Presentation

CLIMATOLOGICAL METHODS OF ESTIMATING EVAPOTRANSPIRATION BY RADIATION METHOD

Muhammad Faisal Abbas 2013-ag-4405 B.sc Agri. Engg (A1)

University of Agriculture, Faisalabad

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.

EVAPOTRANSPIRATION

(ET) is a term used to describe the sum of

evaporation and plant transpiration from the

earth's land surface to atmosphere.

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.

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

Factors affecting evapotranspiration

Weather parameters

Crop factors

Management and environmental conditions

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.

METEOROLOGICAL FACTORS DETERMINING ET

Solar radiation

Air temperature

Air humidity

Wind speed

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

ESTIMATION OF ET0

It is estimated fromET0=c (W – Rs)

WhereET0=reference crop evapotranspiration,

mm/dayRs =solar radiation at the ground level, mm/day

W = weighing factor

c = adjustment factor

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.

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.

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.

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

. 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.

. 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.

. The radiation method is considered

superior to Blaney–Criddle method.

It has proved valuable particularly in humid regions.