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Precipitation

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Single strongest variable driving hydrologic processes

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Formed water vapor in the atmosphere

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As air cools its ability to ‘hold’ water decreases and some turns to liquid or ice

Hydrologic Cycle http://www.ec.gc.ca/Water/en/nature/prop/e_cycle.htm

Causes of Precipitation

U.S. Annual Precipitation

Illinois. Annual Precipitation Average annual precipitation pattern for Illinois with average monthly amounts (inches) for selected weather stations, 1901 1980 http://www.sws.uiuc.edu/wsp/climate/ClimateToday_precipmap1.asp

Forms of Precipitation

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Rain - Rain is liquid precipitation that reaches the surface in the form of drops that are greater than 0.5 millimeters in diameter

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Drizzle - Drizzle is liquid precipitation that reaches the surface in the form of drops that are less than 0.5 millimeters in diameter.

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Snow - Snow is an aggregate of ice crystals that form into flakes.

Forms of Precipitation

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Hail

Hail is dense precipitation ice that is that least 5 millimeters in diameter

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Graupel - Graupel forms in the same way as hail except the diameter is less than 5 millimeters.

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Freezing Fog - Freezing fog is a fog composed of supercooled water drops that freeze just after they wet the earth's surface

Illinois Annual Snowfall Average annual snowfall pattern for Illinois with average monthly amounts (inches) for selected weather stations.

http://www.sws.uiuc.edu/wsp/climate/ClimateToday_precipmap2.asp

Snow Measurement

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Determine the water equivalent 5%-60% of snow depth may be water equivalent- “density”

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Snow pillows use antifreeze solution and pressure measurement to measure water equivalent

Location of Gages

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Gauges measure point rainfall

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True precipitation unaffected by surroundings-winds, trees, buildings

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Clearance distance 2 times height of object For large areas multiple gauges are needed for more accurate estimates

Placement of Rain Gauges

Gauges are affected by wind pattern, eddies, trees and the gauge itself, therefore it is important to have the gauge located and positioned properly.

• 1m above ground level is standard all gauges in a catchment should be the same height • 2 to 4 times the distance away from an isolated object (such as a tree or building) or in a forest a clearing with the radius at least the tree height or place the gauge at canopy level

Placement of Rain Gauges

• shielded to protect gauge in windy sites • or if obstructions are numerous they will reduce the wind speed, turbulence and eddies .

Placement of Rain Gauges

 For sloping ground the gauge should be placed with the opening parallel to the ground  The rainfall catch volume is then divided by the opening area that the rain can enter

Number of Gauges Depends on Storm type

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Frontal storms

(large areas, low intensities) -small number of gauges may be O.K.

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Convective storms

(local, intense, uneven distribution) -denser network needed. Convective storms may have seasonal dominance -need to consider this as well.

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Orographic

rainfall due to mountains (not fronts) -may need denser network than flatter area.

Size of Watershed Acres 40 100 600 Square Miles 5 10 100 300 Number of Gage Sites 2 3 4 10 15 50 100

Methods of Measuring Rainfall: Manual

• Often have a funnel opening into a cylinder gauge.

• Come in a variety of shapes and sizes • Calculate the rainfall ( inches) by dividing the volume of water collected by the area of the opening of the cup. (The gauge marking often accounts for this).

Methods of Measuring Rainfall: Remote

• Tipping bucket rain gauge -The bucket tips when precipitation of 0.2 mm, 0.5 mm, 1.0 mm has been collected. Each tip is recorded by a data logger. • Weather Station - Records rainfall, but also evaporation, air pressure, air temperature, wind speed and wind direction (so can be used to estimate evapo-transpiration) • Radar - Ground-based radar equipment can be used to determine how much rain is falling and where it is the heaviest.

Weather Station

Methods of averaging rainfall data

•Arithmetic average •Theissen polygons •Isohyetal method Although, most of these calculations are done with computer mapping programs, it is still useful to understand these methods.

Isoheyetal method for Mapping Rainfall The most basic method

of representing the spatial distribution.

This is generally the

most accurate

method but is also the most

laborious

•Locate all rainfall stations on a base map and record the rainfall amount.

•Draw isohyets (lines of equal rainfall) by proportioning the distances between adjacent gauges according to differences in catch.

Thiessen Method for Average Rain

Step 1

Thiessen Method for Average Rain

Step 2

Thiessen Method for Average Rain

Step 3

Thiessen Method for Average Rain

Step 4

Interpolating Rainfall Data

• Nearest neighbor • Inverse distance weighted method • Kriging

INTERPOLATION Procedure to predict values of attributes at unsampled points within the region sampled Why?

-Can not measure all locations: •Time •Money •Impossible (physically, legally) deposits - Changing cell size - Missing/unsuitable data - Past date (e.g. temperature) Examples: - temperature - acid rain deposition - soil characteristics - mining: gold