North American Hydroclimatology (I) North American Hydroclimatology (I)

The nature of hydrologic variablesThe nature of hydrologic variables

Hydro Group Seminar

September 12, 2007

Xiaogang Shi and Dennis P. Lettenmaier

HydroclimatologyHydroclimatology

The study of water in the The study of water in the atmosphereatmosphere and on the and on the landland

-David R. Legates-David R. Legates

OutlineOutline

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 Hydrologic Variables in the water budget in the water budget

Precipitation;Precipitation;

Evapotranspiration (ET); Evapotranspiration (ET);

Streamflow; Streamflow;

Storage terms:Storage terms:

Soil moisture Soil moisture SnowSnow Groundwater Groundwater Lake, wetland, reservoir Lake, wetland, reservoir Glaciers and ice sheetsGlaciers and ice sheets

Land surface water balanceLand surface water balance

Schematic of land surface water balance

where Ws represents the water storage; P is Precipitation; E is Evapotranspiration; R is Runoff.

REPt

Ws

EE PP

RR

Atmospheric water balanceAtmospheric water balance

Schematic of atmospheric water balance

where is the horizontal divergence of atmosphere moisture; Wa is total column water flux in the atmosphere

QPEt

Wa.

Q.

Long term water balanceLong term water balance

Schematic of hydrological cycle

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.

EPQR .PPEE

RR(Su el al. 2006)

Atmospheric moisture Atmospheric moisture transport and recyclingtransport and recycling

Atmospheric moisture Atmospheric moisture transport and recyclingtransport and recycling

1.The ocean is the primary 1.The ocean is the primary source of atmospheric source of atmospheric moisture, accounting for moisture, accounting for about 85% of all evaporation about 85% of all evaporation worldwide.worldwide.

2. Atmospheric moisture is 2. Atmospheric moisture is transported horizontally by transported horizontally by the wind, often traveling great the wind, often traveling great distances before distances before precipitating. Along with the precipitating. Along with the local evaporation (or ET), local evaporation (or ET), they play an important role on they play an important role on the contributions of the contributions of precipitationprecipitation

(Source: NOAA)

OceanOcean

1. The evaporation of surface moisture

2. Convergence of the moisture advectionover a long period

Atmospheric moisture source regionsAtmospheric moisture source regions

1. Maritime tropical – 1. Maritime tropical – Caribbean or subtropical Caribbean or subtropical

Pacific source regionPacific source region Warm and humid, can produce heavy Warm and humid, can produce heavy

rains rains

2. 2. Continental tropical - Continental tropical - southwestern desert source southwestern desert source regions regions

Hot and dryHot and dry

3. 3. Maritime polar - North Pacific Maritime polar - North Pacific or North Atlantic source or North Atlantic source regions regions

Cool and moist, follows typical cold front Cool and moist, follows typical cold front into California into California

4. 4. Continental polar and Arctic - Continental polar and Arctic - Canadian source region Canadian source region

Cold and dry, especially in winter. Cold and dry, especially in winter. Responsible for outbreaks of severe Responsible for outbreaks of severe cold weather throughout eastern US cold weather throughout eastern US

(Source: UCSB)

Annual RainfallContributions of theNorth American Monsoon:

NW Mexico: 60-80%Arizona: 35%New Mexico: 45%

North North American American MonsoonMonsoon

North American Monsoon is experienced as a pronounced increase in rainfall from extremely dry May to rainy June–July–August–September (JJAS) monsoon season

(Source: NOAA)( Comrie & Glenn, 1998 )

Schematic illustration of North American Monsoon system

The shading area indicates The shading area indicates precipitation and block arrows precipitation and block arrows indicate convergence zones. indicate convergence zones.

Small arrows show low-level Small arrows show low-level winds, and thick arrows winds, and thick arrows represent low-level jets. represent low-level jets.

One explanation is that it is One explanation is that it is caused by large-scale land-caused by large-scale land-sea surface temperature sea surface temperature contrasts, as well as by land-contrasts, as well as by land-atmosphere interactions atmosphere interactions related to elevated terrain and related to elevated terrain and land surface conditions (e.g. land surface conditions (e.g. soil moisture and vegetation).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 annual mean conditions, the recycling ratio ( 8 ~ 20 %) for L = 1000 km over land.%) for L = 1000 km over land. (Trenberth 1999)

Sources of moisture in MacKenzie (MAGS) and Mississippi River basin (MRB)

MAGS: 1. NPO 2. MAGS 3. CAN 4. ASA1. NPO 2. MAGS 3. CAN 4. ASA

MRB: 1.MRB 2. USMEX 3. NPO 4. GOM 5. NTA 6. CAR1.MRB 2. USMEX 3. NPO 4. GOM 5. NTA 6. CAR

(Bosilovich el al. 2006)

Basin-averaged major moisture sources of precipitationfor 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

(Bosilovich el al. 2006)

1. The recycling ratio is 19.6%, 1. The recycling ratio is 19.6%, meaning that 0.37 mm/day of the meaning that 0.37 mm/day of the water precipitating has come from water precipitating has come from evaporation (total precipitation is 1.9 evaporation (total precipitation is 1.9 mm/day).mm/day).

2. Szeto (2002) computed the 2. Szeto (2002) computed the recycling ratio for MAGS to be 25%.recycling ratio for MAGS to be 25%.

The amount of water from NPO almost doubles the local source during The amount of water from NPO almost doubles the local source during May–July (MJJ). However, other land areas including the rest of May–July (MJJ). However, other land areas including the rest of Canada, and even Asia, also provide significant sources. Canada, and even Asia, also provide significant sources.

Basin-averaged major moisture sources of precipitationfor 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

(Bosilovich el al. 2006)

1. The recycling ratio 1. The recycling ratio is 23.4%. is 23.4%.

2. Trenberth (1999) 2. Trenberth (1999) computed the computed the recycling ratio for recycling ratio for MRB (1800 km MRB (1800 km scales) to be 21%.scales) to be 21%.

Spatial and temporal patterns of precipitation

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.

Seasonal Cycle of Precipitation (mm day-1) Warm Season

Plots courtesy NARRPlots courtesy NARR

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.

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.

Seasonal Cycle of Precipitation (mm day-1) Cold Season

Spatial distribution of precipitation trends (1900-2005)

Green shades indicate a trend Green shades indicate a trend towards wetter conditions over towards wetter conditions over the period, and brown shades the period, and brown shades indicate a trend towards dryer indicate a trend towards dryer conditions.conditions.

over the U.S., total annual over the U.S., total annual precipitation increased at an precipitation increased at an average rate of 6.1 percent per average rate of 6.1 percent per century since 1900, although century since 1900, although there was considerable regional there was considerable regional variability. The greatest variability. The greatest increases came in the East North increases came in the East North Central climate region (11.6 Central climate region (11.6 percent per century) and the percent per century) and the South (11.1 percent). Hawaii was South (11.1 percent). Hawaii was the only region to show a the only region to show a decrease (-9.25 percent)decrease (-9.25 percent)

Data courtesy National Climatic Data CenterData courtesy National Climatic Data Center

Spatial and temporal characteristics of heavy precipitation events over Canada

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.

(Zhang et al., 2001a)(Zhang et al., 2001a)

3838

3030

Increased heavy precipitation in USA

Trends (1910–95) related to the highest daily precipitation amount averaged throughout the year;

Karl et al. 1998

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.

Regional distribution and temporal trends of snow

1.1. MODIS snow maps MODIS snow maps represent maximum represent maximum snow cover in North snow cover in North America during the same America during the same 8-day period (January 17-8-day period (January 17-24) in each of four 24) in each of four consecutive years (2001-consecutive years (2001-2004).2004).

2. Though this 8-day period 2. Though this 8-day period is not necessarily is not necessarily representative of the representative of the entire winter, the maps entire winter, the maps reveal that snow cover reveal that snow cover duration is variable in duration is variable in both space and time.both space and time.

North American snow cover distribution

(Source: NASA)

Snow Data

•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

(T. Mote, 2004)

Temporal trends of snowfall

Knowles et al.2006

1.1. Snowfall records from observations Snowfall records from observations from 1949 to 2004 at NCDC COOP from 1949 to 2004 at NCDC COOP stations.stations.

2. The ratio of winter snowfall liquid 2. The ratio of winter snowfall liquid water equivalent (SFE) to winter total water equivalent (SFE) to winter total precipitation (P) has changed (SFE/P). precipitation (P) has changed (SFE/P). November-March SFE/P has November-March SFE/P has decreased over the vast majority of decreased over the vast majority of stations across the West, although stations across the West, although results are somewhat mixed over the results are somewhat mixed over the interior west including Colorado, Utah interior west including Colorado, Utah and Wyomingand Wyoming

3. Most of the significant changes in SFE 3. Most of the significant changes in SFE were found to be unrelated to were found to be unrelated to changes in total precipitation, so the changes in total precipitation, so the proportion of winter precipitation proportion of winter precipitation falling as rain must have increased falling as rain must have increased during this period.during this period.

Temporal trends in SWE (Snow Water Equivalent)

Mote el al. 2005

1.1. Decreases in April 1 Decreases in April 1 SWE between 1950-SWE between 1950-1997 at the majority of 1997 at the majority of sites;sites;

2. The largest decreases 2. The largest decreases found in western Oregon found in western Oregon and Washington, and and Washington, and northern California;northern California;

3. A number of stations in 3. A number of stations in southern Utah, Colorado southern Utah, Colorado and elsewhere in the and elsewhere in the Southwest, indicated Southwest, indicated increasing trends in increasing trends in SWE.SWE.

Soil moisture

land-atmosphere feedback of soil moisture

…thereby inducing additional precipitation

Precipitation wets thesurface...

…causing soilmoisture toincrease...

…which causesevaporation to increase duringsubsequent daysand weeks...

…which affects the atmosphere moisture balance

Total number = 236 stations in 207 catchments

Dot: insufficient data (136)Plus/Circle: catchment included in analysis (71)

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)

SMMR soil moisture dataset

Avg. # of SMMR data per month (79-87)

SMMR (1978-87): Satellite retrievals (upper 1.25cm, ~140km, ~3 days)

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)

32 km

(NARR)

1. The limitations of observing soil moisture, there have been virtually no 1. The limitations of observing soil moisture, there have been virtually no datasets of soil wetness produced for the whole North America. So we datasets of soil wetness produced for the whole North America. So we calculate surface water storage (S; including snowpack where it exists) as an calculate surface water storage (S; including snowpack where it exists) as an integral of the residual change in a simple water balance relationship. integral of the residual change in a simple water balance relationship.

2. Observed or analyzed gridded data are used to 2. Observed or analyzed gridded data are used to drive the model for a number of months or years.drive the model for a number of months or years.

Model soil moisture of 1993 flood and 1988 drought

1993 1988

Average during 16-31 July

Soil moisture availability for Top 1-meter of soil column

Spatial and temporal distribution of streamflow

The distribution of USGS streamflow gages

Nationally, USGS surface-water data includes more than 8000 Nationally, USGS surface-water data includes more than 8000 real-time gages that describe stream levelsreal-time gages that describe stream levels

Trends (p>0.05) in annual mean daily discharge

The systematic decreases are in the Pacific Northwest, Northern California, and The systematic decreases are in the Pacific Northwest, Northern California, and parts of the Southeast; a broad area with uptrends stretches from the New 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 England to the Lower Colorado, and Mid-Atlantic, Ohio, Tennessee, Upper and Lower Mississippi, Texas-Gulf, Rio Grande, and Great Basin. (1944-1993)Lower Mississippi, Texas-Gulf, Rio Grande, and Great Basin. (1944-1993)

(Lins et al 1999)

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

1. A trend of decreasing annual mean streamflow was found across southern Canada. A small increasing trend in the Great Lakes–St. Lawrence region.

Trends in annual daily streamflow

(Zhang et al., 2001a)(Zhang et al., 2001a)

(Lettenmaier et al., 1994)

1. The research shows the winter flows rise and summer flows drop in the western US.

Stewart et al 2004 Climatic Change

2. Streamflow in the western US is highly seasonal. Earlier timing of snowmelt – longer summer drought

The variations on seasonality and timing of streamflow

Thank YouThank You

Question?Question?

Lake Dillon, Colorado, August 8, 2002Lake Dillon, Colorado, August 8, 2002Lake Dillon, Colorado, August 8, 2002Lake Dillon, Colorado, August 8, 2002