To find ETO by Radiation Method

1. CLIMATOLOGICAL METHODS OF ESTIMATING
EVAPOTRANSPIRATION BY RADIATION
METHOD
 Muhammad Faisal Abbas
 2013-ag-4405
 B.sc Agri. Engg (A1)
 University of Agriculture, Faisalabad

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.  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.
WHY IT SHOULD BE MEASURED

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
ET0=c (W – Rs)
Where
ET0=reference 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.  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.
EMPIRICAL METHODOLOGY FOR ISLAND LOCATIONS

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. .
 The radiation method is considered superior to
Blaney–Criddle method.
 It has proved valuable particularly in humid
regions.