COMPUTATION OF COMPUTATION OF EVAPOTRANSPIRATION BY EVAPOTRANSPIRATION BY

EVAPORATION METHODEVAPORATION METHOD

PREPARED BYPREPARED BYS.JOTHIMANIS.JOTHIMANI

Evaporation methodsEvaporation methods

Piche evaporimeterPiche evaporimeterOpen pan evaporimeterOpen pan evaporimeterSunken screen pan evaporimeterSunken screen pan evaporimeter

PICHE EVAPORIMETER

GENERAL APPLICATION The Piche evaporimeter is used to measure the

rate of evaporation from a wet disc of absorbent paper.

It is used mainly in hot, dry climates where water loss through evaporation must be observed regularly.

Since the results are dependent on wind speed past the disc, as well as the wet bulb saturation deficit, it is almost essential to expose the evaporimeter inside a meteorological screen

CONSTRUCTION

A glass tube graduated in steps of 0.1ml will contain just over 30ml of water.

A metal clip holds the paper disk against the open end, while a ring at the closed end allows the tube to be hung vertically.

INSTALLATION

The evaporimeter is best suspended from a stand available from a hook fixed to the roof of a meteorological screen

USE

Hold the tube vertical, closed end downwards, and fill completely with clean distilled water.

Slide the clip over the end and rotate it until the disc is central with the tube but not quite touching it. With clean, grease-free, fingers or tweezers if available, insert a new disc between the tube and the clip.

Push down the clip to grip the paper firmly. It is recommended for even wetting of the disk

that a pinhole should be made in the centre of the paper disk through the metal disk

Invert the tube and hang up on the stand. Wait a few minutes until the disc is thoroughly moist and

the flow of air bubbles up the tube has practically ceased.

Read the water level and record it alongside the other meteorological data.

At the next observing hour, or more commonly 24 hours later, observe the new water level reading.

If the level is less than half way remove the evaporimeter and refill the tube.

If the paper disc is damaged or is covered with sand or dust, remove it.

The rate of evaporation is usually expressed as the volume of water evaporated per unit area in unit time

Open pan evaporimeterOpen pan evaporimeter

Used to measure the evaporation of water.Used to measure the evaporation of water.The measurement of water evaporation is The measurement of water evaporation is

important in agricultural, hydro-important in agricultural, hydro-meteorological studies, in the design and meteorological studies, in the design and operation of reservoirs, and in irrigation operation of reservoirs, and in irrigation and drainage systems. and drainage systems.

The rate of evaporation is defined as the The rate of evaporation is defined as the amount of water lost by evaporation from a amount of water lost by evaporation from a unit area of surface in unit time. unit area of surface in unit time.

The pan evaporimeter consists of a cylindrical reservoir The pan evaporimeter consists of a cylindrical reservoir made from copper sheet of fixed diameter and depth, made from copper sheet of fixed diameter and depth, filled with water to a few cm below the rim.filled with water to a few cm below the rim.

A fixed-point gauge in a stilling well serves to indicate A fixed-point gauge in a stilling well serves to indicate the level of water in the pan. the level of water in the pan.

A calibrated measuring cylinder is used to add or remove A calibrated measuring cylinder is used to add or remove water at each observation to bring the water level to the water at each observation to bring the water level to the fixed point. fixed point.

The cross-sectional area of the measuring cylinder is The cross-sectional area of the measuring cylinder is such that, the number of millimetres of water added from such that, the number of millimetres of water added from the measuring cylinder divided by 100 gives the amount the measuring cylinder divided by 100 gives the amount of water in millimetres which has evaporated from the of water in millimetres which has evaporated from the pan during a given interval of time. pan during a given interval of time.

The reservoir is covered with wire-mesh netting to The reservoir is covered with wire-mesh netting to protect the pan from birds and animals. protect the pan from birds and animals.

A thermometer suspended from a mounted clamp to the A thermometer suspended from a mounted clamp to the side of the reservoir, records the temperature of the side of the reservoir, records the temperature of the water in the pan water in the pan

Sunken screen pan evaporimeterSunken screen pan evaporimeter

Sharma and dastane(1968) developed the Sharma and dastane(1968) developed the sunken screen pan evaporimeter.sunken screen pan evaporimeter.

The crop coefficient values in a sunken The crop coefficient values in a sunken screen pan evaporimeter was found to be screen pan evaporimeter was found to be 0.95-1.05 in newdelhi. While in a open pan 0.95-1.05 in newdelhi. While in a open pan evaporimeter varied from 0.5-1.3 for evaporimeter varied from 0.5-1.3 for different crops in different locations. different crops in different locations.

The evaporimeter consists of a 60 cm diameter The evaporimeter consists of a 60 cm diameter and 45 cm deep pan made of 20 gauge and 45 cm deep pan made of 20 gauge galvanized iron sheet and a stilling well of 15 cm galvanized iron sheet and a stilling well of 15 cm diameter and 45 cm depth attached to the pan diameter and 45 cm depth attached to the pan with a tube.with a tube.

The pan and the well are painted white and The pan and the well are painted white and screened at the top with 6/20-mesh wire net.screened at the top with 6/20-mesh wire net.

The stilling well has a pointer inside at the The stilling well has a pointer inside at the centre. centre.

The pan with the well is buried in the soil The pan with the well is buried in the soil with 10 cm edge over the soil surface.with 10 cm edge over the soil surface.

The fall of water level in the well during 24 The fall of water level in the well during 24 hour period is taken as the measure of the hour period is taken as the measure of the evaporation rate per day .evaporation rate per day .

Further evaporation rate may be computed Further evaporation rate may be computed from the measured quantity of water from the measured quantity of water added daily to bring the water level tip of added daily to bring the water level tip of the pointer.the pointer.

In order to find out the depth of water In order to find out the depth of water evaporated.evaporated.

The amount of water added is divided by The amount of water added is divided by the area of pan and stilling well taken the area of pan and stilling well taken together.together.

Fresh water is added to bring the water Fresh water is added to bring the water level to the pointer tip level after each level to the pointer tip level after each measurement.measurement.

Tip of the pointer should be at level with Tip of the pointer should be at level with the soil surface while installing the the soil surface while installing the evaporimeter.evaporimeter.

The pan is located in the field with no The pan is located in the field with no obstruction to wind movement over the obstruction to wind movement over the pan.pan.