COMPUTATION OF EVAPOTRANSPIRATION BY EVAPORATION METHOD PREPARED BY

COMPUTATION OF EVAPOTRANSPIRATION BY EVAPORATION METHOD PREPARED BY S. JOTHIMANI

Evaporation methods Piche evaporimeter l Open pan evaporimeter l Sunken screen pan evaporimeter l

PICHE EVAPORIMETER GENERAL APPLICATION l The Piche evaporimeter is used to measure the rate of evaporation from a wet disc of absorbent paper. l It is used mainly in hot, dry climates where water loss through evaporation must be observed regularly. l 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. 1 ml will contain just over 30 ml of water. l 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. l

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

USE l l 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

l l l l 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 evaporimeter Used to measure the evaporation of water. l The measurement of water evaporation is important in agricultural, hydrometeorological studies, in the design and operation of reservoirs, and in irrigation and drainage systems. l The rate of evaporation is defined as the amount of water lost by evaporation from a unit area of surface in unit time. l

l l l The pan evaporimeter consists of a cylindrical reservoir made from copper sheet of fixed diameter and depth, filled with water to a few cm below the rim. A fixed-point gauge in a stilling well serves to indicate the level of water in the pan. A calibrated measuring cylinder is used to add or remove water at each observation to bring the water level to the fixed point. The cross-sectional area of the measuring cylinder is such that, the number of millimetres of water added from the measuring cylinder divided by 100 gives the amount of water in millimetres which has evaporated from the pan during a given interval of time. The reservoir is covered with wire-mesh netting to protect the pan from birds and animals. A thermometer suspended from a mounted clamp to the side of the reservoir, records the temperature of the water in the pan

Sunken screen pan evaporimeter Sharma and dastane(1968) developed the sunken screen pan evaporimeter. l The crop coefficient values in a sunken screen pan evaporimeter was found to be 0. 95 -1. 05 in newdelhi. While in a open pan evaporimeter varied from 0. 5 -1. 3 for different crops in different locations. l

l l l The evaporimeter consists of a 60 cm diameter and 45 cm deep pan made of 20 gauge galvanized iron sheet and a stilling well of 15 cm diameter and 45 cm depth attached to the pan with a tube. The pan and the well are painted white and screened at the top with 6/20 -mesh wire net. The stilling well has a pointer inside at the centre.

The pan with the well is buried in the soil with 10 cm edge over the soil surface. l The fall of water level in the well during 24 hour period is taken as the measure of the evaporation rate per day. l Further evaporation rate may be computed from the measured quantity of water added daily to bring the water level tip of the pointer. l

In order to find out the depth of water evaporated. l The amount of water added is divided by the area of pan and stilling well taken together. l Fresh water is added to bring the water level to the pointer tip level after each measurement. l

Tip of the pointer should be at level with the soil surface while installing the evaporimeter. l The pan is located in the field with no obstruction to wind movement over the pan. l