Green infrastructure, low impact development practices (LIDs), also called stormwater management best management practices (SWM BMPs), are often proposed to restore water balance functions and mitigate impacts or urbanization on runoff and recharge. One argument is that baseflows are lowered due to reduced infiltration and discharges to watercourses. It is a simple textbook theory. What does the data show? The following slide presentation was prepared to respond to the Ontario draft LID guidance manual in early 2017 since water balance impacts have been cited as justification for this infrastructure. Local studies show that baseflows have increased over decades of urbanization, calling into question the need for such measures considering that potential impact has not materialized. As noted in TRCA's Approved Updated Assessment Report under the Clean Water Act, at most gauges there was an upward trend in baseflows which prompted this: "These overall increases to baseflow volumes are contrary to the common thought that increased impervious cover leads to reduced baseflow" - so for those keeping score, data - one, common thought - zero. TMIG also analyzed baseflows in the GTA and noted “The seven-day average consecutive low flow data provides an indication of the observed baseflows within a watercourse, and hence is a suitable measure for determining whether baseflow trends exist in an urbanizing area. The trend analysis identified noticeable baseflow trends in 13 of the 24 recording stations. Of these eight urban and two rural stations exhibited an upward trend, suggesting increasing baseflow.”

1. Urbanization and Baseflow
Impacts - Evidence-based Water
Budget Management and Infiltration
LID / BMP Policy Needs
Robert J. Muir, M.A.Sc., P.Eng.
Manager, Stormwater
City of Markham
January 15, 2017
January 15, 2017 1

2. Baseflows Increase and Decrease Due
to Urbanization
• “For baseflow/low flow, results vary.
Some studies reported increased in
baseflow/low flow [Poff et al., 2006;
Chu, 2013] while others reported
decreased [Rose and Peters, 2001; Cuo
et al., 2008a Braud et al., 2013; Zhou
et al., 2013]”
• Terrestrial Water Cycle and Climate
Change: Natural and Human-Induced
Impacts, American Geophysical Union,
2016, edited by Qiuhong Tang, Taikan
Oki
https://books.google.ca/books?id=Kim9DAAAQBAJ&pg=PA124&lpg=PA12
4&dq=baseflow+increase+in+urban+areas+canada&source=bl&ots=gLJNQ
NUrCz&sig=u_BAH1mnd2thhYTZsZ-
78BbIoH8&hl=en&sa=X&ved=0ahUKEwiF4oj338LRAhUD1mMKHTZVC38Q
6AEITDAI#v=onepage&q=baseflow%20increase%20in%20urban%20areas
%20canada&f=false
January 15, 2017 LID Policy Review - R.Muir 2

3. US Analysis Shows Impervious Area Increase May
Not Result in Measurable Reductions in Base Flow
• “Rapid land development is raising concern regarding the ability of urbanizing
watersheds to sustain adequate base flow during periods of drought. Long
term streamflow records from unregulated watersheds of the lower to middle
Delaware River basin are examined to evaluate the impact of urbanization and
imperviousness on base flow. Trends in annual base flow volumes, seven-day
low flows, and runoff ratios are determined for six urbanizing watersheds and
four reference watersheds across three distinct physiographic regions.
Hydrograph separation is used to determine annual base flow and stormflow
volumes, and nonparametric trend tests are conducted on the resulting time
series. Of the watersheds examined, the expected effects of declining base
flow volumes and seven-day low flows and increasing stormflows are seen in
only one watershed that is approximately 20 percent impervious and has been
subject to a net water export over the past 15 years. Both interbasin transfers
and hydrologic mechanisms are invoked to explain these results. The results
show that increases in impervious area may not result in measurable
reductions in base flow at the watershed scale.”
• Base Flow Trends in Urbanizing Watersheds of the Delaware River Basin,
Brandes et al., Journal of the American Water Resources Association,
December 2005.
• http://onlinelibrary.wiley.com/doi/10.1111/j.1752-
1688.2005.tb03806.x/abstract
January 15, 2017 LID Policy Review - R.Muir 3

4. Toronto and Region Source Protection Area Early
1960’s-2001 Baseflows Increased with Urbanization
Contrary to ‘Common Thought’
• At all but two gauging stations, a positive or
upward trend was observed. These upward
trends vary depending on the watershed,
ranging from 2% in the Don and Etobicoke
watersheds and up to 45% in the Rouge.
These overall increases to baseflow
volumes are contrary to the common
thought that increased impervious cover
leads to reduced baseflow”
• Approved Updated Assessment Report:
Toronto and Region Source Protection Area.
Water Budget and Stress Assessment, 2015,
TRCA
• http://www.ctcswp.ca/wp-
content/uploads/2015/07/RPT_20150723_
Feb2015_TRSPA_PUAR_Chap3_HiRez.pdf
January 15, 2017 LID Policy Review - R.Muir 4

5. Significant Watershed Urbanization From 1966-
1999/2002 Shows Low Vulnerability of Baseflows
• Baseflows increase despite largely conventional design with no infiltration
measures over significant urban expansion areas, sometimes nearly 1000%
expansion (based on 1966 Canada Land Inventory and 1999-2002 SOLRIS
Version 1.2 land cover GIS mapping (compiled in the Ontario Land Cover
Compilation Version 2.0)) – demonstrates insensitivity of baseflows to
urbanization http://www.cityfloodmap.com/2016/08/urbanization-and-
runoff-explain.html
January 15, 2017 LID Policy Review - R.Muir 5

6. TRCA Baseflows Increased 1997-2006 in 6 Systems
Per 2010 Watershed Technical Report
• “The increasing trends are surprising when considering the recent trends in
summer precipitation amounts and their influence on mean baseflow levels.
Precipitation data from Toronto Pearson International Airport between the
years of 1997 to 2006 showed only two summers (2000 and 2003) in which
the normal summer rainfall amount (305.7mm) was reached. “
• Etobicoke and Mimico Creeks Watersheds Technical Update Report, 2010,
TRCA http://www.trca.on.ca/dotAsset/148567.pdf
January 15, 2017 LID Policy Review - R.Muir 6

7. Don River Baseflows Increased with Urbanization
According to 2009 Watershed Plan
• “Historically, increased urbanization and
development, and the establishment of
G. Ross Lord Dam in the Lower West Don
River subwatershed, have had an effect
on both annual and summer baseflow
volumes. The Don River gauge at
Todmorden shows that prior to the
construction of the dam in the 1970s,
average annual baseflow volumes
showed an increasing trend of
approximately 2.5% per year.”
• “Similar to annual trends, average
summer baseflow in the Don also
increased by 2.5% until 1973”
• Don River Watershed Plan Baseflow and
Water UseAssessment – Report on
Current Conditions 2009, TRCA
• http://trca.on.ca/dotAsset/55387.pdf
January 15, 2017 LID Policy Review - R.Muir 7

8. Don River Baseflows Affected by Precipitation and
Water Taking According to 2009 Watershed Plan
• A range of meteorological inputs, water
use outputs and watershed land use
conditions can affect summer baseflows:
“many factors including increased water
withdrawal practices, increased
impervious cover, or short-term trends in
precipitation.”
• Don River Watershed Plan Baseflow and
Water UseAssessment – Report on
Current Conditions 2009, TRCA
• http://trca.on.ca/dotAsset/55387.pdf
January 15, 2017 LID Policy Review - R.Muir 8

9. Rouge River Watershed Overall Natural Levels of
Baseflow Rated “Good” in 2007
• Baseflows are affected by temporary operational activities (16th Ave Trunk
Sewer dewatering) and local water taking. Management focuses on water
taking (demand management) as opposed to recharge management to
manage baseflows.
• Rouge River Rouge River State of the Watershed Report, Surface Water
Quantity, 2007, TRCA http://www.trca.on.ca/dotAsset/37761.pdf
January 15, 2017 LID Policy Review - R.Muir 9

10. GTA Water Balance & Flow Analysis Show MOE 2003
SWM Practices “Appropriate and Effective”
• TMIG completed “A review of policy direction and stream data trends in
the Greater Toronto Area” analyzing precipitation inputs, low flows,
average flows and unit flows over a range of temporal periods.
• “The seven-day average consecutive low flow data provides an indication
of the observed baseflows within a watercourse, and hence is a suitable
measure for determining whether baseflow trends exist in an urbanizing
area. The trend analysis identified noticeable baseflow trends in 13 of the
24 recording stations. Of these eight urban and two rural stations
exhibited an upward trend, suggesting increasing baseflow.”
• “The maximum observed downward trend is less than 1 mm per year,
equivalent to 0.008 mm of annual precipitation capture over the tributary
area.”
• The report concludes that “the introduction of blanket policies in the
absence of scientific and technical support may result in squandering of
our limited resources”
• Water Balance Review and Streamflow Analysis, TMIG, in Technical Bureau
Supplement, Water News, Newsletter of the Canadian Water Resources
Association, September, 2009
January 15, 2017 LID Policy Review - R.Muir 10

11. MNR Climate Change Impact Assessment Predicts
Increasing Groundwater Discharges and Levels
• MNR has assessed groundwater discharges and
groundwater levels at a sensitive groundwater
supply under future climate scenarios using the
Guide for Assessment of Hydrologic Impacts of
Climate Change developed by MNR in partnership
with MOE and Credit Valley Conservation. The
guide is to assess “Ecosystem Impacts and
Quantitative Flow and Water Level Targets”
• Case study median groundwater discharges
increase in all months and available well drawdown
increases for most scenarios.
• Water Budgets and Climate Change Guidance, Web
Application CC Training and Case Study, OCCIAR
and Northern Conservation Authorities, March 26-
27, 2012, Mike Garraway, MNR, Centre of
Excellence for Water Quantity.
• http://www.climateontario.ca/doc/workshop/Adap
tationPlanning/6_M_Garraway.pdf
January 15, 2017 LID Policy Review - R.Muir 11

12. Conclusions
• International, US and Local studies that rely on flow measurements and
scientific analysis indicate that urbanization / impervious area increases
do not adversely affect baseflow conditions – baseflows often increase
and decreases can be negligible (less than 1/100th of a mm per year).
• Water takings and operational factors (treatment plant
decommissioning, construction-related dewatering), as opposed to
urbanization are key drivers affecting local baseflows.
• New, broad-reaching water balance policies beyond MOE 2003 and
other modest local criteria are not required, or scientifically justified, to
manage baseflows.
• Current source control / infiltration targets are sufficient for the purpose
or baseflow management and expanded LID / BMP targets are not
required.
• Local water balance management targets and policies should be set only
based on local scientific study to meet local needs, considering local
risks (e.g., individual source protection quantity management recharge
and demand policies, individual natural feature-based water balance
targets)
January 15, 2017 LID Policy Review - R.Muir 12