1. science
2. weather

ATMOSPHERIC WATER
Basic concepts
TSM 352
7/3/2013
Sid verma
HYDROLOGY | themes | atmospheric water
“Water present in the
atmosphere either as a solid
(snow, hail), liquid (rain) or
gas (fog, mist)”
Topics
• Cloud formation
• Precipitation types
• Measuring precipitation
• Evaporation types
• Areal precipitation
methods
Cloud formation
Relative humidity
reaches 100% at a
certain height
If temperature > 0oC , dew point is reached and
vapor condenses into water droplets
If temperature < 0oC , frost point is reached and
sublimation starts turning water vapor into ice
Cool air cannot hold as much water
vapor as warm air as it’s Saturation
Vapor Pressure is lower
Moist air
Sun’s heat
Moist air
It expands and cools down at
about 1oC/100m
Transpiration
Evaporation
Dry air is less
dense and
therefore it rises
Heated air
Heated surface
Generation of precipitation
Warm cloud
Cold cloud
Water vapor
Water vapor
EVAPORATION
Water
droplets
CONDENSATION
Water vapor
CONDENSATION
Large and heavy
enough to fall ?
YES
RAIN
EVAPORATION
SUBLIMATION
Supercooled Ice crystals
water droplets
CONDENSATION
Large and heavy
enough to fall ?
YES
SUBLIMATION
Water vapor
SNOW
Precipitation types
• Precipitation as a result of local
heating of air at the earth’s
surface is called convective
precipitation
• Active in tropical areas and
interiors of continents
• Precipitation is often local and
intense (thunderstorms)
• When horizontal air currents are
forced to rise over natural barriers
such as mountains, orographic
precipitation occurs
• Precipitation falls on the windward
side
• Leeward side is the other side which
is a precipitation shadow area
Precipitation measurement
A rain gauge (also known as an udometer, pluviometer, ombrometer ) is a type
of instrument used by meteorologists and hydrologists to gather and measure the
amount of liquid precipitation over a set period of time.
The
standard NWS rain
gauge, developed
around the start of
the 20th century,
consists of a funnel
emptying into a
graduated cylinder,
2 cm in diameter,
that fits inside a
larger container
which is 20 cm in
diameter and
50 cm tall.
A weighing-type
precipitation
gauge consists of
a storage bin,
which is weighed
to record the
mass.
The tipping bucket rain gauge consists of a
funnel that collects and channels the
precipitation into a small seesaw-like container.
After a pre-set amount of precipitation falls,
the lever tips, dumping the collected water
and sending an electrical signal.
Areal precipitation
Predicting watershed response to a given precipitation event requires knowledge of the
average rainfall that occurs over the watershed in a specified duration
This involves design of a network of rain gauges
Not many rain gauges are needed specially in flat watersheds
Three basic methods exist to derive areally averaged values from point rainfall data:
• Arithmetic mean
• Thiessen polygon method
• Iso-hyetal method
Areal precipitation can also be estimated based on radar estimates, specially in areas
without the presence of adequate rain gauges
Areal precipitation | arithmetic mean
2.0”
Method is satisfactory when
• Gages are uniformly
distributed
• Individual variations
aren’t far from mean
rainfall
• Applied to smaller
watersheds
• Rainfall distributions are
not variable
1.2”
1.8”
1.0”
Watershed with rain gages and monthly rainfall
for February in inches
𝟏. 𝟖 + 𝟏. 𝟐 + 𝟏. 𝟎
= 𝟏. 𝟑𝟑 𝒊𝒏.
𝟑
Areal precipitation | thiessen polygon
2.0”
Thiessen polygons are built up by
drawing midlines, perpendicular
bisectors, between rain gages on a
map
Value of rainfall measured by a
rain gauge is assigned to the area
surrounding it
Multiplying the rainfall by its
representative area, summing the
products for all rain gages and
dividing by total area gives
weighed average of precipitation
over the area
1.2”
1.8”
1.0”
Watershed with rain gages and monthly rainfall
for February in inches
Areal precipitation | iso-hyetal method
When rainfall is highly variable, or
when high accuracy is required more
rain gages are needed
Iso-hyetal method involves drawing
of contour lines with equal rainfall
depth (iso-hyets)It’s a little arbitrary
to make iso-hyets and may require
experience
30mm
20mm
10mm
Topography and storm patterns are
helpful in making iso-hyets
Rainfall calculation is based on
finding average rainfall between
each pair of contours, multiplying by
the between them, totaling these
products and dividing by total area
Watershed with rain gages and monthly rainfall
for February in inches
Areal precipitation | thiessen polygon
B
Value of rainfall measured by a rain
gauge is assigned to the area
surrounding it
Multiplying the rainfall by its
representative area, summing the
products for all rain gages and
dividing by total area gives weighed
average of precipitation over the area
2.0”
C
1.2”
1.8”
A
D
1.0”
𝑨𝒓𝒆𝒂𝒍 𝒑𝒓𝒆𝒄𝒊𝒑𝒊𝒕𝒂𝒕𝒊𝒐𝒏 =
𝟏𝟑𝟐 𝒌𝒎𝟐 𝒊𝒏
𝟗𝟖 𝒌𝒎𝟐
= 𝟏. 𝟑𝟒 𝒊𝒏
Evaporation types
There are three key types of evaporation:
• Interception loss
Precipitation that is intercepted by vegetation evaporates
back to the atmosphere
• Soil evaporation
Water that evaporates from a wet soil surface
• Transpiration
Water that evaporates through the plant stomata
Potential evaporation is the maximum evaporation rate in
mm/day when moisture content of the soil and vegetation
conditions do not limit evaporation i.e. under unstressed
conditions
Evaporation measurement
A low budget and direct way to estimate
evaporation rate, on days without any precipitation
is to use a pan filled with water and to measure the
height of water in the pan for 2 consecutive days at
exactly the same time
A lysimeter is a device used to obtain values of
potential or actual evaporation
It’s made of steel, concrete or plastic and is dug
into the ground, in which a volume of soil with
some vegetation is hydrologically isolated by
preventing any leakage
The position of water level in the lysimeter is
monitored and regulated by pumping
In weighing lysimeters change in water storage is
determined by the difference of mass of the
lysimeter
Water balance
For any hydrological system, a water budget can be developed to account for various flow pathways and
storage components. The hydrological continuity equation for any system is:
𝐼−𝑄 =
𝑑𝑆
𝑑𝑡
Where
𝐼 = inflow in L3/t
𝑄 = outflow in L3/t
𝑑𝑆
𝑑𝑡
= change in storage per time in L3/t
Evaporation
Precipitation
𝑃 − 𝑅 − 𝐺 − 𝐸 − 𝑇 = ∆𝑆
Surface
Runoff
Groundwater
flow
Change in storage in
specified time period
Transpiration
Water balance | example problem
For a given month a 300acre lake has 15cfs of inflow, 13cfs of outflow and a total storage increase of 16
ac-ft. A USGS gage next to the lake recorded a total of 1.3 in. precipitation for the lake for the month.
Assuming that the infiltration loss is insignificant for the lake, determine the evaporation loss, in inches,
over the lake for the month.
15
𝑃 − 𝑅 − 𝐺 − 𝐸 − 𝑇 = ∆𝑆
1 𝑎𝑐𝑟𝑒 = 43,560 𝑓𝑡 2
𝐸 = 𝐼 − 𝑂 + 𝑃 − ∆𝑆
𝑣𝑜𝑙𝑢𝑚𝑒 = 𝑑𝑒𝑝𝑡ℎ ∗ 𝑎𝑟𝑒𝑎
𝑓𝑡 3
𝑠
𝑎𝑐
43,560 𝑓𝑡 2
12 𝑖𝑛 𝑓𝑡
3600 𝑠 ℎ𝑟 24 ℎ𝑟 𝑑𝑎𝑦
300 𝑎𝑐
30
𝑠
𝑎𝑐
43,560 𝑓𝑡 2
12 𝑖𝑛 𝑓𝑡
3600 𝑠 ℎ𝑟 24 ℎ𝑟 𝑑𝑎𝑦
300 𝑎𝑐
30
𝑰=
13
𝑓𝑡 3
𝑶=
12 𝑖𝑛 𝑓𝑡
300 𝑎𝑐
16 𝑎𝑐 − 𝑓𝑡
∆𝑺 =
𝐸 = 35.70 − 30.94 + 1.3 − 0.64 in.
𝐸 = 5.42 in.
𝑑𝑎𝑦
𝑑𝑎𝑦
𝑚𝑜𝑛𝑡ℎ
1𝑚𝑜𝑛𝑡ℎ
𝑚𝑜𝑛𝑡ℎ
1𝑚𝑜𝑛𝑡ℎ
REFERENCES
•
•
•
•
Introduction to Physical Hydrology, Martin R. Hendricks
Hydrology and Floodplain Analysis, Bedient, Huber and Vieux
National Geographic Magazine
www.wikipedia.org