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Urbanization and Baseflow Impacts - Evidence-based Water Budget Management and Infiltration LID / BMP Policy Needs

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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.”

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Urbanization and Baseflow Impacts - Evidence-based Water Budget Management and Infiltration LID / BMP Policy Needs

  1. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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

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