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COMPONENTS OF
EVAPOTRANSPIRATION
THE WATERSHED ANSWERS BACK
INTERCEPTIONINTERCEPTION
INTERCEPTION = CANOPY INTERCEPTION + LITTER INTERCEPTION ( SOME INTERCEPTION OCCURS ON PAVEMENT AND OTHER HARD SURFACES BUT IS INSIGNIFICANT IN MOST ECOSYSTEMS ). INTERCEPTION BECOMES EVAPORATION
PRECIPITATION
LITTER INTERCEPTION
CANOPY
INTERCEPTION
INFILTRATION
STEMFLOW THROUGHFALL
Gross Rainfall
Throughfall Stem- flow Throughfall
Throughfall
Interception Evaporation
Infiltration
Interception Evaporation
Interception Evaporation
Litter
Three ways of thinking about the results of interception
Pn = P g – I
Pn = P g – (Ic + I l)
Pn = (Th + Sf – I)
P n = net rainfall entering the soil (infiltration)
P g = gross rainfall
I = total interception from canopy and litter
I c = canopy interception
I l = litter interception
Th = throughfall
S f = stemflow BLACK
CHERRY
COTTONWOOD
STEMFLOW
(WHICH
TREE HAS
MORE?)
Soil moisture
Water plus nutrients and other ions and hormones
Root zone
HAWTHORN
RED OAK
WHITE PINE
Less interception and stemflow in winter
WEEPING WILLOW
- A PHREATOPHYTE
“Know, Know Your Oaks”
Sung to the tune of “Row, Row, Row
Your Boat.”
(hand motions for the song are in
blue)
Know, know, know your oaks (tap
your temple with your finger)
This is how they grow— (palms up,
arms out to side)
Red Oaks (hands straight
up in the air)
White Oaks (hands still up
high, but at your sides)
Pin Oaks (hands straight
out to your sides),
Bur Oaks (hands twisted
around in strange,
uncomfortable position),
And acorns down below,
hey!
Percent of gross rainfall available for
infiltration over a 12 month growing season
Throughfall and stemflow interceptors
Stemflow
collectors
STEMFLOW
METERS
Litter interception
The distribution and production of lichens is directly related to stemflow and the nutrients carried in stemflow – “moss” is not always on the north side of the tree!
Foliose lichens
Fruiticose lichens
Crustose lichens
Interception by selected crops – from USDA Soil Conservation Service
Tree hole stemflow habitats
Evaporation and Soil Water
Storage (^) ET = P – Q - S - l
ET = evapo+transpiration
P = precipitation
Q = streamflow
S = change in storage
(= S 2 – S 1 )
l = deep storage or
seepage in and out (lo - li )
ET = E + T
Note: except for minor exceptions, water (and nutrients) cannot be taken up by leaves.
Transpiration: evaporation at the leaf surface
(stomata) causes water to be pulled upward via
hydrogen bonding – vapor pressure gradient.
Max transpiration on warm, sunny, windy day.
Sun drives photosynthesis and wind alters vapor
pressure gradient. (no transpiration at night)
C 3 plant
C 4 plant
CAM plant
•C 3 plants are more efficient under cool and moist conditions than C (^4) and CAM plants. •No specialized anatomy
•Most plants are C 3.
PET (POTENTIAL
EVAPOTRANSPIRATION) VS
AET (ACTUAL EVAPOTRANSPIRATION)
PET methods
Temperature
Thornthwaite
PET = 1.6 (10Ta/I) a
Hammond (Jensen, Blaney-Criddle)
PET = 5.0ps
Temperature + radiation
Penman (Pierce)
PET = Rn+Ea/(+)L
ET = (PET)f(AW/AWC)
Where
AW = available soil water (mm)
= (soil moisture content –
permanent wilting point (-16 bars))
x rooting depth
AWC = available water
capacity (mm) = (field capacity –
permanent wilting point) x rooting
depth of mature vegetation
FIELD CAPACITY can be has been defined in several ways: the maximum water holding capacity of the soil, above which all excess water drains or overflows; an upper limit of water available for transpiration; a water potential of -33 kPa (-1/3 bar); a water potential generally between -10 and -33 kPa (kilopascals (1 kPa ≡ 1000 Pa) depending on soil texture and other properties; when drainage becomes negligible after thorough wetting; 2 or 3 days after a thorough wetting at specified depth at a specified time with drainage from saturation; a specific value of drainage rate, such as 2 mm/d; or a specific value of hydraulic conductivity;
MEASURING PET
CLASS A PAN
EVAPORATORS
EVAPO-
TRANSPIRATION
GAGE
ET Gage A ceramic evaporator at the top of the instrument responds to sun and weather as plants do. Water is drawn from a reservoir. The water level falls in the sight tube one inch for each inch used by plants. Replaceable green canvas covers modify the evaporation rate to simulate ET from field crops or grass. Rain cannot get into the instrument. The rain gauge provided with the meter measures rain separately.
AUTOMATED CLASS A PAN
EVAPORATOR
Level logger
Class A Evaporation Pans
Schematic drawing of a weighing lysimeter
Irrigation
(tipping
bucket)
Change in
soil moisture
(weight, soil
block)
Transpiration
(weight)
SAPFLOW METER
PINE TO CEDAR
HIGH ET LOWER ET
Estivant Pines
Cedar “swamp” typical of those now located near the Estivant Pines
Mean annual evaporation estimated from pans (inches)
PPT is ~ 60 in giving us a surplus of ~ 10 in
BASED ON
PAN DATA
