Monroe L. Weber-Shirk

School of Civil and Environmental Engineering

NYC WatershedsNYC Watersheds

Homework commentsHomework 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 supplywellsCrotonCatskills/Delaware

groundwateroceanstreamsrainfall

NYC Watersheds: Introduction

NYC needs 61 m3/sHow much land is needed to deliver that

much water?How do you choose which land to use for

watersheds?

NYC Water Supply Strategy

QuantityQualityAltitudeProximity

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

PrecipitationWill need to use rain gage data from site close byUS climate data

Evaporation/TranspirationEvaporation data may be available for lakesEvaporation and Transpiration are strongly affected by

Type of vegetationGround coverTemperatureWind _____________Season

Annual Precipitation at Poughkeepsie NY

Snow year (July to June)

60

80

100

120

140

1950 1955 1960 1966 1971 1976 1981 1986 1995

year

prec

ipit

atio

n (c

m)

Annual Precipitation at Poughkeepsie NY

Precipitation cmLowest 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

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1

rank

prec

ipita

tion

(cm)

40 year record

Land Area Required for NYC Watersheds: Detail Approach

Measure the stream flow over time in potential watershedsUnited States Geological SurveyUSGS home pageThe 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)

0

50

100

150

year

cm/y

ear

streamflow precipitationWalton, NY Poughkeepsie

Why is Correlation Between Precipitation and Stream Flow so Poor?

Stations are too far apartEvapo-transpiration

changes with land useTime lag between

precipitation andstream flow

y = 0.4351x + 14.857

R2 = 0.29420

20

4060

80

100

50 70 90 110 130 150

precipitation (cm/yr)

stre

amflo

w (c

m/y

r)

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

0

20

40

60

80

100

0 0.2 0.4 0.6 0.8 1

rank

stre

amfl

ow (

cm/y

r)

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 m3/s demandDrought-year stream flow of 27 cm/yr

= 7130 km2

(flow rate - Q)

(Velocity - V)

(Area - A)

^

Q=VA A=Q/V

361m

s

86400s

day

365.25days

year

year

0.27m

2

2

km

1,000,000m

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 flowsreduce water

consumption

Where Can NYC Get Its Water?

NYC Watersheds: Croton and Catskill

Croton system (1842)12 reservoirs and 3 controlled lakes960 km2 of watershed _____ of the City’s drinking water

Catskill system (1927)2 source reservoirs1461 km2 of watershed _____ of the City’s drinking water

10%

40%

NYC Watersheds: Delaware and Total system

Delaware system (1965)4 source reservoirs2585 km2 of watershed _____ of the City’s drinking water

Total System5000 km2 of watershedour estimate: 7130 km2

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