Estimating Evapotranspiration Trends in the Eastern

United States: 1903-2002

Genevieve NoyceMount Holyoke College

Research and Discover Program

Dr. Lahouari BounouaBiospheric Sciences Branch (614.4)NASA Goddard Space Flight Center

Evapotranspiration, Vegetation and Climate

• Evapotranspiration cools air and changes surface water balance

• Vegetation affects climate through transpiration

• Variability of vegetation/climate interaction over time• Transpiration related to

atmospheric CO2 http://evolution.berkeley.edu

http://www.hbrc.govt.nz

Physiological Responses

• Downregulation: decrease in leaf conductance to conserve water• Observed in some experiments with elevated CO2

(Sellers, Bounoua et al. 1996)

• Model incorporating downregulation predicts less evapotranspiration and higher temperatures (Sellers, Bounoua et al. 1996, Bounoua et al. 1999)

• Runoff increase more than precipitation• Other models showed similar results (e.g. Betts et al. 1997,

Nohara et al. 2006)

• Results confirmed for large basins (Gedney et al. 2006)

Nohara et al., 2006:

19 climate models participating in IPCC AR4

Precipitation: 5.0% increase

Runoff: 8.9% increase

Are these models accurately replicating observations?

• Determine if observed historical trends match predictions• Estimate 20th century evapotranspiration using water

budget method

“Provide information about future changes in global carbon cycling and terrestrial and aquatic ecosystems for use in ecological forecasting and as inputs for improved climate change projections.”

-NASA Carbon Cycle & Ecosystems focus area

Precipitation

Runoff

Evapotranspiration

Aquifer recharge

Surface storage

dt

dSrERP

Study Area and Data Sources

• Precipitation: Dai et al. 1997• Runoff: USGS WaterWatch• Recharge: Wolock 2003, USGS

USGS

Runoff

Aquifer recharge

Withdrawal

Base flow

Base-flow Index

Calculation

VALUE

6 - 15.4

15.5 - 24.9

30.0 - 34.3

34.4 - 43.8

43.9 - 53.2

53.3 - 62.7

62.8 - 72.1

72.2 - 81.6

81.7 - 91.0

Aquifer Recharge

Wolock 2003

Precipitation and Runoff

Precipitation, Runoff and Recharge: Hydrologic Units 1-9

-50

-40

-30

-20

-10

0

10

20

30

40

50

1903-1912 1913-1922 1923-1932 1933-1942 1943-1952 1953-1962 1963-1972 1973-1982 1983-1992 1993-2002

Decade

Dif

fere

nce

fro

m a

vera

ge

(mm

/yr)

Precip y = 6.68x + 899.49

Runoff y = 1.82x + 10.02

Recharge y = 0.79x - 4.36

Evapotranspiration

Evapotranspiration: Hydrologic Units 1-9

y = 4.0671x - 22.369

-50

-40

-30

-20

-10

0

10

20

30

40

50

1903-1912 1913-1922 1923-1932 1933-1942 1943-1952 1953-1962 1963-1972 1973-1982 1983-1992 1993-2002

Decade

Dif

fere

nc

e f

rom

av

era

ge

(m

m/y

r)

Bounoua et al.

Climate Implications

CV IB IB-CV

Global 20.47 20.21 -0.26

All land 22.35 21.78 -0.57

Eastern U.S. 22.85 21.31 -1.54

All land

4.3

4

3

3.5

4

4.5

5

Precip Runoff

% c

han

ge

fro

m c

on

tro

l

Surface Temp. °C

Thanks to: Research and Discover Program

NASA GSFC

Betts, R.A., P.M. Cox, S.E. Lee, and F.I. Woodward. 1997. Contrasting physiological and structural vegetation feedbacks in climate change simulations. Nature 387: 796-9.

Bounoua, L., G.J. Collatz, P.J. Sellers, D.A. Randall, D.A. Dazlich, S.O. Los, J.A. Berry, I. Fung, C.J. Tucker, C.B. Field, and T.G. Jensen. 1999. Interactions between vegetation and climate; Radiative and physiological effects of doubled atmospheric CO2. Journal of Climate 12: 309-24.

Dai, A., I.Y. Fung, and A.D. Del Genio. 1997. Surface observed global land precipitation variations during 1900-88. Journal of Climate 10: 2943-62.

Gedney, N., P.M. Cox, R.A. Betts, O. Boucher, C. Huntingford, and P.A. Stott. 2006. Detection of a direct carbon dioxide effect in continental river runoff trends. Nature 439: 835-837.

Sellers, P.J., L. Bounoua, G.J. Collatz, D.A. Randall, D.A. Dazlich, S.O. Los, J.A. Berry, I. Fung, C.J. Tucker, C.B. Field, and T.G. Jensen. 1996. Comparisons of radiative and physiological effects of doubled atmospheric CO2 on climate. Science 271: 1402-1406.

Nohara, D., A. Kitoh, M. Hosaka, and T. Oki. 2006. Impact of climate change on river discharge projected by multimodel ensemble. Journal of Hydrometeorology 7: 1076-87.

Slayback, D.A., J.E. Pinzon, S.O. Los, and C.J. Tucker. 2003. Northern hemisphere photosynthetic trends 1982-1999. Global Change Biology 9: 1-15.

United States Geological Suvery. 2009. WaterWatch. Accessed 16 June 2009. http://ga.water.usgs.gov/edu/wuir.html

Wolock, D.M. 2003. Base-flow index grid for the conter minous United States. U.S. Geological Survey Open-File Report 03–263, digital dataset. Accessed 16 July 2009. http://water.usgs.gov/lookup/getspatial?bfi48grd