QUIZ EVAPOTRANSPIRATION Flashcards
-the transfer of water into the atmosphere
-occurs when the number of moving molecules that break from the water surface and escape into the air as vapor is
larger than the number that reenter the water surface from the air and become entrapped in the liquid
-The motion of the escaping molecules produces a pressure called _______________
-EVAPORATION (E)
-vapor pressure
Under _________________, the surrounding air is at
saturation vapor pressure and the number of escaping
molecules equals that of returning molecules.
equilibrium condition
the driving force for evaporation is the _____________or ___________ between the water surface and its surrounding air.
vapor pressure difference or deficit
This deficit is influenced mostly by ______________and incident_____________.
-wind movement
- solar radiation
The rate of evaporation is _____________________ to the
difference between the vapor pressure of the water mass and that of the air above the water surface
directly proportional
process by which moisture is
carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere
TRANSPIRATION (T)
The passage of water vapor and other gases occurs through tiny openings called __________ which may number up to 100,000 per cm2 leaf area
stomates
From______________ of the water extracted by plant roots
is used in transpiration.
The annual amount of water loss through transpiration may vary from __________meters
depending upon soil, hydrologic and climatological
conditions as well as type of crop and density of cover.
-95 to 99 %
- 0.50 to 2.0
combination of evaporation
from the soil surface and
transpiration from plants
E + T = ET
EVAPOTRANSPIRATION (ET)
occur simultaneously and there is no easy way of
distinguishing between the two processes
EVAPOTRANSPIRATION (ET)
When the crop is small, water is predominately lost by
____________, but once the crop is well developed and
completely covers the soil, _______________ becomes the
main process.
-soil evaporation
-transpiration
_________________ affecting evapotranspiration are the following:
- radiation
- air temperature
- humidity
- wind speed
Weather parameters
The crop type, variety and development stage should be
considered when assessing the evapotranspiration from
crops grown in large, well managed fields
Differences in resistance to transpiration, crop height, crop
roughness, reflection, ground cover and crop rooting
characteristics result in different ET levels in different types of crops under identical environmental conditions.
Crop characteristics
Factors such as the following:
- soil salinity
- poor land fertility
- limited application of fertilizers
- absence of control of diseases and pests
- poor soil management may limit the crop development and reduce the ET
Management and environmental conditions
Other factors to be considered when assessing ET are the
following:
- ground cover
- plant density
- soil water content
The effect of soil water content on ET is conditioned primarily by the magnitude of the __________ and the_________.
- water deficit
- type of soil
introduced in the late 1940s and 50s by Penman
defined as “the amount of water transpired in a given time by a short green crop, completely shading the ground, of uniform height and with adequate water status in the soil profile”
defined as “the water loss which will occur if at no time
there is a deficiency of water in the soil for use by vegetation” - by Thornthwaite (1943)
POTENTIAL ET (PET)
introduced in 1970s to 80s by engineers and researchers
defined as the rate of ET from a hypothetical reference crop
with an assumed crop height of 0.12 m (4.72 in), a fixed surface resistance of 70 sec/m and an albedo of 0.23, closely resembling the ET from an extensive surface of green grass of uniform height, actively growing, well-watered, and completely shading the ground
used to study the evaporative demand of the atmosphere independently of crop type, crop development and management practices
REFERENCE CROP ET (ETo)
also known as the crop water requirements (CWR) or
consumptive use (CU)
CWR is defined as “the depth of water needed to meet the
water loss through evapotranspiration (ETc) of a
disease-free crop, growing in large fields” (FAO, 1977)
ETo x kc
ACTUAL CROP ET (AET or ETc or ETa)
-the value of the crop coefficient (kc) varies with the
development stages of the crop
Information required on crops:
-Date of sowing
-Length of the total growing season
-Duration of the initial stage (germination to 10% ground cover)
-Duration of crop development stage (from 10% to 80% ground cover)
-Duration of the mid season stage (from 80% ground cover to start of ripening)
-Duration of the late season stage (from start of ripening to harvest)
Crop Coefficient (kc)
-recognizes that water moves away from evaporating
and transpiring surfaces in response to the combined
phenomenon of turbulent mixing of the air and the
vapor pressure gradient
-requires measurement of wind velocity, humidity at two or more elevations
Mass transfer
-heat is required for evaporation of water
-if there is no change in water temperature , the net radiation or heat supplied is a measure of evaporation
Energy balance
-methods developed from experience and field research
-based primarily on the assumption that the energy
available for evaporation is proportional to the temperature
Empirical methods
-tank and lysimeter experiments
-field experiment plots (where the quantity of water applied is kept small to avoid deep percolation)
-soil water studies (large number of samples are taken at various depths in the rootzone)
-analysis of climatological data
-inflow-outflow method for large areas (where yearly
inflow into the area, annual precipitation, yearly outflow from the area and change in ground water level
are evaluated)
Other methods to determine ET
Type of lysimeters:
used in areas where high water table normally exist
1.Non-weighing, constant water table type
Type of lysimeters:
– used in areas of high
precipitation
2.Non-weighing, percolation type
Type of lysimeters:
– provide the most accurate data for short time periods
3.Weighing type
where:
PET
= reference crop potential evapotranspiration (mm/day)
H = daily heat budget at surface (mm/day)
Ea = evaporation due to mass transport (mm/day)
Δ = slope of the saturation vapor pressure curve of air at absolute temperature (mm Hg/ °F)
γ= psychrometric constant = 0.27
Rs = mean solar radiation incident upon the surface (mm/day)
u2 = mean windspeed at 2 meter height (miles/day)
es = saturation vapor pressure (mm Hg)
ea = actual vapor pressure (mm Hg)
r = albedo or reflection coefficient of the surface involved, 0.25 for vegetal cover and 0.05 for free surface
n/N = percent possible sunshine
σ = Stefan-Boltzman constant = 1.9838 x 10-9 mm/day °K4
Penman Equation
Values of a and b that have been established for various
regions in the Philippines
Incident solar radiation (Rs)
Rs = (a + b n/N) Ro
