Transcript Hydroclimatology of North America 1

North American Hydroclimatology (I)
The nature of hydrologic variables
Xiaogang Shi and Dennis P. Lettenmaier
Hydro Group Seminar
September 12, 2007
Hydroclimatology
The study of water in the atmosphere and on the land
-David R. Legates
Outline
1)
Long term water budget;
2)
Atmospheric moisture transport and recycling;
3)
Spatial and temporal patterns of precipitation;
4)
Regional distribution and temporal trends of snow;
5)
Spatial distributions of soil moisture;
6)
Spatial and temporal distribution of streamflow
Hydrologic Variables in the water budget

Precipitation;

Evapotranspiration (ET);

Streamflow;

Storage terms:





Soil moisture
Snow
Groundwater
Lake, wetland, reservoir
Glaciers and ice sheets
Land surface water balance
E
P
Ws
 PER
t
R
where Ws represents
the water storage; P
is Precipitation; E is
Evapotranspiration; R
is Runoff.
Schematic of land surface water balance
Atmospheric water balance
Wa
 E  P  .Q
t
where .Q is the
horizontal divergence of
atmosphere moisture;
Wa is total column water
flux in the atmosphere
Schematic of atmospheric water balance
Long term water balance
E
P
R
R  .Q  P  E
(Su el al. 2006)
For long-term means, the
change of the annual mean
atmospheric water vapor, soil
moisture, and other variables,
can be neglected. So the
atmospheric flux convergence
should be balanced by runoff.
Schematic of hydrological cycle
Atmospheric moisture transport and recycling
Atmospheric moisture transport and recycling
1.The ocean is the primary
source of atmospheric
moisture, accounting for
about 85% of all evaporation
worldwide.
Ocean
2. Atmospheric moisture is
transported horizontally by
the wind, often traveling great
distances before
precipitating. Along with the
local evaporation (or ET),
they play an important role on
the contributions of
precipitation
(Source: NOAA)
1. The evaporation of surface moisture
2. Convergence of the moisture advection
over a long period
Atmospheric moisture source regions
(Source: UCSB)
1. Maritime tropical –
Caribbean or subtropical
Pacific source region
Warm and humid, can produce heavy
rains
2. Continental tropical southwestern desert source
regions
Hot and dry
3. Maritime polar - North Pacific
or North Atlantic source
regions
Cool and moist, follows typical cold front
into California
4. Continental polar and Arctic Canadian source region
Cold and dry, especially in winter.
Responsible for outbreaks of severe
cold weather throughout eastern US
North
American
Monsoon
North American Monsoon is
experienced as a pronounced
increase in rainfall from
extremely dry May to rainy
June–July–August–September
(JJAS) monsoon season
Annual Rainfall
Contributions of the
North American Monsoon:
( Comrie & Glenn, 1998 )
(Source: NOAA)
NW Mexico: 60-80%
Arizona: 35%
New Mexico: 45%
Schematic illustration of North American
Monsoon system
The shading area indicates
precipitation and block arrows
indicate convergence zones.
Small arrows show low-level
winds, and thick arrows
represent low-level jets.
One explanation is that it is
caused by large-scale landsea surface temperature
contrasts, as well as by landatmosphere interactions
related to elevated terrain and
land surface conditions (e.g.
soil moisture and vegetation).
(Source: NOAA)
Atmospheric moisture recycling ratio
Recycling ratio: how much
evaporation in an area contributes
to the precipitation in the same area.
(Trenberth 1999)
North American annual mean recycling ratio
For annual mean conditions, the recycling ratio ( 8 ~ 20
%) for L = 1000 km over land.
(Trenberth 1999)
Sources of moisture in MacKenzie (MAGS) and
Mississippi River basin (MRB)
(Bosilovich el al. 2006)
MAGS: 1. NPO 2. MAGS 3. CAN 4. ASA
MRB: 1.MRB 2. USMEX 3. NPO 4. GOM 5. NTA 6. CAR
Basin-averaged major moisture sources of precipitation
for MAGS
1. The North Pacific
Ocean’s (NPO)
source dominates in
winter and the other
continental sources
play a role in
summer.
2. Maximum moisture
recycling period is
May–July (MJJ)
(Bosilovich el al. 2006)
Moisture balance during May–July (MJJ) for MAGS
1. The recycling ratio is 19.6%,
meaning that 0.37 mm/day of the
water precipitating has come from
evaporation (total precipitation is 1.9
mm/day).
2. Szeto (2002) computed the
recycling ratio for MAGS to be 25%.
(Bosilovich el al. 2006)
The amount of water from NPO almost doubles the local source during
May–July (MJJ). However, other land areas including the rest of
Canada, and even Asia, also provide significant sources.
Basin-averaged major moisture sources of precipitation
for MRB
1. The MRB also has a
a clear annual cycle
of precipitation
recycling with a
maximum in MJJ.
2. The moisture
sources from MRB
dominate
precipitation
recycling in the
summer time.
(Bosilovich el al. 2006)
Moisture balance during May–July (MJJ) for MRB
1. The recycling ratio
is 23.4%.
2. Trenberth (1999)
computed the
recycling ratio for
MRB (1800 km
scales) to be 21%.
(Bosilovich el al. 2006)
Spatial and temporal patterns of precipitation
Seasonal Cycle of Precipitation (mm day-1)
Warm Season
May: Heaviest P in the Gulf Coast and
lower Mississippi Valley.
June: P reaches a maximum over the
Central US, while monsoon rainfall
spreads northward along the western
slopes of the Sierra Madre.
July: Monsoon P shifts northward into
AZ/NM by early July while P decreases in
Central US.
August: Monsoon P reaches a maximum
over SW and then starts to retreat.
September – October: Monsoon P
retreats gradually from SW to Mexico. In
October, the SW are drier while P
increases in the PNW and the north of
California.
Plots courtesy NARR
Seasonal Cycle of Precipitation (mm day-1)
Cold Season
Nov-Dec
Precipitation extends from the
Pacific Northwest southward to
California
Jan- Feb
Rainy season in PNW and
California. The P increases in lower
Mississippi Valley
March-April
Rainfall retreats northward and
lower Mississippi Valley becomes
drier. The precipitation spreads out
the central US and retreats
gradually from PNW and California.
Spatial distribution of precipitation trends (1900-2005)
Green shades indicate a trend
towards wetter conditions over
the period, and brown shades
indicate a trend towards dryer
conditions.
over the U.S., total annual
precipitation increased at an
average rate of 6.1 percent per
century since 1900, although
there was considerable regional
variability. The greatest
increases came in the East North
Central climate region (11.6
percent per century) and the
South (11.1 percent). Hawaii
was the only region to show a
decrease (-9.25 percent)
Data courtesy National Climatic Data Center
Spatial and temporal characteristics of heavy
precipitation events over Canada
38
30
(Zhang et al., 2001a)
The trends in the fraction of annual precipitation occurring in the largest 10% of
daily events with measurable precipitation as derived from 68 adjusted Canadian
stations. Over a large portion of the country, there is a upward trend over the
last 46 yr. It is clearly evident that the upward trend since 1940 was dominated
by the 38 short record stations. The majority of these stations are clustered in
northern Canada.
Increased heavy precipitation in USA
Trends (1910–95) related to
the highest daily precipitation
amount averaged throughout
the year;
Trends are expressed as a percentage of the overall mean of the
highest daily year-month precipitation amount. Statistically significant
trends are highlighted. The national trend is statistically significant at
the a = 0.05 level for the highest daily year-month values.
Karl et al. 1998
Regional distribution and temporal
trends of snow
North American snow cover distribution
1.
MODIS snow maps
represent maximum
snow cover in North
America during the same
8-day period (January
17-24) in each of four
consecutive years (20012004).
2. Though this 8-day period
is not necessarily
representative of the
entire winter, the maps
reveal that snow cover
duration is variable in
both space and time.
(Source: NASA)
Snow Data
(T. Mote, 2004)
•1 by 1 interpolated snowfall data
•interpolated from U.S. National
Weather Service (NWS)
cooperative stations and the
Canadian daily surface observations
•The period of record is 1900-2000
with a daily resolution
Temporal trends of snowfall
Knowles et al.2006
1.
Snowfall records from observations
from 1949 to 2004 at NCDC COOP
stations.
2.
The ratio of winter snowfall liquid
water equivalent (SFE) to winter total
precipitation (P) has changed
(SFE/P). November-March SFE/P has
decreased over the vast majority of
stations across the West, although
results are somewhat mixed over the
interior west including Colorado, Utah
and Wyoming
3.
Most of the significant changes in SFE
were found to be unrelated to
changes in total precipitation, so the
proportion of winter precipitation
falling as rain must have increased
during this period.
Temporal trends in SWE (Snow Water Equivalent)
1. Decreases in April 1
SWE between 19501997 at the majority of
sites;
2. The largest decreases
found in western Oregon
and Washington, and
northern California;
Mote el al. 2005
3. A number of stations in
southern Utah, Colorado
and elsewhere in the
Southwest, indicated
increasing trends in
SWE.
Soil moisture
land-atmosphere feedback of soil moisture
Precipitation
wets the
surface...
…causing soil
moisture to
increase...
…which affects the atmosphere
moisture balance
…which causes
evaporation to
increase during
subsequent days
and weeks...
…thereby inducing
additional precipitation
Observed ground soil moisture dataset
Ground data: Global Soil Moisture Data Bank (upper 5…10cm, point
scale, ~10 days)
Eurasia
USA
(Robock et al,2000)
Total number = 236 stations in 207 catchments
Dot:
insufficient data (136)
Plus/Circle:
catchment included in analysis (71)
SMMR soil moisture dataset
SMMR (1978-87): Satellite retrievals (upper 1.25cm, ~140km, ~3 days)
Avg. # of SMMR data per month (79-87)
SMMR soil moisture not
available
- under dense vegetation,
- close to water surfaces,
- in frozen soil.
(Owe et al.2001)
Model soil moisture: e.g. North America Regional
Reanalysis (NARR)
1. The limitations of observing soil moisture, there
have been virtually no datasets of soil wetness
produced for the whole North America. So we
calculate surface water storage (S; including
snowpack where it exists) as an integral of the
residual change in a simple water balance
relationship.
32 km
2. Observed or analyzed gridded data are used to
drive the model for a number of months or years.
(NARR)
Model soil moisture of 1993 flood and 1988 drought
1993
1988
Soil moisture availability for Top 1-meter of soil column
Average during 16-31 July
Spatial and temporal
distribution of streamflow
The distribution of USGS streamflow gages
Nationally, USGS surface-water data includes more than
8000 real-time gages that describe stream levels
Trends (p>0.05) in annual mean daily discharge
(Lins et al 1999)
The systematic decreases are in the Pacific Northwest, Northern California, and
parts of the Southeast; a broad area with uptrends stretches from the New
England to the Lower Colorado, and Mid-Atlantic, Ohio, Tennessee, Upper and
Lower Mississippi, Texas-Gulf, Rio Grande, and Great Basin. (1944-1993)
Trends in annual daily streamflow
1. A trend of decreasing annual
mean streamflow was found
across southern Canada. A small
increasing trend in the Great
Lakes–St. Lawrence region.
(Zhang et al., 2001a)
2. The trend in streamflow for the
United States during 1948-1988,
Lettenmaier et al. showed negative
trends in the northwest and positive
trends in the middle and northeastern
United States. The results are
consistent with the trends shown for
the 1947-1996 period in Canada
(Lettenmaier et al., 1994)
The variations on seasonality and timing of streamflow
1. The research shows
the winter flows rise and
summer flows drop in
the western US.
2. Streamflow in the
western US is highly
seasonal. Earlier
timing of snowmelt –
longer summer drought
Stewart et al 2004 Climatic Change
Thank You
Question?
Lake Dillon, Colorado, August 8, 2002