Unit 5 - Infiltration, Evaporation, and Heat Flux

Infiltration is defined as

the entry of liquid water into the soil

There are three stages of soil water infiltration:

Stage I: Flux control (Supply control)

Stage II: profile control

Stage III: hydraulic conductivity

Infiltration - Stage I: flux control (supply control (delivery of water to the soil surface))

Controlled by the rate at which water is applied to the surface (flux control); the higher the rate, the shorter stage I

Infiltration - Stage II: profile control

The rate at which the soil absorbs the water is lower than the supply rate; controlled by texture, structure, layering in the profile

Infiltration - Stage III: hydraulic conductivity

As the wetting front increases, the potential gradient tends to one, making the flux of water equal to the conductivity

Evaporation is defined as

the vapor loss from soil or free water directly into the atmosphere

there are three stages of evaporation

Stage I: flux control (demand controlled)

Stage II: profile controlled

Stage III: unsaturated hydraulic conductivity controlled

Evaporation - Stage I: flux control (demand controlled)

The time in stage I decreases with higher atmospheric demand

Controlled by sunlight, heat, wind, humidity > determine the demand for soil moisture from the atmosphere

Evaporation - Stage II: profile controlled

Controlled by texture, organic matter, structure in the profile

Evaporation - Stage III: unsaturated hydraulic conductivity controlled

Unsaturated hydraulic conductivity drops with decreasing water content until the soil reaches an air-dried state

Transpiration:

evaporation from leaves

Evapotranspiration =

evaporation + transpiration

Estimating potential evapotranspiration (PET) - quantifies atmosphere demand for moisture

Penman (1948) developed an equation for PET from weather station data

Data needed includes:

• Air temperature

• Soil temperature

• Wind speed

• Net radiation

• Relative humidity

Electromagnetic (EM) radiation is emitted by

all objects including soil, plants, and the sun

Radiation is electromagnetic energy that exhibits

wave-like characteristics

The rate of emission and the wavelength of radiation depend on

the temperature of the emitter

Steffan - Bolteman Law

Hotter the object, more intense the radiation

The hotter an object is, the more photons emitted per

square meter per hour and the shorter the wavelength

Wiem’s Displacement Law

As temperature increases, wavelength gets shorter

The EM spectrum includes

TV, radio, radar, microwave, infrared, visible, UV, X-rays, gamma rays

Light that reaches the surface of the earth is mainly

Visible light

Near-infrared species

Radiation balance

Earth radiates long-range radiation, mostly far infrared (IR) radiation

Shortwave radiation passes easily through the atmosphere

Longwave radiation (far-IR) does not pass easily through the atmosphere

There are four things that can happen when radiation encounters matter:

Transmitted - pass through things like air, glass, or water

Scattered - bounced off in all directions; bounces off of dust, clouds, fog scatter light

Reflected - most sources reflect a lot of light

Absorbed

When the radiation is transmitted, scattered, or reflected, it remains radiation; however, when it gets absorbed, it is converted into:

Sensible heat - most often

Chemical energy - photosynthesis

Energy budget for soils

Risw - Rsw = Rabs = Hg + Ha + Hl + Rlw

Risw = incoming shortwave radiation

Rsw = reflected shortwave radiation

Rabs = absorbed and used to do something with

Hg = heat the ground

Ha = heat the air above the ground

Hl = energy used to evaporate water (latent heat of vaporization)

Rlw = long wave radiation emitted by the soil

The net radiation absorbed by any surface is the total arriving minus

the reflected shortwave and emitted longwave radiation

The amount of radiation absorbed is also related to the reflectivity of the surface:

Rabs = Risw (1-x)

X = albedo = fraction of sunlight reflected by a structure

Net radiation absorbed can be estimated by measuring the incoming radiation with a pyranometer and estimating

the reflected radiation from the albedo and emitted long-wave radiation from the surface temperature

In soils, the largest energy loss goes

to Hl (latent heat of vaporization; evaporation)

Energy units for solar energy are J m ^(-2) or W hr m ^(-2)

W = Js ^ (-1)

The amount of surface heating can be calculated from the

heat capacity and thermal conductivity of the soil

Heat capacity is the amount of energy needed to

raise the temperature of 1 gram of matter 1 degree celsius

The larger the heat capacity, the less

the soil changes in temperature when it gains or loses a certain amount of heat

Temperatures tend to be less variable in soils that

are wet and dense

Thermal conductivity is the ease

with which a material or object conducts or transmits heat, mostly dependent on water content

Because water has a high heat capacity and is a better conductor than

air, both soil heat capacity and thermal conductivity depend mostly on theta v

The temperature of the soil depends on depth and surface cover

Variability in temperature is reduced at depth

There is a damping and lag of soil temperature with greater depths

Peak temperature occurs later at deeper depths

Factors that affect heat flux and soil temperature

Radiation intensity

Soil color (albedo)

Soil water and air content - impacts heat capacity and thermal conductivity

Soil aspect - south facing vs. north facing slopes are important to know

Wind and effects of soil cover - vegetation at the surface; mulches

Conduction or diffusive transfer is the principle way in which

heat moves in soil, requires a temperature gradient

Convection is the mass flow or transfer of heat with fluid flow

Not too important within a soil

However, important for exchange within atmosphere of heat, water vapor, and CO2

Turbulence accelerates the vertical movement of

heat and water vapor away from the ground

Evaporation from soil and plant transpiration are

greatly increased during sunny, hot, dry, windy weather

Plant cover reduces

wind speed and turbulence

Mulch and vegetation moderate

soil air temperature

Earth radiates

long-range radiation, mostly far infrared (IR) radiation

Shortwave radiation passes

easily through the atmosphere

Longwave radiation (far-IR) does

not pass easily through the atmosphere