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Lecture 10: Transpiration and Evapotranspiration
Key Questions1. What are stomata?
2. What controls the ascent of sap?
3. What controls transpiration?
4. What factors constitute evapotranspiration?
5. How does evapotranspiration control soil water storage?
6. How does logging affect streamflow?
Vegetation influences the timing and magnitude of streamflow in a watershed
Vegetation influences the timing and magnitude of streamflow in a watershed
Vegetation intercepts and stores precipitation (review Lecture 6)
interception/storage
The magnitude of interception and storage is determined by
1. Type and growth stage of the vegetation
The magnitude of interception and storage is determined by
1. Type and growth stage of the vegetation
2. Precipitation characteristics (intensity and duration)
heavy rain
light rain intermittent light rain
How do trees (plants) get their mass?
6CO2 + 6H2O → C6H12O6 + 6O2light
plant matter
1 square centimeter on a leaf or needle has 1000 to 100,000 stomata
stomata range from 20 nm to 50 μm ( 20 x 10-9 to 50 x 10-6 m)
plants draw CO2 in through small openings called stomata
CO2 dissolves in a water bath in the stomata
There is a continuum of water that goes all the way from the roots, through the vascular system (xylem) of the plant to the stomata.
Because of the polar nature of water, it adheres to the xylem cell walls and the hydrogen bonds keep the molecules held together (cohesion) in a continuum.
stomata
Because stomata are open to the atmosphere, they evaporate water.
evaporation
… more water enters the roots which keeps the water flowing.
As molecules leave the stomata …
Flow can be on the order of 70 centimeters per minute.
Ascent of Saphttp://plantcellbiology.masters.grkraj.org/html/Plant_Cellular_Physiology5-Translocation_Of_Water_And_Nutrients.htm
Water can be held in a continuum for hundreds of feet within a tree.
The rise of water in plants is called the ‘Cohesion-Tension Theory’ or sometimes the ‘Adhesion-Cohesion Theory’
The process of evaporation from stomata and the ascent of sap is called transpiration.
Click on the link below and read about transpiration
.
conceptual model of a stomata
leaf
liquid water
vapor
conceptual model of a stomata
The same environmental variables that control evaporation from a lake (or any water surface for that matter), control the evaporation from a stomata.
leaf
conceptual model of a stomata
Heat Inputs (review Lecture 9)
sensible heat
longwave radiation solar radiation
leaf
conceptual model of a stomata
the boundary layer air is always saturated with water vapor which is determined by the leaf temperature
leaf
Diffusion or Vapor Transfer (review Lecture 9)
Variables
ea = actual vapor pressure of the air
esat(TL) = saturation vapor pressure of the boundary layer
esat(Tair) = saturation vapor pressure of the air
TL = temperature of the LEAF
Tair = temperature of the air
esat(TL) - ea is called the vapor pressure deficit
conceptual model of a stomata
Diffusion or Vapor Transfer (evaporation)
If ea < esat(TL) then molecules will diffuse from the boundary layer to the air
ea
esat(TL)leaf
conceptual model of a stomata
Diffusion or Vapor Transfer (evaporation)
ea
esat(TL)leaf
Wind sustains the deficit by sweeping moist air away from the leaf surface if ea < esat(TL) then evaporation continues
stomata close at night
so transpiration slows down dramatically or stops at night
high CO2 concentrations low CO2 concentrations
high vapor pressure deficit low vapor pressure deficit
high leaf temperature lower leaf temperature
low soil water content high soil water content
low opening size larger opening size
Other environmental variables control the stomata opening size, hence the degree of transpiration.
High CO2 concentrations, means smaller stomata opening size, hence less transpiration and more streamflow.
Evapotranspiration (ET) is a term used to quantify all evaporative losses in a watershed
1) Evaporation of water intercepted and stored by vegetation
2) Water transpired by vegetation
3) Water evaporated from soil
ET accounts for about 60 to 70% of the water loss from a slope.
lowers water content in soil
evaporation of water stored by interceptiontranspired water
evaporation of soil water
Potential evapotranspiration (PET) is a maximum ET assuming a sufficient supply of soil water to meet the highest demand by plants.
high water content
Actual evapotranspiration (AET) is the actual ET that occurs under natual field conditions (i.e., variable soil water contents).
low water content
If the soil-water content is low, stomata openings will reduce in size and hence, lower transpiration.
A small sapling might only lose between 0.5 and 1 gallon per day, whereas a very large Douglas-firtree might lose between 50 and 100 gallons per day.
A single acre of forest land, during the course of a growing season can transpire about one million gallons of water (that’s about 3 feet per acre).
Lake Whatcom watershed
lowers water content in soil
evaporation of water stored by interceptiontranspired water
evaporation of soil water
The bottom line is that ET dries out soils.
Models are used to estimate PET and AET
1) Thornthwaite Model is a simple, temperature index empirical model for estimating PET
2) Penman-Monteith is a physically based model that incorporates many meteorological, vegetation, and soil water characteristics.
Vegetation type and distribution
Vegetation and Landcover
ET
(inch
es)
Oct Apr Sep
Modeled Daily ET in the Lake Whatcom Watershed(AET is about 20 inches)
Modeled Monthly AET and AET Contributions in the Lake Whatcom Watershed
Inch
es
Modeled Monthly Contributions as a % of the Total AET in the Lake Whatcom Watershed
Smith Creek Hydrograph, Sept 7--22, 2000
0
10
20
30
40
50
0 50 100 150 200 250
Time (hr)
Dis
char
ge (c
fs)
Smith Creek Stream Gauge
Smith Creek Hydrograph, May 8--16, 2000
0
10
20
30
40
50
0 50 100 150 200 250
Time (hr)
Dis
char
ge (c
fs)
1.2 inches of rain in 24 hours
How does logging affect evapotranspiration and streamflow?
Q
Time
Hydrograph
Q
Forested Basin
Q
Time
Hydrograph
Q
Logged Basin
GROUNDWATER RESPONSE TO PRECIPITATION EVENTS, KALALOCH, OLYMPIC PENINSUALA, WASHINGTON
BY
CASEY R. HANELL
Casey’s research results predict a 27% reduction in ET in the logged basin, which correlates to an increase in streamflow and soil water and groundwater storage.
