/a

4 05/16/1999 16:25 05/17/1999 22:45 n/a n/a n/a

5 06/06/1999 14:24 06/07/1999 04:24 118 80 --

7 07/16/1999 20:34 07/18/1999 02:00 1,770 47 --

8 07/20/1999 13:07 07/21/1999 20:07 84 69 --

9 08/23/1999 08:35 08/24/1999 14:45 82 52 0.08

12 04/19/2000 07:45 04/19/2000 12:40 n/a n/a n/a

13, 14a 04/20/2000 00:18 04/21/2000 10:00 n/a n/a n/a

13a 05/17/2000 12:47 05/21/2000 15:15 278 256 .37

14a 05/26/2000 22:14 06/01/2000 18:13 376 312 .32

15 06/12/2000 05:50 06/15/2000 22:15 n/a n/a n/a

16 07/02/2000 19:36 07/04/2000 21:07 168 128 .16

17 07/10/2000 03:41 07/11/2000 16:00 46 17 .06

18 08/05/2000 11:10 08/07/2000 17:00 98 79 .14

19 08/17/2000 02:23 08/19/2000 06:00 42 25 .07

20 09/11/2000 07:38 09/13/2000 17:00 106 71 .25

21 09/22/2000 09:36 09/24/2000 00:30 38 14 .09

22 01/29/2001 11:08 01/31/2001 03:00 10,000 49 .20

23 02/08/2001 14:47 02/10/2001 16:00 1,410 51 .16

24 02/24/2001 09:21 02/25/2001 11:50 646 60 .21

25 04/10/2001 22:49 04/14/2001 00:20 366 193 .25

26 04/21/2001 00:10 04/22/2001 15:00 396 300 .30

27 05/10/2001 19:40 05/12/2001 21:50 152 113 .22

28 05/20/2001 23:58 05/23/2001 01:27 118 59 .13

29 05/23/2001 01:28 05/24/2001 22:00 206 18 .09

30 06/05/2001 01:29 06/07/2001 12:00 320 21 .05

31 06/11/2001 22:04 06/13/2001 21:10 298 148 .25

32 07/18/2001 07:21 07/20/2001 13:00 132 42 .10

33 08/01/2001 18:00 08/04/2001 23:00 n/a n/a n/a

34 08/24/2001 19:05 08/27/2001 04:00 122 59 .06

35 09/07/2001 12:19 09/11/2001 20:00 98 59 --

36 09/18/2001 23:44 09/22/2001 17:00 78 17 .07

37 09/22/2001 22:34 09/25/2001 06:00 74 32 .08

38 10/22/2001 14:38 10/24/2001 08:09 94 44 .13

39 11/23/2001 22:17 11/25/2001 19:00 82 60 .10

40 02/18/2002 22:13 02/23/2002 21:00 3,050 28 .12

41 03/08/2002 16:53 03/12/2002 19:00 1,050 234 .29

42 04/07/2002 03:39 04/07/2002 06:48 180 26 .08

43 08/04/2002 02:25 08/04/2002 10:33 112 55 .11

44 08/11/2002 16:34 08/11/2002 17:44 104 56 .12

Appendix 2 51
Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(mg/L)

Total suspended 

solids (mg/L)

Total phosphorus

(mg/L)

45 08/21/2002 18:01 08/22/2002 13:00 54 35 0.08

46 09/02/2002 03:46 09/02/2002 11:01 64 42 .08

47 09/28/2002 21:44 09/29/2002 05:29 112 71 --

48 03/04/2004 17:40 03/05/2004 11:28 138 54 .20

49 03/25/2004 20:09 03/26/2004 03:17 188 132 .13

50 05/08/2004 06:31 05/09/2004 00:53 270 179 .24

51 05/10/2004 02:24 05/10/2004 15:05 92 47 .18

52 05/17/2004 20:01 05/18/2004 03:01 164 105 .19

53 05/21/2004 07:16 05/21/2004 08:28 96 56 .14

54 05/29/2004 06:54 05/29/2004 12:05 <50 16 .07

55 05/30/2004 07:58 05/30/2004 10:42 88 54 .07

56 05/31/2004 06:54 05/31/2004 21:00 98 66 .12

57 06/09/2004 21:42 06/10/2004 12:15 <50 26 .10

58 06/10/2004 14:15 06/11/2004 05:00 78 30 .06

59 06/12/2004 00:06 06/12/2004 02:04 <50 19 .07

60 06/16/2004 20:26 06/17/2004 03:46 <50 21 .11

61 06/24/2004 06:04 06/24/2004 11:21 60 24 .10

62 07/03/2004 15:06 07/04/2004 00:04 78 47 .15

63 07/09/2004 11:33 07/09/2004 12:43 100 43 .15

64 07/16/2004 10:01 07/16/2004 16:41 192 117 .22

65 07/21/2004 07:49 07/21/2004 10:36 94 32 .20

66 07/29/2004 13:07 07/29/2004 17:31 104 51 .30

67 08/01/2004 05:59 08/01/2004 07:08 76 51 .11

68 08/02/2004 10:00 08/02/2004 11:43 60 27 .08

69 08/03/2004 17:51 08/04/2004 01:25 134 87 .19

70 08/18/2004 17:14 08/18/2004 18:40 154 105 .22

71 09/01/2004 03:48 09/01/2004 05:39 78 43 .15

72 09/15/2004 09:29 09/15/2004 15:32 120 64 .24

73 10/07/2004 23:24 10/08/2004 06:29 66 16 .11

74 10/22/2004 17:14 10/23/2004 09:23 52 14 .13

75 10/29/2004 22:38 10/29/2004 23:07 214 108 --

76 11/26/2004 22:08 11/27/2004 16:24 <50 11 .07

77 12/05/2004 21:12 12/06/2004 07:29 74 19 .09

78 12/07/2004 00:44 12/07/2004 11:18 <50 17 .07

79 01/01/2005 19:30 01/02/2005 18:00 456 10 .30

80 01/12/2005 06:46 01/13/2005 02:00 690 33 .36

81 03/22/2005 12:00 03/23/2005 19:00 222 32 .43

82 03/24/2005 12:00 03/24/2005 19:00 -- 29 .50

83 03/30/2005 13:31 03/30/2005 18:18 530 364 .52

84 04/06/2005 18:54 04/06/2005 20:38 210 143 .18

Appendix table 2-4. Event mean concentrations for sampled discharge events in the conventional-development basin—

Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; mg/L, milligram per liter; --, not computed; n/a, data not available; <, less than]

52 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix table 2-4. Event mean concentrations for sampled discharge events in the conventional-development basin—

Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; mg/L, milligram per liter; --, not computed; n/a, data not available; <, less than]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(mg/L)

Total suspended 

solids (mg/L)

Total phosphorus

(mg/L)

85 05/11/2005 03:50 05/11/2005 10:04 -- -- .14

86 05/13/2005 00:49 05/13/2005 06:34 110 73 0.13

87 05/18/2005 21:36 05/19/2005 03:14 82 39 .13

88 06/05/2005 01:06 06/05/2005 03:44 176 123 .39

89 06/24/2005 18:38 06/25/2005 04:44 174 74 .27

90 06/30/2005 01:41 06/30/2005 01:45 -- -- --

91 07/21/2005 08:45 07/21/2005 13:28 350 167 .19

92 07/25/2005 00:47 07/26/2005 02:20 90 56 .09

93 08/11/2005 07:12 08/11/2005 23:41 104 57 .13

94 08/26/2005 19:29 08/26/2005 20:30 136 88 .22

95 09/13/2005 16:17 09/13/2005 16:35 -- -- .39

96 09/19/2005 05:07 09/19/2005 07:18 -- -- .17

97 09/22/2005 02:49 09/22/2005 12:29 150 100 .20

98 09/25/2005 09:46 09/25/2005 21:08 80 49 .10

a

 Single discharge event separated into three sampled periods.

Appendix 2 53
Appendix table 2-5. Constituent loads computed for sampled discharge events in the low-impact-development (LID) basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; --, no data]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(pounds)

Total suspended 

solids (pounds)

Total phosphorus 

(pounds)

2 10/05/1998 17:19 10/05/1998 20:07 4 2 --

3 02/11/1999 13:27 02/11/1999 16:30 289 237 --

4 04/22/1999 19:21 04/23/1999 09:52 487 288 0.94

5 05/16/1999 21:56 05/17/1999 19:31 54 17 .10

6 06/30/1999 23:38 07/01/1999 09:52 110 9 --

7 07/17/1999 02:46 07/17/1999 15:10 27 29 --

8 07/20/1999 13:48 07/24/1999 18:40 1,096 787 --

9 08/23/1999 08:36 08/25/1999 10:35 390 160 .41

10 02/23/2000 09:00 02/24/2000 22:00 430 86 .77

11 02/25/2000 18:14 02/26/2000 12:49 363 131 .43

13, 14, 15a 04/20/2000 00:16 04/22/2000 07:42 436 102 .49

16 05/17/2000 12:49 05/19/2000 21:53 10,495 5,892 54.13

17 05/30/2000 06:23 06/01/2000 15:00 5,870 1,501 8.66

18 06/13/2000 15:36 06/15/2000 02:54 4,895 1,489 6.69

19 07/02/2000 19:34 07/03/2000 17:07 141 85 .23

20 07/10/2000 03:40 07/11/2000 02:25 56 22 .07

21 08/05/2000 11:09 08/06/2000 03:54 1,103 823 1.41

22 08/17/2000 02:23 08/18/2000 01:50 44 14 .09

23 09/11/2000 07:37 09/12/2000 14:03 194 140 .39

24 09/22/2000 09:16 09/24/2000 03:07 67 18 .23

25 04/09/2001 23:18 04/10/2001 03:10 137 21 .18

26 04/11/2001 06:01 04/12/2001 13:48 588 186 .81

27 04/21/2001 00:11 04/21/2001 23:55 94 21 .14

28 05/20/2001 23:50 05/22/2001 05:59 355 173 .60

29 05/23/2001 01:34 05/23/2001 21:03 15 2 .02

30 06/05/2001 01:29 06/06/2001 06:40 116 24 .17

31 08/01/2001 18:00 08/03/2001 13:30 23,260 20,670 31.56

32 09/07/2001 19:05 09/09/2001 00:02 254 79 --

33 09/18/2001 23:28 09/20/2001 10:48 184 13 .47

34 09/22/2001 22:34 09/24/2001 15:23 971 298 2.15

35 10/22/2001 14:36 10/23/2001 22:14 48 7 .23

36 11/23/2001 22:17 11/25/2001 13:20 70 8 .21

37 02/18/2002 22:10 02/21/2002 20:19 732 69 .80

38 04/07/2002 15:14 04/09/2002 16:25 168 13 .20

39 03/04/2004 18:04 03/06/2004 12:24 1,241 379 1.52

40 03/25/2004 14:20 03/27/2004 05:25 631 180 .96

41 05/21/2004 07:16 05/24/2004 10:00 13,720 8,003 19.76

42 05/29/2004 06:41 06/01/2004 00:00 360 140 .56

43 07/03/2004 15:03 07/04/2004 12:00 46 13 .11

44 07/16/2004 10:01 07/17/2004 10:00 1,417 831 2.45

45 08/03/2004 17:51 08/04/2004 14:00 788 396 1.90

46 01/12/2005 13:04 01/13/2005 15:38 1,098 37 .64

47 03/21/2005 14:00 03/24/2005 19:10 186 31 .60

48 03/24/2005 10:00 03/25/2005 21:45 55 5 .10

49 03/30/2005 13:33 03/31/2005 17:14 40 6 .06

a

 Single discharge event separated into three sampled periods.

54 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Appendix table 2-6. Constituent loads computed for sampled discharge events in the conventional-development basin.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; --, no data; <, less than]

Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(pounds)

Total suspended 

solids (pounds)

Total phosphorus 

(pounds)

1 10/05/1998 18:01 10/05/1998 19:55 305 276 --

3 04/21/1999 20:58 04/26/1999 03:50 399 126 0.40

4 05/16/1999 16:25 05/17/1999 22:45 919 728 .77

5 06/06/1999 14:24 06/07/1999 04:24 249 169 --

7 07/16/1999 20:34 07/18/1999 02:00 7,681 204 --

8 07/20/1999 13:07 07/21/1999 20:07 332 273 --

9 08/23/1999 08:35 08/24/1999 14:45 229 145 .24

12 04/19/2000 07:45 04/19/2000 12:40 90 30 .07

12a 04/20/2000 00:18 04/21/2000 10:00 880 270 .38

13 05/17/2000 12:47 05/21/2000 15:15 4,028 3,710 5.36

14 05/26/2000 22:14 06/01/2000 18:13 4,910 4,074 4.13

15 06/12/2000 05:50 06/15/2000 22:15 3,000 1,858 2.98

16 07/02/2000 19:36 07/04/2000 21:07 559 426 .53

17 07/10/2000 03:41 07/11/2000 16:00 134 49 .16

18 08/05/2000 11:10 08/07/2000 17:00 492 397 .71

19 08/17/2000 02:23 08/19/2000 06:00 116 69 .20

20 09/11/2000 07:38 09/13/2000 17:00 348 233 .80

21 09/22/2000 09:36 09/24/2000 00:30 67 25 .15

22 01/29/2001 11:08 01/31/2001 03:00 41,704 204 .84

23 02/08/2001 14:47 02/10/2001 16:00 7,231 262 .79

24 02/24/2001 09:21 02/25/2001 11:50 1,260 117 .41

25 04/10/2001 22:49 04/14/2001 00:20 671 354 .46

26 04/21/2001 00:10 04/22/2001 15:00 505 382 .38

27 05/10/2001 19:40 05/12/2001 21:50 392 292 .58

28 05/20/2001 23:58 05/23/2001 01:27 516 258 .58

29 05/23/2001 01:28 05/24/2001 22:00 297 26 .13

30 06/05/2001 01:29 06/07/2001 12:00 1,277 84 .22

31 06/11/2001 22:04 06/13/2001 21:10 3,212 1,595 2.73

32 07/18/2001 07:21 07/20/2001 13:00 321 102 .23

33 08/01/2001 18:00 08/04/2001 23:00 3,954 3,068 6.25

34 08/24/2001 19:05 08/27/2001 04:00 378 183 .19

35 09/07/2001 12:19 09/11/2001 20:00 711 428 --

36 09/18/2001 23:44 09/22/2001 17:00 445 97 .37

37 09/22/2001 22:34 09/25/2001 06:00 296 128 .31

38 10/22/2001 14:38 10/24/2001 08:09 203 95 .28

39 11/23/2001 22:17 11/25/2001 19:00 168 123 .20

40 02/18/2002 22:13 02/23/2002 21:00 17,782 163 .72

41 03/08/2002 16:53 03/12/2002 19:00 1,430 319 .40

42 04/07/2002 03:39 04/07/2002 06:48 225 32 .10

43 08/04/2002 02:25 08/04/2002 10:33 162 80 .16

Appendix 2 55
Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(pounds)

Total suspended 

solids (pounds)

Total phosphorus 

(pounds)

44 08/11/2002 16:34 08/11/2002 17:44 247 133 0.29

45 08/21/2002 18:01 08/22/2002 13:00 103 66 .14

46 09/02/2002 03:46 09/02/2002 11:01 172 113 .23

47 09/28/2002 21:44 09/29/2002 05:29 179 113 --

48 03/04/2004 17:40 03/05/2004 11:28 646 253 .92

49 03/25/2004 20:09 03/26/2004 03:17 571 401 .39

50 05/08/2004 06:31 05/09/2004 00:53 377 250 .34

51 05/10/2004 02:24 05/10/2004 15:05 103 53 .21

52 05/17/2004 20:01 05/18/2004 03:01 244 156 .29

53 05/21/2004 07:16 05/21/2004 08:28 196 114 .28

54 05/29/2004 06:54 05/29/2004 12:05 <82 26 .12

55 05/30/2004 07:58 05/30/2004 10:42 149 91 .11

56 05/31/2004 06:54 05/31/2004 21:00 94 63 .12

57 06/09/2004 21:42 06/10/2004 12:15 <50 26 .10

58 06/10/2004 14:15 06/11/2004 05:00 128 49 .10

59 06/12/2004 00:06 06/12/2004 02:04 <50 19 .07

60 06/16/2004 20:26 06/17/2004 03:46 <64 27 .14

61 06/24/2004 06:04 06/24/2004 11:21 54 22 .09

62 07/03/2004 15:06 07/04/2004 00:04 258 156 .49

63 07/09/2004 11:33 07/09/2004 12:43 70 30 .11

64 07/16/2004 10:01 07/16/2004 16:41 1,004 612 1.16

65 07/21/2004 07:49 07/21/2004 10:36 90 31 .19

66 07/29/2004 13:07 07/29/2004 17:31 54 27 .15

67 08/01/2004 05:59 08/01/2004 07:08 61 41 .09

68 08/02/2004 10:00 08/02/2004 11:43 63 28 .09

69 08/03/2004 17:51 08/04/2004 01:25 384 249 .56

70 08/18/2004 17:14 08/18/2004 18:40 169 115 .24

71 09/01/2004 03:48 09/01/2004 05:39 95 52 .18

72 09/15/2004 09:29 09/15/2004 15:32 162 86 .33

73 10/07/2004 23:24 10/08/2004 06:29 163 40 .27

74 10/22/2004 17:14 10/23/2004 09:23 110 30 .27

75 10/29/2004 22:38 10/29/2004 23:07 234 118 --

76 11/26/2004 22:08 11/27/2004 16:24 <28 6 .04

77 12/05/2004 21:12 12/06/2004 07:29 51 13 .06

78 12/07/2004 00:44 12/07/2004 11:18 <82 28 .11

79 01/01/2005 19:30 01/02/2005 18:00 1,562 34 1.02

80 01/12/2005 06:46 01/13/2005 02:00 2,049 98 1.08

81 03/22/2005 12:00 03/23/2005 19:00 661 95 1.29

82 03/24/2005 12:00 03/24/2005 19:00 -- 42 .73

Appendix table 2-6. Constituent loads computed for sampled discharge events in the conventional-development basin—

Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; --, no data; <, less than]

56 A Comparison of Two Small Basins Undergoing Implementation of Conventional- and Low-Impact-Development Strategies
Sample 

number

Start date

(mm/dd/yyyy)

Start time

(hh:mm)

End date

(mm/dd/yyyy)

End time

(hh:mm)

Total solids

(pounds)

Total suspended 

solids (pounds)

Total phosphorus 

(pounds)

83 03/30/2005 13:31 03/30/2005 18:18 550 378 .53

84 04/06/2005 18:54 04/06/2005 20:38 205 140 0.18

85 05/11/2005 03:50 05/11/2005 10:04 -- -- .36

86 05/13/2005 00:49 05/13/2005 06:34 130 86 .15

87 05/18/2005 21:36 05/19/2005 03:14 80 38 .13

88 06/05/2005 01:06 06/05/2005 03:44 4 3 .01

89 06/24/2005 18:38 06/25/2005 04:44 15 6 .02

90 06/30/2005 01:41 06/30/2005 01:45 -- -- --

91 07/21/2005 08:45 07/21/2005 13:28 823 393 .45

92 07/25/2005 00:47 07/26/2005 02:20 278 173 .29

93 08/11/2005 07:12 08/11/2005 23:41 72 40 .09

94 08/26/2005 19:29 08/26/2005 20:30 85 55 .14

95 09/13/2005 16:17 09/13/2005 16:35 -- -- .19

96 09/19/2005 05:07 09/19/2005 07:18 -- -- .17

97 09/22/2005 02:49 09/22/2005 12:29 144 96 .19

98 09/25/2005 09:46 09/25/2005 21:08 186 114 .24

a

 Single discharge event separated into three sampled periods.

Appendix table 2-6. Constituent loads computed for sampled discharge events in the conventional-development basin—

Continued.

[mm/dd/yyyy, month/day/year; hh:mm, hour:minute; --, no data; <, less than]

Appendix 2 57
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