Humidity Changes and the
Future Water Balance over the
Western U.S.: Divergences
Between Global and
Hydrological model Projections
David W. Pierce 1
Anthony L. Westerling 2
Jared Oyler 3
1S c r i p p s
I n s t i t u t i o n o f O c e a n o g r a p hy, L a J o l l a , C A
2S i e r r a
N ev a d a R e s e a r c h I n s t i t u te
U n i v e r s i t y o f C a l i fo r n i a , M e r c e d
3N u m e r i c a l
Te r r a d y n a m ic S i m u la t i o n G r o u p
U n i v e r s i t y o f M o n t a n a , M i s s o u la
Photo (CC) Gerry Slabaugh@flickr
OBJECTIVES
Show relative humidity (RH) will decline by 2 – 6% this
century in western U.S. (17 global climate models, CMIP-3)
The Variable Infiltration Capacity (VIC) hydrological model
dewpoint is biased low, esp. over coast
When VIC is forced with temperature and precipitation (T,P)
from global models, it does not reproduce the original global
model trend.
Estimate has 5-20% ef fect on runof f.
CHANGE IN REL HUMIDIT Y, %/DECADE
Median of 17 CMIP-3 models (SRES A1B)
12-16 (of 17) models agree on sign
VIC HYDROLOGICAL MODEL
Meteorological algorithms from
MT-CLIM ( H u n g e r fo r d et a l . 1 9 8 9 ;
T h o r n to n a n d R u n n i n g , 1 9 9 9 ; T h o r n to n et
a l . , 2 0 01 )
Uses daily T min , T max , P
T dew = T min + aridity correction
(Kimball et al. 1997)
Evaluated dewpoint simulation
at 74 Global Summary of Day
stations, 1975-2009
DEWPOINT BIAS (C)
KIMBALL ET AL. (1997) ARIDIT Y CORRECTION
Red dots:
observed
Tdew/Tmin
More arid -->
X axis: aridity (daily PET/ 90-day P)
Blue line:
Kimball-97
correction
term
VIC SIMULATED RH TRENDS (SRES A2)
GFDL CM2_1.1 model
Bias correction and statistical downscaling make no difference
RH TREND ERROR (%/DECADE)
Too moist
EST. RUNOFF CHANGES (%) IF FIXED
Routed Upper Colorado R.
flow declines 5%
SUMMARY
Global models: Relative humidity -2 to -6% this century in
spring/summer (depends on model and forcing)
VIC does not reproduce this trend. It can turn global model RH
decreases into RH increases
Simulation does not become arid enough
Makes 5% dif ference in Colorado River flow, 10-20% in
interior western U.S.