EEOS 350: Quantitative hydrogeology

Lecture 3

Precipitation processes

Business

• 10 undergrads, 3 grads successfully enrolled in class

• Everybody have a text book in hand or on order???

Errata: The hard cover version may not have as many errors corrected as the softcover version. Go through your text and make corrections.

Field trip to Nantucket Island

• Proposed date: Nov 9-11 (Fri-Sun, you have Mon off)

• Proposed schedule: leave Friday afternoon or evening– “slow” ferry leaves Hyannis at 2:45pm (arrives

5pm) and 8:00pm (arrives 10:15pm).– All you need to do is bring clothes, etc, and

money for food.– Leave UMB at 12:30pm or 5:30pm.

I would prefer the earlier ferry….

Proposed schedule

• Fri evening: Dinner and settle in • Sat morning: Tour field site • Later Sat morning through Sat afternoon:

collect field data, develop piezometric map, perform single well tests.

• Sat evening: have fun!• Sun morning: aquifer pumping test• Sun at 5:30pm: leave Nantucket, arrive

Hyannis 7:45pm, arrive UMB 9:30pm.

Computing an annual water balance for the Lamprey River

Q

P

Watershed area =212 mi2

Catchment area forstream gage atPackers Fall =183 mi2

Precipitation gageat Durham.

Water balance data• Annual precipitation from the Durham NH rain

gage.– Dimensions: [L/T]– Units: inches per year (in/yr)

• Streamflow from the USGS gage at Newmarket.– Dimension: [L3/T]– Units: cubic feet per second (cfs).

all terms in the water balance must be in the same dimensions and units!! Now what?

Water balance computations

• All water balance terms should be in [L3/T]• Time step is annual (by definition of our

problem), so time unit = year.• Typical volume unit = ft3 or m3 or km3.

– How do we convert Q from ft3/sec to ft3/yr?– How do we convert P from in/yr to ft3/yr?

• First, let’s compute long-term average annual values for P & Q.

Lamprey_P&Q.xls

Precipitation processes

• Why does it rain sometimes and not others?• Why does it rain at all?• How do you know when it’s raining?

– Is there a difference between mist, drizzle and rain?

• How do we measure rain?

Sources for this section: - Physical Hydrology, S. Lawrence Dingman, Prentice

Hall, 1994.- University of Illinois, WW2010 (WeatherWorld 2010) (

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/home.rxml)

Some physical quantities

• Pressure: force applied per unit area (dimensions F/L2 or M/LT2). Typical units: psi, Pa, inHg (why inHg?)

Standard air pressure = 14.7 psi or 101.3 bars or 1013.25 millibars or 29.92 in Hg

Some physical quantities

• Partial Pressure: The pressure exerted by a particular gas within a mixture of gases.

• Vapor pressure,ev: The partial pressure of water vapor over a liquid, measured at equllibrium. Dependent on temperature…why?

• Saturated vapor pressure, esat: Maximum vapor pressure at a given temperature.

• Relative humidity, RH: the ratio of measured vapor pressure to saturated vapor pressure. Typically reported as percent. Dimensions?

Estimating vapor pressure

100

3.237

3.17exp11.6)(

sat

v

sat

ssatv

e

eRH

T

TTe

Tee

Vapor pressure from an evaporating surface (i.e, lake orwet soil) is a function of the surface temperature, Ts

where Ts is in °C. esat can be approximated by

Relative humidity is simply the percent of saturation.

What is the dew point?

• The dew point is the temperature to which the air must be cooled (at constant pressure) in order to reach it’s saturated vapor pressure, i.e., the relative humidity =100%.

• What happens on a hot muggy summer day, when the temperature cools off at night?

• Gets very uncomfortable. Why?• Water droplets form on grass (dew). Why?

Phase changes

• Evaporation: water changes from liquid phase to vapor phase. Requires energy input, is a cooling process. Why?

• Condensation: water changes from vapor phase to liquid phase. Outputs energy.– What are the implications with respect to

condensation in the atmosphere?

Latent heat of vaporization of water = 2260 kJ.kg-1. It takes 2260 kJ of energy to convert one kg of water from liquid to vapor. By the same token, 2260 kJ of energy is given off when one kg of water condenses.

Adiabatic process

• Cooling or heating due solely to pressure changehttp://buphy.bu.edu/~duffy/semester1/c27_process_adiabatic_sim.html

• As a packet of air containing a set amount of water vapor rises, it expands and cools.

• At some point, it will reach the dew point, and water vapor will condense clouds!

How does precipitation occur?

1. Need to cool a moist air mass to its dew point.

need some form of uplift.- convection- convergence- orographic- frontal- cyclonic

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/cld/dvlp/wtr.rxml

How does precipitation occur?

2. Water vapor condenses to form water droplets.

Need condensation nucleii, else the droplets won’t form.

Water is a “wetting fluid”, it prefers to stick to solid surfaces rather than air. So, the presence of condensation nuclei will promote droplet formation.- Dust- Smoke- Sea salt- Particulates of any kind

Why are raindrops round?Because of surface tension. Hydrogen bonds between water molecules are stronger thanbonds to air. Within drop, tension is equal all around. On surface of drop, tension pulls inward towards other molecules.

How does precipitation occur?

3. Droplet growth Needs enough tiny droplets for coalescence Need enough larger droplets for collisions OR need temperature to be below freezing

to cause ice crystal growth.

CLOUD DROPLET: 100m

UPDRAFT6.5 M/SEC

AS THE DROPLET GOES UP WITH THE UPDRAFTIT GROWS LARGER

THROUGH COALESCENCE

CLOUD DROPLET1000 m

AS THE DROPLET FALLS IT COLLIDES WITH OTHER

DROPLETS;THUS,GROWING

LARGER

Raindrop5000 umHow big can raindrops get?

http://www.shorstmeyer.com/wxfaqs/float/dropdeform.html

Coalescence and collision

RAINDROP 5000 m

How does precipitation occur?

4. Need a source of moisture.

the amount of water vapor present will not create a rain event. Need a continuous influx of moist air.

Cold air masses tend to be dry. Warm air masses tend to be moist.

When is it rain and when is it not?

SURFACE

5,000FT ABOVE THE SURFACE

3,000FT ABOVE THE SURFACE

FREEZING LEVEL1,000FT ABOVE THE SURFACE

INVERSION (RISE IN TEMPERATURE WITH HEIGHT)

TOO SHALLOW FOR RAIN TO FREEZE

32

40

47º

28

Droplet falls from cloud

Droplet falls through a shallow freezing layer

Droplet freezes on contact with surface

The Ice Storm of 1998• Ontario Canada

through SE Maine• 25 people died• Millions of trees

damaged• 4 million without power

(after three weeks, 700,000 still without power)

• $2 billion in Quebec, $4-6 billion in US

Precipitation vs elevation

How is rainfall/snowfall measured?

• Cumulative measurements:– Measured cumulative rainfall– Manual reading required– Tough, reliable, cheap

• Incremental measurements (requires a data logger):– Weighing gages– Tipping buckets– Optical

Rain gage placement

Errors in rainfall measurement

Windshields