Lecture 10: Transpiration and Evapotranspiration

Key Questions 1. 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? VegetationVegetation influences influences the the timing timing and and magnitude magnitude ofof streamflow streamflow in in a awatershed 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? light 6CO2 + 6H2O → C6H12O6 + 6O2 plant matter 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

1 square centimeter on a leaf or needle has 1000 to 100,000 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. Because stomata are open to the atmosphere, they evaporate water.

evaporation

stomata As molecules leave the stomata …

Flow can be on the order of 70 centimeters per minute.

… more water enters the roots which keeps the water flowing. Ascent of Sap http://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 a of model conceptual

liquid water liquid

vapor . leaf 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)

longwave radiation solar radiation

sensible heat

leaf

conceptual model of a stomata Diffusion or Vapor Transfer (review Lecture 9)

the boundary layer air is always saturated with water vapor which is determined by the leaf temperature

leaf

conceptual model of a stomata Variables

Tair = temperature of the air

TL = temperature of the LEAF ea = actual vapor pressure of the air esat(Tair) = saturation vapor pressure of the air esat(TL) = saturation vapor pressure of the boundary layer esat(TL) - ea is called the vapor pressure deficit Diffusion or Vapor Transfer (evaporation)

If ea < esat(TL) then molecules will diffuse from the boundary layer to the air

leaf

esat(TL)

conceptual model of a stomata Diffusion or Vapor Transfer (evaporation)

Wind sustains the deficit by sweeping moist air away from the leaf surface if ea < esat(TL) then evaporation continues

leaf

esat(TL)

conceptual model of a stomata stomata close at night

so transpiration slows down dramatically or stops at night Other environmental variables control the stomata opening size, hence the degree of transpiration.

low opening size larger opening size

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 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.

transpired water evaporation of water stored by interception

evaporation of soil water

lowers water content in soil 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).

If the soil-water content is low, stomata openings will reduce in size and hence, lower transpiration.

low water content A small sapling might only lose between 0.5 and 1 gallon per day, whereas a very large Douglas-fir tree 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 The bottom line is that ET dries out soils.

transpired water evaporation of water stored by interception

evaporation of soil water

lowers water content in soil 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 (inches) Modeled DailyET in theLakeWhatcom Watershed Oct (AET isabout20 inches) Apr Sep Modeled Monthly AET and AET Contributions in the Lake Whatcom Watershed Inches Modeled Monthly Contributions as a % of the Total AET in the Lake Whatcom Watershed Smith Creek Stream Gauge

Smith Creek Hydrograph, Sept 7--22, 2000 1.2 inches of rain in 24 hours 50

Discharge (cfs) Discharge 10

0 0 50 100 150 200 250 Time (hr)

Smith Creek Hydrograph, May 8--16, 2000

Discharge (cfs) Discharge 10

0 50 100 150 200 250 Time (hr) How does logging affect evapotranspiration and streamflow? Forested Basin

Hydrograph

Time Logged Basin

Hydrograph

Time

GROUNDWATER RESPONSE TO PRECIPITATION EVENTS, KALALOCH, OLYMPIC PENINSUALA, WASHINGTON

CASEY R. HANELL Q Q

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.