Chapter One Averaging Precipitation over a watershed

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Chapter OneAveraging Precipitation over a
watershed
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Annual Rainfall CONUS lower 48
The NW windward precip., Western interior rain
shadow arid, east dominated by mT versus cP
invasions. Gulf Coast very rainy due hurricanes.
Red Subtropical desert in summer.
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Seasonal Rainfall
The west coast windward precip in fall- winter,
snowmelt in spring, east of cordillera arid with
May rain, eastern US year round precip /-
mid-year rain.
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Precipitation is generally recorded automatically
in increments
Transmitter
Reed switch activated by magnet
http//climate.rutgers.edu/njwxnet/dataviewer-netp
t.php
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For our purposes, a wide can will do
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A Hyetograph
On a rainy day, rainfall intensity varies with
time. Plot called a hyetograph.
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Intensity-duration-frequency (IDF) curves. How
frequently do you get a storm of a particular
intensity and duration? Here is a summary for
Houston. Every five years, on average, they get
an inch of rain per hour lasting 4 hours. Every
100 years they about half an inch of rain per
hour lasting 24 hours.
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Notice only the gages within the watershed are
used for the average.
All gages used
All gages used
There are almost never enough reliable rainfall
records inside the basin you are working on.
Until Nexrad archives improve, three averaging
methods are used taking the mean, Thiessen
method, and the Isohyetal method, similar to
making a contour map.
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For Thiessen, find the rain gauges near your
watershed
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Draw triangles between adjacent rain gages and
draw perpendicular bisectors for each side
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Thiessen Figure E1-4b
This breaks up the area around each gauge into
regions. CONSIDER ONLY the area of each region
that belongs to your watershed. Here the area
around gage D is the largest, then B, then F,
then A, etc. Measure these areas by drawing a
grid onto the map, by tracing/printing them on
gridded paper, or by some other method.
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Add all the small grid areas together to get the
total area of the watershed, then calculate the
fractional area for each gauge. Multiply the
proportionate area for each polygon by the
rainfall for that gage, and add them up for the
weighted average total rainfall over your
watershed.
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NEXRAD (Next-Generation Radar) gives you fine
resolution data with area squares.
These are available real time (current weather) ,
but the online archives are still unwieldy for
past storm events.
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Thiessen Method
We will stop here to work on averaging and
Theissen method.