ATMOSPHERIC WATERBasic concepts

TSM 3527/3/2013

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

Sun’s heat

Heated surface

Evaporation

Transpiration

Heated air

Dry air is less dense and

therefore it rises

It expands and cools down at about 1oC/100m

Moist airMoist air

Cool air cannot hold as much water vapor as warm air as it’s Saturation

Vapor Pressure is lower

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

Cloud formation

Warm cloud Cold cloud

Water vapor

Water vapor Water vapor

Water vapor

Water droplets

EVAPORATION CONDENSATION

Large and heavy enough to fall ?

YES

RAIN

EVAPORATION SUBLIMATION

CONDENSATION SUBLIMATION

Supercooledwater droplets

Ice crystals Large and heavy enough to fall ?

YES

SNOW

CONDENSATION

Generation of precipitation

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

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.

Precipitation measurement

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

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

2.0”

1.8”

1.2”

1.0”

Watershed with rain gages and monthly rainfall for February in inches

𝟏 .𝟖+𝟏 .𝟐+𝟏 .𝟎𝟑 =𝟏 .𝟑𝟑𝒊𝒏 .

Areal precipitation | thiessen polygon

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

2.0”

1.0”

Watershed with rain gages and monthly rainfall for February in inches

1.8”

1.2”

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

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

30mm

20mm

10mm

Watershed with rain gages and monthly rainfall for February in inches

Areal precipitation | thiessen polygon

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”

1.0”

1.8”

1.2”

A

B

C

D

Evaporation types

There are three key types of evaporation:

• Interception lossPrecipitation that is intercepted by vegetation evaporates back to the atmosphere

• Soil evaporationWater that evaporates from a wet soil surface

• TranspirationWater 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=𝑑𝑆𝑑𝑡

Where

= inflow in L3/t = outflow in L3/t

= change in storage per time in L3/t

Precipitation

SurfaceRunoff

Groundwater flow

Evaporation

Transpiration

Change in storage in specified time period

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.

𝐸=𝐼−𝑂+𝑃−∆𝑆

Water balance | example problem

1𝑎𝑐𝑟𝑒=43,560 𝑓𝑡2

𝑣𝑜𝑙𝑢𝑚𝑒= h𝑑𝑒𝑝𝑡 ∗𝑎𝑟𝑒𝑎

𝑰=(15 𝑓𝑡

3

𝑠 )( 𝑎𝑐43,560 𝑓𝑡2 )(12 𝑖𝑛𝑓𝑡 )(3600 𝑠

h𝑟 )(24 h𝑟𝑑𝑎𝑦 )(30 𝑑𝑎𝑦

h𝑚𝑜𝑛𝑡 ) (1 h𝑚𝑜𝑛𝑡 )

300 𝑎𝑐

𝑶=(13 𝑓𝑡

3

𝑠 )( 𝑎𝑐43,560 𝑓𝑡2 )(12 𝑖𝑛𝑓𝑡 )(3600 𝑠

h𝑟 )(24 h𝑟𝑑𝑎𝑦 )(30 𝑑𝑎𝑦

h𝑚𝑜𝑛𝑡 ) (1 h𝑚𝑜𝑛𝑡 )

300𝑎𝑐

∆𝑺=(16𝑎𝑐− 𝑓𝑡 )(12 𝑖𝑛𝑓𝑡 )

300 𝑎𝑐

in. in.

REFERENCES

• Introduction to Physical Hydrology, Martin R. Hendricks• Hydrology and Floodplain Analysis, Bedient, Huber and Vieux• National Geographic Magazine• www.wikipedia.org