Lecture 3: Hydrological Cycle and Processes on the Watershed

define the hydrologic cycle

The natural sequence through which water passes into the atmosphere as water vapor, precipitates to earth, and returns to the atmosphere through evaporation

Precipitation = Runoff + Evaporation and Transpiration +- Storage

Runoff = Precipitation - Evaporation and Transpiration +- Storage

P = RO + ET +- S

RO = P - ET +- S

Where

• P is Precipitation

• RO is Runoff

• ET is Evaporation and Transpiration

• S is Storage

identify the red bubble

cloud formation

identify the red bubble

cloud

identify the red bubble

precipitation

identify the red bubble

evaporation while falling

identify the red bubble

interception

identify the red bubble

infiltration

identify the red bubble

runoff

identify the red bubble

Solar Energy

identify the red bubble

Sun

identify the red bubble

Percolation

identify the red bubble

Evaporation from Surface Water

identify the red bubble

Transpiration

identify the red bubble

Evaporation from soil surface

identify the red bubble

Evaporation

identify the red bubble

Ground Water Table

identify the red bubble

Ocean

identify the red bubble

Evapo-transpiration losses

what is the renewal time for the astmosphere

1-12 Days

what is the renewal time for rivers

10-30 days

what is the renewal time for soil moisture

14-365 days

what is the renewal time for freshwater lakes

1-100 years

what is the renewal time for glaciers

1-16,000

what is the renewal time for polar ice caps

>30,000 years

energy from solar radiation is the

driving force behind the hydrologic cycle

the functional model of energy from solar radiation is So + T + L + G + C + St = 0, where

Where

So = Solar Radiation

T = Thermal Radiation

L = Latent Heat

G = Conduction

C = Convection

St = Storage

If positive, then continual increase in energy

Burn up

“0” implies balance

It does balance over time (ice ages)

where is storage

Plants due to photosynthesis

Fossil fuels

the solar constant

2.0 cal / cm^2 / min (we don’t receive this much because of atmospheric attenuation)

what are the factors that decrease the amount of energy (solar radiation received)

Atmospheric thickness and density

Atmospheric pollutants

Earth’s axis position

Terrain features

define atmospheric thickness and density

Energy coming in at equator has much shorter distance to reach the surface than the poles

define atmospheric pollutants

Particulates reflect heat

CO2 and methane absorb heat

Example: Mount Krakatoa, Indonesia, erupted in 1883

• Global temperatures – 1.2oC lower in 1884

• Did not return to normal until 1888

define Earth’s Axis Position

The tilt of the earth affects how much solar radiation we get

what are terrain features

slope, aspect, and elevation

in the northern hemisphere, north facing slopes are

cooler and wetter than south facing slopes (Impacts evaporation rates, and ultimately water yield)

Orientation of a watershed effects the

Evaporation -> stream flows and groundwater -> water yields

as radiant energy strikes a surface, either of these three things happen

Absorption

Reflection

Transmission

what is albedo

Reflectivity of materials

the values of albedo range from 0-1, the value of 1 is

a perfect reflector - no absorption

the values of albedo range from 0-1, the value of 0 is

a black body, no reflection

how much energy that is received during albedo is converted into heat, which may

impact land management decisions

absorbed energy is

Available to do work

as a body absorbs energy,

its temperature rises and it in turn radiates energy

net radiation is the

difference between incoming and outgoing radiation

specific heat definition - amount of heat necessary to

raise 1 gram of a particular substance 1 degree celsius: units of cal / C / grams

what is water’s specific heat?

1.00

what is lead’s specific heat?

0.03

what is soil’s specific heat?

0.20

sensible heat:

the thermal energy that changes the temperature of an object

temperature is a measure of the

quantity of sensible heat

latent heat -

the thermal energy which does not change the temperature of an object in a sensible fashion

sublimation -

changing directly from solid to vapor

conduction is the transfer of

heat through matter by means of molecular contract

plant tissue and soil are

poor conductors of heat

convection is the

transfer of heat by mass movement of the substance containing the heat (important in watersheds)

name some examples of convection

cold fronts, gulf streams

what are the factors affecting convective heat transfer

wind speed of air

temperature differences and vapor pressure gradients between surface and air

surface roughness and surface area

temperature and vapor pressure gradients both have a

higher energy transfer with greater differences

the active surface is where

energy is either absorbed or reflected

how can we manage the active surface?

alter vegetation, change albedo, modify the hydrologic cycle

where is the active surface on a watershed

about 10-20 meters above the treetops down to 1 meter below the soil surface

what are the fixed characteristics of a watershed

Slope

Aspect

Elevation

Location - with respect to other feathers, both topographic and geologic

Size - position and extent

• Keep in mind how differences respond to incoming solar radiation

what are the variable characteristics of a watershed

land use

vegetation

precipitation is defined as

any product of condensation of atmospheric water vapor that falls under gravity

what two forms of water are not considered precipitation

fog and mist

precipitation is the only significant form of

recharge (water input) to the hydrologic

there is always some water vapor in a gaseous state in the atmosphere, this is called

humidity

humidity varies spatially and temporally,

Higher over water bodies, decreases as you move inland

Decreases with higher latitudes and altitude

Higher over vegetation

Seasonal differences

• Higher during summer, lower during winter

Diurnal differences

• Highest at peak temperature, lowest at sunrise

what are the different ways to express humidity?

relative humidity, specific humidity, absolute humidity, dew point

define relative humidity

Percentage of the water contained in a unit volume of air based upon what could be contained for a given temperature at the saturation point - (%)

define specific humidity

Weight of water/weight of air - (g/kg)

define absolute humidity

Weight of water/unit volume of air - (g/m3)

define dew point

Temperature at which the air becomes saturated with water vapor (F or C)

what are the three causes of cooling

adiabatic cooling, mixing of air masses, contact or radiation cooling

explain adiabatic cooling

Air rises, pressures decrease, expansion

Loss of heat energy => cooling

• EX. Cloud formation and rain

explain the mixing of air masses

Very hot air injected into cool air

May cause clouds or fog but usually not precipitation

• EX. Breath in winter

explain contact or radiation cooling

Moist air comes in contact with cool surface

Causes condensation on a surface but not precipitation

• EX. Condensation on beer can

to form precipitation in a cloud, this requires

Uplift mechanism

Condensation nuclei

Process that makes droplets that fall

• Two theories

  • Collision Coalescence

  • Bergeron Theory

collision coalescence applies to

mostly tropical areas

what is collision coalescence

Small droplets form

Stay suspended

Collide to form larger ones

• Capture in wake, also

Grow too large to be held against gravity

Fall

Ripped apart by convectional currents and air friction –> smaller rain drops

what is the Bergeron Theory (Bergeron-Findeisen process)

Does not apply to tropical areas

• Small ice crystals form

• Water vapor condenses on ice crystals and freezes

• When become heavy enough –> fall

• Melt when falling to become rain

• May also include aspects of Collision Coalescence

what falls through warm air?

rain

what falls through air below freezing?

snow or snow pellets

what falls, but turbulence pushes it back up, then falls again?

hail (or layered frozen droplets)

what is cloud drip or fog drip

Vegetation may cause fog to condense and result in falling drops

Not a major source of rain

what are the characteristics of precipitation

Form – snow, rain, sleet, hail

Areal distribution – areal extent

Frequency distribution – timing of occurrences

Storm types – orographic, convectional, frontal

Intensity – amount per unit of time

what are the different storm precipitation types?

orographic, convectional, frontal storms

explain orographic storms

Mountainous country

Created by air rising over mountains, cooling, then precipitating

Moderate to high intensity for short periods

• But not as high as convectional storms

Totals may be very high

Duration may be long – several days to a week

Seldom any thunder and lightning

What do we call the resulting rainfall pattern?

• Rain Shadow

explain convectional storms

Summer thunderstorms (Spring or Fall)

Created by uneven heating of air

• Sets up air currents

Usually of local extent

Intensity may be very high

Short duration

Usually with thunder and lightning, maybe hail

Flash floods are common

explain frontal storms

Cold Front

• Cold air moves under warm air

• Thunder and lightning and hail on leading edge

• Usually a narrow band

• Intensity high on leading edge

• Total amounts may be high

Warm Front

• Gentle to moderate, widespread precipitation

• Duration usually longer than with a cold front

• Seldom thunder and lightning

• Total amounts may be high

what are the ways to measure water vapor (humidity)

Psychrometer

Hygrometer

Chemical hygrometer

Others

• Dew point hygrometer

• Hygrothermograph

what are the ways to measure rainfall?

Non-Recording Gauge

• Standard U.S gauge – most common

  • 20” tube inside

  • Holds only 2.0” of rainfall

  • Ability to measure to 0.01”

  • Overflow for holding larger amounts and snow

• Sacramento gauge – more storage, used in remote areas, old style

Recording Gauge

• Tipping bucket gauge

• Weighing gauge – rain falls into a bucket on a scale and this records on chart

  • You should have a standard gauge to check against recording gauges

how to install gauges

Gauge opening 36” above ground in open area

Make sure it is level

If put on post, bevel the top to prevent splashing into the gauge

No obstructions within 30° angle of gauge (or better) with radius of open space being twice the height of nearest obstruction.

Site selection

• Elevation - account for difference in terrain

• Aspect - N, S, E, W

• Accessibility - must be convenient to read

Number of gauges—adequate to do the desired job accurately

Measure Snow

• Snow gauge

• Graduated ruler

  • Must be placed in areas where snow will not accumulate in drifts

how to measure snow moisture content

Put known amount of warm water in rain gauge

Record water level

Add known depth of snow

Let melt, then record water level

Subtract to get moisture content

what is interception

Process in which rainfall or snowfall is caught by the vegetative canopy and redistributed

Redistribution:

• Throughfall

• Stemflow

• Evaporation from vegetation

Difficult to calculate directly

what is one way to estimate interception

Gross Precipitation – all precipitation falling

Net Precipitation – precipitation reaching the ground under the canopy

(Gross Precipitation) – (Net Precipitation) = Interception

Pg = gross precipitation

Pn = net precipitation

Ic = canopy interception loss

Il = litter interception

Th = throughfall

Sf = stemflow

Interception total: I = Ic + Il

The amount reaching the forest floor = Th + Sf

Interception by canopy (overstory+understory): Ic = Pg – Th – Sf

Net precipitation Pn = Th + Sf – Il

Amount of Intercepted loss varies with:

Type and density of vegetation

Atmospheric conditions during and following precipitation

Type and amount of precipitation

explain the type and density of vegetation

Leaf surface

• Rough and hairy will hold more water

Bark

• Rough bark will hold more water than smooth bark

Density – little or no vegetation vs. thick stand

Type and Form – Deciduous trees vs. Conifers

atmospheric conditions that impact interception rates

Evaporation rate

• Higher evaporation rate => more Interception loss

• BEFORE can be disturbed

Wind

• Decreases Interception

  • Blowing drops off leaves

  • NOT – wind blowing rain onto vegetation

• Increases Interception loss

  • Higher evaporation rate

Studies – trees on edge have higher Interception loss

• More likely to have higher evaporation rate due to wind

• Throughfall not different

type and amount of rainfall that can be collected when factoring interception

Rainfall

• Light rainfall

  • Loss may be up to 100%

• Large storms

  • Loss typically 10 to 25% or higher

Snow

• More complex than rainfall

  • Interception and Redistribution of snow

    • Throughfall to ground

    • Increase in cohesion of flakes within snow mass

    • Dropping of snow from branches

    • Blowing of snow from branches

    • Running of snow melt down tree trunk

    • Evaporation of snow melt