Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering NYC Watersheds

Homework comments  Office hours today and Wednesday  Equations: solve first using symbols.  Substitute numbers (with units) as the last step  Make sure you answer all parts of every question  Work in teams!  Each person should attempt the homework before meeting together  Final solution should be checked by a different team member than the person writing the solution  This will eliminate careless mistakes!

Overview  Quantity of water needed by NYC  Potential sources of water  ______________  _____________  History of NYC water supply  wells  Croton  Catskills/Delaware groundwater ocean streams rainfall

NYC Watersheds: Introduction  NYC needs 61 m 3 /s  How much land is needed to deliver that much water?  How do you choose which land to use for watersheds?

NYC Water Supply Strategy Quantity Quality Altitude Proximity  Choose streams to meet 4 criteria  _________  Build dams on streams to provide storage (reservoirs)  Build aqueducts to carry the water from the reservoirs to NYC

Land Area Required for NYC Watersheds (The Big Picture)  Where does the water in the streams come from?___________  How could we estimate the average stream flow?  ______________________ Precipitation Rain gage Direct stream measurements

Hydrologic Cycle

Data Requirements for Predicting Stream Flows  Precipitation  Will need to use rain gage data from site close by  US climate data  Evaporation/Transpiration  Evaporation data may be available for lakes  Evaporation and Transpiration are strongly affected by  Type of vegetation  Ground cover  Temperature  Wind  _____________ Season

Annual Precipitation at Poughkeepsie NY Snow year (July to June)

Annual Precipitation at Poughkeepsie NY Precipitationcm Lowest year: 66 First quartile: 92 Median: 106 Third quartile: 115 Highest year: 137 Mean: 103 For comparison, Binghamton’s mean precipitation is 94 cm/year 40 year record

Land Area Required for NYC Watersheds: Detail Approach  Measure the stream flow over time in potential watersheds  United States Geological Survey  USGS home page  The National Atlas of the United States of America  Calculate the annual water yield per watershed area (cm/year)

Stream Flow Vs. Precipitation Snow year (July to June) Walton, NY Poughkeepsie

Why is Correlation Between Precipitation and Stream Flow so Poor?  Stations are too far apart  Evapo-transpiration changes with land use  Time lag between precipitation and stream flow

Stream Flow vs. Precipitation Estimates for the Catskills 60% ground water flow  Approximately _____ of the rainfall leaves the watershed as stream flow  The majority of the remaining rainfall leaves the watershed as transpiration/evaporation  A small amount of water leaves the watershed as _________________

Annual Stream Flow at Walton NY Stream Flow(cm/yr) Lowest year:27 First quartile:53 Median:58 Third quartile:70 Highest year:92 Mean:60

Drought Year Watershed Sizing  Estimate the Watershed size required for NYC based on:  61 m 3 /s demand  Drought-year stream flow of 27 cm/yr = 7130 km 2 (flow rate - Q) (Velocity - V) (Area - A) ^ Q=VA A=Q/V

Analysis Assumptions  Reservoirs can store water to even out flow throughout a year  We assumed drought intensity is same in all NYC watersheds  We assumed all watersheds have same drought ___________  What about NYC’s ability to ____________ __________ during a drought? stream flows reduce water consumption

Where Can NYC Get Its Water?

NYC Watersheds: Croton and Catskill  Croton system (1842)  12 reservoirs and 3 controlled lakes  960 km 2 of watershed  _____ of the City’s drinking water  Catskill system (1927)  2 source reservoirs  1461 km 2 of watershed  _____ of the City’s drinking water 10% 40%

NYC Watersheds: Delaware and Total system  Delaware system (1965)  4 source reservoirs  2585 km 2 of watershed  _____ of the City’s drinking water  Total System  5000 km 2 of watershed  our estimate: 7130 km 2 50% MAP

Summary  We found how much land NYC needs to supply their water based on stream flow data  We found where the land is located and saw where the reservoirs are located  Coming up…  How big do the reservoirs have to be to provide adequate storage?  How does the water get from the reservoirs to NYC?

NY Map

NYC Watersheds

Pepacton Schoharie Ashokan Neversink Rondout Cannonsville Catskill/Delaware Watersheds