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Where Will the Water Go?Hydrologic Impacts of Climate Change

David Purkey, SEI and

Richard M. VogelDepartment of Civil and Environmental Engineering

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SEI Climate Change SymposiumTufts University

November 30, 2007

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• Previous national water resource assessments were completed 30-40 years ago:Wollman and Bonem, 1971; Water Resources Council 1968, 1978National Water Commission, 1973

• Methods introduced here apply to local, regional, national and global Climate and Water Assessments

• Water Availability Is Impacted by Climate, Land Use and Water Use and their Interactions and

Changes

Background and Motivation I

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Many recent innovations enable us to perform water resource assessments at extremely fine spatial and temporal scales.

Intellectual quest for an analog to the ‘Mach number’ or ‘Reynolds number’ for hydroclimatic systems

Background and Motivation II

Background and Motivation

III

Balancing Water for Humans and Nature

by

Malin Falkenmarkand

Johan Rockström

2004

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Methodology for a National/Global Water Census

Many of the following ideas arise from a collaboration with Peter Weiskel (USGS) and others resulting in:

Weiskel, P.K., R.M. Vogel, P.A. Steeves, P.J. Zarriello, L.A. DeSimone and K.G. Ries, III, Water-Use regimes: Characterizing direct human interaction with hydrologic systems, Water Resources Research, 43, W04402, 2007

and several other papers in progress.

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P

Traditionally, water availability is defined in terms of NET water balance of a watershed

P – ET = SWout

* water availability = runoff * reflects both the traditional water-supply perspective, and an aquatic-focused ecological perspective P = Precipitation;

ET = EvapotranspirationSWout = Surface-water runoffAssume that GWin = GWout = 0

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Consider total instead of net water balance…

P = SWout + ET * considers both: “green water” (ET) demands of terrestrial ecosystems, including rainfed agriculture, and “blue water” (SWout) demands of aquatic ecosystems and human withdrawals.

See Falkenmark and Rockström, 2004

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From watersheds to hydrologic units …

SWin + P = SWout + ET

* Considers landscape position,

as well as climate.

* considers both green and blue water

Recent GIS datasets (or gridded models) are

essential to this approach: (i.e. National

Hydrography Dataset, PRISM Climate Data, etc.)

Unit 1

Unit 2

Hydroclimatic Regimes 4 Extreme End-members Arise From Total Water Balance

headwatersource

headwaterno-flow

terminal flow-through

terminal sink

P P

ET SW + GW

ET

SW + GW

SW + GW

(from Weiskel, Vogel and others., in prep.)

Example fromNew England

Potential Water Availability(= P + SWin) for each of 308 HUC-12’s of the Conn. Riverwatershed (mean annual)

Map by Sara Brandt, using regional hydrologic equations of Vogel and Wilson (1996)

Paper on hydroclimatic regimesto appear as Weiskel, Vogel and others, in preparation, 2007

p + (swin + gwin) = et + (swout + gwout) = 1

- Land-atmosphere fluxes (P, ET) - Landscape fluxes (GW, SW)

hydro- system

Now, lower case denotes the normalized water balance:

Map of Potential Water Availability

for the African Continent

From MS Thesis by Sara Freeman

Tufts University 2007

Hydroclimatic regime plot

Shows relative magnitudes of vertical and horizontal fluxes

Deerfield River, MA, HUC-12

ConnecticutRiver basin, hydroclimaticregimes(for 308 HUC-12’s)

ET / PVery humid 0 – 0.33Humid 0.33 – 0.66Sub-humid 0.66 – 1.0Semi-arid 1.0 – 1.5Arid 1.5 – 3.0Very arid > 3.0

(data compiled by S.. Brandt using Vogel et al regressions)

humidvery humid sub-humid

semi-arid

arid

very arid

= et

= p

hydroclimatic pathway

headwaters

mouth

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Integrating human water useinto the water balance …

SWin + P + Hin

= SWout + ET + Hout

(see Weiskel and others, 2007)

Hout = withdrawals Hin = return flows + imports

…a water balance with three flux classes:

- Land-atmosphere fluxes (P, ET) - Landscape fluxes (GW, SW) - Human fluxes (Hin, Hout)

A new conceptual model of the terrestrial water balance:

hydro- system

Water-use Regimes: 4 end-member (EXTREME) regimes

surcharged churned

undeveloped depleted

Hin Hin Hout

SW + GW

SW + GW

SW + GW

(from Weiskel, Vogel and others 2007)

Hout

Central Valley Aquifer

P ET

P - ET P - ET

P - ET

Water-use regime plot

Shows relative magnitudes of withdrawals versus return flows and of human vs. natural fluxes.

(Weiskel, Vogel and others, 2007)

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Selected Water-Use RegimesWatersheds

From Weiskel, Vogel and others., 2007

Nor

mal

ized

Im

por

ts

+R

etu

rn F

low

s

Normalized Withdrawals

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Selected water-use regimesAquifers

From Weiskel, Vogel and others., 2007

Nor

mal

ized

Ret

urn

Flo

ws

Normalized Withdrawals

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Seasonal (Monthly) Water Use Regimes

Upper Charles River Aquifer,

Massachusetts1989-1998

Regimes are sensitive to seasonal climate and

water use variations

Based on transientsimulations of

Eggleston (2003)

Normalized Withdrawals

Nor

mal

ized

Ret

urn

Flo

ws

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A Water Resource Development Pathway

Mississippi River Alluvial Aquifer,

Predevelopment 1918to 1998

Water use regimes are subject to trends

Based on transientsimulations of Reed

(2003)Normalized Withdrawals

Nor

mal

ized

Ret

urn

Flo

ws

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Sustainable Water-Use Regimes

A rich topic forfuture research

For example relative Net demand RND

RND>0.2 impliesSTRESS

out

inout

SW

HHRND

Con

stan

t RN

D

Normalized Withdrawals

Nor

mal

ized

Ret

urn

Flo

ws

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Green Water Management Potential

Green water management strategies are most attractive in hydrologic units with

high water use intensity AND high green water

availability

An Indicator of Green

Water Management

Potential

From MS thesisSarah Freeman

Tufts University2007

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Traditional focus has been on net water balance of

watersheds•Focus was on blue-water demands of humans and aquatic

ecosystems•Traditional water assessments did not fully incorporate humans into the water balance•Focus was on watersheds, whereas water availability also depends upon location WITHIN watershed

Total water balance of hydrologic units offers a more comprehensive view of hydroclimatology

Summary

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Water Resource Assessments Must Focus on Hydrologic Units (HU’s) and total water balance because:

•1- Total water balance focuses on blue and green-water demands of humans (e.g., rainfed

agriculture) and terrestrial ecosystems•2-Water is managed in hydrologic units •3- Spatial datasets are gridded which is consistent

with HU’s•4-Integrated water balance is needed for full incorporation of humans into water cycle

Summary

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Climate Elasticity of Streamflow

Sankarasubramanian, Vogel and Limbrunner, Climate Elasticity of Streamflow in the United States, Water Resources Research, 2001.