Soils and Hydrology chapter 10:evapotranspiration

What is evapotranspiration the sum of?

Evaporation, Interception
Transpiration

Evaporation –

the movement of water
from a water body or the soil directly
to the atmosphere.

Interception –

water caught on
vegetative surfaces (leaves and
branches) during rainfall and
evaporated before ever reaching the
soil surface

Transpiration –

the movement of water
through the stoma from leaves to the
atmosphere.

The Latent heat of vaporization

the energy per gram of evaporated water (580 cal/g at 25C).

what increase evaporation speeds

vapor pressure deficit (VPD) and wind speed

What 3 things cause/are needed evapotranspiration in plants.

1. Open stomata for CO2

2. Pull of evaporation is how water flows through plant

3. needed for cooling

Water Budget Equation

P = ET + R
P:precipitation

R:runoff

ET:Evapotranspiration

Consumptive Use:

a use that does not send the water back to

the stream, either sent to the atmosphere via

evapotranspiration or out of the basin by export. Ex:water plants, cooling powerplants.

Non-consumptive

water uses go

back to the river. Ex:washing machine, shower

Eddy covariance

-

measures ecosystem-atmosphere exchanges of water, energy,

and carbon (typically in a forest). [its a big fake tree tower thing]

(GRACE)

Gravity Recovery and Climate Experiment

Box lysimeter

for measuring soil

evaporation by water budget

method.

evaporation equation

Evap = rainfall - outflow

Potential Evapotranspiration (PET) is the…

What measures it?

maximum possible ET on any given day based
on atmospheric conditions.

An evaporation pan

Actual Evapotranspiration (AET) is
the

amount of ET that occurs each
day given the constraints of
available soil moisture and
vegetative condition.

when does AET=PET

If the soil

is at field capacity, and the

vegetation is lush and thick,

What is the upper limit of flow through stomata openings in trees?

70 cm per minute

3 steps of Water Budget Equation

Step 1. Convert average discharge into an annual volume.

Step 2. Convert annual volume into an annual depth by dividing by basin area.
Volume = depth x area so
depth = Volume/area ft

Step 3. Apply conservation of mass

How is the Blaney-Criddle formula used

for calculating daily
AET from pan evap data, soil moisture data,
and plant condition

Blaney-Criddle formula

AET = Kc · Ks · PET

AET is actual evapotranspiration

– PET is potential (max) evapotranspiration

– Kc is a crop factor - changes with time

– Ks is a soil factor - changes with soil

moisture

• Ks = F / S, where

–S is the Maximum Available Water

• F is the Actual Available Water

We calculate the soil storage
using:

F = P - (Q + AET)

Water Budget Procedure:(SLIDE 28)

• Find the initial water storage in the root zone

• set equal to the field capacity, F(1) = S

• appropriate in the spring after soaking rains

• Calculate the soil factor

• Ks = F / S

• Calculate the AET = Kc Ks PET

• Subtract AET from the soil storage, F' = F - AET

• If rainfall, then add, F'' = F' + P

• Subtract drainage and runoff if soil is too wet

• if F'' > S, then Q = F'' - S, and F''' = S

• Carry over soil moisture to next day

• say from end of day 1 to beginning of day 2

• F(2) = F'''(1)

AET: Blaney-Criddle formula