Estimating ET

1
Estimating ET Type of method used will be
determined by 1. Type of surface (e.g. open
water vs. leaf) 2. Availability of water for
evaporation 3. Value of )Q/)t 4. Value of Aw 5.
Amount of air-advected energy Free-water and
lake evaporation - start with simplest situation,
evaporation from an open water body - generally
not possible to generate evaporation equations
for a lake using meteorological data alone
because Aw and )Q/)t vary significantly from lake
to lake -annual values exceptions in lakes with
residence time lt 1 year - theoretical concept of
free-water evaporation (FWE) developed assumes
no advection or changes in heat storage - depends
only on overlying climate actual lake
evaporation can be calculated from FWE for
particular lakes
2
Water Balance Approach E W SWin GWin -
SWout - GWout - ?V W is precip on lake SW is
surface water GW is ground water ?V is lake
storage change during the period of
consideration - in theory simple, in practice
not easy
3
Energy-Balance Approach or, by inserting the
Bowen ratio The energy balance approach has
many of the same detractions as the water-balance
approach, but with the energy balance, at least
we can a) eliminate some variables, and
associated error b) use regional climatic data to
estimate some of the radiation components
4
Shortwave Radiation KKin(1-a) Kin is incoming
SW radiation, and a is the albedo of the
surface Kin is the amount of SW radiation hitting
the ground. It is conceptually thought of
as ? latitude J day of year ß slope a
aspect C fraction of sky covered by clouds F
fraction of sky covered by forest canopy Kin
can be measured directly using pyranometers, or
an empirical relation can be used albedo can
also be directly measured, or a constant assumed
depending on the surface type. For water, a
typically ranges between 0.05 to 0.10, but with
low solar angles can be very high (up to 0.6)
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Long-Wave Radiation Net long-wave rad is equal
to the LW flux coming in from the atmosphere,
minus the amount reflected from the surface and
the amount radiated from the surface where ew
emissivity of the water surface eat
effective emmisivity of the atmosphere s the
Stefan-Boltzman constant (1.17x10-7
cal/cm2/day/K4) temperatures are in Celcius eat
is a function of humidity and cloud cover and can
be estimated as ea is in mb
6
Conduction to the Ground In the case of lakes
can be considered negligible Water-Advected
Energy cw is specific heat of water w is
average precipitation rate SW and GW are surface
and groundwater inflows and outflows (in volume
per time per unit area) temperatures are in
celcius
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Change in Stored energy V is lake volume T
is average lake (reservoir) temperature subscripts
1 and 2 are values at start and end of time
period Bowen Ratio Use of the Bowen ratio has
the main advantage that it eliminates the need
for measuring wind speed Summary of Energy
Balance Approach - suffers some of the same
problems as water balance approach - better
suited to longer (7 days ) time periods to get a
maximum accuracy of 5
8

• Others
• Combination (Penman) Approach
• Mass Transfer Approach
• Read in handout

9
Pan-Evaporation Approach - simple concept...set
a bathtub out an measure the water loss
EW-(V2-V1) W precip V storage loss
measured to high precision
10
Pans generally over-estimate real evaporation and
coefficients are applied for correction so that
daily free-water evaporation (mm/day) can be
calculated as apan pan evaporation
(mm/day) P atmospheric pressure (mb) vpan is
mean wind speed 15 cm above the pan
(km/day) Tspan is water surface temperature
(EC) is plus when Tspan gt Ta and minus when
Tspan lt Ta "apan is a factor to account for
energy exchange at the edge of the pan (see
equation 7-40 in text) Corrections to the
evaporative loss are only needed for calculating
short-term (e.g. daily) values since the errors
cancel out over the long term (e.g. annually)
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Moving to the more complex...ET When we consider
ET, the complications of interactions with
vegetative cover is added Transpiration and
Interception Loss - Transpiration is evaporation
from the vascular system of plants into the
atmosphere - transpiration is a physical process
(not metabolic) driven by water content
gradients - interception loss is that precip that
is caught by the canopy and evaporated directly
to the atmosphere
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Potential ET Potential Evaporation (PET) is the
rate of ET that occurs under the prevailing solar
inputs and atmospheric properties, if the surface
is fully wet. Actual ET is the amount really
removed. PET is a theoretical concept that
defines the drying power of the climate or
local meteorological conditions, but actual ET is
affected by 1. albedo 2. Maximum leaf
conductance 3. atmospheric conductance 4.
presence or absence of intercepted water
Methods for estimating PET are classed as 1.
Temperature-based use only air
temp Hammond PETH is in cm/day D is the
length of the day in hours ?vsat(Ta) is the
saturation absolute humidity at the mean daily
temperature in g/m3
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2. Radiation-based use net rad and air temp
and pressure Priestley and Taylor recall
that gamma is the psychrometric constant 3.
Combination based on Penman-Monteith Equation -
this equation was derived from the early
combination (Penman) equation, expressed in terms
of conductance, adding terms for water vapor
conductance of the atmosphere and vegetative
canopy. Read in Dingman Chapter 4. Pan pan
evaporation often very similar to short
vegetation ET
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Direct Measurement of ET Water-balance
approaches Lysimeters -Artificially enclosed
volumes of soil that have a representative
vegetative cover, outflows and inflows of water
can be measured, and changes in storage can be
measured by weighing -accurate for low-lying
vegetation, but very difficult for large (e.g.
forest vegetation) Soil moisture Balance - Total
ET is monitored by precise measurement of
rainfall and soil water content throughout the
root zone - a lysimeter without walls - can be
useful for larger vegetation an is more natural
than lysimeters, however obtaining accurate soil
moisture profiles is difficult
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• Land-Area Water Balance
• ETW-Q-Gout
• - Problems in accurate assessment of components
  and ensuring storage change is negligable
• error associated with storage change is minimal
  when mostly soil water involved
• Review Dingmans evaporatranspiration chapters
  section on Turbulent Transfer Methods