CLIMATOLOGICAL METHODS OF ESTIMATING EVAPOTRANSPIRATION BY RADIATION METHOD

1
CLIMATOLOGICAL METHODS OF ESTIMATING
EVAPOTRANSPIRATION BY RADIATION METHOD
2
EVAPOTRANSPIRATION

• Evaporation accounts for the movement of water to
  the air from sources such as the soil, canopy
  interception, and water bodies.
• Transpiration accounts for the movement of water
  within a plant and the subsequent loss of water
  as vapour through stomata in its leaves.

3
EVAPOTRANSPIRATION

• (ET) is a term used to describe the sum of
• evaporation and plant transpiration from
  the
• earth's land surface to atmosphere.

4
UNITS OF ET

• It is normally expressed in millimeters (mm) per
  unit time.
• The rate expresses the amount of water lost from
  a cropped surface in units of water depth.
• The time unit can be an hour, day, decade, month
  or even an entire growing period or year.

5
WHY IT SHOULD BE MEASURED

• Evapotranspiration is one of most important
  factors from the agricultural engineering point
  of view.
• In order to plan the proper irrigation scheduling
  at the upland field, to quantify the soil water
  consumption accurately by evapotranspiration is
  prerequisite.
• Evapotranspiration is an important part of the
  water cycle.

6
Factors affecting evapotranspiration

• Weather parameters
• Crop factors
• Management and environmental conditions

7
ET COMPUTED FROM METEOROLOGICAL DATA

• Owing to the difficulty of obtaining accurate
  field measurements, ET is commonly computed from
  weather data.
• A large number of empirical or semi-empirical
  equations have been developed for assessing crop
  or reference crop evapotranspiration from
  meteorological data.

8
METEOROLOGICAL FACTORS DETERMINING ET

• Solar radiation
• Air temperature
• Air humidity
• Wind speed

9
RADIATION METHOD

• It is developed by Makkink in 1957.
• Recommended where weather data is not sufficient
  to use penman method.
• ESSENTIAL CLIMATIC DATA
• 1.Air temperature
• 2.Sunshine or radiation

10
ESTIMATION OF ET0

• It is estimated from
• ET0c (W Rs)
• Where
• ET0reference crop evapotranspiration, mm/day
• Rs solar radiation at the ground level, mm/day
• W weighing factor
• c adjustment factor

11
HARGREAVES RADIATION FORMULA

• Solar Radiation data derived from air temperature
  differences

where Ra extraterrestrial radiation,Tmax
maximum air temperature,Tmin minimum air
temperature,kRs adjustment coefficient.
12
SOLAR OR SHORTWAVE RADIATION (RS)

• As the radiation penetrates the atmosphere, some
  of the radiation is scattered, reflected or
  absorbed by the atmospheric gases, clouds and
  dust.
• The amount of radiation reaching a horizontal
  plane is known as the solar radiation, Rs.
• Because the sun emits energy by means of
  electromagnetic waves characterized by short
  wavelengths, solar radiation is also referred to
  as shortwave radiation.

13
EXTRATERRESTRIAL RADIATION (Ra)

• The radiation striking a surface perpendicular to
  the sun's rays at the top of the earth's
  atmosphere, called the solar constant, is about
  0.082 MJ m-2 min-1.
• The local intensity of radiation is, however,
  determined by the angle between the direction of
  the sun's rays and the normal to the surface of
  the atmosphere.
• This angle will change during the day and will be
  different at different latitudes and in different
  seasons.
• The solar radiation received at the top of the
  earth's atmosphere on a horizontal surface is
  called the extraterrestrial (solar) radiation,
  Ra.

14

EMPIRICAL METHODOLOGY FOR ISLAND LOCATIONS

• For island locations, where the land mass has a
  width perpendicular to the coastline of 20 km or
  less, the air masses influencing the atmospheric
  conditions are dominated by the adjacent water
  body in all directions.
• The temperature method is not appropriate for
  this situation.

15
.

• Where radiation data from another location on the
  island are not available, a first estimate of the
  monthly solar average can be obtained from the
  empirical relation
• Rs 0.7 Ra - b (51)
• Where
• Rs solar radiation MJ m-2 day-1,Ra
  extraterrestrial radiation MJ m-2 day-1,b
  empirical constant, equal to 4 MJ m-2 day-1.

16
.

• This relationship is only applicable for low
  altitudes (from 0 to 100 m).
• The empirical constant represents the fact that
  in island locations some clouds are usually
  present, thus making the mean solar radiation 4
  MJ m-2 day-1 below the nearly clear sky envelope
  (0.7 Ra).
• Local adjustment of the empirical constant may
  improve the estimation.
• The method is only appropriate for monthly
  calculations. The constant relation between Rs
  and Ra does not yield accurate daily estimates.

17
FOR KUMULUR

• CALCULATION OF Rs
• Tmax
• Ra
• Tmin
• kRs

18
ESTIMATION OF ET0

• C
• W
• Rs
• Then
• ET0 c (W Rs)

19
.

• The radiation method is considered superior to
  BlaneyCriddle method.
• It has proved valuable particularly in humid
  regions.

20
By M.Lavanya