© 2011 - The University of Arizona

Precipitation

CLIMATE is what you expect, WEATHER is what you get – R.A. Heinlein

Arizona Water Issues 1

© 2011 - The University of Arizona

What’s Important

• Basic Concepts:– Measurement– Patterns– Factors affecting rain– Factors affecting floods– Drought

• In-class Activity:– Impact of Orographic Precipitation

Arizona Water Issues 2

How important is rainfall?

• 640 ac/mi2• => 89.6 Mac• Precip ~ 1 ft/yr• => 89.6 Maf• Where does it go?• Runoff ~ 1.8 Maf• => Evap. ~ 98 %

Arizona Water Issues © 2011 - The University of Arizona 3

AZ ~ 140,000 mi2

Measuring Precipitation

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“Tipping bucket”

www.weathershack.com/images

rainlog.org

How would you read this?

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2.5 1.0

Assume water level is here

© 2011 - The University of Arizona

Questions• How does precipitation vary in time and space?

– What areas get the most precipitation and why?– Why are summer storms different from winter storms?– What causes extreme events? Are they predictable?

• What causes condensation?• How do we measure precipitation?• How does uncertainty in precipitation affect water

resources?

• Where do you find info about weather and climate?

Arizona Water Issues 6

Evaporation & Condensation

• Evaporation– Surface molecules (liquid) escape to

form water vapor (gas)– Enhanced by high temperature & wind

speed ; lower humidity– Requires heat (cools environment)

• Condensation– Water vapor, near saturation, becomes

liquid at dew point– Enhanced by rising air cooling– Gives up heat (warms environment)

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Daily Patterns

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rainlog.org

Daily Patterns

Arizona Water Issues © 2011 - The University of Arizona 9rainlog.org

Monthly Patterns

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Monthly Patterns

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Radar Precipitation Maps

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Annual Patterns

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© 2011 - The University of Arizonahttp://www.wrcc.dri.edu/pcpn/westus_precip.gif

What factors control precipitation in the SW?

Arizona Water Issues 14

© 2011 - The University of Arizona

Source: www-unix.oit.umass.edu

Orographic Precipitation: related to moist adiabatic lapse rate

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http://www.wrcc.dri.edu/pcpn/az.gif

Precipitation trends & Topography

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Arizona Water Issues © 2011 - The University of Arizona

Orographic Pattern to SWE

www.nohrsc.noaa.gov/interactive/html/map.html

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Arizona Water Issues © 2011 - The University of Arizona

Orographic Pattern to SWE

www.nohrsc.noaa.gov/interactive/html/map.html

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© 2011 - The University of Arizona

High Variability in Space and TimePrecipitation influenced by rough mountains.

Winter: Large scale storms fromthe Pacific Ocean. Important for watersupply and snow.

Summer:Complex convectivestorms, very heterogeneous, intense.High evaporation.

Sources of Precipitation% Precipitation in Winter (Oct-Mar)

% Precipitation in Summer (July & Aug)

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Arizona Water Issues © 2011 - The University of Arizona

Where does our precipitation come from?

Moisture from theGulf of MexicoGulf of California

E. Pacific Ocean

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Relative Strength of 2012 Monsoon

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Types of Precipitation

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Convective

Frontal/synoptic

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Recent Water Supply Forecast Map

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www.nwrfc.noaa.gov/westernwater

23http://wateroutlook.nwrfc.noaa.gov/

Arizona Water Issues © 2011 - The University of Arizona

Feb. Water Supply Outlook

www.cbrfc.noaa.gov/wsup/pub2/map/html/cbrfc.2.2008.html24

2008 2010

Arizona Water Issues © 2011 - The University of Arizonahttp://www.hydro.washington.edu/forecast/westwide/

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Storm/Runoff Classification

Arizona Water Issues © 2011 - The University of Arizona 26Source: AHIS, Hirshbeck

Storm/Runoff Statistics

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© 2011 - The University of Arizona

Tropical Storm-related floodsOct 1983 STORM PRECIPITATION TOTALS:Eloy: 4.51"Miami: 6.58"Nogales: 9.72"Oracle: 6.76"Safford: 6.36"Tucson: 6.40"

PEAK FLOWS:Gila R near Clifton: 15,300 cfs (10/02/83)Blue R @ Clifton: 24,300 cfs (10/01/83) 7300 (1/21/10)San Francisco R @ Clifton: 90,900 cfs (10/02/83)Eagle Creek above Morenci: 36,400 cfs (10/02/83)Gila R @ head of Safford Valley: 132,000 cfs (10/02/83)Gila R @ Calva: 150,000 cfs (10/03/83)San Carlos R near Peridot: 10,300 cfs (10/01/83)San Pedro R near Tombstone: 13,600 cfs (10/02/83)San Pedro R near Redington: 25,400 cfs (10/02/83)Aravaipa Creek near Mammoth: 70,800 cfs (10/01/83)San Pedro R @ Winkelman: 135,000 cfs (10/01/83)Gila R @ Kelvin: 100,000 cfs (10/02/83)Santa Cruz R @ Nogales: 16,200 cfs (10/02/83)Santa Cruz R @ Continental: 45,000 cfs (10/02/83)Santa Cruz R @ Tucson: 52,700 cfs (10/02/83)Rillito Creek near Tucson: 29,700 cfs (10/02/83)Santa Cruz R @ Cortaro: 65,000 cfs (10/02/83)

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CBRFC – real-time forecasts

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Factors Affecting Flooding

• Persistence and intensity of precipitation• Soil moisture conditions• Surface cover – forested to bare• Permeability – sandy/tilled to compacted

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Arizona Water Issues © 2011 - The University of Arizona

Palmer Drought Severity Index (PSDI)

• The PDSI drought index responds to abnormally wet or dry weather conditions and classifies the conditions on a scale from -6 to 6. • The equation for the index was empirically derived from the monthly temperature and precipitation scenarios of 13 instances of extreme drought in western Kansas and central Iowa and by assigning an index value of -4 for these cases. Conversely, a +4 represents extremely wet conditions. • The index is a sum of the current moisture anomaly and a portion of the previous index to include the effect of the duration of the drought or wet spell. The moisture anomaly is the product of a climate weighting factor and the moisture departure.

•The weighting factor allows the index to have a reasonably comparable local significance in space and time. •The moisture departure is the difference of water supply and demand.

•Supply is precipitation and stored soil moisture and demand is the potential evapotranspiration, the amount needed to recharge the soil, and runoff needed to keep the rivers, lakes, and reservoirs at a normal level.

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Arizona Water Issues © 2011 - The University of Arizona

PDSIThe PDSI is based on rainfall,

temperature and historic data, and is computed based on a complex

formula devised by W.C. Palmer in 1965.

Although the Palmer is the main drought index used by the U.S.

government, it is slow to detect fast-emerging droughts, and does not reflect snowpack, an important

component of water supply in the western United States.

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Drought Indicators

© 2011 - The University of ArizonaArizona Water Issues 34

Arizona Water Issues © 2011 - The University of Arizona 35http://droughtmonitor.unl.edul

Arizona Water Issues © 2011 - The University of Arizona 36http://droughtmonitor.unl.edul

Arizona Water Issues © 2011 - The University of Arizonahttp://droughtmonitor.unl.edul

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Arizona Water Issues © 2011 - The University of Arizonahttp://www.drought.unl.edu/dm/monitor.html

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© 2011 - The University of Arizona

Drought Monitor – historical

droughtmonitor.unl.edu/archive.htmlArizona Water Issues 39

http://www.youtube.com/watch?v=_1vLaEVmG34&feature=player_embedded

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http://droughtmonitor.unl.edu/archive.html

Drought over western North America from 800 to 2100 (Schwalm, Nature Geosci 5, 551–556 (2012))

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Reconstructed summer PDSI from 800 to 2006, five-year mean

Normalized CMIP5 summer precipitation from 1900 to 2100, five-year mean

Dry Times Ahead(Overpeck and Udall, Science 25 June 2010, V.328(5986) pp. 1642-1643)

• Reduced late season snowpack• Drying faster/more than climate

change models indicate• Colo. River flow decrease 20%

by 2050?• Chance of loss of Powell/Mead

storage now 3 in 10

• Recommendation:– Learn to live with less– Climate change adaptation

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Arizona Water Issues © 2011 - The University of Arizona

SW Climate Outlook

http://www.climas.arizona.edu/forecasts/swconditions.html

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AZ Cooperative Extension

Arizona Water Issues © 2011 - The University of Arizona 44cals.arizona.edu/climate