Review of the New York City Department of Environmental Protection Operations Support Tool for Water Supply (2018) / Chapter Skim
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5 Use of OST in a Changing Climate
5 Use of OST in a Changing Climate
Pages 131-158
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From page 131... ...
To adapt to future changes in precipitation and streamflow patterns, the New York City Department of Environmental Protection (NYC DEP) and other water utilities need statistical methods, simulation modeling, and other analytical tools to help them assess trends over recent decades, project a range of possible future conditions, and estimate impacts of these future conditions on water quantity and water quality, most notably turbidity in the case of New York City's system.
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From page 132... ...
These broader-scale assessments suggest that the New York City water supply system will experience changing hydrologic conditions under a changing climate, although with large uncertainties about the effects and their direction (wetter vs. drier)
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From page 133... ...
Formal attribution approaches, however, have not established a significant connection between increased riverine flooding and human-induced climate change. Observed Trends in Regional and Local Extreme High Precipitation and Streamflow Several assessments, including some by the NYC DEP, and individual studies have analyzed observed hydrologic changes for the specific area of the New York City water supply watershed; these assessments also include projections of future trends.
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New York depending on decades with accelerated rate of and Frei, precipitation; events greater than State station; range increase since the mid-1990sc 2013 or equal to the 95th percentile of 55–121 years
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From page 135... ...
, and cold Mean magnitude of extreme During warm season, magnitudes (May–November) events were 10-40% higher in the 2000s season streamflow than during the 1980s; cold-season values peaked during the 1970s and 1980s; thus warm-season magnitudes of extreme hydrologic events were greater in the last two decades than during previous decades.
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climate extremes index3 O and global projections based on climate models (Kunkel et al., 2013)
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From page 137... ...
GLOBAL- AND REGIONAL-SCALE PROJECTIONS OF CLIMATE CHANGE For purposes of providing context for considering impacts of climate change on the NYC water supply and specifically OST's applicability, this section summarizes some of the salient conclusions from recent global, regional, and more localized studies of future climate impacts. It is important to distinguish these projections, which are made at various scales with varying relevance to the NYC watershed, from the recent observations previously discussed.
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From page 138... ...
. A considerable body of work has addressed how future climate change would affect water resources specifically for New York City, starting in 2001 with the Metro East Coast Assessment (Major and Goldberg., 2001)
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From page 139... ...
The Metro East Coast Assessment did project the potential for increasing droughts in the New York City watershed region (Major and Goldberg, 2001) , as did the ClimAID study, which concluded that there was potential for increasing late warm-season droughts throughout New York State (Horton et al., 2011)
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TABLE 5-2 Projected Changes in New York City Watershed Components Due to Climate Change 140 Number of GCMs, Projected Change Emissions Scenarios, Variable Time of Year Timeslice(s) Downscaling Method HWMs Reference Snowmelt Earlier 2081–2100 9 GCMs, SWAT-WBa Pradhanang et Change factor method al., 2013 Earlier 2046–2065 3 GCMs, GWLF-VSAb Matonse et al., 2081–2100 Delta method 2013 Streamflow Increase November–March, 2081–2100 9 GCMs, SWAT-WBa Pradhanang et decrease April Change factor method IHAc al., 2013 Streamflow Positive trends in 3-day peak flows; decrease in 2028–2082 16 GCMs, VICd Demaria et al., magnitude of 7-day low flows and mean base 2 Emission scenarios, 2016 flows; length of low-flow season likely to extend Quantile-mapping by mid-century despite increased precipitation approach as atmospheric demand increases Runoff Increase in winter, 2046–2065 3 GCMs, GWLF-VSAb Matonse et al., slight decrease in spring 2081–2100 3 Emissions scenarios, 2013 Delta method Droughts More frequent 2020s 2 GCMs PDSIe Major and 2050s Delta method Goldberg, 2001 2080s Droughts Short-duration warm season droughts more 2020s 16 GCMs, PDSIe Horton et al., likely than not become more common; extreme 2050s Delta method 2011 multiyear droughts uncertain 2080s
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gA two-dimensional, longitudinal/vertical, hydrodynamic and water quality model. hOASIS - Options AnalysiS in Irrigation Systems.
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From page 142... ...
Adaptive management practices can help to improve water resources decision making under changing climate conditions, even in the face of population growth, changes in water demand, and land-use changes. Regular Updates and Reanalysis of Trends in Climate and Streamflow Data A key part of adaptive management is ongoing data collection and analysis to improve and update decisions.
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From page 143... ...
To build confidence in using selected GCMs, RCMs, and hydrologic models to project future water supply conditions in OST, the OST team needs to compare the observed climate and hydrology with climate and ydrology h hindcast simulations at appropriate scales. As discussed in Chapter 2, hindcasting refers to the practice of establishing the skill of mathematical simulation models to replicate past conditions.
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From page 144... ...
, but here the Committee elaborates further on methods used in recent studies and those that could be used in the future. NYC DEP Climate Change Integrated Modeling Project Since the NYC DEP's Climate Change Plan's release in 2008, the NYC DEP has utilized some components of OST in simulation mode for climate change studies (Owens and Frei, 2017)
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, Phase II studies will focus on extreme events, using improved models and model linkages, and will make use of updated climate data and scenarios. Approaches to Projecting Future Climate Conditions at a Watershed Scale There are multiple, complementary approaches to projecting future changes in temperature, precipitation, and other climate variables.
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From page 146... ...
In addition, the use of sensitivity tests and stress tests, which are applicable to any of these approaches to generating climate projections, is discussed. Approach 1: GCMs, RCMs, and Hydrologic Models In Approach 1, GCMs and RCMs provide projections of precipitation and temperature, which are used as inputs to a hydrologic model that in turn provides streamflow inputs to OST.
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. Using ensembles of climate and hydrologic models as drivers to OST analyses would enable climate change planning to be based on a better understanding of the range of potential impacts on supply and quality in the NYC watershed region.
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In addition, this approach does not account for how continuing increases in greenhouse gas forcing may alter future climate processes, including larger-scale dynamics (e.g., changing circulation patterns and storm tracks) , which may alter the trajectory of changes in the NYC watershed region in the coming decades.
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From page 149... ...
Vulnerability Assessments Vulnerability assessments, sometimes referred to as stress tests, are another useful way of investigating the impacts of climate change on water supply and quality, and can be done with any of the three approaches to climate projections already discussed. In one approach, critical levels of hydrologic changes (defined by their impacts on operations)
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(2014) , OST was run 252 times across a wide range of possible future climate, demand, regulatory, and flow conditions.5 Performance of the water supply system was assessed in terms of percent drought days and percent alum days for a "future without action" and three future strategies (increased capacity of the Catskills Aqueduct, various operational changes, and supply augmentation from groundwater sources)
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From page 151... ...
Representatives from the groups that currently gather hydrologic data could join together to form a Climate Resiliency Indicators and Monitoring Working Group that would be able to link the many sources of climate change-related information for both water supply and quality and to analyze trends of key variables for the entire New York metropolitan region. NYC DEP's efforts could be usefully coordinated into a rigorous r egional process linked by agreed-on approaches, data, scenarios, models, and interactions.
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From page 152... ...
As OST is used in simulation mode in future climate change studies, it will be important to consider a range of approaches as inputs to OST, including climate and hydrologic models, historical climate analogs, and current conditions and trends. Together, climate modeling and observational approaches provide effective and complementary inputs to near- and long-term water supply planning for the region.
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From page 153... ...
NYC DEP should consider coordinating with other NYC and regional agencies to create and update a Climate Resiliency Indicator and Monitoring System for the New York metropolitan region and assess climate change. This will track, on an ongoing basis, the frequency of occurrence of weather conditions that pose the highest risks to the water supply system.
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Climate Change Impacts in the United States: The Third National Climate Assessment, J
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In: Climate Change Impacts in the United States: The Third National Climate Assessment.
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From page 156... ...
2013. Suspended sediment source areas and future climate impact on soil erosion and sediment yield in a New York City water supply watershed, USA.
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From page 157... ...
2013. Streamflow responses to climate change: Analysis of hydrologic indicators in a New York City water supply watershed.
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